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Using Resin Traps for Assessment of Nitrogen Leaching in Agricultural Production Systems

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Using Resin Traps for Assessment of Nitrogen Leaching in Agricultural Production Systems
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Reno, Kari
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Journal of Undergraduate Research

Volume 8, Issue 1 - September/ October 2006



Using Resin Traps for Assessment of Nitrogen Leaching in
Agricultural Production Systems

Kari Reno


ABSTRACT


Nitrogen is one of the most limiting nutrients for global crop production. However, it is becoming

increasingly important for farmers to design nutrient management plans that will maximize yield while

reducing fertilizer costs and potential groundwater contamination associated with nitrogen (N) leaching below

the root zone. To help determine optimal fertilizer application rates, several contributing factors need to be

studied, including soil leaching and root nitrogen uptake capacity of specific agricultural production systems. In

this project we studied the feasibility of using resin traps for assessing N-leaching in sweet corn (Zea mays). We

used 5.2 cm wide and 44 cm tall PVC columns filled with a sandy soil including 10 g Rexyn 500 resin placed in

the middle section of the soil columns. After application of ammonium nitrate (AN) corresponding to field

applications rates of 0, 50, 100, and 200 kg N/ha and an equilibration period of 2 days, residual soil N was

leached with 3.3 pore volumes of water. As the N application rate increased, the efficacy of nitrate-N (NO3-

N) recovery by the resin material decreased from 98.6 to 61%. Ammonium (NH4-N) recovery from the resin

material was relatively low, decreasing from 22.5 to 7.3% with increasing AN rates. The amount of N bypassing

the resin trap was on the order of 5.6-12.1% and 1.3-10.0% for ammonium and nitrate, respectively. We

concluded that the ion-exchange resin was most efficient in recovering nitrate from the soil column when AN

was applied at a low rate. Poor ammonium recovery may be related to ammonium being absorbed by the

soil surrounding the resin material which was not included in our N analysis. It is concluded that interaction

between soil and ammonium may interfere with the extraction process N for short-term N leaching

determination. However, for field studies this may be less of an issue, since ammonium converts to nitrate within 1-

2 weeks, and this readily leachable form of nitrogen therefore will be the most prevalent N form under

Florida conditions. Our findings are important, because they point out the limitations of our approach and will help

us to design better assessment tools that will be able to recover a higher percentage of N leached below the

active root zone. We plan to repeat these studies using KNO3 as an N source to avoid ammonium retention by the

soil complex and the interference of transformation processes such as nitrification on fertilizer nitrate recovery by

the resin. This may be more representative of long-term leaching dynamics under field conditions.



INTRODUCTION


With the world population nearing 6.5 billion at the end of 2004, food producers are being challenged to continue

to maximize crop yields while minimizing production costs. At the same time, they are also being forced to adhere





to stricter regulations governing fertilizer use through implementation of nutrient management plans. Nitrogen (N),

a naturally occurring compound, is found in all ecosystems in various forms. There are three major natural inputs

to the soil N cycle: atmospheric deposition, biological N fixation, and weathering and decomposition (Pierzynski et

al. 1989).



The major losses of N from the soil are due to crop removal and leaching. However, under certain

conditions, inorganic N can be converted to gases and lost to the atmosphere (Havlin et al. 1990). This

occurs through denitrification and ammonia (NH3) volatilization, which can increase with higher temperatures. Loss

of N from the active root zone (leaching) can be appreciable, especially for sandy soils and it can be induced

by excessive irrigation or high rainfall events. Runoff may be induced by high intensity rainfall events especially

with fine textured and/or compacted soils. Environmental concerns about N arise when one of the N

transformations results in the conversion and concentration of N in a form that can adversely affect the health

or quality of an organism or ecosystem (Pierzynski et al 1989). This is the case for both leaching and

volatization, which cause pollution and have become major concerns because of groundwater contamination

and increased atmospheric levels of N.



Most transformations of N are dependent on how much N is utilized by plants. Ammonia volatization occurs as NH3

is lost from the soil as a gas due to the use of surface application of ammonia fertilizers on high pH soils with

low cation exchange and under hot and dry conditions (Havlin et al 1990). Nitrification can also occur as NH3

is converted to N03-N via nitrifying bacteria in the soil. This can increase N leaching, as more nitrate is available

for flushing below the root zone. Denitrification occurs when soils become anaerobic due to waterlogged

conditions. Inorganic N is converted to N2 and N20 via anaerobic organisms that break down NO2 -and NO3 -

The rate of denitrification is affected by many factors including organic matter, soil moisture, pH, aeration

and temperature (Havlin et al 1990). There is concern that increased fertilizers may substantially

increase denitrification and N20 emissions from soils, which has been identified as a greenhouse gas that

destroys the ozone layer.



Fertilizer use efficiency will be critical to minimize leaching and will require an improved understanding of

the processes that control crop N uptake efficiency and leaching. The efficiency and utilization of N by plants

varies among cultivar type and crop type, but there are other variables that affect the role of N in crop

production. Environmental factors affect the amount of N taken up by a crop and how efficiently it is used. If

too much N is applied, there can be residual N left in the soil. The amount of residual N depends on factors such

as plant uptake capacity, nutrient requirements, release from the previous soil, soil organic matter and soil

water content, temperature and N in rainfall, and irrigation rates.



Adequate N supply is crucial to plants in almost every aspect, from seedling development and emergence

to photosynthesis and yield formation. Crop yield is typically closely related to nutrient supply and/or

availability. Understanding the way plants use N in each part of the life cycle is the key to maintaining and

improving crop yield (Sinclair et al. 1989). N supply not only affects plant growth in terms of biomass but also

the size and proportion of organs and their structure (Pearman et al. 1977; Greenwood et al. 1991; Lemaire et





al. 1992; Jeuffroy et al. 1997) Increased N supply increases the number of cells per leaf and their size, and as

a result, the way proteins are synthesized. This allows for efficient growth and expansion of cells.



However, excessive N application may have adverse effects on both crop production and the environment.

Studies have shown that the efficiency of N absorption decreases as the amount of N applied increases. It is not

only important to supply the right amount of nitrogen to a crop, but also at the right time. The most important

time for N supply is during stem elongation and fruit or ear formation. Excess N can cause excess growth and

weak stems and therefore result in lodging, an increase in pests and weeds and can cause leaching of N into

the groundwater. It can also prevent the filling in of grains and flowering.



According to the National Academy of Sciences, "The nitrogen applied to corn in synthetic fertilizer exceeded

that removed in the grain (corn) by 50 percent or more every year since 1968" (NRC 1993; NRDC 1994).

These calculations did not include residual N left on fields from previous crops. According to the Academy

committee, overall N application including all types of fertilizer and exceeds crop needs by approximately 33

percent, or eight billion pounds of excess nitrogen per year (NRC 1993). The most important way to improve

N management is to account for all residues and manure then supplement crops with additional N if

necessary. However, most farmers do not consider residual N nor use an N budget approach when

calculating fertilizer rates.



Nitrate Problems in Florida


In Florida alone, the production of citrus and vegetables are each billion-dollar industries annually (McNeal et

al. 1995). Maintaining high crop production on Florida sandy soils requires continuous application of both

fertilizer and irrigation. Most soils have low organic matter and poor water and nutrient retention, leading to

high leaching rates and creating a vicious cycle as farmers over-fertilize to make up for the low inherent soil

fertility and continuous loss of applied nutrients. With increased fertilizer use, high nitrate levels result in increased

N leaching and/or runoff, thereby threatening water resources. When lakes, streams, rivers and

groundwater resources are contaminated, it eventually leads to poor water quality in many areas and it

potentially results in permanently impacted potable drinking water supplies.



Nitrate contamination poses numerous health risks to humans and animals. It also disrupts healthy

ecosystems. High nitrate levels are attributed to several illnesses including methemoglobenemia, a condition

where levels of methemoglobin become elevated in the bloodstream, causing decreased oxygen transport.

This condition can lead to brain damage and death, especially in infants (Pierzynski et al 1989). Elevated

nitrate levels have also been associated with gastrointestinal problems, stomach cancers, birth defects, and

thyroid dysfunction.



Hazardous levels of nitrogen originating from nitrogenous fertilizers are being found in numerous

locations throughout the state, from the Suwannee River Basin to West-Central Florida, where citrus flatwoods

and shallow-rooted vegetables are grown. According to the Environmental Protection Agency, 10 parts per




million (ppm) is considered the federal limit for nitrate in drinking water. Along the Suwannee River, nitrate

levels above this level have been found in many areas. This has prompted necessary action by government

agencies to reduce the amount of nitrates flowing into the Suwannee River. It has been shown that the majority

of nitrates entering the watershed are from septic tanks and agricultural operations. The river is now on the

state's impaired water body list. There are several other rivers and ecosystems around the state with

similar problems.



In lakes around Florida, eutrophication has become a major environmental problem due to increased fertilizer

use and introduction of other non-point sources into watersheds surrounding lake bodies. Eutrophication has

caused the accelerated growth of algae, depletion of dissolved oxygen levels, increased turbidity and a

general degradation of water quality around Florida. This threatens to eradicate keystone species in

lake ecosystems, which can cause imbalances in other fragile environments around the state.



BACKGROUND/OBJECTIVE


Although many methods have been used to monitor N leaching dynamics in agricultural systems, there is

still argument as to which method yields the best data. The goal in any method is to assess the effects of

farm management practices on potential groundwater loading rates. The objective of this study was to develop

a better method for assessing the effects of farm management practices on nitrogen loading of

groundwater resources underlying agricultural production systems. In the past the following methods have been

used to assess N leaching: Monitoring wells (saturated zone), soil coring, suction lysimeters, and resin

traps (rhizosphere).



Monitoring wells are used frequently to assess nitrogen in both vegetable production and animal

agricultural operations. Wells are placed in the saturated zone using a multi-level sampling port approach

to determine N levels at different depths. Using monitoring wells greatly decreases operational variability

during sampling events and also gives accurate measurements of actual groundwater conditions. Suction

lysimeters are devices that are placed in the unsaturated zone during the growing season. A vacuum is applied to

a collection cup where a water sample is taken and analyzed. This method only provides a concentration of

nutrients and doesn't give actual loading rates. Soil coring is another method common in agricultural production. It

is relatively easy to do; however, it is labor intensive, doesn't provide loading rates and causes more disturbances

in the surrounding soils.



The device tested in this experiment is referred to as a "resin trap," which uses an ion-exchange resin to catch

ions as they filter down through the soil column. This method has many advantages. Traps can be buried in a

field for an entire growing season after which the resin can be extracted and analyzed for nutrients. With a good

resin trap design, this method allows scientists to calculate potential N loading rates of groundwater resources as

all of the nitrogen leaving the rhizosphere is forced to pass through the resin filter. This allows for more

accurate assessment of N loading rates. Once loading rates are assessed, recommendations can be made to

improve nutrient management. Developing management tools to help farmers assess, plan and manage

nutrient applications is one of the ways to help decrease degradation to the environment, while helping meet






the world's future food needs. These plans also enable farmers to minimize financial burdens associated

with excessive fertilizer costs and decreased yields.



METHODS AND MATERIALS


Sixteen resin traps were constructed using 44 cm high PVC pipe with a diameter of 5.2 cm, polyester filter

screen, and river rocks (Figure 1). In order to duplicate field conditions, predetermined amounts of soil were

added to the top and bottom portions of the columns, with a solid layer of Rexyn 500 (Fisher Scientific), an

ion-exchange resin, in between each soil layer. In the bottom portion of each tube, a polyurethane filter screen

was attached using adhesive tape. Small washed river rocks were placed at the bottom of the resin trap to

enhance drainage and to prevent the sand from washing out of the bottom of the column. After rinsing the rocks

with deionized water, 572.3 g of soil was added (approximately 19 cm from top of rocks) and 10 g of Rexyn 500

resin was placed in a solid layer on top of the soil. The top 20.2 cm of the tube was then filled with 504.9 g of

soil. Four reps of four fertilizer treatments were used in a randomized design and the columns were placed into

a wooden frame.



Ammonium nitrate granules were applied at four rates, equivalents of 0, 50, 100, and 200 kg/ha to each

column translating to fertilizer application rates of 0, 0.03 g, 0.06 g, and 0.012 g NH4NO3 per column. The same

day the fertilizer was added, the columns were leached using 50 mL of deionized water. On the second day,

another 40 mL of deionized was leached through the columns to dissolve any remaining fertilizer. On the third

day, the columns were flushed using 850 mL of deionized water. The leachate that passed through the resin trap

was collected and filtered through Whatman #42 filter paper, then put into 20 mL polyethylene scintillation vials.

The resin cores were extracted along with soil above and below the resin layer to total 200 g. The resin/soil

mixtures were weighed and placed into sixteen individual 2 L large-mouth amber Nalgene� bottles with 1200 mL

of 2M KCI and were shaken for 100 minutes. Each resin/soil mixture was filtered through Whatman #42 filter

paper and put into 20 mL polyethylene scintillation vials. The filtered leachate and resin extracts were analyzed

for NH4-N and N03-N. NH4-N was analyzed on a Technicon AAII using EPA method number 351.2. N03-N

was analyzed on a Rapid Flow Analyzer (RFA) using EPA method number 353.2.









soil
- 202 cm


4.7cm - ""' - Soil/resin mix





- 19cm
soil


Rocks I - 4.- cm


Figure 1. Schematic drawing of the resin trap used for determination of N leaching.




RESULTS/DISCUSSION


When the resin was extracted and analyzed, there was an average of 5.031 mg of N03-N, and an average of

1.150 mg of NH4-N intercepted by the resin at the application rate of 50 kg/ha or 5.1 mg NH4 N03 per column

(Table 1). This translates to an average recovery of 98.6% for N03-N and 22.5% for NH4-N at the 50 kg/ha

rate (Table 2). Subsequently, the trend of recovery decreased as the N-fertilizer application rate increased. At

the 100 kg/ha rate or 10.2 mg NH4N03 per column, an average of 8.6 mg of N03-N (84.2%) and an average of

1.131 mg NH4-N (11.1%) was recovered by the resin. The average recovery at 200 kg/ha or 20.4 mg NH4NO3

per column was 11.27 mg of N03-N (55.2%) and 1.48 mg NH4-N (7.3%). The average total recovery of

both ammonium and nitrates in the resin was 6.81 mg of NH4NO3 applied at the 50 kg/ha rate or 10.2 mg per

column (60.6%), 9.716 mg of NH4NO3 applied at the 100 kg/ha rate or 20.4 mg per column (47.6%) and 12.75

mg of NH4NO3 applied at the 200 kg/ha rate or 40.8 mg per column (31.3%).



Table 1. Average leaching and recovery of N03-N and NH4-N leaching, interception by resin layers and fertilizer N recovery

as affected by N-rates. N application was 0, 5.1,10.2, and 20.4 mg N03-N and NH4-N per column.


N-rate Nitrogen leached
(kg N/ha) (mg N/column)

N03-N


Nitrogen intercepted
(mg N/column)

NH4-N Total-N N03-N


Nitrogen recovery
(mg N/column)

NH4-N Total-N N03-N


NH4-N Total-N


0 0176 0380 0556 0611 0962 1573 0787 1 342 2129

50 0512 0617 1 129 5031 1 150 6181 5543 1 767 7310

100 0134 0738 0872 8585 1 131 9716 8719 1 869 10588

200 0331 1 149 1480 11270 1480 12750 11601 2629 14230






Table 2.

Average leaching and recovery percentages of N03-N and NH4-N leaching, interception by resin layers and fertilizer

N recovery as affected by N-rates.

N-rate Nitrogen leached Nitrogen intercepted Nitrogen recovery
(kg/ha) (% apple) (% apple) (% appt)

N03-N NH4-N Total-N NO3-N NH4-N Total-N NO3-N NH4-N Total-N

0 N/A N/A N/A N/A N/A N/A N/A N/A N/A

50 100 121 11 1 986 225 606 1086 346 716

100 13 72 43 842 11 1 476 855 183 519

200 16 56 36 552 73 312 568 129 348


Analysis of the leachate showed that at the application rate of 50 kg/ha or 5.1 mg NH4N03 per column, there was

an average of 0.520 mg (10.4%) of N03-N and an average of 0.617 mg of NH4-N (12.1%) that bypassed the

resin layer and ended up in the leachate. When the leachate was analyzed at the application rate of 100 kg/ha

or 10.2 mg NH4N03 per column, there was an average of 0.134 mg of N03-N (1.3%) and 0.738 mg NH4-N

(7.2%) that bypassed the resin layer and ended up in the leachate. At the 200 kg/ha rate or 20.4 mg NH4N03

per column, an average of 0.331 mg N03-N (1.6%) and 1.149 mg (5.6%) NH4-N bypassed the resin layer and

ended up in the leachate.



The average total amount of NH4N03 leached through the columns was 1.137, 0.872, and 1.48 mg of NH4NO3 for

the 50, 100, and 200 kg N/ha rate (10.2, 20.4, and 40.8 mg AN per column), respectively. This corresponds to

11.3, 4.3, and 3.6% of the total amount of fertilizer N that was applied.



Based on the above results, it is concluded that ammonium was lost due to volatilization or retained by the soil

above or below the resin trap. Alternatively, it may also have been converted to nitrate too quickly to be

accounted for. The total recovery rates of nitrate and ammonium in the resin are a good indication that the

resin could be a valuable nitrogen assessment tool. Although the rates of recovery decreased with

increased application of fertilizer, there is an obvious trend. A possible reason the resin did not capture all of the

ions could be because of the retention capacity of the individual resin beads, where some beads may

become saturated on one side and not on the other, thereby limiting the use of the whole resin bead surface.

This could change the way the fertilizer solution interacts with the resin matrix.



CONCLUSIONS


It is concluded that although the resin material appeared to be relatively effective in intercepting nitrate at

lower rates, the recovery of ammonium was rather poor. Although only 1-10% of the applied fertilizer bypassed

the resin trap, it appeared that appreciable amounts of NH4-N may have been retained by the soil surrounding

the resin trap, which was not included in the soil analysis. A possible solution to this problem would be to fill





the entire column with a homogenous soil/resin mix, which would duplicate field conditions better and allow

more interaction of the fertilizer solution with the resin. However, this approach would also have some

disadvantages since some times the soil from either the top or bottom of the resin trap can be lost during

excavation of the resin trap, especially if the soil is fairly dry. Another possible solution would be to take samples

of both the top and bottom portions of the soil column. In our experiment, only 200 g of soil was used to extract

the resin core. It would be desirable to analyze both the soil sections below and above the resin trap for NH4-N

or N03-N in addition to an analysis of the soil/resin mixture or the leachate so that all of the N in the system can

be accounted for. One of the limitations of using ammonium nitrate as an N-fertilizer source for a short-

term experiment was that initially at least 50% of the nitrogen was in the ammonium form. It is expected that

if leaching was initiated after one week instead of two days, nitrogen recovery would have been much greater. It

may be desirable to use KNO3 as an N source instead or wait for a minimum of 7-14 days before leaching to

mimic field conditions, which we aim to do in future studies. It is anticipated that current studies will contribute

to improved design and use of resin traps for monitoring N leaching which will be critical to assess the

effectiveness of Best Management Practices in assisting farmers to attain their nutrient management goals.






REFERENCES


1. Environmental Protection Agency. (2002). National Primary Drinking Water Regulations. (Title 40 Code of

Federal Regulations, Part 141, 425).

2. Greenwood D.J., F. Gastal, G. Lemaire, A. Draycott, P. Millard, and J.J. Neeteson. (1991). Growth rate and N%

of filed grown crops: theory and experiments. Annals of Botany, 67, 181-190.

3. Havlin, J.L, J.D. Beaton, S.L. Tisdale, and W. Nelson. (1997). Soil Fertility and Fertilizers. (6th Ed., pp. 87-153).

New York: Prentice Hall.

4. Jeuffroy M.H., J.M. Meynard. Azote: production agricole et environment. In: Morot-Gaudry J.F. (ed)

(1997). Assimilation de I'azote chez les plants, 369-380.

5. Lemaire G., M. Khaity M, B. Onillon, J.M. Allirand, M. Chartier, and G. Gosse. (1992). Dynamics of accumulation

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6. McNeal, B.L., Stanley, C.D., Graham W.D., Gilreath, P.R., Downey, D., and Creighton, J.F. Nutrient-Loss Trends

for Vegetable and Citrus Field in West-Central Florida: I. Nitrate. (1995). Journal of Environmental Quality, 24,

95-100.

7. National Research Council. Board on Agriculture. (1994). Soil and Water Quality: An Agenda for Agriculture.

(Natural Resources Defense Council 1992-93 Update). National Washington, DC: Academy Press.

8. Pearman I, S.M. Thomas, and G.N. Thorne. (1977). Effects of nitrogen fertilizer on growth and yield of spring

wheat. Annals of Botany, 41, 93-108

9. Pierzynski, G.M., J.T. Sims, and G.F. Vance. (2000). Soils and Environmental Quality. (2nd Ed.) Washington, D.





C.: CRC Press.

10. Sinclair TR and T. Horie. (1989). Leaf nitrogen, photosynthesis, and crop radiation use efficiency: a review.

Crop Science, 29, 90-98


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