Journal of Undergraduate Research
Volume 8, Issue 1 - September/ October 2006
Using Resin Traps for Assessment of Nitrogen Leaching in
Agricultural Production Systems
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.
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
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
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
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.
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
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 Nalgeneï¿½ 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.
- 202 cm
4.7cm - ""' - Soil/resin mix
Rocks I - 4.- cm
Figure 1. Schematic drawing of the resin trap used for determination of N leaching.
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)
NH4-N Total-N N03-N
NH4-N Total-N N03-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
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.
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.
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