NITROGEN UPTAKE AND LEACHING POTENTIAL OF
POLYMER-COATED N FERTILIZERS IN
A TURFGRASS ENVIRONMENT
AMIR A. VARSHOVI
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
I wish to express my sincere appreciation to Dr. Jerry B. Sartain# the chairman of my supervisory committee/ for his continued support and friendship during my graduate study. The opportunity he gave me to pursue the graduate degree while working for him is greatly appreciated.
Sincere appreciation is also extended to Dr. L.B. (Bert) McCarty for his friendship/ encouragement/ and support. I would further like to thank the other members of my supervisory committee/ Drs. Donald A. GraetZ/ Brian L. McNeal/ and W. Lamar Miller/ for their teaching/ advice# support/ and friendship. I would also like to thank Dr. Jack L. Fry# Dean of the College of Agriculture, for his assistance and encouragement throughout the past several years.
Appreciation and sincere thanks are forwarded to Ms. Maria Buchanan for her endless help throughout this study during the past four years and for her friendship.
I would also like to thank Brian Owens, Martin Sandquist# and Shawn Rogers for their assistance in the field/ and Bill Pothier for 15N analysis.
A final thank-you goes to my wife/ Nahid/ and to our sons, Alex and Zak/ who endured this journey and sacrificed their time while supporting me to its completion.
TABLE OF CONTENTS
LIST OF TABLES.................... V
LIST OF FIGURES................... viii
ABSTRACT ........................ X
CHAPTER 1: INTRODUCTION ............... 1
CHAPTER 2: LITERATURE REVIEW ............. 4
Fate of Nitrogen in a Turfgrass Environment .................. 4
Nitrogen Transformations in the Soil 4
Nitrogen Uptake ............ 9
Leaching of Applied Nitrogen ...... 11
Other Losses of Nitrogen........ 19
USGA Golf Green Profile........ 19
Nitrogen Fertilizers .......... 22
Fertilizer Recovery Methods ...... 28
Ceramic Cup Samplers....... 29
Difference Method ........ 30
15N Tracer Method......... 31
CHAPTER 3: NITROGEN RELEASE RATE OF REACTIVE-LAYER COATED
UREA, AMMONIUM SULFATE, AND POTASSIUM
Introduction .............. 40
Materials and Methods......... 41
Results and Discussion ...... 43
Summary and Conclusion ......... 55
CHAPTER 4: THE EFFECT OF REACTIVE-LAYER COATED UREA
AND AMMONIUM SULFATE ON GROWTH AND N UPTAKE
OF BERMUDAGRASS.............. 56
Materials and Methods ......... 57
Results and Discussion ......... 58
Summary and Conclusion ......... 74
CHAPTER 5: UPTAKE AND LEACHING OF REACTIVE-LAYER
COATED 15N AMMONIUM SULFATE AND UREA
Materials and Methods ......
Results and Discussion ......
Summary and Conclusion ......
CHAPTER 6: THE FATE OF APPLIED NITROGEN IN A USGA
GOLF GREEN .............
Materials and Methods ......
and Discussion ......
Summary and Conclusion ......
CHAPTER 7: SUMMARY AND CONCLUSION .......
APPENDIX: WEATHER DATA ............
REFERENCE LIST .................
BIOGRAPHICAL SKETCH ...............
LIST OF TABLES
Fertilizer material specifications .
Nitrogen treatment specifications (1992) .............
Cumulative growth of bermudagrass following N application (1992)
Cumulative N uptake by bermudagrass following N application (1992) .
Visual quality rating of bermudagrass
following N application (1992)
Nitrogen treatment specifications (1993) ............
Cumulative growth of bermudagrass following N application (1993) .
Cumulative N uptake by bermudagrass following N application (1993) .
Visual quality rating of bermudagrass following N application (1993) .
Selected soil chemical and physical properties of the bermudagrass site
Total N and C:N ratios in the bermuda-
grass root-zone soil
Selected soil chemical properties of the bermudagrass root-zone soil .
Nitrogen treatment specifications
Growth of Tifway bermudagrass following N application (1992)
Nitrogen uptake calculated using the tracer and difference methods .
Percent recovery of applied 15N
labeled N after 90 days (1992)..... 104
Growth of Tifway bermudagrass following N application (1993) .
Nitrogen uptake by Tifway bermudagrass following N application (1993) .
Percent recovery of applied 15N labeled N (1993) ......
Particle size analysis of root-zone
Physical properties of root-zone mix
Nitrogen treatment specifications (1992) ............
Growth of Tifgreen bermudagrass
following each N application (1992)
Root length, and root and stolon dry weight (1992) ............
Average N concentration of Tifgreen Bermudagrass following each N application (1992) ........
Nitrogen uptake by Tifgreen bermudagrass following each N application (1992) .
Nitrogen uptake in stolons and
roots (1992).............. 141
Visual quality rating of Tifgreen bermudagrass following each N application (1992) ......
Nitrate leached to the 4 6cm depth after each N application (1992) ......
Ratio of N uptake to N leaching
Growth of Tifgreen bermudagrass following each N application (1993) ......
Nitrogen uptake by Tifgreen bermudagrass following each N application (1993) .
Table 6-14. Table 6-15.
Stolon and root dry weight, and root length (1993) ...........
Nitrogen uptake in roots and stolons
Average N concentration of Tifgreen bermudagrass following each N application (1993) ........
Visual Quality Rating of Tifgreen bermudagrass following each N application (1993) .......
Figure 2-1 Figure 3-1 Figure 3-2 Figure 3-3
LIST OF FIGURES
Nitrogen cycle in a turfgrass ecosystem
Reactive-layer coating process.....
Effect of temperature on % N release .
Effect of coating thickness and temperature on N release from CAS: macro-size ...............
Effect of coating thickness and temperature on N release from CAS: micro-size ...............
Effect of coating thickness and temperature on N release from C Urea: macro-size ...............
Effect of coating thickness and teuton N release from C Urea: micro-size ...............
Effect of coating thickness and temperature on N release of CKN03: micro-size ...............
Open-end lysimeter with multi-depth sampler ................
Effect of N substrate and coating thickness on N03-N concentrations at the 30cm depth ................
Effect of N substrate and coating thickness on N03-N concentrations at the 90cm depth................
Effect of N substrate and coating thickness on N03-N concentrations at the 12 0cm depth...........
Close-end lysimeter packed with a USGA golf green profile .........
Concentrations of N03-N at the 20cm and 46cm depths: AS treatment group
Concentrations of N03-N at the 20cm and 46cm depths: urea treatment group .
Concentrations of N03-N at the 2 0cm and 4 6cm depths: KN03 treatment group .
Nitrogen transformation of applied AS and Urea ...............
Nitrogen uptake and leaching (1992): AS treatment group .
Nitrogen uptake and leaching (1992): urea treatment group
Nitrogen uptake and leaching
(1992) : KN03 treatment group
Nitrogen uptake and leaching
(1993) : AS treatment group .
Nitrogen uptake and leaching (1993): urea treatment group
Nitrogen uptake and leaching (1993): KN03 treatment group
Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
NITROGEN UPTAKE AND LEACHING POTENTIAL OF
POLYMER-COATED N FERTILIZERS IN
A TURFGRASS ENVIRONMENT
Amir Varshovi August 1995
Chairperson: Jerry B. Sartain
Major Department: Soil And Water Science
Establishing and maintaining quality turfgrass requires nitrogen (N) fertilization. However, potential leaching losses of N present environmental and economic concerns. Nitrogen losses deprive turfgrass of an essential nutrient and serve as a potential pollutant to surface and ground waters. Studies were conducted to investigate the effect of polymer-coated (NH4)2S04 (AS) urea, and KN03, with varied coating thicknesses and sizes, on uptake and leaching of applied N to bermudagrass (Cvnodon dactylon) grown on sandy soil and the factors effecting the N release.
A laboratory study was conducted to investigate the effect of temperature, coating thickness, and N substrate on the release of N from the encapsulated fertilizers into
deionized H20. An increase in temperature from 25 C to 35C
increased N released from 30% to 56% for coated AS with 4.7% coating (CAS-4.7), and from 70% to 78% for coated urea with 4.8% coating (CUrea-4.8). Data suggest a differential increase in solubility of the N substrates. Increases in coating thickness reduced N release differently, depending on N substrate and granule size.
In a field study, using open-ended lysimeters equipped with multi-depth samplers, the N uptake and leaching of uncoated and polymer-coated 15N AS and urea applied to bermudagrass growing on sandy soil were determined. Nitrogen uptake recovery using an 15N tracer method and a difference method increased as a result of coating, except for CAS-9.8. Coated Urea-10 produced the highest N uptake, followed by CUrea-4.8 and CAS-4.7. Nitrate-N concentrations in the intermediate vadose zone (IVZ) at the 30 cm depth, and the saturated zone at 90 and 120 cm, were affected by applied N, coating and coating thickness.
In another lysimeter study, the N sources were applied to lysimeters packed with a sand-based golf green profile. Nitrogen uptake and N leached to the 4 6 cm depth were determined. Nitrogen uptake ranged from 2 6% (KN03) to 69% (CAS-
7.5). Nitrogen leached from the CAS and CUrea treatments ranged from 0.3% (CAS-15) to 7.7% (CAS-7.5) of applied N.
This stud/ documents the effectiveness of the polymer-coated N sources in minimizing N leaching and increasing N uptake in a turfgrass environment.
CHAPTER 1 INTRODUCTION
The turfgrass industry in the United States is a multi-billion dollar industry that is in demand of high quality turfgrass. Nitrogen (N) is one of the most important nutrients in turfgrass culture. Maintaining a high quality turfgrass often requires the application of large quantities of N for plant uptake (Snyder et al., 1984; Sartain, 1988). One of the primary goals of N management is to improve the use efficiency of applied N. Low efficiency of applied N is due to N losses from soil/plant systems through leaching and/or gaseous losses during the growing season (Bock, 1984). There is a growing concern that N fertilizers are potential nitrate (N03~) pollution sources, contaminating groundwater and surface waters. Nitrate is, in fact, considered to be one of the most widespread groundwater contaminants (Pye et al., 1983). Leaching of nitrate from fertilizers applied to turfgrass areas is proposed to be a major source of groundwater pollution in areas uhere turfgrass is a major land use (Flipse et al., 1984).
Attention is focused on the leaching of N03" from N fertilizers for two main reasons: first, N03" is the most mobile form of N, and will move freely with the soil solution
if not taken up by the plant; second, its presence in drinking water supplies in excess of EPA-established drinking water standards (10 mg/1 N03"-N) presents health hazards (Exner et al., 1991). Leaching potential may be minimized or eliminated through the use of available technology and management practices.
Optimizing plant uptake and minimizing N losses may be accomplished through the use of controlled-release N sources. Recent advances in polymer coating technology has resulted in production of thin layer polymer-coated N fertilizers such as reactive-layer coated N sources. The thin-layer coating makes these products more cost effective and the nature of the polymer and its reaction with the N substrate makes the coating less susceptible to physical and microbial degradation. Information is needed regarding the effectiveness of controlled release N fertilizers such as reactive-layer coated N fertilizers in optimizing uptake efficiency and minimizing leaching potential of applied N in turfgrass environments.
The objectives of this study were:
1) to quantify plant uptake of N from applied polymer-coated
2) to monitor NH4-N and N03-N concentration in the intermediate vcidose zone (IVZ) and the saturated zone;
3) to deterir Lne and quantify the effect of N sources and
varying coating thicknesses on NH4-N and N03-N leaching through a golf green profile; and
to evaluate the effectiveness of coating thicknesses in assuring N supply to turfgrass and minimizing leaching losses.
CHAPTER 2 LITERATURE REVIEW
Fate of Nitrogen in a Turfgrass Environment Understanding the nitrogen (N) cycle is paramount for maintaining a high quality turfgrass while minimizing the effects of N fertilization on the environment. Fate of N in a turfgrass environment includes several major categories of the N cycle. These include: 1) plant uptake; 2) soil retention and microbial immobilization; 3) runoff and leaching into groundwater and surface waters; and 4) losses to the atmosphere (Petrovic, 1990). Figure 2-1 is an illustration of the N dynamics which occur in a turfgrass ecosystem.
Nitrogen Transformations in the Soil There are two basic forms of N within the soil: organic and inorganic. Nitrogen can also undergo numerous reactions, with the primary soil reactions including mineralization, immobilization, nitrification, denitrification and volatilization (Tisdale et al., 1993).
Through the process of mineralization, organic N present in the soil is transformed by soil heterotrophic microorganisms to NH4+. The NH4+ produced during mineralization or supplied through fertilization may take a variety of paths.
NH4 NH4 Plant
Figure 2-1. Nitrogen cycle in a turfgrass ecosystem
It may be oxidized to N02" and then to N03" during the
process of nitrification, it may be taken up by plant, it may be further utilized by microorganisms in the soil, it may be "fixed" in the interlayers of clay minerals, or it may be lost to the atmosphere at high pH through the process of volatilization (Tisdale et al., 1993).
Inorganic N present in the soil or produced by mineralization can also be used by soil microorganisms through microbial immobilization. Soil microorganisms require both carbon (C) and N to grow and multiply. When an organic material with a high C:N ratio is incorporated into the soil, soil microorganisms use the readily available N for their growth processes. This immobilized N is temporarily tied up and is unavailable for plant uptake (Paul and Clark, 1989).
Starr and DeRoo (1981) studied N immobilization and uptake by a mixture of Kentucky bluegrass (Pao pratensis L.) and red fescue (Festuca rubra L.) in a 3-year field study using labeled and unlabeled N source at 195 Kg ha'1yr"1 for the first 2 years and 180 Kg ha"1 for the third year. They reported uptake of 20% from the first fertilizer application and 35% from the second application. Most of this uptake occurred within 2 to 3 weeks after application, with uptake falling to near zero after that. They suggested that large amounts of fertilizer N were immobilized in the thatch by the microbial population associated with this layer. This resulted in low total efficiency of fertilizer N uptake into
the above-ground portion of the grass. Watson (1987) also
conducted a study wherein the effect of immobilization on N recovery by perennial ryegrass was observed. Seven weeks after the application of N, 13% of applied urea was found in the soil organic N pool compared to 17% in the case of NH4N03.
The two-phase process of nitrification also plays a significant role in the fate of N. Ammonium is first oxidized to N02~. This reaction can be represented by (Tisdale et al.,
2NH4+ + 302 -------------> 2N02" + 2H20 + 4H+
The second phase involves further oxidation of N02" to N03"' in a reaction represented by the following.
2N02" -I- 02-------------> 2N03~
Since microbial activity is involved in the nitrification process, the rate and degree of nitrification are greatly influenced by soil temperature, moisture, Ph, microbial population, and supply of NH4+. The N03" produced by nitrification may also take different paths. These paths include plant uptake, assimilation by microorganisms, leaching, or denitrification and resulting loss to the atmosphere (Tisdale et al., 1993).
Denitrif ication is the process of reduction of N03" to gaseous forms of N (N20, NO, and N2) by soil microorganisms.
The rate of denitrification is affected by several factors. It generally occurs where oxygen (0P) is absent or present in
only low concentrations, such as when the soil is moist and has a high water holding capacity, in aggregates which are water saturated, or near sizeable clumps of actively decomposing organic material. Carbon is also a necessary component of the denitrification reaction. Thus, it is typically assumed that denitrification does not occur at an appreciable rate in groundwaters (Firestone, 1982).
Another nechanism for gaseous loss of applied N is through the process of ammonia (NH3) volatilization. This path of N loss may be significant when urea fertilizers are used. The rate at which NH3 is volatilized is dependent on pH and urease enzyme activity, which hydrolyzes urea to ammonia-cal form (Torello et al., 1983). Torello et al. (1983) reported that NH3 volatilization rates were reduced when sulfur-coated urea was used, compared with uncoated urea. Losses were also smaller from prilled urea than from liquid urea which was spray applied. In another study, Torello and Wehner (1983) reported volatilization losses of 39% for urea applied to thatch and 9% for urea applied to soil cores.
The interaction of these processes constitutes the study of fate and transformations of N complex. Studies of the distribution of N in a turfgrass environment are limited. Most studies in this area have further limited to individual paths of N. More research is needed to investigate the fate
and transport of N in a turfgrass ecosystem as a whole.
Nitrogen is an essential nutrient for turfgrass growth and maintenance. Thus, plant uptake is a major sink for N. Plants absorb N in the forms of NH4+ and/or N03" (Barraclough and Smith, 1987). However, N03" can normally be found in higher concentrations in agricultural soils than NH4+ because of nitrification (Tisdale et al., 1993). Ammonium-N, on the other hand, is a preferred source of N by some plants because internal energy is saved when this form of N is taken up by the plant (Taiz and Zeiger, 1991).
Efficient N management for optimum plant growth and protection of the environment requires increasing use-efficiency of applied N, for example, optimization of plant uptake. However, plant uptake of N is affected by numerous factors. These include N available for uptake, N source and rate, soil moisture and temperature, and plant species (Petrovic, 1990).
Amount and form of available N affect plant uptake. Barraclough et al.(1985) studied N uptake of ryegrass using single- and double-labeled 15N NH4N03. The results of their 3-year study snowed that average annual recoveries of N were 99, 76, and 50% of applied N at 250, 500, and 900 Kg ha'1, respectively. They also found that at 250 Kg N ha"1, about 70% of the N in the crop was derived from the ammonium pool compared with 64 and 59% at 500 and 900 Kg N ha"1, respectively. They
suggested that, at the lower application rates, preferential
uptake of ammonium was occurring. However, as N supply exceeded crop requirements, nitrate became the major N source.
Plant species also take up N in varying amounts and at different rates. Plants differ in the amounts of N they require for growth and maintenance. In comparing the study conducted by Barraclough (discussed above) with a study conducted by Sheard et al. (1985), differences in N uptake by different plant species were observed. Sheard et al. reported a recovery of 60% of the applied N during season-long clipping trials of Penncross creeping bentgrass (Aarostis palustris Huds.) when fertilized at 240 to 287 kg ha"1 yr"1.
Nitrogen source, in particular, plays an important role in N uptake efficiency. Numerous studies have investigated the effect of different N fertilizers on plant uptake. Mikkelsen et al* (1994) investigated the uptake efficiency of selected controlled-released fertilizers (CRF). They reported that both N application rate and fertilizer source had a significant effect on the N concentration of stems and leaves. Plants receiving a daily application of NH4N03 had the highest tissue N concentration for the entire experiment, with the results for other materials decreasing in the following order:
NH4N03> SCU> Osriocote> Prokote> IBDU> Ureaformaldehyde> Oxamide. Among the controlled-release sources tested, coated N fertilizers were more effective in increasing tissue N concentrations than uncoated sources when averaged over the
entire experiment. In another study conducted by Watson
(1987) two water-soluble N sources, urea and NH4N03, were applied to perennial ryegrass to evaluate N uptake. He reported that 53% of applied N from NH4N03 was recovered in the clippings, compared to 31% recovery from urea.
It is the interaction of these factors which determines net uptake of N. Best management practices should take into consideration the relative weight of each factor affecting uptake at specific turfgrass locations.
Leaching of Applied Nitrogen
Total world consumption of fertilizer N in 1990-91 was 78 million tons per annum, with a projected yearly increase of 2 to 3% (Shaviv and Mikkelsen, 1993) Golf courses alone account for more than 1.3 million acres of managed turfgrass in the United States (Cohen et al., 1993), requiring approximately 3 00,000 tons of N annually. Once N is added to the soil, it undergoes various transformations within the N cycle, including nitrification. If nitrate is not taken up by the plant, it may move below the root zone. Once below the root zone, it will eventually leach into groundwater, thereby contaminating groundwater and surface water supplies. Thus,
groundwater quality has become a focal point of concern with regard to N fertilization and management.
Numerous studies have shown that the most common source of nitrate pollution of groundwater and surface waters is agriculture ( Halberg, 1987; Keeney, 1982; Pratt, 1984). Much of the nitrate pollution arising from agricultural use
can be attributed to heavy fertilization in intensively row-cropped settings, in intensively irrigated-grain agriculture, in the irrigation and fertilization of shallow-rooted vegetable crops, and locally in intensive animal feeding and handling operations (Keeney, 1986). Leaching in relation to fertilization/nutrient management has been documented as well for golf courses and home lawns (De- Roo 1980; Keeney, 1986; Morton et al., 1988).
Groundwater is a major source of drinking water in the United States (CAST, 1985), with nitrate pollution of groundwater being steadily on the rise (Hallberg, 1987) This rise in nitrate contamination has caused concern over potential health hazards associated with this type of groundwater pollution. Nitrate accumulation in groundwater becomes a health risk when the water is subsequently consumed in high enough amounts by humans and animals. These health risks include methemoglobinemia, cancer, and possibly others (CAST, 1985; Keeney, 1986). For these reasons, public health standards for nitrate in public drinking water supplies in the U.S. have been set by the United States Environmental Protection Agency (EPA) at 10 mg L"1 N03-N (Keeney, 1986) .
Nitrate is a highly soluble and very mobile plant nutrient. It is these qualities that facilitate plant uptake. However, nitrate's high solubility and mobility also make it
very susceptible to leaching through the soil (Keeney, 1982).
Nitrate can enter ground water and surface waters from a
variety of natural and human sources. Natural sources include soil N, N-rich geologic deposits, and atmospheric deposition (Madison and Brunett, 1985). Human sources include fertilizer application, feedlot drainage, dairy and poultry production, mineralization of N by cultivation, and leaching of soils as a result of the application of irrigation water (Madison and Brunett, 1985) Once N is converted to nitrate in the soil, it remains in nitrate form unless it is removed by plant uptake, denitrified or leached.
There are two main mechanisms that control the movement of nitrate through the soil. The first and most important mechanism is convection or mass flow of nitrate with moving soil solution. A second mechanism of transport is by diffusion. Diffusion involves the process whereby solute become less concentrated under the influence of molecular-scale collisions. However, under saturated conditions, convection is the main mechanism for nitrate transport to the ground water. Diffusion is limited to relatively small distances (e.g. near plant roots) compared to convection (Keeney, 1986).
Nitrate movement within the soil is affected by many factors. These include spatial variability, soil type, infiltration rate, irrigation rate, N source and rate, and climate (Keeney, 1986; Peterson and Frye, 1989). Spatial variability in the soil affects the rate at which nitrates will be transported through the soil in any given area. For example, a singe worm hole, decayed-root channel, or restric-
tive layer of soil can drastically effect the rate at which nitrate is leached. Thus, spatial variability makes it very difficult to determine infiltration rates for large areas.
Van de Pol et al. (1977) conducted a study to investigate the impact of spatial variability on infiltration rates. They added a pulse of chloride to water applied to a clay soil by steady-state trickle irrigation, and monitored downward movement of the pulse at 24 locations in an 8 m x 8 m field plot. Even though the application rate was uniform, they observed a log-normal distribution of apparent vertical solute velocities, with a coefficient of variation of 61%.
Soil type also greatly influences nitrate leaching rates. Soils have long been known to evidence vertical and horizontal
variability in their chemical, physical, and biological properties (Kevney, 1986). Biological and physical factors interact strongly to affect many processes involved in nitrate leaching. For example, soil texture and structure affect water and oxygen diffusion, thereby affecting in turn the presence of anaerobic microsites along with subsequent denitrification rates and amounts (Keeney, 1986). Variability also affects water infiltration rates, leaching rates, and biological tran3formations including plant uptake and mineralization.
Reike and Ellis (1974) conducted a study to investigate N03* movement in a sandy (91% sand) soil profile. They applied 290 kg N ha'1 as NH,N0, each spring for two years, with
periodic soil samples being taken to a depth of 60 cm. Fertilizer N application increased N03" concentrations at the 45 to 60 cm depth when compared with unfertilized plots on only two of 2 0 sampling occasions during the study. However, soil N03" levels were elevated at a depth of 30 cm for the fertilized plots throughout the experiment.
Infiltration rates also play an important role in nitrate leaching, because nitrate normally moves with percolating water. Therefore, the rate and direction of nitrate movement in the soil is roughly related to the concentration of the nitrate in soil solution, as well as, the direction and rate of water movement (Keeney, 1986). Infiltration rates are affected by many factors including soil slope, vegetative or plant cover, stability of soil aggregates, amount (antecedent level) of soil water at the start of a rainfall event, and all factors affecting the size and continuity of soil pores (Peterson and Frye, 1989).
Dowdell and Webster (1980) conducted a lysimeter study to investigate leaching losses of fertilizer N applied to perennial ryegrass. They applied 400 kg N ha'1 of calcium nitrate labeled with 15N. Approximately 2 to 5% of the 15N was lost to drainage in the winter after application. Losses due to drainage in subsequent years were drastically diminished, only accounting for about 0.1% of the 15N. Concentrations of nitrate ranged between 4 and 16 mg N L"1.
Irrigation and rainfall have a direct impact on leaching
of nitrate. Most leaching occurs when soil moisture exceeds evapotranspiration rates (Peterson and Frye, 1989). Snyder et al. (1984) investigated the effect of irrigation and N source on leaching of N from a sandy profile covered with bermudagrass. Ammonium nitrate and sulfur-coated urea (SCU) were applied at a rate of 98 kg N ha"1 to their plots. Each plot was then irrigated either on a standard daily schedule or by tensiometer-activated irrigation. Nitrogen was also applied in the irrigation water, and samples of soil water were taken daily. Samples were analyzed for the amount of N leached past the root zone. Results showed that 0.3 to 56% of the applied N had leached. These results were highly influenced by irrigation schedule, N source, and time of the year. February and March evidenced the highest leaching, with less in April and May and the least in June and July. Snyder et al. (1984) concluded that the decline in leaching was most likely a result of increased plant growth and evapotranspiration rates, with irrigation rates also heavily influencing leaching losses. Leaching losses were 2 to 28 times higher for plots irrigated on a daily schedule than for plots coupled to tensiometer sensor. Plots with the NH4N03 treatment had a leaching rate 2 to 3.6 times higher than plots with the SCU treatment.
Nitrogen source and rate play a critical role in the leaching of nitrate to the groundwater. Numerous studies have been conducted to investigate the impact of slow-release
fertilizers in reducing leaching potential. Mikkelsen et al. (1994) conducted a greenhouse study to evaluate N leaching loss of six controlled-release N fertilizers and one soluble N fertilizer on container-grown Euonymus patens Rehd. They reported that coated fertilizers generally had less leaching
losses compared to the non-coated fertilizers. Daily application of NH4N03 resulted in a constant rate of N loss. After the first two weeks following fertilizer application, virtually all of the leached N was in the N03" form. Nitrogen leaching losses were all increased by doubling the fertilizer application rate from 1 kg N m"3 to 2 kg N m"3. Total N leaching losses from all sources were greater during the first 15-week period following fertilizer application, and were determined to represent as much as 24% of the applied N. However, in the 11-week period following a second fertilizer application, total N leached was determined to be less than 11% of the applied N.
The rate at which N is applied to turfgrass also affects the amount of N leached. Exner et al. (1991) conducted a study to investigate deep nitrate movement in the unsaturated zone of a simulated urban lawn fertilized with different rates of N. In this study turfgrass plots were treated with NH4N03 at rates of 0, 1.0, 1.5, 2.0, and 2.4 kg N 100 m"2. The turf received approximately 64 0 mm of water during the 34-day study. The researchers reported that as much as 95% of the nitrate applied in late August leached below the turfgrass
root zone. They also reported that nitrate concentrations in the leachate ranged from 34 to 70 mg L"1 N03~-N.
There are many management practices which help to control N leaching and consequently reduce groundwater contamination. These practices include selecting the best N source, applying appropriate N rates, proper irrigation scheduling and maintenance of good soil cover (Peterson and Frye, 1989) Numerous studies have baen conducted to investigate the effectiveness of differing management practices. Shirmohammadi et al. (1991) studied the long-term effects of different management practices on nitrate N loadings to a shallow, unconfined, groundwater system. Two representative watersheds in the coastal plain physiographic region of Maryland were selected for this study, with soils in these watersheds having moderate infiltration capacity. Results from their study indicated that best management practices (BMPs) used in conjunction with winter cover (barley) reduced nitrate leaching to a groundwater system. They also found that turfgrass reduced surface runoff of water and N, while increasing leaching losses of water and nitrate N.
Other Losses of Nitrogen
Other potential losses of N in a turfgrass system include sediment losses and runoff. However, sediment losses and runoff from turfgrass sites are minimal. Gross et al. (1990 and 1991) investigated the effect of liquid and granular fertilizer N on runoff volume and sediment losses from various
tall fescue (Festuca arundinacea Schreb.) mixed with Kentucky bluegrass. Runoff losses of total N were higher from the liquid and granular treatments than from the control plots. However, there were no differences between treatments. The results of their studies also indicated that nutrient and sediment losses from turf via runoff is very low compared with agronomic row crops.
In another study conducted by Watschke, runoff from turfgrass sites was investigated (Petrovic, 1990). The results from silt loam turf sites with slopes ranging from 9 to 12% showed that one rainfall event produced runoff during the two year study.
USGA Golf Green Profile
Leaching may be a problem for many sports turf sites. Golf greens, in particular, have a high potential for leaching because of the accompanying high sand profile which is designed for rapid removal of excess water. Golf course green construction utilizes multiple soil layers in order to optimize soil moisture retention and water movement. The United States Golf Association (USGA) recommends the use of a 3 0.5 to 3 5.5 cm sand-based topsoil overlying a 5 to 10 cm coarse sand (choker) layer and a 10 cm gravel bed with underlying tile drainage (McCarty and Cisar, 1993). These multiple layers are designed on the basis of the physics of
layered soils to optimize moisture content of the root zone
mix and water flow through the golf green profile and into the drainage system.
Each layer serves a specific function in the retention and movement of water through the profile. The root-zone mix layer is designed for a relatively high infiltration rate. The moisture content of this layer then increases as it approaches thv. choker layer. As the moisture content increases it creates a near-saturated zone, generally referred to as the "perched water table" zone. The perched water table zone is, therefore, a part of the root-zone mix. The depth of this zone is a function of the texture of the root-zone mix layer relative, to the choker layer's texture and depth (McCarty and Cisar, 1993).
The choker layer or coarse sand layer has multiple functions. This layer is designed to prevent migration of finer texture root zone mix into the gravel layer. However, its primary function is to regulate water flow into the gravel layer and subsequent flow into the drainage tiles. The choker layer is responsible for creating the perched water table as well (McCarty and Cisar, 1993). This plays an important role in regulating the water content of the root-zone mix layer. However, the necessity of this intermediate layer of coarse sand has been questioned.
The gravel layer has several functions in the golf green profile. This layer is responsible for the rapid movement of
infiltrating water to the tile lines. The gravel layer also
helps prevent a rising water table from saturating the root zone and preventing salt movement from the subsoil into the root zone. Additionally, it acts as a buffer between the moist root zone and dry sub-grade soil, preventing the dry sub-grade soil from extracting water from the root zone. Finally, if an intermediate choker layer is not used, the gravel layer serves the functions of the intermediate choker layer as well (McCarty and Cisar, 1993).
Rapid removal of excess soil water is a vital attribute of putting greens. Drainage tiles have been embedded in the gravel layer for the removal of this excess soil water. Drainage tiles help prevent the greens from holding too much water under heavy rainfall or over-irrigation conditions. This helps in turn to minimize soil compaction and protect the golf green (Unger, 1971).
Several studies have been conducted in order to asses plant uptake and leaching characteristics in this type of profile. Brown et al. (1982) conducted a study investigating the interaction of N sources and soil texture in effecting nitrate leaching from a USGA golf green profile and a sandy loam soil green profile covered with bermudagrass. Leaching losses were 22% from NH4N03, 9% from activated sewage sludge, and less than 2% from IBDU and ureaformaldehyde where the root-zone mixture contained more than 80% sand. On the other
hand, greens constructed with sandy loam soil had losses of
9%, 1.7% and less than 1%, respectively.
Gaines and Gaines (1994) also conducted a study to investigate N03" retention by USGA greens mixtures compared to other soil textural sequences. Samples of sand, USGA greens mixture, loamy sand, and sandy clay loam, were used to compare nitrate retention by the several materials. Their results showed that soil texture affected the retention of N03"-N in
the sand, which absorbed the least amount of N03'-N by 119 mg kg"1, followed by the USGA greens mixture at 125 mg kg"1, loamy sand at 149 mg kg"1, and sandy clay loam at 173 mg kg"1. They
also reported that more N03'-N was released from the first 50 ml of sand percolate (at 63%) followed by the USGA greens mix percolate at 58%, loamy sand percolate at 4 6%, and sandy clay loam percolate at 37%.
High-sand golf green profiles require improvement in
nutrient and water retention. Through the use of the proper selection of fertilizer materials such as controlled release nutrient sources, N03"-N leaching can be reduced.
Nitrogen Fertilizers Sources of N used for turfgrass production and maintenance are numerous and have widely varying release characteristics. In general, N sources can be divided into two categories. The first category includes soluble N sources
which are generally referred to as "quickly available N". The second category includes insoluble organic and soluble encapsulated N sources, which are generally referred to as "slow-release or controlled release N".
Soluble N Sources Quick-release N is water soluble and usually contains N in the form of N03" or NH4+. As discussed earlier, N03" is readily available for plant uptake but is also very susceptible to leaching through the soil. Ammonium N is less likely to leach but is also rapidly transformed to N03" through the process of nitrification. Quickly-available N sources require more frequent application in order to maintain turfgrass at a high quality. The products classified in this fertilizer group include soluble organics such as urea, and soluble inorganic salts such as (NH4)2S04 and NH4N03.
Urea is a product commonly used in the turfgrass industry. It is a water-soluble organic compound, containing 45 to 4 6% N. Before the N can be absorbed by the turfgrass it must undergo hydrolysis. Urea hydrolysis is an enzymatic reaction
carried out by urease which transforms the N into NH4+ within a few days (Turner and Hummel, 1992)
Numerous studies have been conducted to determine the characteristics of this product. Studies have found that urea has a rapid release rate but is available for plant uptake only for a short time (Landschoot & Waddington, 1987; Mosdell et al., 1986; Spangenberg et al., 1986). One disadvantage associated with urea is its high foliar-burn potential (Johnson & Christians, 1984; Moberg et al., 1970). In a study conducted by Wesley et al. (1985) the rate of urea hydrolysis was investigated. They found that maximum N uptake occurred
within 24 hours after urea application with the recovery rate of applied N being 31 to 61% after 72 hours.
The most common inorganic salts used by the fertilizer
industry include (NH4)2S04 and NH4N03. Inorganic salts have a high salt index which results in a high potential for foliar burn (Monteith & Bengston, 1939). Like urea, inorganic salts are quickly available for plant uptake but have a limited duration as such (Waddington et al., 1985). Most of the N applied is ^;aken up by the plant within 4 to 6 weeks after N application (Starr and DeRoo, 1981)
Many studies have been conducted to investigate the effects of different inorganic salt fertilizers. In a study conducted by Sartain (1985), the effect of (NH4)2S04 on thatch buildup was investigated. He found that thatch accumulated most quickly when (NH4)2S04 was used as the N source, and concluded that thatch buildup was the result of the acidifying effect of (NH4)2S04 and consequent reduction in microbial activity.
Controlled-Release Fertilizers (CRFs) The terms 11 control led-release" and "slow-release" fertilizers are often used interchangeably. However, a distinction can be made between these two N sources. Controlled-release fertilizers are generally synthetic organic N sources with low solubility and/or encapsulated synthetic organic or inorganic N sources.
The rate of N-release from CRFs can be manipulated by varying the degree of solubility, particle size, and nature and
thickness of the coating used for encapsulation. Environmental factors may influence the rate of release but do not control it. Slow-release fertilizers (SRFs), on the other hand, encompass a variety of natural organic sources, with an release rate controlled by microbial activity and environmental factors affecting the N mineralization of these materials (Turner and Hummel, 1992).
Both SRFs and CRFs have been utilized in turfgrass production and maintenance for many years. Golf courses and nurseries, in particular, have utilized these materials to help satisfy their need for a long-lasting N source with a lower potential for leaching. The agronomic properties of these N sources have been investigated and are well-documented (Carrow and Johnson, 1989; Landschoot and Waddington, 1987; Sartain, 1990; Waddington and Duich, 1976; Woolhouse, 1974). In general, these materials extend the supply of N to the plant and support a steady rate of growth compared to quickly-available sources of N. This has three advantages. First, the N source does not have to be applied as frequently. Second, the response to applied N and consequently the quality
of turfgrass is maintained for a longer period of time. Finally, losses of applied N may be reduced (Turner and Hummel, 1992).
Numerous studies have been conducted comparing and assessing the benefits from SRFs and CRFs. Sartain (1985) conducted a study comparing milorganite, a SRF, with (NH4)2S04
and IBDU (a synthetic organic CRF). He found that, when milorganite was used, the quality of bermudagrass was not affected by N ,source when compared with IBDU and (NH4)2S04. However, growth rates and N recovery were higher for IBDU and (NH4)2S04 compared to milorganite. In contrast, Barrios et al. (1982) evaluated the effects of milorganite in warmer climates, where milorganite produced higher growth rates and N recoveries when compared with other quickly-available and slow-release fertilizers.
Arminger et al. (1948) evaluated the response of turf-grasses to urea-formaldehyde (UF), a synthetic organic SRF. They suggested that the response of turfgrass is highly dependent on the ratio of urea to formaldehyde, and found that ureaform produced with a 1.3:1 ratio (38% N, 65 to 70% water insoluble) was slowest to release in the field.
Hummel (1989) evaluated four Escote RCUs with release rates of 70, 100, 150, and 270 days. Turfgrass response was
highly correlated with release rates with the 70 day material producing the most rapid response with the shortest duration. The 270 day material released its N too slowly to produce a turf of acceptable quality.
Controlled-release and slow-release fertilizers should grow in demand in the future, not only for their agronomic benefits but, more importantly, for their place in nutrient management strategies and environmental pollution control. Snyder et al. (1984) investigated the influence of N source
and rate on leaching potential of N applied to a sandy profile. The higher rate of 78kg N/ha applied bimonthly, had leaching losses of 9.3 and 5% of the applied N from calcium nitrate and IBDU, respectively. Ureaform, SCU and urea leached less than 1% of the total amounts applied. Low leaching losses from urea were suspected to be a result of high volatilization rates on the alkaline soils to which the urea was applied. In another study conducted by Sartain (1990), selected SRFs were investigated to determine their N-release and leaching characteristics. He found that the form of N leached and the rate at which it leached depended on the substrate and solubility of the material. The quantity of N leached during this experiment was lowest for nitroform (33%) and highest for reactive-layer coated urea (RLCU) (95%), with intermediate rates for coated ammonium sulfate (88%), Nutra-lene (81%), sulfur-coated urea (SCU) (75%), and IBDU (52%). Substantial quantities of N03" were not present in the leachate until after 64 days. This suggestes, that the absence of N03" prior to 64 days was the result of the absence of any plants, and subsequent rhizosphere microbial populations, in the experimental pots. While the environmental importance and benefits of SRFs and CRFs are believed to be significant they have not been fully assessed. More long-term studies need to be conducted to further evaluate the environmental impacts of these materials.
Fertilizer Recovery Methods
Efficient N management requires an understanding of the
fate of applied N in the soil/plant system. The recovery and
quantification of applied N fertilizer is a difficult task,
due to the dynamic nature of the N cycle in the complex
soil/turfgrass system. Numerous research has been conducted
to evaluate and compare different methods of N recovery in the
soil/plant environment. The most common techniques employed include the use of lysimetery, ceramic cup samplers, soil core
sampling, and the difference and tracer methods of calculation
The use of lysimetery is an important research tool used
to investigate the fate of N under a range of management
practices, soil types and turf varieties. A lysimeter is a
container which physically isolates a soil/air/water environ-
ment hydrologically from surrounding soil (Bailey, 1984).
There is no set design for lysimeters. Their design varies
with the intended purpose, as they can be used to address a
variety of research objectives.
There are two main types of lysimeters: weighing and
percolation lysimeters. Both types have several subtypes
within the category. For instance, weighing lysimeters
include the hydraulic load-cell type (Hanks and Shawcroft,
1965), a floating type (King et al., 1956), mechanical
weighing systems with automatic readout (Harrold and Dreibelb-
is, 1958) and strain gauge load cells (Sammis, 1981) Weighing lysimeters vary greatly in size and expense. Weighing lysimeters can be as simple as pots that are weighed at various time intervals, or as complex as lysimeters equipped with automatic weighing scales, moisture sensors and drainage systems. The cost can vary from as little as the price of a plastic pot to as much as $16,000 each (Keeney, 1986).
Percolation lysimeters also vary in size and price, according to research objectives. These types of lysimeters are most often used to measure solute leaching. De Nobili et al. (1992) conducted an 15N field study using lysimetric tanks to investigate the fate of oxamide and urea applied to turfgrass. Initial N absorption, during the first 40 days after fertilization, was higher for urea (23.5%) than for oxamide (12.1%) but after 64 days, absorption efficiencies were similar (11%) for both fertilizers. Leaching was much greater from the urea-fertilized soil (1.57 g) compared with losses from oxamide-fertilized soil (0.05 g). Total N fertilizer remaining in the soil at the end of the experiment was 27% of the applied urea and 40% of the applied oxamide.
Ceramic Cup Samplers
Ceramic cup samplers have also been used to monitor soil
water since the early 1900,s (Debyle et al., 1988). The use
of ceramic cups has been established as a research tool for
sampling soil solutes in the saturated and unsaturated
(vadose) zone. A tube is attached to the end of the ceramic
cup and suction is applied, causing water to completely fill the pores of the ceramic cup and drip into its interior. In order to construct a device that will sample soil water in the vadose zone, it is necessary to use a porous cup that contains pores small enough so that the air in the soil, under atmospheric pressure, cannot enter even though a full vacuum is created within the sampler (Everett and McMillion, 1985). Under these conditions, only water from the capillary spaces in the soil will flow through the pores in the porous cup and into the sampler (Everett and McMillion, 1985).
Many studies have been conducted utilizing ceramic cup samplers to measure nutrient contents of soil solutions. For example, Hook and Kardos (1978) and Hook and Burton (1979) estimated nitrate loss below the root zone at municipal sewage irrigation sites using porous cups. Mansell et al. (1980) also utilized ceramic cup samplers to measure the effect of shallow and deep tillage on solute movement beneath citrus. Difference Method
Recovery of applied N fertilizer by the plant can be estimated using the difference method. In this method,
apparent recovery of applied N fertilizer is estimated by subtracting the N recovery by plants grown on unfertilized control plots from that by plants grown on fertilized plots. Hauck (1971) reported that percent uptake of applied N is
generally overestimated using the difference method, when compared with the 15N tracer method. This is because the
addition of N fertilizer to the soil induces mineralization of soil organic matter. This is often referred to as the "priming effect". When mineralization occurs, N is released and taken up by the plant. The difference method estimation of N-uptake does not differentiate between these two sources of N resulting in an overestimation of fertilizer recovery. Low and Piper (1957) calculated N recovery using the difference method and the 15N tracer method. They found that N uptake calculated by the difference method was greater than the amount calculated by the 15N tracer method. This suggests that plants took up more soil N where fertilizer was applied. Terman and Brown (1968) compared the difference method with another recovery method. They indicated that the difference method is over-simplified and does not effectively characterize the efficiency of applied N. However, due to the high cost of isotope tracer methods in large-scale field experiments, the difference method should be considered as a part of any N recovery study, and results compared with other methods. 15N Tracer Method
Tracer methods allow direct N measurement in N transformations, N uptake by the plant, and fertilizer studies. Without such techniques, N can only be studied indirectly. Thus, many advantages can be derived from using tracer techniques. 15N is often used as tracer in the studying of the N cycle and its inherent processes. The stable isotope 15N is used because it occurs naturally in combination with 14N in an
almost constant ratio of 0.3663 atom % 15N (Naude, 1930). Likewise the chemical properties of UN and 15N are generally
identical, making 15N ideal for the study of N.
However, there are three assumptions which must be made when conducting research using tracer methods. According to Hauck and Bremner (1976) these assumptions are: 1) elements containing two or more isotopes (complex elements) have a constant isotope composition in the natural state; 2) one isotope from the same element cannot be distinguished from another by living systems; and 3) biochemical systems maintain the chemical identity of the isotopes. These assumptions are not valid in every experimental situation. However, they are valid for most tracer studies.
The non-radioactivity of 15N makes this isotope ideal as a research tool for many reasons. First, the use of 15N imposes no time limitation since there is no isotopic decay (Nason and Myrold, 1991) Furthermore, the use of 15N does not cause any ill effects from radiation poisoning, either to the experimenter or the system under study. Finally, there is no disposal problem associated with 15N residues and no permits
are required for its use.
The 15N tracer method offers several advantages over non-tracer methods for research on N. According to Hauck and Bremner (197 6) these advantages include: 1) positive identification of the labeled N as it enters and moves through an
ecosystem; 2) no control treatments are required; and 3)
pre-sampling of the investigation site is not required. There are disadvantages to using the 15N tracer method as well. The largest obstacle to overcome is the initial cost and the cost of equipment needed for 15N analysis. However, as 15N has become widely used as a research technique, these limitations have diminished.
15N has been useful in following the complex interrelationships between organic and inorganic N. Many researchers have used ]5N isotopes in experiments concerning N uptake and transformation in the soil, water and plant system. Research using 15N to measure the dynamics of N cycling can be divided into three categories: 1) use of 15N as a tracer, 2) isotope dilution experiments, and 3) the application of mathematical models to 15N dynamics (Myrold and Tiedje, 1986).
Tracer studies have been used to determine the fate and partitioning of applied 15N. However, tracer studies can only provide a qualitative estimate of process rates. Isotope dilution experiments involve the addition of 15N into a product pool. Dilution of the atom %15N in this pool by natural abundance N from a precursor pool is investigated over time.
These studies have been used to investigate selected N cycle rates in sediments and soils. (Koike and Hattori, 1978; Nishio et al., 1985) The use of 15N to characterize total pool sizes in mathematical N cycle models was first begun by Kirkham and Bartholomew (1955). The application of mathematical models to N cycling in soils allows rates of several N-cycle processes
to be estimated simultaneously. All of these methods using 15N provide the researcher with valuable information.
Walker et al. (1956) determined the fate of 15NH4+ and 15N03" applied to pots in which grass, clover, and grass plus clover were grown. Approximately 3 0% of the applied N was lost through denitrification. The researchers concluded that uptake of soil N by the plant was increased by the application of N fertilizer. This was evidence of the immobilization of 15N by soil organisms as well as the biological interchange of
15N for soil N.
Tyler and Broadbent (1958) investigated uptake of applied 15N. They indicated that much of the applied 15N was immobilized soon after it was applied. Legg and Allison (1959) applied 15N to sudangrass (Sorghum sudanense Piper). They recovered an average of 93.6% of the 15N applied in this study.
Cady and Bartholomew (1960) also conducted a greenhouse study to investigate the uptake and recovery of 15N applied to sudangrass. They found that the addition of an organic amendment decreased the percent of 15N recovered in the grass.
Dilz and Uoldendorp (1961) studied the distribution of N in plant and soil using 15N KN03 applied to grass. They found that differing levels of moisture had no effect on N distribution in sandy and clay soils. However, there was an effect on N recovery in peat soils. The recovery rate of 15N in peat soils was much less than that from clay or sand.
Allison (1964) found that recovery rates for 15N studies
conducted in a greenhouse were higher than recovery rates for lysimeter and field experiments. The recovery rates of applied 15N in greenhouse studies averaged 86%. Much of the research suggested most of the unaccounted-for N was lost due to denitrification. Most field experiments reviewed by Allison showed considerably lower recovery rates.
Carter et al. (1967) conducted a field study in which the recovery rates of applied N fertilizer were investigated. Cylinders were driven into the soil and sudangrass was grown on top. Total recovery of the applied 15N averaged 99%, and no significant differences in recovery rates were found to be caused by treatment, cropping, or time of application.
Westerman et al. (1972) conducted a field experiment to investigate the recovery of N from urea and oxamide 15N using a sorghum-sudan hybrid. The crop was harvested four times during the experiment. Urea 15N evidenced a 51% recovery rate by the plant with 28% left in the soil (0 to 25cm). Oxamide 15N evidenced a recovery rate of 52% by the plant with 31% left
in the soil (0 to 25cm).
Edwards and Hauck (1974) conducted a greenhouse pot
experiment using both 15N-depleted and 15N-enriched material. Fertilizer particles were prepared from (NH4)2S04 with a depleted value of 0.031 atom % 15N and an enriched value of 0.734 atom %. These two treatments were then compared by
measuring plant uptake of the applied 15N forms. They found
no difference in N recovery rates for five harvests taken over an 18 week period at three rates of application.
Dowdell and Webster (1980) conducted a monolith lysimeter study to investigate the uptake of 15N fertilizer by perennial ryegrass swards and losses by leaching. Forty three to 54% of the 15N fertilizer applied was recovered, with between 4.6 and 9.5 % of the original applied 15N being taken up in the
following year and 1% being taken up in the fifth year of the experiment.
Kowalenko (1980) used 15N on Manotick sand to determine
the transport and transformations of N fertilizer in a sandy
field plot. He found that leaching, denitrification, clay
fixation of NH4+, mineralization and immobilization each played a significant role in the transport and transformation of
Strebel et al. (1980) used 15N to determine nitrate and water uptake by spring wheat. The study was conducted on a Udalf formed from loess, with uptake as a function of soil depth and time being determined. Data were collected to compute total vertical water flow, capillary water flow, water flow through the roots, labeled fertilizer recovery, total nitrate content in the soil, and mass flow of nitrate to plant
These values were then compared with N uptake by the plant through different growth stages. They found that 76% of the total N uptake was by mass flow and that 62% of the total water uptake came from the arable layer (0-30 cm). Because of
the high water uptake rates and high N03-N concentrations in the Ap horizon, this layer contributed most to the supply of NO3-N.
Power and Legg (1984) conducted a field lysimeter study utilizing 15N to investigate the fate of N fertilizer applied to grasslands. During this course of the experiment, soil, grass and root samples were collected for analysis. Highly variable precipitation and evapotranspiration rates contributed to a five-fold variation in annual top growth. From 12 to 52% of the 15N applied was recovered in top growth during the initial season, representing about 75% of the total isotope recovery in tops after four additional growing seasons. The roots contained approximately 15% of the applied 15N after the initial growing season, with approximately two thirds of this N appearing in plant tops within three years. 15N content of the soil varied from 29 to 49% of the fertilizer applied, present mostly as organic N after the first season. Power and Legg recovered between 70 and 95% of the 15N applied in the tops, roots, and soil during the five years of the experiment.
Nishio et al. (1985) conducted an experiment in aerobic soil which simultaneously determined the rates of mineralization, immobilization, and oxidation of N in the presence and absence of acetylene using a 15NH4+ dilution technique. The acetylene inhibited nitrification but did not directly affect mineralization and immobilization. They reported that denitrification or immobilization of nitrate was negligible.
Mineralization proceeded much faster than immobilization with net mineralization accounting for 22 to 59% of the gross mineralization. Nitrogen transformation rates were higher at a soil moisture content of 60% than at 40%. They concluded that, in soil, the pool size of nitrogenous compounds is
controlled by several microbial reactions which occur simultaneously. The combined use of the 15NH4+ dilution technique, coupled with kinetic model analysis, provided a novel approach for understanding the dynamics of N transformations.
Impithuska and Blue (1985) conducted a field study using enriched 15NH415N03 to investigate N absorption by three warm-season grasses grown on a Florida Spodosol. Recovery rates in the forages for the average of the two N rates and three 15N applications were: 35% of that applied to Ona stargrass, with 13% present the stolon and root mass and 22% present in the soil; 40% of that applied to Transvala digitgrass with 9% of that present in the stolon and root mass and 17% present in the soil; and 35% of that applied to Pensacola Bahiagrass, with 20% of that present in the stolon and root mass and 20% present in the soil. Results from the study indicated: 1) essentially equal quantities of 15N in the three warm-season grasses; 2) large retention of 15N in these sandy soils; and 3) a large amount of non-fertilizer N in the forages during 1977, the year of 15N application.
Varvel and Peterson (1991) conducted a study using 15N
to investigate N fertilizer recovery and use by grain sorghum
in monoculture and rotational systems. Grain sorghum was grown under rain-fed conditions on a silty clay loam. Broadcast applications of depleted 15NH4N03 were made at 90 and 180 kg N ha"1 in 1985 and 1986. Recovery rate as determined by the isotopic method was higher than for grain sorghum in monoculture at 65% than grain sorghum in rotation at 55%. Fertilizer N recovery rates determined by the difference method ranged from 119% in continuous grain sorghum to 9% in sorghum following oat plus clover. Recovery rates differed for the isotopic and difference methods, which suggests that N fertilizer applied to grain sorghum in various cropping systems was entering different organic soil-N pools.
Nitrogen tracer methods appear to be indispensable in the study of the fate of N in turfgrass ecosystems. For many nutrient cycling and management studies, using the 15N tracer method enables one to better understand and quantify the N present in the turfgrass systems over extended time periods.
NITROGEN RELEASE RATES OF REACTIVE-LAYER COATED UREA, AMMONIUM SULFATE, AND POTASSIUM NITRATE
Nitrogen fertilizer use efficiency by the plant is affected by soil, plant, and environmental conditions. Increasing N uptake efficiency is an environmental and economic necessity. As a result of processes such as leaching volatilization, immobilization, and denitrification, the N fertilizer effect often diminishes quickly. Application of N fertilizers, therefore requires relatively large dosages which may be expensive and may cause environmental pollution. Coating technologies have provided viable alternatives to other synthetic organic and inorganic N sources through the use of controlled-release fertilizers (CRFs) which are more efficient and less costly.
Controlled-release fertilizers provide an extended supply of nutrients through the gradual release of the substrate which they enclose. The controlled-release characteristic minimizes losses of N to the environment and increases uptake efficiency by the plant. Depending on the coating technology used, release rates will vary with coating thickness and membrane material. Recent advances in fertilizer coating technology have- developed a new process which improves the
release rate as well as lowering the cost of the coated product, throuqh the use of an ultra-thin coating. This new process is referred to as Reactive-Layer Coating (RLC) (Moore, 1987).
The RLC process involves placing fertilizer granules in a drum rotating at 10 rpm and heated to 90C (Figure 3-1). A twin nozzle setup consisting of two separate pneumatic spraying nozzles, is positioned in the drum approximately 5 cm from the subst.rate of rolling granules. From one nozzle a thin layer of diphenylmethane diisocyanate is sprayed on the fertilizer. This reacts with the NH2 groups of the urea, forming a reactive layer coating and leaving excess unreacted NCO groups. (It is unknown how diphenylmethane diisocyanate reacts with (NH4)2S04 and KN03) Hot (100C) aromatic polyester polyol is sprayed from the second nozzle on the product, reacting with the free NCO groups to form a water-insoluble sealing layar which allows water vapor to enter the granule through diffusion. The thickness of the coating layer is built up by the simultaneous spray from both nozzles.
The objectives of this study were to determine the effects of temperature, coating thickness, substrate and granular size on RLC urea (CUrea), ammonium sulfate (CAS), and potasium nitrate (CKN03) release rates.
Materials and Methods
A laboratory study was conducted using RLC urea, AS, and
KN03 with different coating thicknesses and granule sizes
(Table 3-1). Duplicate five gram samples of each fertilizer material were placed in plastic containers containing 50 ml of deionized water. The plastic containers were maintained at 25C and 35C for 63 days. Containers were shaken prior to sampling in order to homogenize the released N within the solution, solutions were sampled on the 3rd and 7th days, and then weekly for the remaining experimental period. All samples were analyzed for NH4-N and N03-N using an air segmented Rapid Flow Analyzer (RFA) unit. The RLC urea materials were also analyzed for urea-N using a colorimetric
Table 3-1. Fertilizer material specifications.
Fertilizer % Coating Weight Particle Size (U.S. Std. sieve) |
CAS 4.7 -6+10 (macro)
CAS 9.8 -6+10
CAS 7.5 -14+30 (micro)
CAS 15 -14+30
CUrea 4.8 -6+10 (macro)
CUrea 10 -6+10
CUrea 10.5 -14+30 (micro)
CUrea 21 -14+30
CKNO^ 3 -10+14 (micro)
CKNO, 6 -10+14
method (Bremner, 1982). At termination of the study, all fertilizer materials were dried and weighed for gravimetric determination of substrate release. Statistical analysis of
data was performed using Statistical Analysis System (SAS)
procedures. The least significant difference (LSD) method was used for means separation.
Results and Discussion
The release rate of encapsulated controlled-release N fertilizers is affected by several factors. These include temperature, moisture, chemical, biological and photo degradation of the coating, coating thickness, nature of the coating, fertilizer particle size, and distribution density, along with, the (temperature dependent) solubility of the N sub-strate that is being released. The impact of these factors on N release rates can be controlled through the selection of a coating type and thickness that will be most effective for the environment in which it will be applied. Reactive-layer coated materials investigated in this study showed that the rate of N release is affected by temperature, coating thickness, size, and N substrate. Effect of Temperature
Temperature effects on release rates of N from the RLC materials investigated were evident by comparing N release at 25C and 35C (Figure 3-2). Increasing temperature increases the rate of diffusion of water vapor, as well as the solubility of the substrate resulting in an increase in the rate of N release. Hauck (1972) suggested that the first phase of nutrient release from coated fertilizers is the diffusion of water vapor from the surrounding environment through the coating into the granule. The second phase, after the
OH-R-OH + OCN-R'-NCO
Spray Nozzles 1 2
Urea i it* *
Rotating Drum (temp. 90 C)
1. OCN-R-NCO + H2N-C-NH2
^ Reactive Layer
2. Polyol + excess Diisocyanate
Figure 3-1. Reactive-layer coating process.
condensation of water inside the granule and the buildup of hydrostatic pressure, is the outward flow of concentrated fertilizer solution. Thus, temperature and solubility of the N substrate are the determining factors in the release rate of N from the fertilizer granule.
The increase in temperature from 25C to 35C increased release from 30% to 56% for CAS-4.7. The release rate from CAS-9.8 was doubled as well, but only from 3% to 6% as a result of increasing temperature. Release rates from C Urea-4.8 and C Urea-10 increased from 70% to 78% and from 38% to 51%, respectively. Christianson (1988), studied the effect of temperature on the release rate of urea from RLCU (7.6% coating weight) in a 5-week incubation study with temperatures ranging from 9 C to 3 0 C. He found that when temperature increased from 9 C to 3 0 C, the release rate of urea increased from 17% to 72%, respectively.
The difference in release rates between these two substrates can be attributed to differences in their solubility characteristics. The solubility of ammonium sulfate is lower than that of urea at the temperatures over which this experiment was conducted. Therefore, increasing temperature had a more profound affect on the solubility of AS than on urea, resulting in a larger increase in the release rate of N.
The increase in temperature, increased percent N release from the micro-size materials as well. However, the increase
CAS-7.5 CAS-15 C Urea-10.5 C Urea-21 C KN03-3 CKN03-6
Figure 3-2. Effect of temperature on % N released.
was smaller in magnitude compared to the macro-size materi-als, with the exception of CAS-15 and CKN03-6. These exceptions were most likely the result of a combination of the low solubility and high coating thickness on these two substrates compared to urea. In general, the percent release from the micro-size materials was higher than for their macro-size counterparts, even though the percent coating weight for the micro-size materials was generally higher. This is a result of the smaller particle size of these materials which, increases surface area and subsequently increases percent
Effect of Coating Thickness
Increasing coating thickness resulted in extended diffusion time and therefore, slower release rates and less N released during the 63-day experimental period (Figures 3-3 through 3-7). In general, increasing coating thickness resulted in a slower rate of release of N from the capsules, with the exception of the urea materials. As a result of the slower release rates, the more thickly coated materials released less N overall. However, although N release for both coating thicknesses increased at the higher temperature, the heavily coated materials had a higher rate of increase in N release. This resulted in a smaller difference in overall N release between the two coating thicknesses during the latter
part of the experimental period at the higher temperature.
Macro-size Granules at 25 C
Macro-Size Granules at 35 C
28 35 Time (days)
Figure 3-3. Effect of coating thickness and temperature
N release from CAS: macro-size.
g 3.5 o
Micro-Size Granules at 25 C
Micro-Size Granules at 35 C
g 3.5 o
Figure 3-4. Effect of coating thickness and temperature on
N release from CAS: micro-size.
Macro-Size Granules at 25 C 4.5 i-
Macro-Size Granules at 35 4.5 i-
3 7 14 21 28 35 42 49 56
Figure 3-5. Effect of coating thickness and temperature
N release from C Urea: macro-size.
Micro-Size Granules at 25 C
Micro-Size Granules at 35 C
Effect of coating thickness and temperature N release from C Urea: micro-size.
Micro-Size Granules at 25 C
Micro-Size Granules at 35 C
Figure 3-7. Effect of coating thickness and temperature on
N release from CKN03: micro-size.
Coated KN03-3 did not exhibit the slow-release characteristics that the other coated materials exhibited. Nitrogen release was essentially complete by the end of the first week of the experiment. This suggests that the 3% coating weight was not sufficient to regulate N release. However, increasing
the coating on KN03 from 3% to 6% had a profound effect on release rates. The total N released from CKN03-6 at 25C was the lowest compared to all other micro-size treatments.
A coating effect was also observed among the urea materials for overall N release (Figure 3-2). However, the release patterns did no": seem to reflect these differences (Figure 3-6) This may have been the result of NH3 volatilization from the urea materials, as was suggested by the high pH (9 to 10) of the solution sampled.
In general, the N release patterns observed were indicative of the two stage release process described by Christian-son (1988). During the first phase of N release, release rate is regulated by the rate of the dissolution of the N substrate within the granule and its subsequent movement through the coating wall. When dissolution of the N substrate is com-
, the rate of diffusion begins to decrease as the concentration of N decreases inside the granule during the second stage of N release. This explains the non-linear shapes of the N release curves shown in Figures 3-3 through 3-7.
Effect of N Substrate
Nitrogen substrates used in this study affected both the release rates and the quantities released. This is primarily the result of differences in their solubilities. In order for a substrate to diffuse through the coating membrane, it must first enter into solution, so solubility has a higher effect on the rate and quantity at which a substrate enters solution. Urea was the most soluble N substrate used. It reaches 100% solubility (w/v) at 17C (Perry and Chilton, 1973). Ammonium sulfate has a solubility of 75% at 20C. However, as temperature increases the solubility of ammonium sulfate increases to 78% at 30C and 81% at 40C. Potassium nitrate, on the other hand, has a solubility of 32% at 20C, increasing to 46% and to 64% at temperatures of 30 C and 40 C, respectively. Therefore, temperature and subsequently the solubility of the different N substrates, influenced the rate and quantity of N released. High solubility of urea (100% at 17C) suggests that varying temperature did not affect the solubility of urea as it did ammonium sulfate and potassium nitrate.
The reaction of these three N substrates with the coating may have also influenced the conductivity of the membrane and therefore the release rates of the N. Moore (1987) showed that the polymer used in reactive-layer coatings chemically reacts with -NH2 groups of urea and forms a reactive layer which affects the kinetics of the release. However, no information is available which suggests that the same reaction
occurs with inorganic N substrates such as ammonium sulfate and potassium nitrate. The data suggest that the reaction which takes place when urea is coated may have played a role in increasing the release of urea in addition to its solubility. However, coating thickness is the primary factor regulating the rate of N release.
Summary and Conclusions Reactive-layer coated N fertilizers showed different N release rates and subsequent total amount of N released. Factors affecting these differences were temperature, coating thickness, particle size and N substrate. A 10C increase in temperature increased total N released, ranging from 8% to 50% for the macro-size materials and from 3% to 28% for the micro-size materials. The ranges in N release were affected by coating thickness and N substrate, with increasing coating thickness decreasing the total percent of N released in all cases. However, the magnitude of the decrease depended on the N substrate.
This study demonstrated that the release of N can be optimized through the manipulation and consideration of different factors. Consideration of environmental factors such as temperature (climate and season) and selection of different coating thicknesses could optimize N release and subsequent use efficiency by the plant, and minimize losses to the environment.
CHAPTER 4 THE EFFECT OF REACTIVE-LAYER COATED UREA AND AMMONIUM SULFATE ON GROWTH AND
N UPTAKE OF BERMUDAGRASS
The turfgrass industry in the United States is a multi-billion dollar per year industry. Turfgrass use includes residential lawns, golf courses, parks and sports facilities, roadways, and educational and research facilities. Nitrogen is the primary nutrient required for turfgrass production and maintenance. This translates into several million tons of N applied to turfgrass annually.
Golf courses alone account for more than 1.3 million acres of managed turfgrass in the United States (Cohen et al., 1993), requiring approximately 300,000 tons of N annually. Therefore, the efficient use of N is essential for both economic and environmental reasons.
Controlled-release N fertilizers are used as a management tool to prolong N availability to turfgrass and to increase the use efficiency of the applied N. The objectives of the following two studies were to investigate the growth, N uptake, and quality response of bermudagrass to reactive-layer coated ammonium sulfate and urea with varying coating thicknesses.
Experiment 1: 1992
Materials and Methods This field study was conducted on established Tifway bermudagrass at the G.C. Horn Turfgrass Field Laboratory in Gainesville Florida between June and September, 1992. Treatments (Table 4-1) were arranged in a randomized complete block design and replicated three times.
Table 4-1. Nitrogen treatment specifications (1992).
Treatment N Substrate % Coating
(NH4)2S04 (AS) 0
Coated AS (CAS-4.7) (NH4)2S04 4.7
Coated AS 1 (CAS-9.8) 9.8
Coated Urea (CUrea-4.8) UREA 4.8
Coated Urea (CUrea-10) 10
Each treatment was applied to a 2 x 3 meter turf plot maintained at a 2 cm mowing height. Treatments were applied at 1.5 Kg N 100 m'2. All plots received phosphorus and potassium at a rate of 0.5 and 0.25 Kg 100 m"2, respectively. Plots receiving urea treatments also received sulfur in the form of calcium sulfate in order to balance the sulfur present in the AS treatments applied to other plots. Grass was mowed at a
height of 2 cm and all clippings were collected on a weekly
basis, for a total of 15 harvests during the 104-day experiment. Clippings were weighed for growth rates and digested for total Kjheldal nitrogen (TKN) analysis (Bremner and Mulvaney, 1982), using an air segmented Rapid Flow Analyzer (RFA). Visual quality ratings were taken biweekly for turfgrass quality and evaluations. Statistical analysis of data was performed using Statistical Analysis System (SAS) procedures. The least significant difference (LSD) method was used for means separation.
Results and Discussion Growth of Bermudagrass
Bermudagrass growth was affected by several factors throughout the experimental period including the available N pool, temperature, daylight, and rainfall. However, N substrate and coating thickness had the most profound effects on growth rates. Growth responses over the 104-day experiment are presented in Table 4-2.
Coated Urea-10 produced the highest total bermudagrass growth, followed by CAS-4.7 and C Urea-4.8. Plots receiving uncoated urea and ammonium sulfate produced similar overall
growth and an identical growth pattern. Bermudagrass growth was lowest in response to CAS-9.8, because the release rate of N was insufficient to sustain growth. Bermudagrass growth and N release from coated fertilizers were highest during the month of July for all treatments, because of optimum daylight, temperature and increased rainfall.
Ammonium sulfate treatments. Growth of bermudagrass during the first 32 days was not influenced by coating ammonium sulfate (AS). During this 32 day period, coated ammonium sulfate (CAS) with 4.7% and 9.8% coating produced 27% and 28% of total bermudagrass growth, respectively, for the 104 day experiment. However, the bermudagrass growth in response to uncoated ammonium sulfate (AS) during the 32 period was 3 6% of total growth. Cumulative growth of bermudagrass at 65 days for the CAS-9.8 treatment was lower than the other two AS treatments. The CAS-9.8 treatment had a slower N release rate than the other two AS treatments (Chapter 3).
Table 4-2. Cumulative growth of bermudagrass
following N application (g plot"1) .
Days After Application
Treatment 32 65 104
AS 501a* 1080a 1381b
CAS-4.7 507a 1303a 1848a
CAS-9.8 322a 726b 1153b
CV** 24 17 14
Urea 509a 1118a 1435b
CUrea-4.8 605a 1307a 1718ab
CUrea-10 569a 1373a 1876a
CV 20 15 13
*Mean values within a treatment group and column followed by the same letter are not significantly different according to LSDQ5; **Coefficient of variation (%).
The percent of total growth at 65 days differed according to fertilizer coating thickness. The CAS-9.8 treatment produced 63% of total bermudagrass growth during the 65 day period, whereas CAS-4.7 produced 71% of the total growth during this same period. However, AS produced 78% of the total growth during this time. This suggests a faster rate of depletion for this uncoated substrate.
Bermudagrass growth slowed in the latter part of the experiment in response to the uncoated AS treatment, apparently because of depletion of the N source. Total growth of bermudagrass in response to uncoated AS was 1381 g plot"1. Overall growth of bermudagrass in response to CAS-4.7 was highest (1848 g plot"1) at 104 days after N application and lowest for CAS-9.8 (1153 g plot"1).
The growth data indicate that coating AS increases total growth of bermudagrass over extended periods of time. However, if the coating is too thick, the N will fail to release rapidly enough and growth could be even lower in response to a heavily coated material than the uncoated material. Peacock and DiPaola (1992) found a similar result in their study comparing the effect of coated urea treatments with varying coating thicknesses on bermudagrass growth. The thickly coated urea treatment (12.5%) produced a significantly lower growth when compared to growth produced by the more thinly coated urea treatments and uncoated urea.
Urea treatments. The growth of bermudagrass during the first 32 days exhibited no differences to the coating of urea. During this period, coated urea (C Urea) with 4.8% and 10% coating and uncoated urea produced 30%, 35% and 35% of total growth of bermudagrass, respectively, for the 104 day experiment. Cumulative growth of bermudagrass at 65 days still showed no differences in response to the different coating thicknesses, with the percentages of total growth equalling 73% in response to C Urea-10, 76% in response to C Urea-4.8, and 78% in response to uncoated urea. By the 87th day of the experiment, however differences in growth were detected between C Urea-10 and uncoated urea. These differences persisted throughout the remainder of the 104 day experiment.
The growth data demonstrate that coating urea increased bermudagrass growth over time. Total bermudagrass growth was highest in response to C Urea-10 (1876 g plot"1) followed by C Urea-4.8 (1718 g plot'1) and then uncoated urea (1435 g plot"1). Although bermudagrass growth in response to uncoated urea was similar initially, growth dropped off as the applied N source was depleted. This data is consistent with the
findings of Mikkelsen et al. (1994) in a study they conducted comparing coated and uncoated N sources. They found that the coated materials were more effective in increasing yields of stems and leaves than uncoated materials because these
materials were better able to release nutrients in synchrony
with plant demand.
Bermudagrass N uptake pattern in response to the treatments was similar to that of growth (Table 4-3). Nitrogen substrate and coating thickness of applied N fertilizers affected total N uptake by the bermudagrass during the 104 day experiment.
Ammonium sulfate treatments. Bermudagrass N uptake during the first 32 days was not affected by differences in coating thickness among the ammonium sulfate treatments. During this 3 2 day period, coated ammonium sulfate (CAS) with 4.7% and 9.8% coating resulted in 30% and 26% of total N uptake by bermudagrass, respectively, for the 104 day
Table 4-3. Cumulative N uptake by bermudagrass
following N application (g plot"1) .
Days After Application
Treatment 32 65 104
AS 16. 68a* 35.34ab 44.82b
CAS-4.7 18.62a 46.45a 63.81a
CAS-9.8 9.85b 21.69b 37.63b
CV** 35 20 16
Urea 17.96a 39.30a 49.18b
CUrea-4.8 22.16a 48.03a 61.06ab
CUrea-10 19.74a 50.79a 67.87a
CV 23 15 13
*Mean values within a treatment group and column followed by the same letter are not significantly different according to LSDQ5; **Coefficient of variation (%) .
experiment. However, the N uptake by the bermudagrass during the 32 day period in response to uncoated ammonium sulfate (AS) was 38% of total N uptake. Cumulative N uptake by bermudagrass at 65 days showed a difference between CAS-9.8 and the other two AS treatments. The CAS-9.8 treatment did not have an N release rate that was sufficient to produce an N uptake similar to that of the other two AS treatments during this 65 day period. There was no difference in N uptake between AS and CAS-4.7.
The percent of total N uptake at 65 days differed according to coating thickness of the fertilizer. The CAS-9.8 treatment resulted in an N uptake of 22 g N plot'1 by the bermudagrass. This represents 58% of the total N uptake by bermudagrass during the 104 day experimental period. Cumulative N uptake of bermudagrass at 65 days for plots receiving CAS-4.7 was 46 g N plot"1, or 73% of the total N uptake over the 104 day experimental period. However, application of uncoated AS resulted in an N uptake of 35 g N plot"1. This represents 79% of the N uptake during entire the experimental period for this treatment.
At the end of the 104 day experimental period, differences in N uptake were observed among the ammonium sulfate treatments. Nitrogen uptake by bermudagrass in response to uncoated AS slowed in the latter part of the experiment, as a result of depletion of the applied N source. Total N uptake
by bermudagrass was highest in response to CAS-4.7 ( 64 g N
plot'1) at 104 days after N application, and lowest for CAS-9.8 (38 g N plot'1) Total N uptake by bermudagrass was 45 g N plot"1 in response to uncoated AS.
The N uptake data showed that coating ammonium sulfate with a 4.7% coating increases total N uptake by bermudagrass over extended periods of time. However, increasing the coating to 9.8% resulted in a substantial decrease in overall N uptake compared to CAS-4.7. Nitrogen uptake patterns revealed that, after 65 days, N uptake by bermudagrass in response to uncoated AS slowed because of the depletion of uncoated AS due to high N uptake during the first 65 days and higher losses of this uncoated N compared to the coated treatments. Starr and DeRoo (1981) found similar results in a study in which they looked at N uptake of (NH4)2S04 by Kentucky bluegrass-red fescue (Festuca rubra L.). They found that N uptake of this uncoated N source primarily took place within the first 3 weeks after application. They further stated that most of the N uptake which took place from 3 to 9 weeks was derived from the soil N pool.
Urea treatments. Bermudagrass N uptake during the first 32 days exhibited no differences in response to coating urea. Cumulative N uptake by bermudagrass at 65 days still showed no differences in response to the different coating thicknesses. Total bermudagrass N uptake was 68 g N plot'1 in response to C Urea-10 or 75% of total N uptake, 61 g N plot"1 or 76% of total N uptake in response to C Urea-4.8, and 49 g N plot"1 or
78% of total N uptake in response to uncoated urea. No differences in bermudagrass N-uptake were detected until the 80th day of the experiment. At this time, differences in N uptake were observed between C Urea-10 and uncoated urea. These differences persisted throughout the remainder of the 104 day experiment.
In a study conducted by Christianson (1988) comparing N release from reactive-layer coated urea with varying coating thicknesses, the rate of N release varied depending on coating thickness. He found that the granules with a thicker coating released urea at a slower rate. These findings would suggest that N uptake by bermudagrass should vary depending on the coating thickness of the N source applied to the turfgrass. However, N uptake data for the 1992 field experiment generally showed no differences in N uptake from the urea treatments. Quality Ratings
One of the primary functions of N fertilization in turfgrass is the production of a high quality turf. A subjective measure of turf quality is turf color and density. One of the objectives of the utilization of controlled release N fertilizers is to prolong the quality of turf without need for frequent applications of N. During the 104 day field experiment, quality ratings of bermudagrass were recorded on a bi-weekly basis on a scale of 1 to 9, with 9 representing a
superior quality turf, 1 representing brown or dead turf, and 5.5 representii.g minimum acceptable quality turf (Table 4-4).
Ammonium sulfate treatments. Bermudagrass quality was affected by treatment application throughout the experiment. Coated AS-4.7 produced the highest quality turfgrass, followed by uncoated AS. Coated AS-9.8 produced the lowest quality turfgrass, which is in agreement with the N uptake and growth data. Hummel (1989) obtained visual quality results similar to those produced by CAS-9.8 when resin-coated urea-270 (RCU-270) was applied to Kentucky bluegrass. He concluded that RCU-270 released N too slowly to produce turfgrass color comparable to the other treatments used in his experiment.
However, all treatments produced an acceptable quality
Table 4-4. Visual quality rating of bermudagrass
following N application.
Days Aftei : Appli cation
Treatment 14 28 42 56 70 84 98
AS 8. 3a 8.7a 9.0a 8.0b 8.0b 7.3a 7.3a
CAS-4.7 8. 0a 8. 3a 9.0a 9.0a 9.0a 6.7a 6.7a
CAS-9.8 7.7a 7.0b 8.0b 7.3c 7.3c 6. 3a 6.7a
CV** 9 6 0 4 4 8 8
Urea 8. 0a 8. 0a 8. 3a 7.7a 7.3a 7.7a 7.7a
CUrea-4.8 8.7a 8.3a 8.3a 7.7a 7.3a 6.7a 6. 3a
CUrea-10 7.7a 8. 0a 8.3a 8. 0a 8. 0a 7.3a 7.0a
CV 9 4 7 6 6 8 11
*Mean values within a treatment group and column followed by the same letter are not significantly different according to LSDni-; **Coefficient of variation (%) .
of turfgrass. Differences in turfgrass quality between CAS-9.8 and the other two treatments were detected at 28, Table(4-4) and 42 days after N fertilizer application. Coated AS-4.7 sustained the highest quality turf through the 70th day of the experiment. During this same time period, the effect of the other two treatments on turf quality declined and no differences between these two treatments could be detected subsequently. After the 70th day all treatment affects on turf quality diminished, resulting in no differences in turf quality.
Urea treatments. Unlike the ammonium sulfate treatments, differences in turfgrass quality for plots receiving the urea treatments were not detected until the 42nd day of the experiment. Both coated urea treatments showed differences when compared to uncoated urea at this time. However, these differences were not sustained for the remainder of the experiment. Overall, C Urea-10 produced the highest quality turfgrass, followed by C Urea-4.8 and uncoated urea. Bermudagrass quality response to the urea treatments followed the same trend as growth and N uptake for this treatment group.
Experiment 2; 1993 Materials and Methods This field study was conducted on established Tifway
bermudagrass (Cynodon transvaalensis Burtt-Davy X C. dactylon (L) Pers.) at the G.C. Horn Turfgrass Field Laboratory in
Gainesville, Florida between the months of June and August, 1993. Treatments (Table 4-5) were arranged in a randomized complete block design, replicated three times.
Table 4-5. Nitrogen treatment specifications (1993).
Treatment % Coating
Coated Urea (CUrea-6) 6
Coated Urea (CUrea-12) 12
Each treatment was applied to a 2 x 3 meter turf plot maintained at a 2 cm mowing height. Treatments were applied at a rate of 1.5 Kg N 100 m"2. All clippings were collected on a weekly basis, with a total of 8 harvests being collected during the 65 day experiment. Clippings were weighed to determine growth and digested for total Kjheldal nitrogen (TKN) analysis, using a Rapid Flow Analyzer (RFA). Visual quality ratings were taken weekly for turfgrass quality evaluation. Statistical analysis of data was performed using Statistical Aralysis System (SAS) procedures. The least significant difference (LSD) method was used for means separation.
Results and Discussion
Bermudagrass growth was determined during the 65 day
period by collecting all grass clippings on a weekly basis.
Cumulative growth of the "Tifway" bermudagrass is summarized
in Table 4-6. Bermudagrass growth, 21 days after treatment application, was highest for uncoated urea, followed by
C Urea-6 and C Urea-12. Uncoated urea produced a growth 5 times higher than C Urea-6 and 11 times higher than C Urea-12 during this time period. Growth by 29 days following treatment application differed for each treatment. Uncoated urea continued to have the highest growth among the treatments, producing 13 times higher growth than C Urea-12 during this time period. An increase in release of N from C Urea-6 narrowed the margin of difference in growth between this treatment and uncoated urea. However, uncoated urea still produced 2 times as much bermudagrass growth.
Table 4-6. Cumulative growth of bermudagrass following
N application (g plot"1)*.
------- Days After Application ---------
Treatment 12 21 29 36 43 50 57 65
Urea 39a 374a 575a 773a 944a 1106a 1200a 1289a
CUrea-6 9b 70b 291b 584a 830a 1056a 1200a 1310a
CUrea-12 14b 34b 45c 92b 176b 293b 388b 490b
CV** 38 15 21 17 15 14 12 11
*Mean values within each column followed by the same letter
are not significantly different according to LSD05. **Coefficient of variation (%) .
Midway through the experiment, bermudagrass growth was at 59% of its total growth in response to uncoated urea, followed by
45% of the total growth for bermudagrass receiving C Urea-6. Bermudagrass receiving C Urea-12, on the other hand, was only at 19% of its total growth by the 65th day of the experiment. Statistical analysis revealed that uncoated urea and C Urea-6 produced the same cumulative bermudagrass growth at this stage, and continued to produce the same cumulative growth throughout the remainder of the experiment.
Fifty days after application of uncoated urea, bermudagrass growth was at 86% of its total growth for the experiment, followed by 81% for C Urea-6. Coated Urea-12, on the other hand, had produced only 60% of the total bermudagrass growth during this same time period. At the end of the 65 day period, cumulative growth of bermudagrass in response to both uncoated urea and C Urea-6 was the same, but the total growth in response to C Urea-12 was only 37% of that produced by C Urea-6. However, 40% of the total growth produced by C Urea-12 was produced during the last 15 days of the experiment. This indicates the increased (or at least sustained) release of N from this heavily coated material during the latter part of the experiment. Nitrogen Uptake
Nitrogen uptake for the applied urea treatments during the 65 day experiment is summarized in Table 4-7. Uptake of applied N by the bermudagrass followed the same pattern as
growth. Uncoated urea resulted in the highest initial N uptake, followed by the two coated treatments. This trend was
maintained throughout the first 29 days. Statistical analysis of data for the third harvest indicated an increase in N uptake for bermudagrass receiving C Urea-6 material, which was sustained throughout the remainder of the experiment. Forty three days after treatment application, N uptake for bermudagrass receiving C Urea-6 surpassed that by bermudagrass receiving uncoated urea. This would suggest an increase in available N from the C Urea-6 treatment and a depletion of available N from the uncoated urea.
Bermudagrass receiving C Urea-12 had very low N uptake compared to the other two treatments, throughout the experiment. The data suggest that N uptake may have been impeded by
Table 4-7. Cumulative N uptake by bermudagrass following
N application (g plot"1) .
------- Days After Application --------
Treatment 12 21 29 36 43 50 57 65
Urea a 1.95 a 14.5 a 20.7 a 27.2 a 31.6 a 35.7 a 38.0 a 40.5
CUrea-6 b 0.21 b 2.64 b 11. 6 a 23.9 a 31.9 a 38.9 a 43.7 a 47.6
CUrea-12 b 0.29 b 0.75 c 0. 99 b 2.18 b 4.47 b 7.74 b 10.7 b 14.4
CV** 25 23 30 13 16 15 15 29
*Mean values within each column followed by the same letter
are not significantly different according to LSDQ5. **Coefficient of variation (%) .
the slow N-release rate for this material. A similar observation was made by Hummel (1989) during a study to evaluate differentially coated RLC urea materials. He concluded that the heavily coated material (RCU-270) released N too slowly to produce sufficient N uptake and growth.
Percent of total N uptake at 21 (N2) and 43 days after N application clearly identifies the effect of coating on N uptake by bermudagrass. Cumulative N uptake at 21 days showed 3 6% of the total N uptake for bermudagrass receiving uncoated urea and 5.5% of total N uptake for the two coated treatments. However, after 43 days, the N uptake values were 78%, 67% and 31% of total N uptake by bermudagrass receiving uncoated urea, C Urea-6 and C Urea-12, respectively. Overall N uptake was highest for bermudagrass receiving C Urea-6 and uncoated urea, even though uncoated urea had the highest N uptake for the first half of the experiment.
One explanation for the lower N uptake produced by the thickly coated urea treatment (12%) is the effect of soil moisture on the release of N. Less rainfall occurred during the 1993 experimental period, possibly influencing the release of N from the coated materials. The volumetric water content of the soil, effects the release process directly as it effects the diffusion of N through the coating membrane. Therefor, the greater the soil moisture content, the greater
the release of N from the coating resulting in more N available for plant uptake.
Bermudagrass visual quality ratings are summarized in Table 4-8. Uncoated urea produced the highest visual quality of bermudagrass for the first 21 days of the experiment.
Table 4-8. Visual quality rating of bermudagrass following
------- Days After Application -----
Treatment 12 21 29 36 43 50 57 65
Urea a 8.0 a 8.3 a 8.0 b 7.7 b 8.0 b 6.7 c 6.0 c 6.0
CUrea-6 b 6.0 b 6.7 a 8.0 a 9.0 a 9.0 a 8.0 b 7.0 a 8.0
CUrea-12 b 6.3 b 5.7 b 5.7 c 6.3 c 7.3 a 7.7 a 7.7 a 7.3
C. V. ** 4.9 8.4 4.6 6.1 4.1 6.3 4.8 4.9
*Mean values within each column followed by the same letter
are not significantly different according to LSDQ5. **Coef f icient of variation (%) .
Visual quality for bermudagrass receiving C Urea-6 matched that of bermudagrass receiving uncoated urea 29 days after application. This is most likely the result of an increase in available N from the C Urea-6. Thirty six days after treatment application, visual quality for bermudagrass receiving uncoated urea began to diminish as the available N from this source was depleted. Coated Urea-12 produced a lower-visual
quality bermudagrass for the first 43 days of the experiment.
However, visual quality of the bermudagrass receiving C Urea-12 increased during the last three weeks of the experiment.
This corresponds to findings made by Peacock and DiPaola (1992). In a study comparing the effects of different coating thicknesses for RLC urea on visual quality of bermudagrass, they observed that urea treatments with greater than 10% coating had a delayed effect on visual quality. They recommended that a soluble N source be mixed with these heavily coated materials to increase the visual quality of the bermudagrass more rapidly after application.
Summary and Conclusions
Bermudagrass growth and N uptake in response to coating urea differed from those obtained coating AS. No differences in growth or N uptake were detected in response to the two coated urea treatments until late in the experiment. However, differences between the two coated AS treatments were detected at 65 days. The data suggest that these differences are related to different release rates of the encapsulated N. Findings from the N release-rate study discussed in Chapter 3 revealed that urea has a higher release rate compared to AS with the same coating thickness. This is the result of the higher solubility of urea compared to AS, along with the more profound effect of temperature on urea's solubility and subsequent release. Uncoated AS and urea produced similar
cumulative growth and N uptake in bermudagrass.
Coating these N substrates resulted in an overall increase in growth and N uptake by bermudagrass with the exception of CAS-9.8. Coating AS with a 9.8% coating obstructed N release and resulted in overall lower N uptake for this treatment. In general, however, coating the N substrate resulted in increased growth and N uptake compared to the uncoated treatments. These increases reflect a direct influence of coating on release of the N substrate, resulting
in a higher growth and N uptake.
Visual quality of the bermudagrass was also affected by coating the N substrates. Coated AS-4.7 produced the most uniform and highest quality bermudagrass throughout the experiment. Uncoated AS produced its highest quality rating at 42 days and then steadily declined throughout the remainder of the experiment. This represents the normal pattern of quality for uncoated materials. With the exception of CAS-9.8, the ammonium sulfate group produced higher quality bermudagrass when compared to the urea treatments. One explanation is that the urea treatments produced a higher growth and N uptake resulting in a dilution of N concentrations in the turfgrass. This dilution effect resulted in a lower quality turfgrass. Bermudagrass response to the urea treatment group was not as profound as for the ammonium sulfate treatments. All the urea treatments produced the same quality of bermudagrass regardless of coating, suggesting that the high release rates of both coated urea treatments produced
the same quality turfgrass as uncoated urea. However, bermudagrass quality was acceptable throughout the experiment for all treatments. Quality ratings never reached the minimum acceptable level of quality for any of the treatments. Experiment 2
Differences between the two coated urea treatments were observed early in the experiment. Coated Urea-12 produced lower N uptake and growth than the other two urea treatments. Data indicate that the 12% coating on urea impeded the N release, resulting in lower N uptake and growth. As a result, visual quality of the bermudagrass receiving this treatment was lower compared to bermudagrass receiving the other urea
Coating urea with a 6% coating resulted in higher N uptake, growth and visual quality when compared to C Urea-12. However, no differences in N uptake and growth between uncoated urea and C Urea-6 were detected. One explanation is the low rainfall experienced during the experiment in 1993, resulting in lower leaching losses than would be typical for the uncoated material. Results from the lysimeter study (Chapter 5) indicated that no leaching of N occurred during the 1993 experimental period, as a result of low rainfall.
UPTAKE AND LEACHING OF REACTIVE-LAYER COATED
15N AMMONIUM SULFATE AND UREA
Nitrogen, once introduced into the soil, undergoes several dynamic transformations which constitute the N Cycle. It is these interconnected transformations which make N difficult to study in the soil/plant system. However, there is both an economic, as well as an environmental, need to determine the fate of N in turfgrass ecosystems. Numerous techniques have been used to investigate the fate of N. However, most studies have focused on a single process within the N cycle. In order to examine N transformations as an interacting unit it is necessary to trace N through the various components of the N cycle.
Through the use of lysimetery and 15N tracers, information regarding the partitioning of applied N in various ecosystems can be gained. These research tools can provide information regarding N uptake, soil retention, and leaching characteristics of applied N to turfgrass.
Nitrate contamination of groundwater has led to increasing public concern and consequently a need for more research investigating the fate of applied N fertilizers. Research is
focused on better understanding of N in the soil/plant system with an emphasis on increasing the use efficiency of N and reducing its leaching potential. Controlled-release fertilizers, and particularly Reactive-Layer Coated N sources, have been designed to extend the supply of N to the plant through
controlled release of the nutrient, consequently reducing leaching through higher uptake efficiency.
A field lysimeter study using 15N labeled material was designed with the following objectives: 1) to determine the effect of 15N labeled (NH4)2S04 and urea with different coating thicknesses on growth of, and N uptake by bermudagrass; 2) to quantify bermudagrass uptake of N from applied N sources; 3) to investigate leaching characteristics of the applied N sources and the influence of coating type and coating thickness on N leaching; and 4) to determine NH4-N and N03-N concentrations in the unsaturated and saturated zones underlying the fertilized lysimeters, including their impact on shallow groundwater.
Materials and Methods A field lysimeter study was conducted at the G.C. Horn Turfgrass Field Research Laboratory at the University of Florida in Gainesville, Fl. Twenty four open-ended lysimeters were constructed from PVC pipe 61 cm in diameter and cut to 122 cm length (Figure 5-1) A 15m X 15m area of Tifway bermudagrass (Cynodon transvaalensis Burtt-Davy X dactylon (L) Pers.) established on Arredondo Fine Sand (siliceous,
Ceramic Cup Sampler
Figure 5-1 Open-end lysimeter with multi-depth sampl
hyperthermic Grossarenic Paleudults) was selected for this study. The location of each lysimeter was prepared for installation through the removal of core samples, 1.25 cm in diameter, around the perimeter of the lysimeter and to a depth of 100 cm. Each lysimeter was hydraulically pressed to a depth of 120 cm. Installation was done in such a manner as not to disturb the soil column contained within the lysimeter. This lysimeter depth was chosen because the water table at the experimental site fluctuates approximately between the 120 cm and 90 cm depth.
Two types of devices were used for sampling leachate from within the lysimeter. A ceramic cup sampler 6 cm in diameter and 5.4 cm in length was used to sample soil solutions from the intermediate vadose zone at the 3 0cm depth. In anticipation of a fluctuating water table, a multi-depth sampler was also constructed in order to sample the saturated zone. The sampler was constructed from PVC pipe 5 cm in diameter and 90 cm long. Four sampling ports were installed in the PVC pipe to take water samples at the 45, 70, 95 and 120 cm depths when ever the soil was saturated. Polyethylene tubing was connected to the four sampling ports and the tubing was pulled to the surface. A core sample extending through the center of each lysimeter was extracted using a hydraulic core sampler equipped with a plastic liner for maintaining intact core samples. These core samples were sectioned and analyzed for pre-study chemical and physical analysis (Tables 5-1, 5-2 and
5-3) The sampler was then placed in the center of the lysimeter where the core sample was removed. The multi-depth sampler was placed so that the sampling ports were located 45 cm, 70 cm, 95 cm, and 120 cm from the soil surface. A layer of soil was then placed on top of the multi-depth sampler and the ceramic cup sampler was positioned at 3 0 cm below the surface. A slurry of Arredondo fine sand was poured around the samplers so that there would be maximum contact with the soil. All sampling tubes were brought to the edge of the lysimeter and pulled to the surface through holes drilled 15 cm from the top of the lysimeter.
Labeled 15N (NH4)2S04 and urea (5 atom % 15N) were pellet-ized by the Tennessee Valley Authority (TVA) in order to facilitate the Reactive-Layer Coating process. Ammonium sulfate and urea were coated with two different coating
thicknesses (Table 5-4). In addition to the coated treatments, both substrates were applied in uncoated form as well. These 6 treatments were applied to the bermudagrass in 4 replications, using a complete randomized block design. Treatments were applied at 1.5 kg N 100 m"2 yr"1 with the labeled 15N materials being applied once, in the first year.
Study design, construction, and installation was completed in December of 1991, with the study being initiated in May of 1992. Irrigation was maintained to meet estimated
evapotranspiration (ET) rates. During the growing season,
Table 5-1. Selected soil chemical and physical properties
of the bermudagrass site.
Depth (cm) TKN (mg kg-1) PH Bulk Density (g cm-3) Porosity (%)
0-15 543* (318-698)+ 6.5 (6.1-6.8) 1.26 (1.05-1.52) 52 (43-60)
15-30 216 (148-275) 6.4 (6.1-6.8) 1.49 (1.37-1.63) 44 (39-48)
30-60 135 (63-148) 6.4 (6.2-6.6) 1.50 (1.35-1.62) 43 (39-49)
60-90 117 (63-148) 6.1 (5.8-6.6) 1.61 (1.45-1.69) 39 (36-45)
*Mean of 24 samples; +Range of values;
Table 5-2. Total N and C:N ratios in the
bermudagrass root-zone soil.
Depth (cm) TKN (mg kg-1) C:N
0-5 1371* (1000-1730)* 13.1 (11.6-15.0)
5-15 470 (310-650) 17.0 (14.0-21.3)
* Mean of 24 samples; +Range of values;
Table 5-3. Selected soil chemical properties of bermudagrass
Depth (cm) Ca (mg kg"1) Mg (mg kg"1) K (mg kg'1) P (mg kg"1)
0-5 1508* (1184-1948)* 113 (83-163) 41.8 (24.8-56.0) 204 (155-282)
5-15 942 (568-1196) 80 (44-108) 28. 0 (15.2-50.0) 230 (201-277)
*Mean of 24 samples; +Range of values;
rainfall and irrigation, and soil and ambient air temperatures were recorded (APPENDIX). Bermudagrass was maintained at a height of 2.5 cm. Grass clippings were collected on a weekly basis for 90 days during the bermuda- grass growing period. Clippings were analyzed for total Kjheldal nitrogen (TKN) (Bremner and Mulvaney, 1982) and 15N using a Rapid Flow Analyzer and a mass spectrometer (Hauck, 1982),
Table 5-4. Nitrogen treatment specifications.
Treatment N Substrate % Coating
(NH4)2S04 (AS) (NH4)2S04 0
Coated AS (CAS-4.7) 4.7
Coated AS (CAS-9.8) 9.8
Urea UREA 0
Coated Urea (CUrea-4.8) 4.8
Coated Urea (CUrea-10) 10
A manifold connected to a vacuum pump was used to collect soil solution samples at depths of 3 0 cm, 95 cm and 12 0 cm simul-
taneously. These samples were analyzed for
N03-N and NH4-N
Samples collected from the lysimeters receiving urea treatments were also analyzed for urea during the first two weeks
of the experiment.
Soil, thatch, and root samples were
collected at the end of each growing season and analyzed for TKN and 15N.
Bermudagrass N recovery was determined using the 15N tracer method according to Hauck and Bremner (1976) and also the difference method (chapter 2). Recovery of applied N in soil and thatch was determined by the 15N tracer method. Statistical analysis of data was performed using Statistical Analysis System (SAS) procedures. The least significant difference (LSD) method was used for means separation.
Results and Discussion
Tifway bermudagrass growth was affected by the N substrate and coating thickness of the applied N fertilizer (Table 5-5) Coated urea with a 10% coating (C Urea-10) and coated ammonium sulfate with a 4.7% coating (CAS-4.7) produced the highest bermudagrass growth, followed by coated urea with a 4.8% coating (C Urea-4.8). Uncoated AS and urea had similar growth patterns, with AS producing a slightly higher overall growth. Coated AS with a 9.8% coating (CAS-9.8) produced the lowest bermudagrass growth.
Bermudagrass growth is affected by several factors, including the available N pool, the growth cycle of bermudagrass, temperature and rainfall (Hummel and Waddington, 1984). Growth data suggest that these factors produced an overall net effect on the growth rates according to the magnitudes and degrees of interaction between the respective factors .
Ammonium sulfate treatments. During the first 3 0 days after treatment application, differences in growth were
observed in response to the three AS treatments. Uncoated AS produced the highest growth during this time period. Lysimeters receiving uncoated AS produced 58% of their total growth during the first 3 0 days of the experiment. This may be the result of the quick availability of applied N in this substrate form. Coated AS-4.7 produced the second highest rate of growth among the AS treatments, followed by CAS-9.8.
Table 5-5. Growth of Tifway bermudagrass following
N application (g lysimeter"1) .
Treatment Monthly Growth at 30d 60d 90d Total Growth
AS 18.7a+ 11. lb 2.6b 32.3a
CAS-4.7 8.9b 17. 3a 10.0a 36.2a
CAS-9.8 2.7c 4.9c 4.7b 12.3b
Urea 16.9a 8.9b 4.0b 29.8a
CUrea-4.7 11.4b 15.5a 7.4a 34.3a
CUrea-10 8.8b 17.2a 10.3a 36. 3a
+ Mean values within a treatment group and column followed by the same letter are not significantly different according to LSDnc.
Bermudagrass growth in response to these two treatments was affected by the different coating thicknesses. Application of CAS-4.7 resulted in 25% of the total bermudagrass growth during this 30 day period. Coated AS-9.8 produced 22% of the
total bermudagrass growth during this same period. The higher
growth in response to CAS-4.7 probably reflects the higher release rate of N with this thinner coating thickness compared
During the second 3 0 days, CAS-4.7 produced the highest bermudagrass growth among the AS treatments, resulting in 48% of total growth. Uncoated AS followed producing 34% of its total bermudagrass growth during this 3 0 day interval. Coated AS-9.8 again produced the lowest bermudagrass growth. During the 60 to 90 day period of the bermudagrass growing season, no differences in growth between uncoated AS and CAS-9.8 were detected. Uncoated AS produced the lowest growth, accounting for only 8% of total growth. However, coated AS-9.8 produced 38% of its total bermudagrass growth during this same time period. No differences were detected between these two treatments because growth resulting from uncoated AS decreased substantially, and the growth rate for bermudagrass receiving CAS-9.8 was maintained. Bermudagrass growth in lysimeters receiving CAS-4.7 decreased, resulting in only 27% of the total seasonal bermudagrass growth. However, CAS-4.7 maintained a reasonable growth rate during this period.
Urea treatments. During the first 30 days after treatment application, differences in growth were observed between the uncoated treatment and the two coated urea treatments. Uncoated urea produced the highest growth during this time period, with lysimeters receiving uncoated urea producing 57% of their total growth during the first 30 days of the experi-
ment. No differences among the two coated urea treatments were detected. Application of C Urea-4.8 resulted in 33% of total seasonal bermudagrass growth during this 3 0 day period, while C Urea-10 produced 24% of total seasonal bermudagrass growth during this same period. Though C Urea-4.8 produced a higher growth rate than C Urea-10, these differences were not significant.
During the second 3 0 days, growth resulting from application of uncoated urea was reduced, for the N supply from this
uncoated N source was nearly depleted as a result of high initial uptake. Therefore, uncoated urea only produced 30% of its total growth during this period, resulting in the lowest growth among the three urea treatments. While the coated urea treatments produced the highest growth, statistical analysis revealed no differences between them. Bermudagrass growth in response to coated Urea-4.8 was 45% of total seasonal growth during this period, while C Urea-10 produced 47% of total seasonal bermudagrass growth during this same 30 day period.
During the 60 to 90 day period, the coated urea treatments again produced the highest growth of bermudagrass, though no differences between the two treatments were detected. Coated Urea-4.8 produced 22% of total seasonal bermudagrass growth, while C Urea-10 produced 29% of total seasonal bermudagrass growth. Lysimeters receiving uncoated urea produced the lowest growth, with only 13% of total growth being produced during the final 30 days of the season.
Tifway bermudagrass N uptake was determined using both the Difference and tracer methods. Both methods revealed that Tifway bermudagrass N uptake was affected by coating thickness and subsequent N release rate of the applied N fertilizer, as well as by growing conditions. Similar results were reported by Hummel and Waddington (1981) and Hummel and Waddington (1984) The N uptake pattern for bermudagrass was similar to that of growth. Both coated urea treatments produced the highest N uptake, followed by CAS-4.7. Uncoated urea and AS produced the same N uptake by bermudagrass. Coated AS-9.8 produced the lowest N uptake throughout the bermudagrass growing season.
Ammonium sulfate treatments. During the first 3 0 days after fertilization, differences in N uptake were observed in response to the three AS treatments (Table 5-6). Using both the Difference and Tracer methods, uncoated AS produced the highest growth. This is consistent with the findings of Waddington et al. (1985). In their study initial "flushes" of growth and N uptake were observed in response to the application of (NH4)2S04.
However, the effects from the applied N quickly waned in this case, due to the soluble nature of these materials. Bermudagrass receiving uncoated AS produced 67% of its total
N uptake during the first 30 days of the experiment, according
to the 15N Tracer method. When data were calculated using the
Table 5-6. Nitrogen uptake calculated using the 15N tracer
and difference methods (mg N lysimeter'1) .
Treatment NUP*30 NUP60 NUP90
15N Diff. 15N Diff. 15N Diff.
AS 969 a+ 845 a 370 b 424 b 115 c 129 c
CAS-4.7 497 b 345 b 800 a 725 a 537 a 525 a
CAS-9.8 77 c 107 c 185 c 253 c 228 b 325 b
Urea 965 a 732 a 360 b 324 b 145 c 210 c
CUrea-4.8 752 b 547 b 931 a 698 a 420 b 405 b
CUrea-10 393 C 434 b 1062a 887 a 724 a 709 a
* NUP3 0, NUP60 and NUP90=monthly N uptake at 30, 60 and 90 days.
+ Mean values within a treatment group and column followed by the same letter are not significantly different according to
Difference method, N uptake for this period was 63% of the total N uptake. The high initial N uptake is the result of the rapid availability of applied N in this substrate form. Coated AS-4.7 produced the second highest N uptake among the AS treatments, followed by CAS-9.8. Bermudagrass N uptake in response to these two treatments was affected by coating thickness. Based on the 15N Tracer method, application of CAS-4.7 resulted in 28% of total bermudagrass N uptake during this 30 day period. This percentage was slightly different, 24%, when N uptake data were calculated using the Difference method. Coated AS-9.8 produced 15.8% of total bermudagrass N uptake according to the 15N Tracer method and 18% according to the Difference method during this same period. The higher N uptake in response to CAS-4.7 reflects the higher release rate