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Enhanced efficiency fertilizers, potato production, and nitrate leaching in the Wisconsin Central Sands

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Maintaining yield goals while reducing nitrate‐nitrogen (NO3‐N) leaching to groundwater is a challenge for potato (Solanum tuberosum) production in the Wisconsin Central Sands as well as across the United States. The objectives of this study were to quantify the effect of conventional and enhanced efficiency nitrogen (N) fertilizers on NO3‐N leaching, crop yield, and N uptake in potatoes. We compared five N treatments, which include a 0 N control and 280 kg ha⁻¹ as ammonium sulfate and ammonium nitrate (AS/AN), polymer‐coated urea (PCU), urea with a urease inhibitor (Urea+UI), or urea with a UI and a nitrification inhibitor (Urea+UI+NI). The study occurred on grower fields during the 2009, 2010, and 2011 growing seasons, and NO3‐N leaching was measured with equilibrium tension lysimeters. PCU resulted in a reduction in NO3‐N leaching and an increase in yield compared to AS/AN in a year with large early‐season rainfall; Urea+UI also reduced NO3‐N leaching in this year. In 2010, large plot‐to‐plot variation and 250 kg ha⁻¹ of additional N applied by the grower masked our ability to see differences among fertilized treatments. In 2011, a year with less intense rainfall events, no differences among treatments were observed. Collectively, these results show a potential benefit to PCU, but these benefits are only realized under specific seasonal weather conditions. Overall, the percentage of applied N lost to leaching during the growing season and removed in biomass was relatively low, suggesting substantial amounts of NO3‐N leaching outside of the growing season.
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Received: 18 June 2024 Accepted: 12 December 2024
DOI: 10.1002/jeq2.20672
TECHNICAL REPORT
Groundwater Quality
Enhanced efficiency fertilizers, potato production, and nitrate
leaching in the Wisconsin Central Sands
Tracy Campbell Matthew Ruark Edward Boswell Birl Lowery
Department of Soil Science, University of
Wisconsin–Madison, Madison, Wisconsin,
USA
Correspondence
Tracy Campbell, Department of Soil
Science, University of Wisconsin–Madison
1525 Observatory Drive Madison, WI
53706, USA.
Email: tacampbell@wisc.edu
Assigned to Associate Editor Tyler A. Groh.
Funding information
Wisconsin Potato and Vegetable Growers
Association
Abstract
Maintaining yield goals while reducing nitrate-nitrogen (NO3-N) leaching to ground-
water is a challenge for potato (Solanum tuberosum) production in the Wisconsin
Central Sands as well as across the United States. The objectives of this study were
to quantify the effect of conventional and enhanced efficiency nitrogen (N) fertiliz-
ers on NO3-N leaching, crop yield, and N uptake in potatoes. We compared five N
treatments, which include a 0 N control and 280 kg ha1as ammonium sulfate and
ammonium nitrate (AS/AN), polymer-coated urea (PCU), urea with a urease inhibitor
(Urea+UI), or urea with a UI and a nitrification inhibitor (Urea+UI+NI). The study
occurred on grower fields during the 2009, 2010, and 2011 growing seasons, and
NO3-N leaching was measured with equilibrium tension lysimeters. PCU resulted in
a reduction in NO3-N leaching and an increase in yield compared to AS/AN in a year
with large early-season rainfall; Urea+UI also reduced NO3-N leaching in this year.
In 2010, large plot-to-plot variation and 250 kg ha1of additional N applied by the
grower masked our ability to see differences among fertilized treatments. In 2011, a
year with less intense rainfall events, no differences among treatments were observed.
Collectively, these results show a potential benefit to PCU, but these benefits are
only realized under specific seasonal weather conditions. Overall, the percentage
of applied N lost to leaching during the growing season and removed in biomass
was relatively low, suggesting substantial amounts of NO3-N leaching outside of the
growing season.
1INTRODUCTION
Across the US Midwest, agricultural regions are challenged
with maintaining profitable crop yields while simultaneously
reducing the amount of nitrogen lost to the environment. The
Wisconsin Central Sands (WCS), a region characterized by
Abbreviations: AN, ammonium nitrate; AS, ammonium sulfate; FWMC,
flow weighted mean concentration; NI, nitrification inhibitor; PCU,
polymer-coated urea; UI, urease inhibitor; WCS, Wisconsin Central Sands.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original
work is properly cited.
©2025 The Author(s). Journal of Environmental Quality published by Wiley Periodicals LLC on behalf of American Society of Agronomy, Crop Science Society of America, and
Soil Science Society of America.
its sandy soil, high hydraulic conductivity, and shallow depth
to groundwater, exemplifies many of these challenges. The
WCS is the primary potato-producing region in the state,
and this crop requires large amounts of nitrogen (N) fertilizer
and frequent irrigation to reduce drought stress and produce
large high-quality yields. As a result, groundwater nitrate-
nitrogen (NO3-N) concentrations can exceed drinking water
standards with 20%–25% of local domestic drinking water
wells exceeding 10 mg L1s (Masarik et al., 2018).
J. Environ. Qual. 2025;1–13. wileyonlinelibrary.com/journal/jeq2 1
2CAMPBELL ET AL.
Split application of N fertilizer for potato production
is standard practice. Previous studies have demonstrated
a reduction in NO3-N leaching when fertilizer is applied
in split applications with limited impact on potato yield
(Kelling et al., 2015;Vos,1999; Waddell et al., 2000). How-
ever, even with split N applications, large rainfall events can
still lead to excessive NO3-N losses in sandy soils, especially
later in the growing season (Sexton et al., 1996). Polymer-
coated urea (PCU) can be used as an alternative or supplement
to split applications to reduce NO3-N leaching losses. The
release of N from PCU fertilizers is controlled by soil temper-
ature and moisture (Gandeza et al., 1991; Golden et al., 2011),
which slows the release of available N to match crop N needs
more closely. Studies conducted to evaluate PCU on potatoes
found increased yields over traditional fertilizers (Pack et al.,
2006; Wilson et al., 2009; Zvomuya & Rosen, 2001). Several
studies in Minnesota demonstrated the added benefit of reduc-
ing NO3-N leaching in potato cropping systems using PCU
over traditional fertilizers in sandy soils (Wilson et al., 2010;
Zvomuya et al., 2003). However, Clément et al. (2020) found
PCU led to similar or greater NO3-N losses compared to split
applications of conventional fertilizers, concluding that PCU
may only be beneficial if substantial rainfall occurs within a
few weeks after planting.
Nitrification inhibitors (NIs) can also improve the syn-
chrony of fertilizer N availability and crop N uptake by killing
or inhibiting the bacteria Nitrosomonas, which limits the con-
version of ammonium to NO3-N (Ruark et al., 2018; Trenkel,
2010), reducing the time NO3-N is susceptible to loss. A
global meta-analysis across a diverse range of crops found
that while NIs increased crop productivity and nitrogen use
efficiency in relation to the control, the level of success var-
ied considerably but was greatest for crops receiving high
amounts of nitrogen fertilizer and grown on coarse-textured
soil (Abalos et al., 2014). However, previous research on pota-
toes in Wisconsin found that use of NIs reduced harvested
US No. 1 tubers in 4 out of 6 site years when used with
ammonium-based fertilizer (Kelling et al., 2011), limiting its
feasibility as a best management practice for this crop.
Since there is an extra cost associated with enhanced
efficiency fertilizers, evaluation of both agronomic and envi-
ronmental impacts needs to be executed on farmer fields,
across multiple growing seasons, and using quantitative
approaches to inform farmer decisions. Most studies quan-
tifying NO3-N leaching losses under potato production have
used suction cup samplers (e.g., Clément et al., 2020; Wilson
et al., 2010; Zvomuya et al., 2003). While this approach has
advantages for comparative studies in terms of ease of instal-
lation and sampling, it does require a water budget approach
to determine fluxes such as NO3-N leaching. Equilibrium ten-
sion lysimeters are recognized as a more accurate method
for flux quantification, as they maintain natural drainage pat-
terns with little disturbance to the soil profile (Brye et al.,
Core Ideas
Polymer-coated urea (PCU) fertilizer reduced
NO3-N leaching when large rainfall events were
concentrated early in the season.
Enhanced efficiency fertilizers did not consistently
reduce NO3-N leaching for all study years.
PCU had no negative impacts on potato yield
across all years.
NO3-N leaching exceeded 80 kg ha1during two
sampling events in the study.
Nitrogen budget approaches indicate large amounts
of unaccounted-for N in potato cropping systems.
1999). The objective of the study was to quantify the effect of
conventional and enhanced efficiency N fertilizers on NO3-
N leaching, crop yield, and N uptake for potatoes in sandy
soil. To do so, this research was conducted on farmer fields in
the WCS and used equilibrium tension lysimeters to quantify
seasonal NO3-N leaching losses.
2MATERIALS AND METHODS
2.1 Study sites
This study was conducted in 2009, 2010, and 2011 in a differ-
ent commercial potato field each year. Two fields (2009 and
2010) were operated by the same grower near Grand Marsh,
WI (48˚5434N, 89˚4039W), and one field (2011) was
near Coloma, WI (44˚0121N, 89˚3551W). The soil in all
fields was classified as Plainfield loamy sands (mixed, mesic
Typic Udipsamments). This soil was formed from glacial till
overlain by glacial outwash representing the bed of glacial
Lake Wisconsin, and it is generally composed of deep, well-
drained sands with massive structure and rapid or very rapid
saturated hydraulic conductivity (Bockheim & Hartemink,
2017;Hart&Lowery,1996).
2.2 Experimental design
The study was designed as a randomized complete block
design replicated three times with five N fertilizer treat-
ments. Plot sizes were 3.66 by 6.10 m, encompassing four
potato rows. The N fertilizer treatments consisted of: (i)
no experimental N fertilizer applied (0 N), (ii) ammonium
sulfate–ammonium nitrate (AS/AN) representing a conven-
tional N fertilizer, (iii) urea with a urease inhibitor N-(n-
butyl)-thiophosphoric triamide (Agrotain, Agrotain Interna-
tional LLC) (Urea+UI), (iv) urea impregnated with N-(n-
butyl)-thiophosphoric triamide and dicyandiamide (SuperU,
CAMPBELL ET AL.3
Agrotain International LLC) (Urea+UI+NI), and (v) a PCU
(ESN, Agrium Advanced Technologies, Agrium, Inc.).
A-grade Russet Burbank (Solanum tuberosum L.) pota-
toes were cut in halves or thirds and mechanically planted
by the growers at 0.9 m row spacing with a seed density
of 36,600 seeds ha1. The potatoes were planted on April
17, 2009; April 15, 2010; and May 1, 2011. For the PCU
treatment, nitrogen fertilizer was applied once at a rate of
280 kg ha1between 26 and 34 days after planting in 2009,
2010, and 2011. For the AS/AN, Urea+UI, and Urea+UI+NI
treatments, nitrogen fertilizer was applied in split applica-
tions, with the first application occurring on May 18, May
19, and May 26 in 2009, 2010, and 2011, respectively; the
second fertilizer application occurred on June 3, June 6, and
June 17 in 2009, 2010, and 2011, respectively. Fertilizer was
incorporated into the soil through hilling, except for the sec-
ond application in 2011. All treatments, including the 0 N
unfertilized treatments, received 32, 39, and 0 kg N ha1of
(NH4)2SO4at planting in 2009, 2010, and 2011, respectively.
During the 2009 and 2010 sampling season, the cooperat-
ing growers applied 113 and 250 additional kg ha1of N
through fertigation. As a result, the total N applied in the
fertilized treatments was 425, 569, and 280 kg·ha1of N
in 2009, 2010, and 2011; and 145, 289, and 0 kg·ha1N
were applied in the unfertilized treatments in 2009, 2010, and
2011, respectively. A surfactant consisting of 10% alkoxy-
lated polyols and 7% glucoethers (Irrigaid, Aquatrols) was
applied at emergence and at the time of tuber initiation
fertilization in all 3 years. Aside from the experimental N
applications, growers followed their own crop management
plans for soil preparation, irrigation, pesticide application,
and non-N fertilization for the duration of the three growing
seasons.
2.3 Pan lysimeters
In each of the 15 experimental plots, leachate was collected
using custom-fabricated equilibrium tension pan lysimeter as
described by Brye et al. (1999). The lysimeters measured
25.4 cm wide by 76.2 cm long by 15.2 cm tall, providing a
capacity of at least 120 mm of drainage water, and included
a porous stainless-steel top, a sampling line, and a vacuum
line. Lysimeters were installed at a depth of 1 m below potato
hilltop on May 11, 2009, April 27, 2010, and May 19, 2011,
prior to plant emergence. Holes were hand dug, and soil was
repacked by hand on top of and around the lysimeter after
installation. Care was taken to place soil back at the depths
at which it was removed (i.e., a horizon soil was placed back
at the soil surface). The lysimeters were interconnected, and
the entire system was connected to a 1/12 hp. vacuum pump
(model 900-13-58, Thomas) that maintained a constant and
continuous suction consistent with 100 cm of water, equiva-
lent to the field moisture capacity of the study soils (Hart &
Lowery, 1996).
Drainage water was collected under vacuum from the out-
flow tube of each lysimeter approximately every 7 days from
May 27 through September 24. in each of the 3 years, hereafter
referred to as “sampling season.” At each sampling event, the
total drainage volume within each lysimeter was measured,
and a subsample was collected and placed into a 500-mL plas-
tic Nalgene bottle. Samples were transported to the laboratory
in an ice-filled cooler, filtered through a 0.45 μm filter within
24 h after collection, and stored at 4˚C until analysis.
Drainage water samples were analyzed for NO3-N by the
single vanadium chloride, microplate method described in
Doane and Howarth (2003), with each sample ran in triplicate.
Colorimetric analysis and absorbances were read by a Bio-Tek
Powerwave (BioTek Instruments Inc.) microplate reader. The
only exception was that samples from May 28, 2009, were
analyzed for NO3-N using an ion chromatograph (Dionex
DX500). A subset of samples was re-analyzed after the project
was completed due to extremely high values; these samples
were analyzed using a SEAL AQ2 automated discrete ana-
lyzer (SEAL Analytical Inc.). In all cases, where the NO3-N
concentration was below that of 0.5 mg L1(the lowest point
of the standard curve), a value of 0.25 mg L1was assigned.
Drainage was converted from volume to area by dividing by
theareaoflysimeter.NO
3-N leaching for each sample col-
lection was calculated as the product of NO3-N concentration
and drainage. All events in a sampling season were summed
to determine sampling season cumulative leaching. Sampling
season flow weighted mean concentrations (FWMC) were
calculated by dividing the seasonal leaching by the total
season drainage.
At each farm, precipitation was measured using a tip-
ping bucket rain gauge (model RG3, Onset Computer Corp.)
connected to a datalogger (Model 10X, Campbell Scien-
tific). However, in 2009, National Oceanic and Atomospheric
Administration precipitation records recorded at Hancock
Agricultural Research Station, approximately 30 km from the
study site, were used instead due to data inconsistencies in
the tipping bucket method. Irrigation amounts were reported
by the growers.
2.4 Plant analysis
Tubers from the two center rows of each plot were mechani-
cally harvested on September 15, 2009; August 30, 2010; and
September 23, 2012. The tubers were graded into US No. 1,
undersize (not retained on a 4.8-cm screen), and cull (off-
shape, green, diseased, or blemished). The US No. 1 tubers
were electronically graded into size categories. Ten potatoes
from the 170 to 283 g size class were collected and analyzed
for specific gravity with a PW-2050 (Weltech International
4CAMPBELL ET AL.
Ltd) analyzer and calculated using weight in air divided by
weight in water. Total N in tubers was determined using dry
combustion with a Leco CNS-2000 analyzer (Leco Corp.).
Tuber dry matter was calculated as a function of specific
gravity (Kellock, 1995; Henderson, 2000). Total N uptake in
tubers was calculated by multiplying tuber dry matter by tuber
N content and by tuber yield.
2.5 Calculations and statistics
Two N budget calculations were conducted. First, the poten-
tially leachable N was calculated as the difference between
total N applied and N removed in the tuber. Second, the
unaccounted-for N (remaining N) was calculated as the dif-
ference between total N applied and the sum of N removed in
biomass and the measured NO3-N leaching.
To compare nitrogen fertilizer treatment effects on season
totals, data from each year was analyzed independently. We
used a linear mixed effects model followed by analysis of vari-
ance for cumulative sampling season measurements of NO3-N
leaching, FWMC, crop yield, N removed in tuber, poten-
tially leachable N, and unaccounted-for N; fertilizer treatment
was treated as a fixed effect and block was treated as a ran-
dom effect. Because of a lack of homogeneity in variance
for FWMC and NO3-N leaching values between treatments,
data were log transformed for the 2010 sampling season. Fol-
lowing analysis of variances, differences in treatment means
were determined using Tukey’s post hoc analysis means
comparison with a p-value of <0.1 considered significant.
To determine the impact of nitrogen treatment on NO3-N
leaching across sampling events, we used a mixed-effects lin-
ear model with fertilizer treatment and sampling event and the
interaction between fertilizer treatment and sampling event
serving as fixed effects and block as a random effect. Each
year was analyzed independently and log transformed due to
non-normality. All analysis was completed in RStudio version
4.2.2 (Posit team, 2023) using the lme4 (Bates et al., 2015) and
emmeans (Length, 2023) packages.
3 RESULTS
3.1 Drainage, irrigation, and rainfall
In total, there were 15, 14, and 14 sampling events during the
2009, 2010, and 2011 sampling seasons, respectively. Average
seasonal drainage (across all plots) was 274, 287, and 167 mm
for 2009, 2010, and 2011, respectively; this led to water recov-
ery efficiencies [drainage/(irrigation +rainfall)] of 0.32, 0.37,
and 0.30. In 2009, the highest amount of drainage was mea-
sured during the June 9, 2009, sampling event, with 55 mm
across plots recorded. In the week leading up to the June 9
sampling event, 46 mm of rainfall occurred over 3 days. Dur-
ing 2010, the greatest amounts of drainage were measured on
July 21 and July 28, 2010, which each followed a week receiv-
ing >75 mm of rainfall. The largest amount of drainage from
a single sampling event during the 2011 sampling season was
lower than the previous 2 years; the largest drainage event,
when averaged across plots, was 35 mm recorded on both
June 24 and July 22, 2011. The 2009 sampling season received
297 mm of rainfall and 549 mm of irrigation; in 2010, 607 mm
of rainfall occurred and 155 mm of irrigation, and the 2011
sampling season measured 267 mm rainfall and 278 mm irri-
gation. Based on the timing and intensity of the rainfall events,
and the differences in management by all three cooperating
growers, each year can be considered a separate study that
reflects: (i) rainfall-induced leaching occurring early in the
growing season and with additional N applied by the grower
(2009), (ii) rainfall-induced leaching occurring in the middle
of the growing season and with additional N applied by the
grower (2010), and (iii) a low rainfall intensity season with
no additional N applied by the grower (2011). Each year was
analyzed separately and considered under the context of these
environmental and management scenarios.
3.2 Leaching events
Across all five N fertilizer treatments and across all sampling
events, NO3-N concentration per sampling event ranged from
0.16 mg L1to 192 mg L1, with a mean value of 40.7 mg
L1. For control treatments (0 N), the average N concentra-
tion per sampling event ranged from 0.25 to 63.4 mg L1
across the 3 years of the study. Mean NO3-N concentrations
across sampling events exhibited a general trend in all 3 years
with increased concentrations coincident with the first large
rainfall events and a subsequent declining trend as the season
progressed. Similar to NO3-N concentrations per sampling
events, average NO3-N leaching per event (averaged across all
treatments and plots) also ranged widely from 0.01 to 91.7 kg
ha1.
Based on our linear mixed effects model, which was ana-
lyzed separately by year, there were significant effects of
fertilizer treatment and sampling time each year, but there was
no significant interaction between the treatments. Despite no
significant interaction between sampling event and fertilizer
treatment, the observed differences between fertilizer treat-
ments within specific sampling events are still of interest,
as large leaching events occurring during specific sampling
events likely influenced our cumulative results. During the
2009 sampling season, most NO3-N leaching occurred during
the first month of the growing season, which coincided with
a large rainfall event also occurring during the early grow-
ing season (Figure 1a,b). Most notably, on June 9, 2009, the
largest NO3-N leaching of the year was measured. The AS/AN
CAMPBELL ET AL.5
FIGURE 1 NO3-N leaching across five fertilizer treatments (a, c, and e) and irrigation and rainfall patterns (b, d, and f) for the 2009 (a–b),
2010 (c–d), and 2011 (e–f) sampling seasons. Treatment differences for nitrate leaching, averaged across sampling events, are provided in the inset
table, different letters indicate significant differences at α=0.1 (Tukey HSD) and are listed from smallest to largest average value. AS/AN,
ammonium sulfate/ammonium nitrate; NI, nitrification inhibitor; PCU, polymer-coated urea; UI, urease inhibitor.
6CAMPBELL ET AL.
treatment leached 81.9 kg ha1of NO3-N, which was more
than four times greater than the 0 N (16.9 kg ha1) treatment
and three times greater than PCU (30.7 kg ha1) treatment
(Figure 1a). Additionally, all fertilized treatments except for
the PCU treatment lost more NO3-N than the 0 N control
treatment, with the Urea+UI and Urea+UI+NI leaching 50.1
and 41.7 kg ha1of NO3-N, respectively (Figure 1a). The
peak in NO3-N leaching measured during the June 9, 2009,
sampling event occurred 6 days after a second application of
fertilizer (for the fertilized, non-PCU treatments) and followed
38 mm of rainfall (Figure 1b). A similar trend in NO3-N leach-
ing across the fertilizer treatments was also observed during
the June 17, 2009, sampling event. During this sampling
period, 41.1, 17.9, 16.0, 5.7, and 3.0 kg ha1of NO3-N were
leached under the AS/AN, Urea+UI, Urea+UI+NI, PCU,
and 0 N fertilizer treatments, respectively, with the AS/AN
treatment leaching more than seven times that of the PCU
(Figure 1a).
On average, event NO3-N losses were greater throughout
the 2010 sampling season compared to 2009 because of the
continued fertilizer application by the cooperating farmer in
addition to higher amounts of rainfall throughout the sampling
season. After a week of extreme rainfall (94 mm), 91.7 kg
ha1of NO3-N was leached during the July 21, 2010, sam-
pling period under the AS/AN treatment, compared to only
7.54 kg ha1lost under the 0 N treatment (Figure 1c,d).
Under conditions vulnerable to NO3-N leaching, as observed
on July 21, 2010, the PCU and inhibitor treatments leached
approximately 60%–75% less NO3-N in comparison to the
conventional AS/AN approach. However, the large estimated
NO3-N leached under the AS/AN was due in part to the
wide variability in drainage collected across the three plots,
which measured 100, 49, and 41 mm, as well as the vari-
ability in NO3-N concentration, which measured 120, 228,
and 92 mg L1. Additionally, a sampling event earlier in the
growing season (July 1, 2010), did not result in large dif-
ferences among fertilized N treatments, although they had
2.6–4.2 times greater amounts of leaching compared to the
control (Figure 1c).
During the 2011 sampling season, considerably less NO3-
N was applied, and considerably less NO3-N was lost during
peak leaching events in comparison to the previous sampling
seasons. While sampling season rainfall quantities were com-
parable in 2009 and 2011, in contrast to the 2009 sampling
season, the largest NO3-N leaching event was not observed
until the end of June, on June 24, 2011 (Figure 1e). This
sampling event occurred 6 days after a fertilizer application
event and included two rainfall events ranging between 18 and
22 mm and resulted in 35.6, 30.8, 27.5, and 20.17 kg ha1of
NO3-N leached under the Urea+UI, PCU, 0 N, AS/AN, and
Urea+UI+NI fertilizer treatments, respectively (Figure 1e,f).
In contrast, a different pattern in leaching was observed on
July 22, where 30.3, 19.0, 10.6, 9.6, and 5.9 kg ha1of NO3-
N leached under the Urea+UI+NI, AS/AN, Urea+UI, PCU,
and 0 N treatments, respectively (Figure 1e).
3.3 Sampling season NO3-N leaching and
FWMC
There was a significant effect of treatment on NO3-N leach-
ing and FWMC concentrations in 2009 and 2010, but not
in 2011. In 2009, during sampling season, NO3-N leaching
was significantly affected by nitrogen fertilizer treatment and
was approximately fivefold greater in the AS/AN treatment
as compared to the 0 N treatment (Table 1; Figure 2). Sam-
pling season NO3-N leaching in the Urea+UI+NI treatment
did not differ significantly from the AS/AN treatment. PCU
had the lowest total mean NO3-N leaching of the added nitro-
gen treatments. In 2009, the AS/AN had the greatest FWMC
values, and all other N treatments were not different from the
0 N treatment (Table 1; Figure 2). In 2010, despite the wide
range in seasonal NO3-N leaching across treatments, only the
AS/AN and Urea+UI treatments leached significantly more
NO3-N than the 0 N treatment. The same effects were deter-
mined for FWMC in 2010. We were unable to determine
differences in growing season NO3-N leaching and FWMC in
2011, despite there being a nearly twofold difference among
treatments (Table 1; Figure 2).
3.4 Tuber yield and quality
In 2009, there was a significant effect of N treatment on total
and US No. 1 yields, with the only difference being between
PCU and 0 N (Table 2). No yield effects were determined in
2010 or 2011. In 2009, tuber specific gravity was significantly
higher under the 0 N and Urea+UI+NI treatments, and no
differences were found in 2010 and 2011. The PCU treatment
had greater N content compared to the urea treatments and the
0 N in 2009, and the PCU and AS/AN treatment had greater N
content compared to the urea treatments and the 0 N in 2010
(Table 2); no differences between treatments were observed
in 2011.
3.5 Nitrogen budget
The potentially leachable N, calculated as the difference
between N applied and N removed in tuber, was generally
much greater than the actual amount of NO3-N leached.
Notable exceptions were in 2009, where the sampling season
NO3-N leaching in AS/AN represented 84% of the potentially
leachable N (Table 3). In contrast, the sampling season NO3-
N leaching from the PCU treatment only represented 31%
of the potentially leachable N. When averaged across all N
CAMPBELL ET AL.7
TABLE 1 Measurements of NO3-N leaching (kg ha1), flow weighted mean concentration (FWMC) (mg L1), irrigation (mm), rainfall (mm)
and drainage (mm) for five nitrogen fertilizer treatments across 2009 (549 mm irrigation and 297 mm rainfall), 2010 (155 mm irrigation and 607 mm
rainfall), and 2011 (278 mm irrigation and 267 mm rainfall) sampling seasons.
Treatment NO3-N leaching (kg ha1) FWMC (mg L1) Irrigation (mm) Rainfall (mm) Drainage (mm)
2009
0 N 29.5a 11.6a 549 297 256
AS/AN 152c 67.6b 549 297 243
PCU 50.4a 17.8a 549 297 288
Urea+UI 80.6ab 34.5a 549 297 240
Urea+UI+NI 107bc 33.0a 549 297 330
2010
0N 71.3a 29.6a 155 607 227
AS/AN 260b 86.3b 155 607 265
PCU 177ab 71.8ab 155 607 213
Urea+UI 245b 85.0b 155 607 262
Urea+UI+NI 132ab 60.3ab 155 607 223
2011
0 N 518 35.5 278 267 148
AS/AN 83.7 60.3 278 267 143
PCU 73.0 58.2 278 267 127
Urea+UI 67.6 44.3 278 267 154
Urea+UI+NI 97.5 38.6 278 267 218
Note: Different letters indicate statistically significant differences (α=0.1).
Abbreviations: AS/AN, ammonium sulfate/ammonium nitrate; NI, nitrification inhibitor; PCU, polymer-coated urea; UI, urease inhibitor.
treatments, the sampling season N represented 44% and 50%
of the potentially leachable N in 2010 and 2011, respectively
(Table 3). The amount of N remaining reflects the amount
that is unaccounted for in our known quantities of the bud-
get (i.e., input, leaching, and removal), which can represent
the combination of N remaining in the system, unaccounted
for inputs, and unaccounted N losses. In 2009, where an
additional 145 kg ha1of N was applied by the grower, the
AS/AN treatment only had an N unaccounted value of 29.2 kg
ha1, suggesting we captured most of the N applied in our
measurements (Table 3). In contrast, the other N treatments
had much more N left unaccounted for (>100 kg ha1). The
unaccounted-for N in 2010 was two to three times greater than
those in 2009, and values in 2011 were similar to or slightly
less than those in 2009 (Table3). When the unaccounted-for N
was calculated as a percent of the total N applied, most values
ranged between 25% and 43%.
4 DISCUSSION
4.1 Enhanced efficiency fertilizers
PCU was most effective at reducing NO3-N leaching when
large rainfall events were isolated to the early portion of the
growing season. In 2009, the PCU treatment resulted in a
total NO3-N leaching comparable to the 0 N treatment and
threefold lower than the split application of AS/AN. The 2009
growing season is characterized by large rainfall events occur-
ring 10 days after PCU application. In contrast, the 2011
growing season N fertilizer application rates were similar
to University of Wisconsin-Madison Extension recommen-
dations (Laboski et al., 2012), but PCU did not result in
less NO3-N leaching due to leaching inducing rainfall events
occurring later in the growing season. This suggests that there
is a capacity for PCU to reduce NO3-N leaching, but only early
in the growing season, at times when the N is still mostly pro-
tected in the polymer coating. In 2010, under conditions of
high N applications (twice the recommended rate), there were
also no differences between PCU and other fertilizer treat-
ments; use of PCU with overapplications of N did not lead
to any reductions in NO3-N leaching. Previous researchers
have reported reductions in NO3-N leaching in potato pro-
duction systems with PCU compared to split applications of
other fertilizer sources (Wilson et al., 2010; Zvomuya et al.,
2003). While others, such as Clément et al. (2020), report
no difference in NO3-N leaching between PCU and other N
fertilizers. These findings suggest that the benefit of PCU
fertilizers to water quality may only become apparent under
excess irrigation and rainfall during certain points of the grow-
ing season (Bero et al., 2014), in combination with moderately
high nitrogen fertilizer inputs (Quemada et al., 2013).
8CAMPBELL ET AL.
TABLE 2 Agronomic yield measured by total yield (Mg ha1), US No. 1 (Mg ha1), and percent of US No. 1 >170 g (%), specific gravity, and
tuber N (%) grown across five nitrogen fertilizer treatments for the 2009, 2010, and 2011 sampling seasons.
Treatment Total yield (Mg ha1) US No. 1 (Mg ha1) US No. 1 >170 g (%) Specific gravity Tuber N (%)
2009
0 N 63.3a 55.6a 50.7 1.092b 1.26a
AS/AN 75.7ab 66.3ab 57.0 1.084a 1.46ab
PCU 78.5b 68.3b 59.3 1.084a 1.55b
Urea+UI 68.7ab 61.3ab 48.7 1.085a 1.30a
Urea+UI+NI 67.0ab 57.3a 42.7 1.089b 1.30a
2010
0N 46.1 31.0 49.0 1.067 1.21a
AS/AN 39.1 25.1 38.7 1.067 1.52b
PCU 49.1 33.7 47.7 1.068 1.56b
Urea+UI 39.4 23.8 33.3 1.068 1.29a
Urea+UI+NI 35.2 23.9 42.0 1.065 1.31a
2011
0 N 34.5 30.0 26.0 1.073 1.34
AS/AN 43.3 39.2 57.3 1.074 1.43
PCU 40.1 37.1 41.7 1.076 1.45
Urea+UI 39.4 34.8 55.3 1.077 1.38
Urea+UI+NI 42.8 38.8 39.3 1.077 1.40
Note: Different letters indicate statistically significant differences (α=0.1).
Abbreviations: AS/AN, ammonium sulfate/ammonium nitrate; NI, nitrification inhibitor; PCU, polymer-coated urea; UI, urease inhibitor.
TABLE 3 Nitrogen budget estimates are based off the amount of N fertilizer applied (kg ha1), the potential leachable N, the amount of N lost
through leaching (kg ha1), and the amount of N removed in tuber biomass (kg ha1) across five nitrogen fertilizer treatments.
Treatment
N applied
(kg ha1)
Potential leachable
N(kgha
1)
NO3-N leaching N removed Unaccounted-for N
kg ha1%kgha
1%kgha
1%
2009
0 N 145 41.5a 29.5a 20.3 187b 129 71.0c 49.0
AS/AN 425 191bc 161c 38.0 235ab 55.2 29.2b 6.9
PCU 425 160b 50.4a 11.9 265a 62.3 110ab 25.8
Urea+UI 425 228c 80.6ab 19.0 197b 46.3 148a 34.7
Urea+UI+NI 425 225c 107bc 25.1 200b 47.0 119ab 27.9
2010
0N 289 184a 71.3a 24.7 105 36.3 113b 38.9
AS/AN 569 457b 260b 45.7 112 19.7 197ab 34.6
PCU 569 424b 177ab 31.1 145 25.5 247ab 43.4
Urea+UI 569 473b 245b 43.0 96.2 16.9 228ab 40.1
Urea+UI+NI 569 484b 132 23.3 84.2 14.8 352a 62.0
2011
0N 0 91 a 51.8 NA 91.2 NA 143b NA
AS/AN 280 157 b 83.7 29.9 124 44.1 72.8a 26.0
PCU 280 161 b 73.0 26.1 119 42.6 87.8a 31.3
Urea+UI 280 169 b 67.6 24.1 111 39.6 101a 36.2
Urea+UI+NI 280 156 b 97.5 34.8 124 44.2 58.7a 21.0
Note: N applied as fertilizer was considered the only input and NO3-N leached and N removed from biomass were considered the only outputs. The percentage of NO3-
N leached, N removed, and N unaccounted for were determined relative to the amount of nitrogen fertilizer applied. Different letters indicate statistically significant
differences (α=0.1).
Abbreviations: AS/AN, ammonium sulfate/ammonium nitrate; NA, not applicable; NI, nitrification inhibitor; PCU, polymer-coated urea; UI, urease inhibitor.
CAMPBELL ET AL.9
FIGURE 2 Mean cumulative seasonal NO3-N leaching (kg ha1) (a–c) and mean cumulative seasonal flow weighted mean concentration
(FWMC) (mg L1) (d–f) represented by line and plot level averages represented by individual points across five nitrogen fertilizer treatments and
three sampling seasons, 2009 (a and d), 2010 (b and e), and 2011 (c and f). Different letters indicate statistically significant differences in seasonal
mean NO3-N leaching (kg ha1) or FWMC (mg L1)atα=0.1 (Tukey HSD). AS/AN, ammonium sulfate/ammonium nitrate; NI, nitrification
inhibitor; PCU, polymer-coated urea; UI, urease inhibitor.
Potato yields in the PCU treatment were the highest of all
fertilizer treatments and the only treatment to result in sig-
nificantly greater yields in comparison to the 0 N treatment in
2009. However, these findings were inconsistent across years,
and there was no yield benefit associated with PCU fertil-
izer during the 2010 and 2011 growing seasons. Past research
conducted under comparable conditions similarly found little
difference in potato yield across conventional and PCU treat-
ments (Bero et al., 2014; Cambouris et al., 2016). In contrast,
others have reported the use of control release fertilizers to
increase crop yields (Hopkins et al., 2008;Liegel&Walsh,
1976; Pack et al., 2006; Zvomuya et al., 2003), which help
support our 2009 findings. The discrepancy in the relationship
between potato yield and PCU fertilizer across field seasons
may be partially explained by differences in rainfall, irriga-
tion, and the amount of nitrogen fertilizer applied in each year
of the study. More specifically, past research has proposed the
idea that PCU may have a greater beneficial impact on crop
yield when N application rates are below the recommended
amount (Bero et al., 2014). Previous work in the region on
corn cropping systems emphasizes the ability of rainfall pat-
terns to hinder the effectiveness of PCU fertilizer. Rui et al.
(2019) found that though PCU fertilizer tended to be both
agronomically and environmentally advantageous over other
fertilizer treatments, the effectiveness of PCU was reduced
under high rainfall years.
While PCU reduced NO3-N leaching compared to the
conventional AS/AN approach under certain environmental
and management conditions with no cost to yield, our results
also indicate that the PCU treatment left more N unaccounted
and available for leaching outside of the growing season.
Recent studies conducted on similar soils support this find-
ing, demonstrating that certain PCU fertilizers had greater
amounts of residual soil N after crop harvest in comparison
to conventional split application approaches (Clément et al.,
2019; Venterea et al., 2011). As a result, it is likely that
the PCU treatment would see continued NO3-N leaching
throughout the fall and spring (Clément et al., 2021). Past
10 CAMPBELL ET AL.
research in the same region as our study found that 54% of
NO3-N leaching in potato cropping systems occurs outside
of the growing season (Masarik, 2023), when leaching can
be exacerbated due to high rainfall and low evaporative
transpiration (Shrestha et al., 2010). Moving forward, longer
monitoring of NO3-N leaching is needed to fully capture
impacts of nitrogen fertilizer on NO3-N leaching inside and
outside of the potato growing season.
The impact of nitrification and urease inhibitors on NO3-N
leaching and crop yield was inconsistent across years. In 2009,
the treatment incorporating both a nitrification and urease
inhibitor (Urea+UI+NI) leached significantly more NO3-N
than the 0 N treatment. However, the treatment incorporat-
ing only a urease inhibitor (Urea+UI) was not statistically
different from the 0 N control. In contrast, in 2010 the
Urea+UI+NI treatment was not significantly different than
the control, while the Urea+UI treatment leached substan-
tially more NO3-N than the 0 N treatment. In 2011 there
was no statistically significant difference in NO3-N leach-
ing across any of the treatments. A meta-analysis conducted
by Quemada et al. (2013) evaluating the effectiveness of NIs
on NO3-N leaching across a range of cropping systems also
found the effectiveness to vary considerably, reducing NO3-
N leaching between 5% and 30%. Similar to NO3-N leaching,
there was little discernible difference in crop yield with the
use of inhibitors across all 3 years of the study, though in
2009, the Urea+UI+NI treatment did result in significantly
less yield than the PCU treatment. Kelling et al. (2011)also
found inconsistent impacts on crop yield, with 3 site years
resulting in increased crop yield associated with the use of
inhibitors, and 4 site years associated with a decrease in crop
yield.
4.2 Nitrogen budgets
NO3-N leaching under potato production can be quite sub-
stantial regardless of the fertilizer used. Previous studies in
this region have quantified annual NO3-N leaching under
potato at 224 kg ha1(Kraft & Stites, 2003). Our growing
season-only leaching amounts ranged from 22% to 72% of
that estimation across the 2009 and 2011 growing season.
While total leaching may be similar, conventional fertilizers
still pose the largest risk of leaching from a single event.
The amount of NO3-N leached during a single sampling time
exceeded 75 kg ha1twice in the AS/AN treatment, repre-
senting a significant portion of the applied N. Arriaga et al.
(2009) also showed the potential for large NO3-N losses in
this region, with peak losses nearing 50 kg ha1day1.
Overapplication of N on potato in the WCS will have dis-
proportionately large influences on NO3-N concentrations in
groundwater. As part of our on-farm research, growers applied
additional N through fertigation and were unable to prevent
this application from occurring on our research plots. In 2010,
the collaborating grower applied an additional 250 kg ha1of
N through fertigation, leading to a total amount of N applied
that was double that of recommended guidelines. As a result,
the NO3-N leaching amounts were double (or more) than
those in other years. These extra N applications reflect the
lack of economic disincentive to over-application of N. Often,
especially for high-value crops such as potato, the cost of
applying excess nitrogen fertilizer to reduce the risk of crop
yield loss outweighs the additional cost of fertilizer (Mitchell,
2004; Mitchell et al., 2021) without considering the impacts
to the environment and human health.
During the 2009 and 2011 growing seasons, when the 0 N
treatments received 145 kg ha1and 0 kg ha1, respectively,
more N was removed from the system either as NO3-N leach-
ing or potato biomass than the amount of nitrogen fertilizer
applied. Previous research also found more N removed than
applied for unfertilized potato, with tuber N removal ranging
from 63 to 73 kg ha1of N (Errebhi et al., 1998), slightly
lower than our estimated 91 kg ha1NO3-N in 2011. Saffigna
and Keeney (1977) also observed a mismatch in the nitrogen
added and nitrogen removed from the system based on a sim-
ilar N budget approach. These findings can be explained by
unaccounted-for N inputs. The NO3-N in irrigation water can
provide a substantial amount of NO3-N to the crop if the NO3-
N concentration in irrigation water is high and if the crops are
well irrigated (Delaune & Trostle, 2012; Cahn et al., 2017;
Campbell et al., 2023). Based on recent work in the WCS by
Campbell et al. (2023), we estimated 92, 27, and 50 kg ha1
of NO3-N may have been applied through irrigation water in
2009, 2010, and 2011, respectively, only partially explaining
the unaccounted-for N in 2011. While residual soil N is usu-
ally low in sandy soil, it is possible that soil N supplied the
crop with unaccounted N. Alternatively, we may have under-
estimated the decomposition of previous plant residues, such
as soybean residue from the previous crop. Past research in
the region has indicated the ability for N to rapidly mineralize
in the sandy soil (West et al., 2016), which may help account
for the unaccounted-for nitrogen in our N budget calculations.
Despite discrepancies in the N budget, the equilibrium ten-
sion lysimeters offered an effective tool for capturing N fluxes
while maintaining accurate estimates of the water budget.
4.3 Limitations
While our findings highlight the benefits of PCU fertilizer
over the conventional AS/AN treatment under certain weather
and management conditions, high levels of plot-to-plot vari-
ability in drainage amount and NO3-N leaching may have
interfered with our ability to determine statistical differences
across fertilizer treatments. In 2009, seasonal drainage val-
ues at the plot level ranged from 188 to 406 mm. During the
CAMPBELL ET AL.11
2010 sampling season, the plot-to-plot variability was smaller,
falling between 223 and 385 mm. Similar to 2009, the 2011
sampling season resulted in a wide variability in cumulative
drainage values across plots, ranging from 75 to 364 mm.
Wide variability in drainage of potato cropping systems has
been documented previously; Nocco et al. (2018) also found
wide variability in recharge under potato cropping systems,
which was not explained by variability in soil texture. Addi-
tional studies using lysimeters have indicated that because of
the hills and furrows in potato cropping systems, there may be
greater spatial variability in drainage (Gee et al., 2009; Herath
et al., 2014), and similarly, Saffigna et al. (1976) found wide
variability in infiltration under potato canopies. While vari-
ability is expected when measuring between treatments, we
also observed high levels of variability within fertilizer treat-
ments. Most notably, during the 2010 sampling season, the
PCU treatment demonstrated high levels of plot-to-plot vari-
ability, with cumulative NO3-N leaching measuring 342, 119,
and 118 kg ha1. Even when standardized based on drainage
quantity, the FWMC values were also highly variable under
the PCU treatment, with plots measuring 115, 43, and 46 mg
L1during the 2010 sampling season. Previous research con-
ducted by Bero et al. (2014) experienced similar challenges,
with high levels of variability impeding clear statistical dif-
ferences in NO3-N leaching between fertilizer treatments.
Additionally, the variability in seasonal rainfall amount and
rainfall patterns, as well as the variability in the amount of
nitrogen fertilizer applied between study years, added to the
inherent variability in our results.
5CONCLUSION
Our results highlight the challenges of managing potato
production for yield and quality goals while minimizing
groundwater contamination. PCU fertilizer provided clear
reductions in NO3-N leaching and increases in potato yield,
but only in 1 year of the study. In this year, N was applied
moderately above the recommended rate, and leaching induc-
ing rainfall occurred only during the early growing season.
Results were less consistent under other environmental and
management conditions, and PCU did not reliably outperform
the other fertilizer treatments in terms of yield or NO3-N
leaching under such conditions. Environmental and man-
agement conditions influence the effectiveness of enhanced
efficiency fertilizers and should be considered by growers in
the decision-making process, especially considering climate
change projections. While PCU fertilizers can reduce NO3-N
leaching relative to conventional fertilizers, NO3-N leaching
was still substantial, with 50 to 177 kg ha1leached during
the 4-month sampling season. However, overapplication of
nitrogen may be the more pressing management issue, as in
two of the 3 years, >400 kg ha1of NO3-N was applied by
growers.
AUTHOR CONTRIBUTIONS
Tracy Campbell: Formal analysis; validation; visualiza-
tion; writing—original draft; writing—review and editing.
Matthew Ruark: Conceptualization; data curation; funding
acquisition; investigation; methodology; writing—original
draft; writing—review and editing. Edward Boswell:Formal
analysis; validation; visualization; writing—original draft;
writing—review and editing. Birl Lowery: Conceptual-
ization; data curation; funding acquisition; investigation;
methodology; writing—review and editing.
ACKNOWLEDGMENTS
This work was funded by the Wisconsin Potato and Vegeta-
bles Grower Association (WPVGA) and from the WPVGA
Chair award. The authors would like to thank Ana Tapsieva,
Bonner Karger, David Evans, and Mack Naber for helping
with lysimeter installation and data collection. The authors
would also like to express our gratitude to the cooperating
farmers.
CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.
ORCID
Tracy Campbell https://orcid.org/0009-0005-1578-2409
Matthew Ruark https://orcid.org/0000-0002-8678-6133
Edward Boswell https://orcid.org/0000-0002-2644-4043
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Chapter
Several efforts have been made to stratify the soils of Wisconsin into general regions based on location (northern, eastern, central, southwestern, southeastern, western) and on broad vegetation cover (forested, prairie) and texture of the soil parent materials (sandy, loamy, silty, silty over rock, etc.). In this chapter, we follow this approach, but in subsequent chapters, our approach examines the distribution of soil taxa (orders, suborders, great groups , subgroups, families, and series) and establishes soil regions based on great groups. Here, we will follow the soil regions from Hole (1976) and modified by Madison and Gundlach (1993) shown in Fig. 2. 6.