Full Text - American Society of Agronomy

12 Pages·2015·3.07 MB·English

Published January 13, 2015
Agronomy, Soils & Environmental Quality
Split Application of Urea Does Not Decrease and May Increase
Nitrous Oxide Emissions in Rainfed Corn
Rodney T. Venterea* and Jeffrey A. Coulter
ABStRACt
Modifi cation of N fertilizer application timing within the growing season has the potential to reduce soil nitrous oxide (N O)
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emissions but limited data are available to assess its eff ects. We compared cumulative growing season nitrous oxide emissions
(cN O) following urea applied to corn (Zea mays L.) in a single application (SA) at planting or in three split applications (SpA)
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over the growing season. For both SA and SpA, granular urea was broadcast and incorporated at six fertilizer N rates in the corn
phase of a corn–soybean [Glycine max (L.) Merr.] rotation and in a continuous corn system over two growing seasons. Daily
N O fl ux was measured using chambers on 35 dates in 2012 and 40 dates in 2013 and soil nitrate-N concentration was measured
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weekly. Split application did not aff ect grain yield and did not reduce cN O. Across N rates and rotations, cN O was 55% greater
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with SpA compared with SA in 2012. Increased cN O with SpA in 2012 likely resulted from a prolonged dry period before the
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second split application followed by large rainfall events following the third split application. Across years and rotations, SpA
increased cN O by 57% compared with SA when the maximum N rate was applied. Exponential relationships between cN O and
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fertilizer N rate explained 62 to 74% of the variance in area-based cN O and 54% of the variance in yield-based cN O. Applying
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urea to coincide with periods of high crop N demand does not necessarily reduce and may increase N O emissions.
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Modifi cation of N management practices to reduce season could improve the synchrony between soil N availability
emissions of N O has been identifi ed as a strategy for reducing and crop N demand and reduce the amount of soil N avail-
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the greenhouse gas footprint of agricultural cropping systems able for conversion to N O. However, the benefi ts of altered
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(Ogle et al., 2014). Altering the timing of N fertilizer applica- N fertilizer timing on N O emissions are in question due to the
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tion is frequently mentioned as a potentially eff ective practice limited number of studies and their inconsistent results.
for reducing all forms of reactive N loss from fertilized soil A few studies have compared single early-season N appli-
including N O (Smith et al., 2007; Robertson and Vitousek, cations to split N applications distributed over the growing
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2009; Ribaudo et al., 2011). For the majority of large-scale crop season. Zebarth et al. (2012) found no eff ect of single vs. split
production systems, N fertilizer is oft en applied well before application timing on N O emissions in a potato (Solanum
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the crop has developed to a stage where the N can be effi ciently tuberosum L.) system. Burton et al. (2008) found reduced N O
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assimilated. A survey of corn producers in Minnesota reported emissions with split application in one of two growing seasons.
that only 9% of growers applied N fertilizer aft er planting Split N application reduced N O emissions in sugarcane in
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(Bierman et al., 2012). Th e demand of the corn plant for N is Australia when 200 kg N ha–1 was applied, but did not aff ect
low during early growth stages but increases and remains high N O when 100 kg N ha–1 was applied (Allen et al., 2010). One
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for several weeks into the growing season (Abendroth et al., process-based N O emissions model that accounts for crop N
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2011). Th erefore, applying N later in the growing season or uptake and soil N transformations predicted that N O emis-
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in multiple split applications distributed across the growing sions will decrease as the number of N fertilizer applications
during the growing season increase (Hu et al., 2012) while
R.T. Venterea, USDA-ARS, Soil and Water Management Unit, 1991 Upper another model was relatively insensitive to single vs. split appli-
Buford Circle, St. Paul, MN 55108 and Dep. Soil, Water, and Climate Univ. of cation (Del Grosso et al., 2009). While not examining split
Minnesota, St. Paul, MN 55108; and J.A. Coulter, Dep. Agronomy and Plant
application per se, other studies have compared single N appli-
Genetics, Univ. of Minnesota, St. Paul, MN 55108. Mention of trade names or
commercial products in this publication is solely for the purpose of providing cations applied early vs. later in the growing season. Phillips et
specifi c information and does not imply recommendation or endorsement by al. (2009) found no signifi cant diff erence in N O emissions
the U.S. Department of Agriculture or the University of Minnesota. Received 5 2
following urea applied to corn 6 wk before planting compared
Aug. 2014. *Corresponding author ([email protected]).
Published in Agron. J. 107:337–348 (2015)
doi:10.2134/agronj14.0411 Abbreviations: CC, continuous corn; cN O, cumulative nitrous oxide
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Available freely online through the author-supported open access option. emissions; cN O-y, yield-based cumulative nitrous oxide emissions; CS,
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Copyright © 2015 by the American Society of Agronomy, 5585 Guilford corn–soybean; dN O, daily nitrous oxide fl ux; EF, fertilizer-induced nitrous
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Road, Madison, WI 53711. All rights reserved. No part of this periodical oxide emissions factor; SA, single application; SMC, soil moisture content;
may be reproduced or transmitted in any form or by any means, electronic or SN, soil nitrate-nitrogen concentration; SNI, soil nitrate-nitrogen intensity;
mechanical, including photocopying, recording, or any information storage and SNI-y, yield-based soil nitrate-nitrogen intensity; SpA, split application; SOC,
retrieval system, without permission in writing from the publisher. soil organic carbon; ST, soil temperature.
Agronomy Journal • Volume 107, Issue 1 • 2015 337
to 3 d before planting. Similarly, Zebarth et al. (2008) found into two split-split-plots, which were randomly assigned to a
no difference in N O emissions following ammonium nitrate single application (SA) at planting or to three split applications
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applied to corn at emergence compared to growth stage V6. (SpA) distributed over the growing season.
Drury et al. (2012) found that urea application to corn at For the SpA treatments, one-third of the total N was applied
growth stage V6 decreased N O by 33% compared to pre-plant in each of three applications. The first application occurred at
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application in a conventional tillage system, but application planting coinciding with the SA treatment. The second and
timing had no effect on N O in no-tillage or zone-tillage third applications occurred when the crop was at vegetative
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systems. To date, no studies have evaluated effects of single vs. growth stage V6 and V14, respectively, which correspond with
split N fertilizer application on N O emissions in corn produc- high rates of N accumulation in plant tissue (Abendroth et al.,
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tion systems in the United States. 2011). The V6 applications were made on 3 July 2012 and 9 July
The total rate of N fertilizer applied to the field is usually 2013 and the V14 applications were made on 24 July 2012 and
the most reliable predictor of N O emissions (Shcherbak et al., 28 July 2013. Urea was hand-applied and immediately incor-
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2014). Differences in the quantity and/or quality of crop residues porated into the soil manually using metal garden rakes with
from prior growing seasons or other residual effects of cropping 100-mm long tines. Corn was planted at 79,000 seeds ha–1
history may also affect N O emissions (Mosier et al., 2006; on 15 May 2012 (Cropland 3337VT2P) and on 16 May 2012
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Drury et al., 2008; Halvorson et al., 2008; Omonode et al., (Pioneer P0193AM) using a John Deere model 7100 Max-
2011). Thus, N fertilizer application timing effects may be influ- Emerge planter. After physiological maturity (on 24 Sept. 2012
enced by the N rate as well as the crop rotation. No studies to and 18 Oct. 2013), corn ears were harvested from plants in the
date have evaluated N application timing effects on N O emis- middle four rows of each split-plot. Ears were initially dried at
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sions across a wide range of N rates or in different crop rotations. 35°C, shelled, and further dried for 3 d at 65°C and weighed to
The objective of this study was to examine the effects of obtain dry grain yield.
single vs. split fertilizer application on cumulative N O emis- Each main plot was 27.4 m (36 rows with 0.76-m row spac-
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sions and soil N availability in continuous corn (CC) and ing) wide by 61 m long, and each split-split plot was 4.57 m (six
corn–soybean (CS) cropping systems across a range of N rates rows) wide by 4.57 m long. Areas within the inner four rows of
and over two consecutive growing seasons on a silt loam soil each split-split plot were used for collection of gas, soil and plant
in Minnesota. Our aim was to investigate a practice that has samples. The same set of three main plots was used both years in
the potential to reduce N O emissions, and not necessarily the CC rotation; in these plots, the plot areas were relocated each
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to simulate a practice that is currently common in the region. year to avoid re-using the same ground. In the CS rotation, a
Our general hypothesis was that split application would reduce different set of main plots was used each year in accordance with
N O emissions with an expectation of interactions of timing the crop rotation. The main plots used in both rotations were
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with N rate, rotation, and/or year. selected from the same long-term tillage treatment (designated as
“conservation tillage”). According to the design of the long-term
MAtERiAlS AnD MEthODS
study, this tillage regime differed depending on the crop grown
Site Description and Experimental Design each year, with fall stalk chopping followed by fall disk-ripping
The experiment was conducted in long-term research plots at occurring after corn and no fall tillage occurring after soybean.
the University of Minnesota Research Station in Rosemount, Thus, the rotation treatments of the long-term study and of
MN (44°45¢ N, 93°04¢ W). The soil is a naturally drained this 2-yr study also have a tillage component embedded within
Waukegan silt loam (fine-silty over sandy or sandy-skeletal, them. Both rotations received secondary tillage (tandem disking)
mixed, superactive, mesic Typic Hapludoll) containing before planting each spring.
220 g kg–1 sand, 550 g kg–1 silt, and 230 g kg–1 clay with pH
nitrous Oxide Emissions
(in 0.01 M CaCl ) of 6.0. The 30-yr (1984–2013) mean annual
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precipitation and temperature are 748 mm and 7.7°C, respec- Soil-to-atmosphere N O fluxes were measured using static
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tively (MCWG, 2014). The plots used in this study are part of chambers (Venterea et al., 2010) on 35 dates between 12 April
a long-term experiment with tillage and rotation treatments and 5 September in 2012 and on 40 dates between 1 May and
in place since 1991 (Venterea et al., 2010). A 2-yr experiment 29 September in 2013. Before planting, fluxes were measured
(2012 and 2013) was conducted using a randomized complete weekly in two locations within each main plot. Following
block, split-split plot design with rotation as the main effect, planting, fluxes were measured twice per week at one location
fertilizer N application rate as the split-plot effect, and fertilizer in each split-split-plot. Approximately 4500 individual N O
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N application timing as the split-split-plot effect. Each year, flux measurements were collected over the 2-yr study period.
three main plots in a CC rotation and three main plots in the In each split-split-plot, one stainless steel chamber anchor (0.50
corn phase of a CS rotation were randomly subdivided into six by 0.29 m) equipped with a 20-mm wide by 5-mm deep flange
split-plots; one split-plot (the control) received no N fertilizer, around its perimeter was installed to a depth of 0.07 m with
while the other five split-plots received one of five fertilizer N the flange resting directly on the soil surface. On each sam-
rates (50, 90, 130, 170, or 210 kg N ha–1) which brackets the pling day between 1000 and 1300 h local time, insulated and
recommended rates for this region of Minnesota (Randall et vented chamber tops (0.50 by 0.29 by 0.10 m high) each also
al., 2008). Long-term management of these plots has used the equipped with a 20- by 5-mm flange around their perimeter
same rate of N application (146 kg N ha–1) to both rotations to were sealed to the anchors by attaching the chamber flange to
avoid confounding rotation with N management. Each of the the anchor flange using binder clips. Gas samples were collected
five split-plots receiving fertilizer N were further subdivided using 12-mL polypropylene syringes 0, 0.5, 1, and 1.5 h after
338 Agronomy Journal • Volume 107, Issue 1 • 2015
sealing the chamber. Samples were immediately transferred to applied, and expressing the result as a percentage (Venterea et
glass vials sealed with butyl rubber septa (Alltech, Deerfield, al., 2012). Soil nitrate-N concentrations for each sampling date
IL) and analyzed within 1 wk using a headspace autosampler were used to determine soil nitrate-nitrogen intensity (SNI) by
(Teledyne Tekmar, Mason, OH) connected to a gas chromato- trapezoidal integration vs. time (Burton et al., 2008). Yield-
graph (model 5890, Agilent/Hewlett-Packard, Santa Clara, based nitrous oxide emissions (cN O-y) and yield-based soil
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CA) equipped with an electron capture detector. The equip- nitrate-nitrogen intensity (SNI-y) were calculated by divid-
ment was calibrated with analytical grade standards (Scott ing cN O and SNI, respectively, by grain yield (Mosier et al.,
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Specialty Gases, Plumsteadville, PA) each day when samples 2006). Data were analyzed at P < 0.05 using the MIXED pro-
were analyzed. Gas concentrations in molar mixing ratios were cedure of SAS (SAS Institute, 2006). To maintain a balanced
converted to mass per volume concentrations using ideal gas experimental design, data from the non-fertilized control
law and air temperatures at sampling. Fluxes of N O were cal- treatment, which lacked a fertilizer N application timing, were
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culated from the rate of change in chamber N O concentration excluded from tests of fixed effects and mean comparisons but
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using the restricted quadratic regression procedure (Parkin et were included in regression and correlation analyses. Year, crop
al., 2012) and the chamber bias correction method to account rotation, fertilizer N rate, and fertilizer N application timing
for suppression of the surface-atmosphere concentration gradi- were considered fixed effects and block and interactions with
ent (Venterea, 2010; Venterea and Parkin, 2012). block were considered random effects. Residuals were assessed
for normality and common variance using the UNIVARIATE
Soil and weather Measurements
procedure of SAS and scatterplots of residuals vs. predicted
During each N O flux sampling period soil temperature values (Kutner et al., 2004). Data for cN O emissions, cN O-y,
2 2 2
(ST) and soil moisture content (SMC) were measured in two of SNI, SNI-y, and emissions factors (EF) did not meet the
the 11 split-split-plots within each main plot. Soil temperature assumptions of normality and common variance, so data for
was measured by inserting a temperature probe (Fisher, Hamp- these dependent variables were logarithm base 10 transformed
ton, NH) to the 0.05-m depth within 1 m of the chamber. before statistical analysis.
Soil cores were collected from midway between the row and Mean comparisons were made using independent pairwise
mid-row positions (referred to as the one-fourth-row position) t tests at P ≤ 0.05 using the PDIFF option in the MIXED
within 1 h of each flux measurement period using 0.05-m procedures of SAS (SAS Institute, 2006). When the main
diam. brass rings to a depth of 0.05 m. Gravimetric water effect of fertilizer N rate or interactions among fertilizer N rate
content was determined by drying at 105°C. Additional soil and other fixed effects were significant at P ≤ 0.05, linear and
samples were collected weekly for analysis of extractable soil nonlinear regression equations were developed to describe the
N concentration. On each sampling date, two cores from each response of the dependent variables to fertilizer N rate using
split-split-plot were collected, one from the mid-row position the MIXED and NLIN procedures of SAS (SAS Institute,
and one from the one-fourth-row position, each to a depth of 2006), respectively. Several regression models were evaluated
0.15 m, and composited into a single bag. Samples were placed based on scatterplots of residuals vs. predicted values (Kutner
in a cooler and delivered to the lab within 2 h where approxi- et al., 2004), and selected regression models were significant
mately 10-g subsamples were extracted in 40 mL of 2 M KCl, at P ≤ 0.05. The agronomic optimum fertilizer N rate for corn
filtered (Whatman no. 42), and analyzed for total nitrite-N grain yield were predicted by setting the first derivative of the
(NO ––N) plus nitrate-N (NO ––N) using the Greiss–Ilosvay fit quadratic regression equation to zero
2 3
method with cadmium reduction (Mulvaney, 1996) modi- Linear associations between dN O, SMC, ST, and SN, and
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fied for use with a flow-injection analyzer (Lachat, Loveland, between cN O and SNI were evaluated for each combina-
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CO). Extractions and analyses were performed within 24 h tion of year, crop rotation, and fertilizer N application timing
of sample collection using procedures designed to minimize with Pearson’s correlation coefficient (r) at P ≤ 0.05 using the
potential losses of NO – during sample processing (Stevens CORR procedure of SAS (SAS Institute, 2006). Because SN
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and Laughlin, 1995). The sum of NO – plus NO – concentra- was measured weekly and dN O was measured twice weekly
2 3 2
tions on a dry weight basis are reported here and referred to as and not necessarily on the same day as SN, linear interpola-
“soil nitrate-nitrogen” (SN). Gravimetric SMC over the 0- to tion was used to estimate SN for association with dN O data.
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0.15-m depth was also determined in the soil samples collected Linear multiple regression models with dN O as dependent
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weekly. A weather station located 1 km from the plots was used variable and SMC, ST and SN as potential independent predic-
to collect air temperature and precipitation data which were tors were evaluated for each combination of year, crop rotation,
recorded at 30-min intervals and converted to daily average and and fertilizer N application timing using the REG procedure
daily totals, respectively. of SAS (SAS Institute, 2006). Models for all combinations of
one, two, or three of these predictors were evaluated and final
Data Analysis models were selected based on the adjusted R2, Mallow’s Cp,
Daily N O fluxes (dN O) measured on each sampling date Akaike’s, and Schwarz’ Bayesian criteria (Kutner et al., 2004).
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were used to determine cumulative growing season N O emis- Selected regression models and all parameter estimates were
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sions (cN O) by trapezoidal integration vs. time (Parkin and significant at P < 0.001.
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Venterea, 2010). The fertilizer-induced N O emission factor
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(EF) was calculated for each treatment by subtracting cN O
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in the control treatment of each main plot from cN O in each
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treatment of that plot, dividing by the amount of fertilizer N
Agronomy Journal • Volume 107, Issue 1 • 2015 339
RESultS grain yield to fertilizer N rate (R2 = 0.89, Fig. 2), with maxi-
weather mum yield occurring at 155 kg N ha–1.
Total precipitation amounts during April through September
Nitrous Oxide Flux and Soil Nitrate-
were similar in 2012 (646 mm) and 2013 (591 mm) compared
nitrogen Concentration
to the 30-yr mean of 542 mm (Table 1, Fig. 1a). In 2012, more
of the precipitation occurred in larger rainfall events. There were Daily mean N O fluxes ranged from below 10 to approxi-
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9 d in 2012 during which >31 mm of precipitation was recorded mately 1000 µg N m–2 h–1(Fig. 3). Several episodic increases
compared to only 1 d in 2013. Periods with lower than normal followed by decreases in dN O were observed at different peri-
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rainfall and dry soil conditions occurred during the latter half of ods during each growing season. Soil nitrate-N concentration
both growing seasons. In 2012, a dry period persisted from 2 wk ranged from below 1.0 to approximately 90 mg N kg–1 (Fig. 4).
before until 1 wk following the V6 urea application (i.e., 21 June Segregated by year, rotation, and timing, dN O was positively
2
through 12 July). During this period, the seasonal maximum ST correlated with SN in all but one case, positively correlated
values (>28°C) and minimum SMC values were observed, with with SMC, and negatively correlated with ST in some cases
SMC at 0 to 0.05 m remaining below 0.12 g H O g–1 on four (Table 3). Significant regression models using combinations
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consecutive sampling dates (Fig. 1b and 1c). A second dry period of SMC, ST, and/or SN as predictors were obtained, with R2
in 2012 started on 16 August, 3 wk following the V14 applica- values ranging from 0.08 to 0.42 (Table 3). When included in
tion, after which only 18 mm of rain was recorded compared to multiple regression models together with SMC and SN, ST had
the 30-yr mean of 72 mm for the month of September. Soil mois- a positive association with dN O.
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ture content steadily declined over the last four sampling dates of
Cumulative nitrous Oxide Emissions
the 2012 season. In 2013, SMC steadily declined starting 1 wk
(cn O and cn O-y)
after the V14 application on 28 July; only 98 mm of precipitation 2 2
was recorded for the months of August and September com- Crop rotation had a significant effect on cN O (Table 2).
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bined, compared to the 30-yr mean of 180 mm. Across years, application timings, and fertilizer N rates, cN O
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in the CC rotation (1.57 kg N ha–1) was significantly greater
grain Yield than in the CS rotation (1.05 kg N ha–1). There were signifi-
Fertilizer N rate was the only factor that had a significant cant rate × timing and year × timing interaction effects on
effect on corn grain yield (Table 2). Across years, crop rotations, cN O (Table 2). Across crop rotations and N rates, the SpA
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and N application timings, there was a quadratic response of treatments in 2012 had greater cN O compared with the SA
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treatments in 2012 and compared with the SpA treatments
Table 1. Rainfall patterns by month and growing season. in 2013 (Table 4). Across growing seasons and crop rota-
Period 2012 2013 30-yr mean† tions in treatments receiving the maximum N fertilizer rate
April 80 146 71 (210 kg N ha–1), cN O was greater with SpA (2.47 kg N ha–1)
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May 188 148 99 compared with SA (1.57 kg N ha–1). However, N applica-
June 153 117 103 tion timing had no significant effect on cN O across grow-
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July 117 82 89 ing seasons and crop rotations in treatments receiving
Aug. 92 62 108 <210 kg N ha–1. Accordingly, the response of log-transformed
Sept. 14 36 72 cN O to fertilizer N rate was fit to a linear regression model
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Total 646 591 542 (r2 = 0.74) for the SA treatments, and to a quadratic regres-
† Calculated for 1984–2013 using data available at http://climate.umn.edu/doc/ sion model (R2 = 0.62) for the SpA treatments (Fig. 5a). When
historical.htm.
Table 2. Significance of F values for fixed sources of variation.†
Source of variation Corn grain yield cN O EF cN O-y SNI SNI-y
2 2
___________________________________________ P > F ____________________________________________
Year 0.081 0.224 0.200 0.310 0.844 0.204
Crop rotation (rotation) 0.628 0.039 0.029 0.095 0.022 0.350
N rate (rate) 0.003 <0.001 0.542 <0.001 <0.001 <0.001
N application timing (timing) 0.227 0.010 0.054 0.003 0.001 0.009
Year × rotation 0.257 0.215 0.066 0.103 0.052 0.191
Year × rate 0.732 0.073 0.505 0.227 0.336 0.721
Year × timing 0.610 <0.001 <0.001 <0.001 <0.001 <0.001
Rotation × rate 0.645 0.680 0.663 0.562 0.249 0.569
Rotation × timing 0.265 0.485 0.173 0.481 0.769 0.647
Rate × timing 0.539 0.039 0.156 0.162 0.007 0.012
Year × rotation × rate 0.591 0.245 0.035 0.346 0.137 0.606
Year × rotation × timing 0.544 0.265 0.160 0.435 0.105 0.050
Year × rate × timing 0.138 0.534 0.961 0.417 0.132 0.252
Rotation × rate × timing 0.986 0.922 0.789 0.951 0.709 0.531
Year × rotation × rate × timing 0.887 0.720 0.629 0.617 0.555 0.599
† Statistical analysis of cumulative nitrous oxide emissions (cN O), nitrous oxide emissions factor (EF), yield-scaled cumulative nitrous oxide emissions (cN O-y), nitrate-
2 2
nitrogen intensity (SNI) and yield-scaled nitrate-nitrogen intensity (SNI-y) are based on logarithm base 10 transformed data.
340 Agronomy Journal • Volume 107, Issue 1 • 2015
cumulative N O emissions were expressed on a yield-scaled significant differences in EF by crop rotation and fertilizer N
2
basis, there was a significant year × timing interaction effect rate were present in 2012 but not in 2013 (Table 5). In 2012,
on yield-based cumulative nitrous oxide emissions (cN O-y). EF was greater in the CC than in the CS rotation at all N
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Consistent with the cN O result, cN O-y was greater with rates except 130 kg N ha–1; differences by fertilizer N rate
2 2
SpA compared with SA only in 2012, and cN O-y in the SpA were observed but were not consistent across crop rotations.
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treatments was greater in 2012 than 2013 (Table 4). In contrast Regression analysis of EF vs. N rate did not generate significant
with cN O, crop rotation had no significant effect on cN O-y, models for any combination of year and crop rotation. There
2 2
and there was no rate × timing interaction effect. The response was also a significant year × timing interaction effect on EF.
of log-transformed cN O-y to fertilizer N rate across all treat- Consistent with cN O and cN O-y, across crop rotations and
2 2 2
ments was fit to a quadratic regression model (R2 = 0.62) (Fig. 5b). N rates, the SpA treatments in 2012 had greater EF than the
SA treatments in 2012 and the SpA treatments in 2013 (Table
Fertilizer-Induced Emissions Factors
4). In 2013, EF was greater with SA than SpA.
There was a significant year × crop rotation × N rate
interaction effect on EF (Table 2). Across timing treatments,
Fig. 1. (a) Daily and cumulative precipitation, (b) soil moisture content, and (c) air and soil temperature. Downward-pointing arrows indicate dates of
planting (P) and fertilizer N application (F). Single application treatments received 100% of their N at F1, and the split-application treatments received
three equal applications at F1, F2, and F3.
Agronomy Journal • Volume 107, Issue 1 • 2015 341
Soil Nitrate-Nitrogen Intensity
Crop rotation had a significant effect on SNI (Table 2).
Across years, timings, and fertilizer N rates, SNI in the CS
rotation (2.45 g N d kg–1) was significantly greater than in
the CC rotation (1.80 g N d kg–1). There was also a significant
rate × timing interaction effect. Across growing seasons and
crop rotations in treatments receiving the maximum N fertil-
izer rate (210 kg N ha–1), SNI was greater with SA (4.51 g N
d kg–1) compared with SpA (2.93 g N d kg–1). Timing had no
significant effect on SNI in treatments receiving <210 kg N
ha–1. Accordingly, the response of log-transformed SNI to fer-
tilizer N rate was fit to a linear regression model (r2 = 0.89) for
the SA treatments, and to a linear-plateau regression model for
the SpA treatments (Fig. 6a). There was also a significant year ×
timing interaction effect on SNI. In 2012, SNI was greater
with SpA compared with SA, while in 2013 the opposite pat-
Fig. 2. Response of corn grain yield to fertilizer N rate, across years,
tern of differences by timing was observed; and within the SA
crop rotations, and fertilizer N application timings.
Fig. 3. Daily nitrous oxide fluxes (dN O) in continuous corn (CC) and corn–soybean (CS) rotations receiving single (SA) and split (SpA) fertilizer
2
applications. Downward-pointing arrows indicate dates of planting (P) and fertilizer N application (F). Single application (SA) treatments received
100% of their N at F1, and split application (SpA) treatments received three equal applications at F1, F2, and F3. Note different scales on right and left
vertical axes.
342 Agronomy Journal • Volume 107, Issue 1 • 2015
treatments, SNI was greater in 2013 than 2012, but within Accordingly, the response of SNI-y to fertilizer N rate was fit to
the SpA treatments, there was no difference by year (Table 4). a linear regression model (r2 = 0.90) for the SA treatments, and to
There was a significant correlation between SNI and cN O a linear-plateau regression model for the SpA treatments (Fig. 6b).
2
for all combinations of year, timing, and crop rotation (Table
DiSCuSSiOn
3). When SNI was expressed on a yield-scaled basis, there was
a significant year × crop rotation × timing interaction effect grain Yield
on SNI-y (Table 4). In 2012, SNI-y was greater with SpA Grain yield increased in response to fertilizer N input,
compared with SA in the CC rotation only. In 2013, SNI-y but regression analysis indicated a quadratic response curve
was greater with SA compared with SpA in both rotations; with maximum yield occurring at an N rate of 155 kg N ha–1.
and SNI-y was greater in the CS compared to CC for both the Applying urea in three split applications at planting, V6, and
SA and SpA treatments. There was a significant rate × timing V14 vs. a single application at planting did not affect grain
interaction effect on SNI-y consistent with the SNI results. yield. Widely varying effects of split N application on corn
Fig. 4. Soil nitrate nitrogen concentrations (SN) in continuous corn (CC) and corn–soybean (CS) rotations receiving single (SA) and split (SpA)
fertilizer applications. Downward-pointing arrows indicate dates of planting (P) and fertilizer N application (F). Single application (SA) treatments
received 100% of their N at F1, and split application (SpA) treatments received three equal applications at F1, F2, and F3.
Agronomy Journal • Volume 107, Issue 1 • 2015 343
Table 3. Correlation of daily nitrous oxide flux (dN O) with soil moisture content (SMC), soil temperature (ST), and soil nitrate-nitrogen concentra-
2
tion (SN), correlation of cumulative nitrous oxide emissions (cN O) with soil nitrate-nitrogen intensity (SNI), and multiple regression of dN O vs.
2 2
SMC, ST, and SN. Pearson’s r and P values are shown for correlation analyses. Model coefficients (b , b , b , and b ) and R2 are shown for multiple
0 1 2 3
regression analyses.†
Correlation
dN O vs. Multiple regression¶
2
SMC ST SN cN O vs. SNI SMC ST SN
2
Year Rotation‡ Timing§ r b b b b R2
0 1 2 3
2012 CC SA 0.26*** –0.05ns# 0.21*** 0.64** –0.14 3.23 0.028 0.227 0.19
SpA 0.34*** –0.07ns 0.54*** 0.83*** –0.58 3.63 0.031 0.588 0.42
CS SA 0.22*** –0.06ns 0.14** 0.59** 0.97 1.40 ns 0.127 0.08
SpA 0.38*** –0.14*** 0.42*** 0.65** 0.63 2.40 ns 0.353 0.29
2013 CC SA 0.47*** –0.08* 0.40*** 0.71*** 0.61 2.91 ns 0.259 0.33
SpA 0.45*** –0.13** 0.23*** 0.70** 0.75 2.58 ns 0.173 0.24
CS SA 0.47*** –0.19*** 0.13** 0.73*** 0.75 2.65 ns 0.076 0.24
SpA 0.46*** –0.18*** –0.06ns 0.61** 0.75 2.62 ns 0.091 0.22
* P < 0.05.
** P < 0.01.
*** P < 0.001.
† dN O, cN O, SN, and SNI were logarithm base 10 transformed before analysis.
2 2
‡ CC, continuous corn; CS, corn–soybean.
§ SA, single application; SpA, split application.
¶ Multiple regression results are for models in the form: dN O = b + bSMC + b ST + b SN. All models were significant at P < 0.001 and all parameter estimates (b, b
2 0 1 2 3 1 2
and b) were significant at P < 0.001.
3
# ns, not significant.
yield have been reported, including no effects (Randall et al.,
1997) increased yield in some but not all growing seasons
(Jaynes, 2013); and decreased yield (Jaynes and Colvin, 2006).
The lack of a yield response to timing across a range of fertilizer
N rates in the current study suggests that application timing
Table 4. Cumulative nitrous oxide emissions (cN O), N O emissions fac-
2 2
tor (EF), yield-scaled nitrous oxide emissions (cN O-y), nitrate-nitrogen had no overall effect on crop utilization of fertilizer N across
2
intensity (SNI), and yield-scaled nitrate-nitrogen intensity (SNI-y) as af- the whole growing season. It is possible there was more efficient
fected by year, crop rotation, and fertilizer N application timing†.
crop N utilization associated with the first SpA application
N application timing
compared to the larger single SA application at planting which
Year Crop rotation Single Split
could have promoted greater N loss via leaching of soluble N
cN O, kg N ha–1
2 below the undeveloped root zone. However, lower N losses
2012 Avg.‡ 1.20aB§ 1.86aA
and more efficient crop N utilization associated with the first
2013 1.18aA 1.01bA
SpA application could have been offset by greater N losses
and/or less efficient crop N utilization of the mid-season (V6
EF, %
and V14) applications. Lack of timely rainfall and relatively
2012 Avg. 0.54aB 0.98aA
low SMC during specific periods before and after the V6
2013 0.55aA 0.37bB
and V14 applications each year may have resulted in limited
cN O-y, g N Mg–1 mobilization of fertilizer N throughout the soil profile which
2
2012 Avg. 110bA 180aB may have restricted root N uptake. Volatilization of ammonia
2013 181aA 153aA (NH ) may have also contributed to lower crop N utilization
3
SNI, g N day kg–1 Table 5. Nitrous oxide emissions factor (EF) as affected by year, crop
2012 Avg. 1.91bB 2.12aA rotation, and fertilizer N rate.†
2013 2.79aA 1.69aB Fertilizer 2012 2013
N rate CC‡ CS CC CS
SNI-y, mg day kg–1 Mg–1 kg N ha–1 ____________________EF, % _________________
2012 CC 178aB 225aA 50 1.63aA§ 0.12cB 0.41aA 0.41aA
CS 169aA 178aA 90 1.03abA 0.21bcB 0.55aA 0.41aA
130 0.90 bA 0.46abA 0.64aA 0.25aA
2013 CC 264bA 155bB 170 0.98abA 0.28abcB 0.60aA 0.39aA
CC 694aA 406aB 210 1.26abA 0.70aB 0.45aA 0.47aA
† Statistical analysis of cN2O, EF, cN2O-y, SNI, SNI-y are based on logarithm † Statistical analysis is based on logarithm base 10 transformed data, and back-
base 10 transformed data, and back-transformed means are reported. transformed means are reported.
‡ Averaged across continuous corn (CC) and corn–soybean (CS) rotations. ‡ CC, continuous corn; CS, corn–soybean.
§ Within a column for cN2O, EF, cN2O-y, and SNI, and within a column and year § Within a column, means followed by the same lowercase letter are not signifi-
for SNI-y, means followed by the same lowercase letter are not significantly cantly different at the 0.05 probability level. Within a row for a given year, means
different at the 0.05 probability level. Within a row, means followed by the same followed by the same uppercase letter are not significantly different at the 0.05
uppercase letter are not significantly different at the 0.05 probability level. probability level.
344 Agronomy Journal • Volume 107, Issue 1 • 2015
Fig. 5. Response to fertilizer N rate for (a) cumulative nitrous oxide emissions (cN O) for single (SA) and split (SpA) application timings across years
2
and crop rotations, and (b) yield-scaled cumulative N O emissions (cN O-y) across years, crop rotations, and fertilizer application timings. Regression
2 2
analyses were conducted using individual observations. Note that y axes use log scale.
following the mid-season applications due to warmer condi- timing with respect to growth stage between growing seasons,
tions which can promote NH loss (Jones et al., 2013). We since prolonged dry periods occurred in 2012 when the crop
3
incorporated the urea following each application consistent was at V6 and in 2013 when the crop was at V14. Relying on
with recommended practices and with the majority of Minne- weather forecasts also risks high NH losses in the event of
3
sota farmers according to a recent survey (Bierman et al., 2012). lower than predicted rainfall amounts. Thus, mechanical incor-
Even though mechanical incorporation following mid-season poration of urea represents a best possible case for minimizing
applications may be impractical or require the use of high-clear- NH losses under the given conditions; nonetheless, significant
3
ance equipment, we decided to incorporate after every applica- NH losses may have occurred.
3
tion to reduce the potential for NH losses. In the absence of
3
nitrous Oxide Emissions
incorporation, rainfall of at least 0.5 inches (13 mm) occurring
in a single event within 24 to 48 h of application is also recom- Previous studies examining N application timing effects on
mended as a means to incorporate urea and to reduce NH N O emissions have observed varying results, including no effects
3 2
losses (Jones et al., 2013). However, relying on expected rainfall of delayed or split application (Zebarth et al., 2008; Phillips et al.,
risks delaying the application; in the current study, this practice 2009); reduced N O emissions with delayed application under
2
likely would have resulted in later application and inconsistent some but not all conditions (Drury et al., 2012); or reduced N O
2
Fig. 6. Response to fertilizer N rate for (a) soil nitrate-nitrogen intensity (SNI) and (b) yield-scaled soil nitrate-nitrogen intensity (SNI-y) for single (SA)
and split (SpA) application timings across years and crop rotations. Regression analyses were conducted using individual observations. R2 values are
not shown for linear-plateau models because residuals do not always sum to zero for nonlinear regression models (Kutner et al., 2004). Note that y
axes use log scale.
Agronomy Journal • Volume 107, Issue 1 • 2015 345
emissions with split application in some but not all growing other types of nonlinear responses of cumulative N O emissions
2
seasons (Burton et al., 2008). This is the first study to report an to N rate (e.g., Van Groenigen et al., 2010; Kim et al., 2013).
increase in cumulative N O emissions with split compared to a Increasing exponential responses similar to the 1° model imply
2
single early-season N application, which was observed in 2012 that with each marginal increase in N fertilizer input, there is a
(across crop rotations and N rates) and in the highest fertilizer disproportionately larger marginal increase in N O emissions.
2
N rate (across rotations and growing seasons). Greater cN O The 2° model found here for cN O with SpA is nearly identical
2 2
with SpA across rotations and N rates in 2012 was likely caused to the 1° model for cN O with SA at N rates ≤90 kg N ha–1, but
2
by wide fluctuations in SMC and rainfall occurring before and diverges from the 1° model at higher N rates. As far as we know,
following the V6 and V14 applications. A prolonged dry period 2° exponential responses of cN O or cN O-y to N rate have not
2 2
persisted from about 2 wk before until 1 wk following the V6 been previously reported. Increasing exponential responses of
application, which (as discussed previously) may have inhibited N O emissions to N rate also imply that EFs will increase with
2
the movement of applied N through the soil profile and limited N rate (Shcherbak et al., 2014). However, an increasing response
crop N uptake. Soil N availability was then supplemented by of EF to N rate was not found in this study. The EFs for each
additional N from the V14 application, which was followed by a individual treatment were calculated by subtracting cN O values
2
series of rainfall events, including the largest event of the season obtained for the control treatment in each main plot from the
(60 mm) the following day, and three events exceeding 20 mm cN O values of each treatment within that plot. This procedure
2
occurring 5, 10, and 22 d later (these four events accounted for is required to calculate variances that allow for examination of
26% of total growing season precipitation). It is likely that these treatment effects (Venterea et al., 2012). Thus, the EF data set has
rainfall events combined with high levels of soil N created condu- a different content and structure than the original cN O data set
2
cive conditions for the large and prolonged pulses in dN O that used to generate the exponential response curves. Thus, an expo-
2
occurred following the V14 application in 2012. nential response of cN O to N rate does not necessarily imply
2
Increased ST later in the season may have contributed that EFs increase monotonically with N rate.
to some extent to enhanced N O emissions following the Across growing seasons, application timings, and N rates,
2
midseason N applications. Phillips et al. (2009) found a trend cN O was 50% greater in CC compared with the CS rota-
2
(P = 0.103) for greater N O emissions when urea was applied tion. In both years, the CC/SA treatments exhibited increases
2
3 d before planting compared to 6 wk before planting and in dN O above baseline levels following rainfall events in
2
noted that greater temperatures as well as SMC following the early August 2012 and mid-August 2013, but this was not
later application may have promoted greater microbial activity observed in the CS/SA treatments (Fig. 3). Soil nitrate-N
and N O production compared to the earlier application. The concentrations were relatively high in both the CC and
2
negative correlation observed here between cN O and ST is CS treatments during these events (Fig. 4). Across growing
2
possibly an artifact of a consistent negative correlation (r = seasons, application timings and N rates, SNI was greater
–0.55) between SMC and ST, that is, when soils were wetter with CS than CC, which could have resulted from greater
and more conducive for N O production, they were also cooler amounts of N released from the previous year’s crop (N-fixing
2
as the result of increased evaporative cooling. This explanation soybean) residue. Thus, the SN and SNI data suggest that
is consistent with the observation that when ST was included some factor other than soil N availability was responsible
in multiple regression models together with SMC and SN, for greater cN O in CC compared to CS. It is possible that
2
there was a positive association between ST and dN O. Includ- the greater mass of crop (i.e., corn) residue inputs from the
2
ing ST as a third predictor variable explained an additional 5.7 previous growing season in the CC rotation compared with
and 4.9% of the variance in dN O for the 2012 CC/SA and the soybean residue inputs in the CS rotation resulted in
2
2012 CC/SpA treatments, respectively (Table 3); ST was not greater levels of soluble organic carbon (SOC) in the CC
included in the final selected model for the remaining treat- rotation. Differences in tillage regime between rotations (i.e.,
ment combinations because it did not substantially improve the fall tillage occurred following the previous crop in CC but
overall model, explaining <2.5% of additional variance in dN O. not in CS) may have also promoted greater levels of SOC in
2
The models describing log-transformed cN O as a function the CC rotation. It has been shown that SOC can promote
2
of fertilizer N rate (Fig. 5) are equivalent to models describing N O production via denitrification (Burford and Bremner,
2
non-transformed N O emissions (y) as functions of fertilizer 1975) and/or nitrification (Venterea, 2007). A previous study
2
N rate (x) in the forms at the same site as the current study observed greater soil CO
2
emissions with CC than CS, which supports the hypothesis
y =b10b1x that labile SOC was greater in CC than CS. It is also possible
0 that variation in SOC availability during the growing season
may have differed between rotations; that is, SOC may have
and been more available in CC compared to CS during July and
August. Previous studies have found significant differences in
y =b 10b1x+b2x2 N2O emissions, but the differences were in opposite direc-
0
tions; Drury et al. (2008) found greater N O emissions with
2
CC and Mosier et al. (2006) found greater N O emissions
2
where b , b , and b are positive constants; we refer to these with CS. Differences in microbial community structure
0 1 2
models as “first-order” (1°) and “second-order” (2°) exponential, or function may also have been involved. Further study is
respectively. Previous studies have reported 1° as well as linear and needed to explain crop rotation effects on cN O and SNI.
2
346 Agronomy Journal • Volume 107, Issue 1 • 2015