Published March 11, 2015
Journal of Environmental Quality
Special Section
Improving Nitrogen Use Efficiency in Crop and Livestock Production Systems
Nitrous Oxide Emissions from Anhydrous Ammonia, Urea, and Polymer-Coated Urea in Illinois Cornfields Fabián G. Fernández,* Richard E. Terry, and Eric G. Coronel
C
orn grain production is a major agricultural activity in the United States, with 34 million ha harvested in 2011 (USDA–NASS, 2012). Nitrogen fertilization is needed to optimize grain production and economic returns (Sawyer et al., 2006). However, N fertilization normally increases soil surface N2O emissions from agricultural systems (Snyder et al., 2009). In 2010, 6.3% (428.4 Tg CO2 Eq.) of the total U.S. greenhouse gas emissions were attributed to agricultural activities, with methane (CH4) and N2O being the primary gases (USEPA, 2012). This report also indicated that the largest source of N2O emissions in the United States (207.8 Tg CO2 Eq. [670 Gg N2O]) in 2010 was related to soil management activities and fertilizer application. This is important because N2O has approximately 300 times more potential to capture infrared radiation than CO2 (Intergovernmental Panel on Climate Change, 2007). Although N2O is a potent greenhouse gas, it is the most poorly quantified greenhouse gas in cropland (Mosier et al., 1996; Venterea et al., 2009). For these reasons, efforts should be devoted to better quantify agriculture-induced N2O emissions. In recent years, possibly as a result of increased demands for ethanol production, fluctuation in corn and soybean [Glycine max (L.) Merr.] prices, and other factors, the traditional corn– soybean rotation in much of the midwestern United States has increasingly become continuous corn. For example, with a constant 8.7 million cultivated hectares in Illinois, corn and soybean were nearly equal in land use in 2000, but since then corn cultivation has steadily increased at the expense of soybean, and in 2012 corn was planted in 58% of cropland (USDA– NASS, 2012). Similar trends have been observed for other regions of the United States (USDA–NASS, 2012). It is possible that greater N fertilization and greater amounts of plant residue produced with continuous corn relative to a corn–soybean rotation could increase greenhouse gas emissions (Drury et al., 2008; Wilson and Al-Kaisi, 2008). To date, however, the few studies quantifying soil surface N2O emissions in continuous corn systems have produced contrasting results ( Jacinthe and Dick, 1997; Drury et al., 2008; Omonode et al., 2011). In addition to cropping system management, N source can have an important impact on the amount of N2O emitted. The
Abstract The use of alternative N sources relative to conventional ones could mitigate soil-surface N2O emissions. Our objective was to evaluate the effect of anhydrous ammonia (AA), urea, and polymer-coated urea (ESN) on N2O emissions for continuous corn (Zea mays L.) production. Corn received 110 kg N ha-1 in 2009 and 180 kg N ha-1 in 2010 and 2011. Soil N2O fluxes were measured one to three times per week early in the growing season and less frequently later, using vented non–steady state closed chambers and a gas chromatograph. Regardless of N source, N2O emissions were largest immediately after substantial (>20 mm) rains, dropping to background levels thereafter. Averaged across N sources, 2.85% of the applied N was lost as N2O. Emission differences for treatments only occurred in 2010, the year with maximum N2O production. In the 2010 growing season, cumulative emissions (in kg N2O–N ha-1) were lowest for the check (2.21), followed by ESN (9.77), and ESN was lower than urea (14.07) and AA (16.89). Emissions in 2010 based on unit of corn yield produced followed a similar pattern, and N2O emissions calculated as percent of applied N showed that AA losses were 1.9 times greater than ESN. Across years, relative to AA, ESN reduced N2O emissions, emissions per unit of corn yield, and emissions per unit of N applied, whereas urea produced intermediate values. The study indicates that, under high N loss potential (wet and warm conditions), ESN could reduce N2O emissions more that urea and AA.
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
F.G. Fernández, Dep. of Soil, Water, and Climate, Univ. of Minnesota, S233 Soils, 1991 Upper Bufford Circle, St. Paul, MN, 55108; R.E. Terry, Plant and Wildlife Sciences, Brigham Young Univ., 255 WIDB, Brigham Young University, Provo, UT 84602; E.G. Coronel, Dep. of Crop Sciences, Univ. of Illinois, 1102 South Goodwin Ave., Urbana, IL. 61801. Assigned to Associate Editor Barbara Amon.
J. Environ. Qual. 44:415–422 (2015) doi:10.2134/jeq2013.12.0496 Received 12 Dec. 2013. *Corresponding author ([email protected]).
Abbreviations: AA, anhydrous ammonia; ESN, polymer coated urea.
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most extensively studied N source comparison in the United States (although it amounts to a few studies) has been between the effects of urea and AA. These studies have shown greater N2O emissions with AA than urea (Breitenbeck and Bremner, 1986; Thornton et al., 1996; Venterea et al., 2005, 2010; Fujinuma et al., 2011). A 1-yr comparison showed greater N2O emissions with AA than with urea ammonium nitrate (Venterea et al., 2005). In a recent review, Snyder et al. (2009) indicated that the use of nitrification and urease inhibitors and controlled-release N fertilizers has the potential to increase N use efficiency and, as a consequence, to reduce N2O emissions. However, studies that evaluate the agronomic impact of these technologies (especially controlled-release N technology) on corn and their potential to reduce N2O emissions are scant. In a 3-yr study in a clay loam Mollisols in Ontario, Canada, Drury et al. (2012) observed that polymer-coated urea reduced N2O emissions relative to urea in one of the years only. Similarly, Sistani et al. (2011), in 1 yr of a 2-yr study in a silt loam Alfisols in Kentucky, showed N2O reductions with ESN relative to urea. In a study in Colorado under irrigation in a semiarid temperate climate in a clay loam soil, Halvorson et al. (2010b) found more consistent results, with ESN reducing N2O emissions relative to urea in a no-till system with continuous corn, but there were no differences in a conventionally tilled system. Other studies by Halvorson et al. (2010a, 2011) indicated substantial reduction in N2O emissions with ESN compared with urea. In contrast, in a much wetter location in Minnesota but with greater natural drainage in a silt loam soil, Venterea et al. (2011) showed no difference in N2O emissions between ESN and urea regardless of tillage system (conventional and no-till). One possible explanation to these contrasting reports is that the clay loam soil in Colorado may retain more water and possibly increase the potential for N2O generation relative to the more permeable silt loam soil in Minnesota. To our knowledge, no studies have compared N2O emissions between AA and ESN for corn production. This is surprising because AA is the most common N source in much of the Midwestern United States, but the amount of ESN being applied for corn production is increasing (A.D. Blaylock, personal communication, 2011) and has been shown in some studies to have the potential to increase yield and reduce N2O emissions. Our main objective was to evaluate the effects of AA, urea, and ESN on N2O emissions for continuous corn production. Additional objectives were (i) to evaluate grain yields to quantify the potential benefits of using ESN relative to the conventional N sources AA and urea and (ii) to relate soil-surface N2O emissions for these N sources on a grain yield and amount of applied N basis.
Materials and Methods Site Description and Experimental Design Field experiments were conducted from 2009 to 2011 at the Crop Sciences Research and Education Center near Urbana, Illinois (40°3¢11² N; 88°13¢47² W). The region has a temperate climate, with a 30-yr (1980–2009) mean temperature of 11.2°C and rainfall of 1044 mm yr–1. The soils were a somewhat poorly drained Flanagan silt loam (fine, smectitic, mesic Aquic Argiudolls) and a poorly drained Drummer silty clay loam (fine416
silty, mixed, superactive, mesic Typic Endoaquolls). In 2009 and 2011, the site consisted of more than 70% Flanagan series, with the remainder a Drummer series. In 2010, the entire site consisted of Flanagan soil. The site has subsurface tile drainage. In the top 18 cm of soil depth, soil organic matter ranged between 35 and 36 g kg-1, and soil pH (1:1 soil/water) ranged between 5.9 and 6.5. Total inorganic N (NO3– plus NH4+) present in the top 30 cm of the soil before treatment application in the spring was low: 13, 7, and 6 mg kg-1 for 2009, 2010, and 2011, respectively. The study was established on fields with corn as the previous crop, and every fall the soils were chisel plowed. The study was set up in a randomized complete-block design with four replications. The treatment consisted of a control (check) with no N applied and three N sources: AA (82–0-0), urea (46–00), and ESN (44–0-0) (Agrium Advanced Technologies) (N-P-K) applied at the rate of 110 kg N ha-1 in 2009 and at the rate of 180 kg N ha-1 in 2010 and 2011. The application rate was increased for the last 2 yr to more closely represent typical farmer practices. All applications were done within 4 wk of planting. In 2009, urea and ESN were applied on 1 June and AA on 2 June. In 2010, AA was applied on 30 April and urea and ESN on 20 May. The 20-d delay between applications in 2010 was caused by rain and persistent wet conditions right after the AA application. In 2011, AA was applied on 12 May and urea and ESN on 19 May. Anhydrous ammonia was injected 15 cm below the soil surface centered between crop rows (38 cm from adjacent corn crop rows); urea and ESN were broadcast applied by hand with a spin-spreader and incorporated by shallow tillage. We obtained ESN directly from the manufacturer, and special care was taken in handling the product to prevent damage of the polymer coating (Beres et al., 2012). Each treatment was applied on a 6 by 23 m plot. Corn was planted on 76-cm row spacing at a rate of 79,000 seeds ha-1 using a John Deere 7200 Max Emerge vacuum planter with Yetter trash movers and openers. Plots were kept weed free using glyphosate [isopropylamine salt of N-(phosphonomethyl) glycine]. Hybrid DeKalb 60–18 was planted on 17 June 2009, and hybrid Pioneer 0916XR was planted on 25 May 2010 and on 19 May 2011. The planting delay in 2009 was related to wet soil conditions in April and May combined with setbacks due to resource availability during the establishment year of the study.
Measurements Soil surface N2O fluxes were measured using a vented non– steady state closed chamber similar to those used by Parkin and Hatfield (2010). Rings constructed from PVC (30 cm in diameter and 10 cm tall) were installed in the soil to a depth of approximately 6 cm in each plot to serve as the base for flux chambers. Rings stayed in place for the entire sampling period except when they were removed for planting operations. Because AA was banded, care was taken to ensure that the rings were installed to proportionately represent the fertilized band and the rest of the soil between crop rows. Anhydrous ammonia movement in the soils used in this study has been well characterized. In this study, AA was applied at soil-water content between 80 and 90% of field capacity (-0.33 bar), in which AA moves laterally approximately 10 cm from the point of injection (Touchton et al., 1978; Hoeft et al., 2000). This dispersion distance represents 26% of the total soil surface Journal of Environmental Quality
between crop rows. Accordingly, we calculated the segment of the soil surface area affected by the N fertilization band needed to be included inside the rings to represent 26% of the inter-row soil surface. This resulted in rings being installed approximately 1 cm off the AA knife track and 7 cm off the crop row. The proximity to the crop row also allowed us to capture the potentially high crop row effect on N2O fluxes (Kessavalou et al., 1998) without interfering with the development of brace roots. Finally, the application of AA can be more variable than dry fertilizers because AA is in gaseous form at the point of application. However, the coefficients of variance of N2O emissions indicate that variability for AA was similar to other N sources in this study. Averaged across years, the coefficient of variance was 1.96, 2.47, 2.81, and 3.13 for the unfertilized check, AA, urea, and ESN, respectively Flux chambers were made from PVC (30 cm diameter and 10 cm tall), sealed on one end with a PVC sheet, and fitted with a butyl rubber septum (through which a syringe needle could be inserted to collect gas samples) and a small inlet to vent the chamber. The open end of the chamber was fitted with a rubber skirt that, when folded over the ring base, created an air-tight connection. Except for the rubber skirt, the chamber was covered with reflective tape. In 2009, N2O emissions were measured between 29 June and 23 November. Due to setbacks in the installation of chambers and resource constraints, soil N2O flux measurements were delayed until the end of June 2009, nearly a month after N treatment application. Measurements were taken weekly until the end of September and every 2 wk until November. In 2010, N2O emissions were measured between 4 May and 13 September. Measurements were taken two to three times a week. In 2011, N2O emissions were measured between 12 May and 18 October. Measurements were taken two to three times a week until the end of June, weekly in July, every 2 wk in August, and monthly in September and October. In all 3 yr there were exceptions during the more intensive sampling times when less frequent sampling events occurred due to inclement weather conditions. Gas samples were collected between 1000 and 1200 h local time when soil temperatures were close to their daily mean values. Gas samples were collected at 0, 15, and 30 min after chamber deployment with a polypropylene syringes and immediately injected into evacuated glass vials (10 mL) fitted with gray butyl rubber septa. The N2O concentrations in samples were determined within 24 h of collection with an Agilent 6890n gas chromatograph equipped with a 63Ni electron capture detector and a stainless steel column (0.3 cm outside diameter by 300 cm long) packed with Porapak Q (80–100 mesh). Based on the change in chamber headspace concentration pattern (accounting for soil surface area and chamber volume), linear and nonlinear regressions were used to calculate soil N2O fluxes (Livingston and Hutchinson, 1995). At each N2O gas sampling time, soil temperature at 5 cm depth was measured using a digital soil temperature thermometer (Model 71119, Gempler’s), and volumetric soil water content for the top 5 cm was measured using ECH2O EC-5 moisture probes and Em-50 digital data loggers (Decagon Devices Inc.). Precipitation and air temperature data were collected from a nearby weather station located at the Research and Education Center.
Estimates of daily N2O emissions between sampling days were calculated by linear interpolation between adjacent sampling dates. Fertilizer-induced N2O emissions as a percentage of the applied N fertilizer for the different N sources were calculated using Eq. [1]. The N2O emissions per unit of corn yield produced were calculated by dividing the cumulative N2O emissions by the grain yield: Fertilizer-induced emission =
æ Total emission of fertilized treatment - Total emission of unfertilized treatment ö÷ çç ÷÷´100 [1] çè ø Nitrogen rate of fertilized treatment Grain yield was obtained by a plot combine harvesting the two center rows of each plot on 1 Dec. 2009, 19 Oct. 2010, and 17 Oct. 2011. Yields were corrected to 155 g kg-1 moisture.
Statistical Analyses Data were analyzed with the MIXED procedure of SAS (SAS Institute, 2009) with year and treatment as fixed effects and blocks and its interactions as random effects. The LS Means for significant fixed effects were further analyzed by the SLICE option in the PROC MIXED procedure of SAS (SAS Institute, 2009). Statistical significance was declared at p < 0.1.
Results and Discussion Environmental Factors Mean air and soil temperature measured in plots during gas flux measurements ranged from 6.8 to 35.9°C, with air temperature being always above soil temperature (Fig. 1). As expected, these temperatures closely followed mean monthly air temperatures. The mean monthly air temperature for the growing season (May through Sept.) was 22.4°C in 2010 and 2.0°C higher than 2009 and 0.8°C higher than 2011. For the same time period, the 30-yr (1980–2009) mean air temperature was 21.2°C (data not shown). There were no differences in soil temperature among treatments. Precipitation during the growing season (May through Sept.) was 547 mm in 2009 and slightly above the 30-yr (1980–2009) mean of 529 mm, whereas precipitation was slightly lower in 2010 (483 mm) and substantially lower in 2011 (385 mm). Precipitation events during the period of soil N2O flux measurement are shown in Fig. 1. During pollination and much of the grain-fill period ( July and Aug.) in 2011, there were only nine rain events totaling 85 mm, compared with the 30-yr normal over the same 2-mo period of 20 rain events totaling 219 mm. As expected, volumetric soil water content was similar across all treatments and was only influenced by the frequency and intensity of rain events (Fig. 1).
Daily Soil-Surface N2O Fluxes
In general, the temporal pattern of soil N2O emissions was similar across N sources but varied substantially in magnitude by year (Fig. 2). The flux values measured in g N2O–N ha-1 ranged from 0.34 to 236 in 2009, from 0.75 to 1086 in 2010, and from 0.74 to 468 in 2011. It is possible that the lag (28 d) between N application and the start of measurement in 2009 resulted in underestimation of N2O emissions. Our assumption is based on (i) the fact that large fluxes were measured soon after N application in the month of June in 2010 and 2011
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Fig. 1. Air and 5-cm depth soil temperature measured at the time of gas flux measurement and mean monthly air temperature (top graphs) and volumetric soil water content for the top 5 cm of the soil measured at the time of gas flux measurement and daily precipitation (bars) during the gas flux measurement period for 2009 to 2011 (bottom graphs). Error bars indicate SE.
and (ii) the high potential for large N2O emissions before flux measurement due to the soil with antecedent high water content receiving 50 mm of rain over a 2-d period in the middle of June, which resulted in water-logged soil conditions. For these reasons and because there was no proper baseline, the 2009 results should be interpreted with caution. However, in all 3 yr the pattern of N2O emissions was similar for the different N sources, indicating that, on a relative basis, N treatment comparisons should be valid. Soil N2O fluxes were near background levels during May with the exception of AA, which showed a few peaks starting on 21 May 2010 (Fig. 2). It is known that N from nitrification can contribute to N2O emissions (Panek et al., 2000). Regardless of treatment, starting at about the middle of July until the end of the growing season, fluxes were very low at approximately background levels. The highest soil N2O fluxes occurred most often after single or 2- to 3-d combined rain events totaling approximately >20 mm during June and July. Although during large spikes in emissions the control treatment remained relatively low, these plots emitted as much as 27 g N2O–N ha-1 d-1 on 6 July 2009, 56 to 79 g N2O–N ha-1 d-1 between 17 June and 6 July 2010, and 41 and 105 g N2O–N ha-1 d-1 on 6 and 16 June 2011, respectively. These data illustrate that a large amount of N can be mineralized and potentially released to the atmosphere as N2O from the organic matter naturally present in these prairie soils. Soil water levels have been reported to be one of the controlling factors of N2O emissions (Bao et al., 2012). The rapid increase in N2O emissions after rain events and soon after N fertilization also agrees with previous findings ( Jacinthe and Dick, 1997; Omonode et al., 2011). Although in general during high-flux periods in 2009 and 2010 peaks for ESN were at least numerically lower than for AA and urea, we did not observe a substantial delay in soil N2O fluxes from ESN relative to the other N sources. We also observed no delays in 418
emissions with ESN relative to the other N sources other than one small-magnitude event at the end of July in 2011 (Fig. 2). This finding contrasts the delay in emissions with ESN reported by Halvorson et al. (2011, 2010a) on an irrigated clay loam Alfisols in Colorado or reported during 1 of 2 yr by Sistani et al. (2011) on a silt loam Alfisols in Kentucky. Although we are unable to explain the contrasting results, it is possible that lower soil organic matter content, greater drainage, and greater rainfall (or irrigation in the case of Colorado) compared with our study site may have caused the difference.
Cumulative N2O Fluxes from Soil
Cumulative daily soil N2O fluxes for the entire growing season show rapid accumulation during June and July consistently for all 3 yr (Fig. 3) and correspond to the large emission peaks during periods of greater soil water content (Fig. 2). In 2009, the most striking difference was for AA, which showed a rapid rise in cumulative daily flux levels, whereas urea and ESN remained low and similar to the unfertilized check (Fig. 3). In 2010, AA showed earlier higher cumulative daily flux levels than urea and ESN, likely reflecting the fact that AA was applied 20 d before the other treatments. There was no delay in emissions from ESN relative to urea in 2010, but in 2011 a modest delay was observed. Cumulative daily flux levels in 2011 for urea and ESN were similar during June, when there was adequate moisture. However, the dry conditions during most of July and August likely slowed the release of the remaining encapsulated urea in ESN and caused ESN to continue to emit slightly more N2O than urea later in the season. Cumulative total N2O emissions for the 2009 and 2011 growing seasons were substantially lower than for the 2010 growing season (Table 1). Although in 2009 we applied 70 kg N ha-1 less than in 2010 and 2011 (and the lower N fertilizer rate could result in reduced emissions), the reason for the low Journal of Environmental Quality
Fig. 2. Soil surface fluxes of N2O–N in the check (no N added) and in plots receiving anhydrous ammonia (AA), polymer-coated urea (ESN), and urea. Error bars indicate SE. The y axis scale is different among years.
cumulative levels in 2009 was most likely an underestimation in flux due to a delay in sampling after fertilization. Furthermore, in 2009 the initial AA flux measurement was 3.8 times greater than ESN and 4.6 times greater than urea. Likely, unaccounted AA fluxes before the first sampling were also elevated above the other treatments, which could result in greater underestimation of the cumulative N2O emissions of AA relative to the other treatments that had initial fluxes comparable to the check. Finally, although the largest peaks in emissions occurred during the month of June (Fig. 2), temperature and precipitation levels, in relative terms, were less conducive to biologically mediated N transformations (nitrification and denitrification) in the soil in 2009 and 2011 than in 2010. During June, there were 13 rain events totaling 108 mm in 2009 and 11 rain events totaling 107 mm in 2011, whereas in 2010 there were 15 rain events totaling 198 mm, and mean monthly air temperature for June 2010 was 23.8 (1°C higher than in 2009 and 2011). These
Fig. 3. Cumulative daily N2O–N emissions for the 2009 to 2011 growing seasons for various N sources: check (no N added), anhydrous ammonia (AA), polymer-coated urea (ESN), and urea. The y axis scale is different among years.
conditions likely resulted in lower N2O emissions in 2009 and 2011 compared with 2010. Nitrogen source–induced differences in growing season total N2O emissions were only observed in 2010 (Table 1). In 2010, the unfertilized check emitted the lowest amount, followed by ESN (9.77 kg N2O–N ha-1). Urea produced 44% more and AA produced 73% more N2O emissions than ESN. Reduced N2O emissions with ESN relative to urea have been observed for irrigated systems in Colorado with strip-till (Halvorson et al., 2011) and in continuous corn under no-till but not under conventional tillage (Halvorson et al., 2010a, 2010b). Sistani et al. (2011) observed similar findings in 1 yr of a 2-yr study. In our study, averaged across years, results for N source were similar to those observed for 2010, except that emission values were not statistically different between urea and ESN (Table 1). Over a 2-yr period, Nash et al.
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Table 1. Cumulative N2O–N emissions and fertilizer-induced N2O–N emissions as a percentage of the applied N fertilizer (check subtracted from treatments) for different nitrogen sources for the 2009, 2010, and 2011 cropping seasons and the combined 3-yr mean. N treatment†
Check AA ESN Urea Mean
2009
Cumulative N2O–N emissions 2010 2011
Mean
N2O–N emissions as % of applied N 2010 2011
2009
———————————— kg N ha−1 ———————————— 1.20 2.21c‡ 1.06 1.49c 3.55 16.89a 3.99 8.14a 1.23 9.77b 3.84 4.95b 0.97 14.07a 3.63 6.22ab 1.74B§ 10.73A 3.13B
Mean
———————————— % ———————————— 2.14 0.03 0.00 0.72B
8.15a 4.20b 6.59ab 6.31A
1.63 1.55 1.43 1.54B
3.97a 1.93b 2.67ab
† AA, anhydrous ammonia; Check, no N added; ESN, polymer-coated urea. ‡ Within a column, values followed by different lowercase letter are significantly different (P < 0.1). § Within a row, mean values followed by different uppercase letter are significantly different (P < 0.1).
(2012) also observed no N2O emission differences between urea and ESN on a poorly drained clay-pan soil in Missouri. Similarly, Venterea et al. (2011) found no difference in total cumulative N2O emissions between ESN and urea in a silt loam Mollisols in Minnesota. Our results indicate that under conditions of high N loss potential, ESN can reduce N2O emissions relative to urea and AA, but in years with less N loss potential, the benefits of ESN to reduce N2O emissions might not be fully realized, especially compared with urea. To our knowledge, this is the first study comparing N2O emissions from AA and ESN for corn production. A study done by Burton et al. (2008) in Manitoba, Canada, compared N2O emissions from a polymer-coated urea and AA but for wheat (Triticum aestivum L.) under much drier and cooler conditions (514 mm annual precipitation and mean annual temperature of 2.6°C). They found no differences in N2O emissions between the two N sources. On the other hand, several studies have compared emissions from urea and AA. Although our study showed a constant trend for greater N2O emissions with AA than urea, the lack of statistical difference contrasts the increase in N2O emissions with AA compared with urea reported for corn production under varied soil characteristics in different states (Iowa, Minnesota, and Tennessee) (Breitenbeck and Bremner, 1986; Thornton et al., 1996; Venterea et al., 2005, 2010; Fujinuma et al., 2011). This contrast likely highlights the complex relationships present in N2O generation and emissions in agricultural systems. Finally, the 3-yr mean clearly illustrates that N applications can substantially increase N2O emissions. Averaged across N sources, we observed more than a 4-fold increase in N2O emissions with N applications compared with the unfertilized
check. This increase in N2O emissions with N fertilization has been well documented by others (Halvorson et al., 2008; Hoben et al., 2011; Smith et al., 2011). Nitrous oxide emissions calculated as percent of applied N showed differences due to N treatment only in 2010, where the highest losses for the 3 yr were also observed (Table 1). Anhydrous ammonia losses were 1.9 times greater than ESN, whereas urea produced intermediate values. Similar results for the different N sources were observed when the data were averaged across all 3 yr. These data indicate that there is substantial potential for reduction of N2O emissions with the use of ESN relative to AA. Across years and N sources, we estimated that 2.85% of the applied N is lost as N2O (Table 1). The Intergovernmental Panel on Climate Change (2007) reported a much smaller value of 1.25%. However, another study conducted in Indiana (approximately 100 km eastnortheast from our study) under similar soils and climate estimated that 2.27% of the applied N was lost as N2O (Smith et al., 2011). Other studies have reported as much as 2.6% loss of the applied N (Sistani et al., 2011), 3.77% loss for urea and 7.33% loss for AA (Thornton et al., 1996), and 3.8% loss for surface-broadcast N fertilizers in a no-till field (Nash et al., 2012). All these values are higher than the value reported by the Intergovernmental Panel on Climate Change (2007) and highlight the fact that N2O emissions can vary substantially under different environments and management conditions. Although yield was always higher for corn receiving N compared with the unfertilized check, grain yield differences in response to N source did not show a consistent pattern from year to year and resulted in no differences due to N source when averaged across years (Table 2). Nonetheless, for the two
Table 2. Corn grain yield and N2O–N emissions per unit of corn yield produced for different nitrogen sources for the 2009, 2010, and 2011 cropping seasons and the combined 3-yr mean. N treatment†
Check AA ESN Urea Mean
2009
Corn grain yield 2010 2011
Mean
———————————— Mg ha−1 ———————————— 3.31c‡ 3.72c 3.79c 3.61b 7.86ab 9.98ab 8.52b 8.78a 7.28b 10.63a 10.00a 9.30a 8.45a 9.17b 9.49ab 9.04a 6.72B§ 8.37A 7.95A
2009
N2O–N emissions per unit of corn yield 2010 2011
Mean
————————— kg N2O–N Mg yield−1 ————————— 0.37 0.64b 0.28 0.43b 0.46 1.73a 0.48 0.89a 0.17 0.93b 0.38 0.49b 0.11 1.56a 0.39 0.69ab 0.28B 1.21A 0.38B
† AA, anhydrous ammonia; Check, no N added; ESN, polymer-coated urea. ‡ Within column, values followed by different lowercase letter are significantly different (P < 0.1). § Within row, mean values followed by different uppercase letter are significantly different (P < 0.1). 420
Journal of Environmental Quality
overall highest-yielding years, ESN produced 16% greater yield than urea in 2010 and 17% greater yield than AA in 2011. The improved yield with ESN over urea in 2010 may be the result of better protection against loss of the applied N with ESN during June, which had high potential for N loss with precipitation 93 mm above the 30-yr normal and mean monthly air temperature 1.5°C above the 30-yr normal. The fact that ESN did not compete as well against urea in 2009 is likely due to a late application on 1 June. Urea provided a readily available source of N, whereas ESN likely did not release N quickly enough to supply the needs of the rapidly developing corn crop. Our grain yield data may indicate that ESN applications are better suited for early-season pre-plant applications when N loss potential of readily available sources may be high, but ESN applications closer to the time of rapid uptake may risk not having an adequate N supply. As with N2O emissions as percent of applied N (Table 1), we observed an overall reduction of N2O emissions per unit of corn yield produced with ESN relative to AA and urea in 2010, the year with the most favorable conditions for N2O loss in our study (Table 2). In 2010, ESN produced 2.9 times more grain yield than the unfertilized check but with similar N2O emissions per unit of grain produced as the check. Averaged across years, ESN produced low N2O emissions per unit of grain, similar to the check and substantially lower than AA, whereas urea produced intermediate values. These data highlight the potential to increase production while minimizing N2O emissions when ESN is used. Others have reported similar results when using this and other enhancedefficiency fertilizers (Halvorson et al., 2010b; Drury et al., 2012; Sistani et al., 2011).
Conclusion Although N2O emissions varied substantially by year, the temporal patterns for N sources were consistent across years, with most emissions occurring during June and July. Across years and N sources, 2.85% of the applied N is lost as N2O. Although cumulative N2O emissions were 4.3 times greater with N fertilization, the unfertilized check emitted 1.49 kg N2O–N ha-1, illustrating the natural potential of these soils to emit N2O. Only in 2010, the year with greater N2O production due to greater rainfalls and warmer temperatures, did we observe differences due to N source where cumulative N2O emissions and emissions based on unit of corn yield produced followed the order ESN < urea = AA. In 2010, soil N2O emissions as percent of applied N showed that AA losses were 1.9 times greater than ESN. Across years, compared with AA, ESN reduced N2O emissions, but the differences compared with urea were not large enough to assign statistical differences due to treatment. Our study shows that, under conditions of high N loss potential, ESN could reduce N2O emissions per unit of applied N and per unit of corn yield compared with the traditional N fertilizers urea and AA. The fact that N2O emissions were different between urea and ESN in 2010 but not across years highlights the need for further investigation on mitigating N2O emissions with different N sources under different environments and growing season conditions.
Acknowledgments The authors thank Agrium Inc., Calgary, Alberta, for supporting this study; the Brigham Young University Soil and Plant Analysis Laboratory for conducting gas analysis; Kristin Greer at the University of Illinois for directing many hours of field work; and Justin Babbel and David Shurtz at Brigham Young University for performing gas chromatography analysis. This work was presented at a conference supported by NSF Research Coordination Network award DEB1049744 and by the Soil Science Society of America, the American Geophysical Union, The International Plant Nutrition Institute, The Fertilizer Institute, and the International Nitrogen Initiative.
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Journal of Environmental Quality
Journal of Environmental Quality
Special Section
Improving Nitrogen Use Efficiency in Crop and Livestock Production Systems
Nitrous Oxide Emissions from Anhydrous Ammonia, Urea, and Polymer-Coated Urea in Illinois Cornfields Fabián G. Fernández,* Richard E. Terry, and Eric G. Coronel
C
orn grain production is a major agricultural activity in the United States, with 34 million ha harvested in 2011 (USDA–NASS, 2012). Nitrogen fertilization is needed to optimize grain production and economic returns (Sawyer et al., 2006). However, N fertilization normally increases soil surface N2O emissions from agricultural systems (Snyder et al., 2009). In 2010, 6.3% (428.4 Tg CO2 Eq.) of the total U.S. greenhouse gas emissions were attributed to agricultural activities, with methane (CH4) and N2O being the primary gases (USEPA, 2012). This report also indicated that the largest source of N2O emissions in the United States (207.8 Tg CO2 Eq. [670 Gg N2O]) in 2010 was related to soil management activities and fertilizer application. This is important because N2O has approximately 300 times more potential to capture infrared radiation than CO2 (Intergovernmental Panel on Climate Change, 2007). Although N2O is a potent greenhouse gas, it is the most poorly quantified greenhouse gas in cropland (Mosier et al., 1996; Venterea et al., 2009). For these reasons, efforts should be devoted to better quantify agriculture-induced N2O emissions. In recent years, possibly as a result of increased demands for ethanol production, fluctuation in corn and soybean [Glycine max (L.) Merr.] prices, and other factors, the traditional corn– soybean rotation in much of the midwestern United States has increasingly become continuous corn. For example, with a constant 8.7 million cultivated hectares in Illinois, corn and soybean were nearly equal in land use in 2000, but since then corn cultivation has steadily increased at the expense of soybean, and in 2012 corn was planted in 58% of cropland (USDA– NASS, 2012). Similar trends have been observed for other regions of the United States (USDA–NASS, 2012). It is possible that greater N fertilization and greater amounts of plant residue produced with continuous corn relative to a corn–soybean rotation could increase greenhouse gas emissions (Drury et al., 2008; Wilson and Al-Kaisi, 2008). To date, however, the few studies quantifying soil surface N2O emissions in continuous corn systems have produced contrasting results ( Jacinthe and Dick, 1997; Drury et al., 2008; Omonode et al., 2011). In addition to cropping system management, N source can have an important impact on the amount of N2O emitted. The
Abstract The use of alternative N sources relative to conventional ones could mitigate soil-surface N2O emissions. Our objective was to evaluate the effect of anhydrous ammonia (AA), urea, and polymer-coated urea (ESN) on N2O emissions for continuous corn (Zea mays L.) production. Corn received 110 kg N ha-1 in 2009 and 180 kg N ha-1 in 2010 and 2011. Soil N2O fluxes were measured one to three times per week early in the growing season and less frequently later, using vented non–steady state closed chambers and a gas chromatograph. Regardless of N source, N2O emissions were largest immediately after substantial (>20 mm) rains, dropping to background levels thereafter. Averaged across N sources, 2.85% of the applied N was lost as N2O. Emission differences for treatments only occurred in 2010, the year with maximum N2O production. In the 2010 growing season, cumulative emissions (in kg N2O–N ha-1) were lowest for the check (2.21), followed by ESN (9.77), and ESN was lower than urea (14.07) and AA (16.89). Emissions in 2010 based on unit of corn yield produced followed a similar pattern, and N2O emissions calculated as percent of applied N showed that AA losses were 1.9 times greater than ESN. Across years, relative to AA, ESN reduced N2O emissions, emissions per unit of corn yield, and emissions per unit of N applied, whereas urea produced intermediate values. The study indicates that, under high N loss potential (wet and warm conditions), ESN could reduce N2O emissions more that urea and AA.
Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
F.G. Fernández, Dep. of Soil, Water, and Climate, Univ. of Minnesota, S233 Soils, 1991 Upper Bufford Circle, St. Paul, MN, 55108; R.E. Terry, Plant and Wildlife Sciences, Brigham Young Univ., 255 WIDB, Brigham Young University, Provo, UT 84602; E.G. Coronel, Dep. of Crop Sciences, Univ. of Illinois, 1102 South Goodwin Ave., Urbana, IL. 61801. Assigned to Associate Editor Barbara Amon.
J. Environ. Qual. 44:415–422 (2015) doi:10.2134/jeq2013.12.0496 Received 12 Dec. 2013. *Corresponding author ([email protected]).
Abbreviations: AA, anhydrous ammonia; ESN, polymer coated urea.
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most extensively studied N source comparison in the United States (although it amounts to a few studies) has been between the effects of urea and AA. These studies have shown greater N2O emissions with AA than urea (Breitenbeck and Bremner, 1986; Thornton et al., 1996; Venterea et al., 2005, 2010; Fujinuma et al., 2011). A 1-yr comparison showed greater N2O emissions with AA than with urea ammonium nitrate (Venterea et al., 2005). In a recent review, Snyder et al. (2009) indicated that the use of nitrification and urease inhibitors and controlled-release N fertilizers has the potential to increase N use efficiency and, as a consequence, to reduce N2O emissions. However, studies that evaluate the agronomic impact of these technologies (especially controlled-release N technology) on corn and their potential to reduce N2O emissions are scant. In a 3-yr study in a clay loam Mollisols in Ontario, Canada, Drury et al. (2012) observed that polymer-coated urea reduced N2O emissions relative to urea in one of the years only. Similarly, Sistani et al. (2011), in 1 yr of a 2-yr study in a silt loam Alfisols in Kentucky, showed N2O reductions with ESN relative to urea. In a study in Colorado under irrigation in a semiarid temperate climate in a clay loam soil, Halvorson et al. (2010b) found more consistent results, with ESN reducing N2O emissions relative to urea in a no-till system with continuous corn, but there were no differences in a conventionally tilled system. Other studies by Halvorson et al. (2010a, 2011) indicated substantial reduction in N2O emissions with ESN compared with urea. In contrast, in a much wetter location in Minnesota but with greater natural drainage in a silt loam soil, Venterea et al. (2011) showed no difference in N2O emissions between ESN and urea regardless of tillage system (conventional and no-till). One possible explanation to these contrasting reports is that the clay loam soil in Colorado may retain more water and possibly increase the potential for N2O generation relative to the more permeable silt loam soil in Minnesota. To our knowledge, no studies have compared N2O emissions between AA and ESN for corn production. This is surprising because AA is the most common N source in much of the Midwestern United States, but the amount of ESN being applied for corn production is increasing (A.D. Blaylock, personal communication, 2011) and has been shown in some studies to have the potential to increase yield and reduce N2O emissions. Our main objective was to evaluate the effects of AA, urea, and ESN on N2O emissions for continuous corn production. Additional objectives were (i) to evaluate grain yields to quantify the potential benefits of using ESN relative to the conventional N sources AA and urea and (ii) to relate soil-surface N2O emissions for these N sources on a grain yield and amount of applied N basis.
Materials and Methods Site Description and Experimental Design Field experiments were conducted from 2009 to 2011 at the Crop Sciences Research and Education Center near Urbana, Illinois (40°3¢11² N; 88°13¢47² W). The region has a temperate climate, with a 30-yr (1980–2009) mean temperature of 11.2°C and rainfall of 1044 mm yr–1. The soils were a somewhat poorly drained Flanagan silt loam (fine, smectitic, mesic Aquic Argiudolls) and a poorly drained Drummer silty clay loam (fine416
silty, mixed, superactive, mesic Typic Endoaquolls). In 2009 and 2011, the site consisted of more than 70% Flanagan series, with the remainder a Drummer series. In 2010, the entire site consisted of Flanagan soil. The site has subsurface tile drainage. In the top 18 cm of soil depth, soil organic matter ranged between 35 and 36 g kg-1, and soil pH (1:1 soil/water) ranged between 5.9 and 6.5. Total inorganic N (NO3– plus NH4+) present in the top 30 cm of the soil before treatment application in the spring was low: 13, 7, and 6 mg kg-1 for 2009, 2010, and 2011, respectively. The study was established on fields with corn as the previous crop, and every fall the soils were chisel plowed. The study was set up in a randomized complete-block design with four replications. The treatment consisted of a control (check) with no N applied and three N sources: AA (82–0-0), urea (46–00), and ESN (44–0-0) (Agrium Advanced Technologies) (N-P-K) applied at the rate of 110 kg N ha-1 in 2009 and at the rate of 180 kg N ha-1 in 2010 and 2011. The application rate was increased for the last 2 yr to more closely represent typical farmer practices. All applications were done within 4 wk of planting. In 2009, urea and ESN were applied on 1 June and AA on 2 June. In 2010, AA was applied on 30 April and urea and ESN on 20 May. The 20-d delay between applications in 2010 was caused by rain and persistent wet conditions right after the AA application. In 2011, AA was applied on 12 May and urea and ESN on 19 May. Anhydrous ammonia was injected 15 cm below the soil surface centered between crop rows (38 cm from adjacent corn crop rows); urea and ESN were broadcast applied by hand with a spin-spreader and incorporated by shallow tillage. We obtained ESN directly from the manufacturer, and special care was taken in handling the product to prevent damage of the polymer coating (Beres et al., 2012). Each treatment was applied on a 6 by 23 m plot. Corn was planted on 76-cm row spacing at a rate of 79,000 seeds ha-1 using a John Deere 7200 Max Emerge vacuum planter with Yetter trash movers and openers. Plots were kept weed free using glyphosate [isopropylamine salt of N-(phosphonomethyl) glycine]. Hybrid DeKalb 60–18 was planted on 17 June 2009, and hybrid Pioneer 0916XR was planted on 25 May 2010 and on 19 May 2011. The planting delay in 2009 was related to wet soil conditions in April and May combined with setbacks due to resource availability during the establishment year of the study.
Measurements Soil surface N2O fluxes were measured using a vented non– steady state closed chamber similar to those used by Parkin and Hatfield (2010). Rings constructed from PVC (30 cm in diameter and 10 cm tall) were installed in the soil to a depth of approximately 6 cm in each plot to serve as the base for flux chambers. Rings stayed in place for the entire sampling period except when they were removed for planting operations. Because AA was banded, care was taken to ensure that the rings were installed to proportionately represent the fertilized band and the rest of the soil between crop rows. Anhydrous ammonia movement in the soils used in this study has been well characterized. In this study, AA was applied at soil-water content between 80 and 90% of field capacity (-0.33 bar), in which AA moves laterally approximately 10 cm from the point of injection (Touchton et al., 1978; Hoeft et al., 2000). This dispersion distance represents 26% of the total soil surface Journal of Environmental Quality
between crop rows. Accordingly, we calculated the segment of the soil surface area affected by the N fertilization band needed to be included inside the rings to represent 26% of the inter-row soil surface. This resulted in rings being installed approximately 1 cm off the AA knife track and 7 cm off the crop row. The proximity to the crop row also allowed us to capture the potentially high crop row effect on N2O fluxes (Kessavalou et al., 1998) without interfering with the development of brace roots. Finally, the application of AA can be more variable than dry fertilizers because AA is in gaseous form at the point of application. However, the coefficients of variance of N2O emissions indicate that variability for AA was similar to other N sources in this study. Averaged across years, the coefficient of variance was 1.96, 2.47, 2.81, and 3.13 for the unfertilized check, AA, urea, and ESN, respectively Flux chambers were made from PVC (30 cm diameter and 10 cm tall), sealed on one end with a PVC sheet, and fitted with a butyl rubber septum (through which a syringe needle could be inserted to collect gas samples) and a small inlet to vent the chamber. The open end of the chamber was fitted with a rubber skirt that, when folded over the ring base, created an air-tight connection. Except for the rubber skirt, the chamber was covered with reflective tape. In 2009, N2O emissions were measured between 29 June and 23 November. Due to setbacks in the installation of chambers and resource constraints, soil N2O flux measurements were delayed until the end of June 2009, nearly a month after N treatment application. Measurements were taken weekly until the end of September and every 2 wk until November. In 2010, N2O emissions were measured between 4 May and 13 September. Measurements were taken two to three times a week. In 2011, N2O emissions were measured between 12 May and 18 October. Measurements were taken two to three times a week until the end of June, weekly in July, every 2 wk in August, and monthly in September and October. In all 3 yr there were exceptions during the more intensive sampling times when less frequent sampling events occurred due to inclement weather conditions. Gas samples were collected between 1000 and 1200 h local time when soil temperatures were close to their daily mean values. Gas samples were collected at 0, 15, and 30 min after chamber deployment with a polypropylene syringes and immediately injected into evacuated glass vials (10 mL) fitted with gray butyl rubber septa. The N2O concentrations in samples were determined within 24 h of collection with an Agilent 6890n gas chromatograph equipped with a 63Ni electron capture detector and a stainless steel column (0.3 cm outside diameter by 300 cm long) packed with Porapak Q (80–100 mesh). Based on the change in chamber headspace concentration pattern (accounting for soil surface area and chamber volume), linear and nonlinear regressions were used to calculate soil N2O fluxes (Livingston and Hutchinson, 1995). At each N2O gas sampling time, soil temperature at 5 cm depth was measured using a digital soil temperature thermometer (Model 71119, Gempler’s), and volumetric soil water content for the top 5 cm was measured using ECH2O EC-5 moisture probes and Em-50 digital data loggers (Decagon Devices Inc.). Precipitation and air temperature data were collected from a nearby weather station located at the Research and Education Center.
Estimates of daily N2O emissions between sampling days were calculated by linear interpolation between adjacent sampling dates. Fertilizer-induced N2O emissions as a percentage of the applied N fertilizer for the different N sources were calculated using Eq. [1]. The N2O emissions per unit of corn yield produced were calculated by dividing the cumulative N2O emissions by the grain yield: Fertilizer-induced emission =
æ Total emission of fertilized treatment - Total emission of unfertilized treatment ö÷ çç ÷÷´100 [1] çè ø Nitrogen rate of fertilized treatment Grain yield was obtained by a plot combine harvesting the two center rows of each plot on 1 Dec. 2009, 19 Oct. 2010, and 17 Oct. 2011. Yields were corrected to 155 g kg-1 moisture.
Statistical Analyses Data were analyzed with the MIXED procedure of SAS (SAS Institute, 2009) with year and treatment as fixed effects and blocks and its interactions as random effects. The LS Means for significant fixed effects were further analyzed by the SLICE option in the PROC MIXED procedure of SAS (SAS Institute, 2009). Statistical significance was declared at p < 0.1.
Results and Discussion Environmental Factors Mean air and soil temperature measured in plots during gas flux measurements ranged from 6.8 to 35.9°C, with air temperature being always above soil temperature (Fig. 1). As expected, these temperatures closely followed mean monthly air temperatures. The mean monthly air temperature for the growing season (May through Sept.) was 22.4°C in 2010 and 2.0°C higher than 2009 and 0.8°C higher than 2011. For the same time period, the 30-yr (1980–2009) mean air temperature was 21.2°C (data not shown). There were no differences in soil temperature among treatments. Precipitation during the growing season (May through Sept.) was 547 mm in 2009 and slightly above the 30-yr (1980–2009) mean of 529 mm, whereas precipitation was slightly lower in 2010 (483 mm) and substantially lower in 2011 (385 mm). Precipitation events during the period of soil N2O flux measurement are shown in Fig. 1. During pollination and much of the grain-fill period ( July and Aug.) in 2011, there were only nine rain events totaling 85 mm, compared with the 30-yr normal over the same 2-mo period of 20 rain events totaling 219 mm. As expected, volumetric soil water content was similar across all treatments and was only influenced by the frequency and intensity of rain events (Fig. 1).
Daily Soil-Surface N2O Fluxes
In general, the temporal pattern of soil N2O emissions was similar across N sources but varied substantially in magnitude by year (Fig. 2). The flux values measured in g N2O–N ha-1 ranged from 0.34 to 236 in 2009, from 0.75 to 1086 in 2010, and from 0.74 to 468 in 2011. It is possible that the lag (28 d) between N application and the start of measurement in 2009 resulted in underestimation of N2O emissions. Our assumption is based on (i) the fact that large fluxes were measured soon after N application in the month of June in 2010 and 2011
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Fig. 1. Air and 5-cm depth soil temperature measured at the time of gas flux measurement and mean monthly air temperature (top graphs) and volumetric soil water content for the top 5 cm of the soil measured at the time of gas flux measurement and daily precipitation (bars) during the gas flux measurement period for 2009 to 2011 (bottom graphs). Error bars indicate SE.
and (ii) the high potential for large N2O emissions before flux measurement due to the soil with antecedent high water content receiving 50 mm of rain over a 2-d period in the middle of June, which resulted in water-logged soil conditions. For these reasons and because there was no proper baseline, the 2009 results should be interpreted with caution. However, in all 3 yr the pattern of N2O emissions was similar for the different N sources, indicating that, on a relative basis, N treatment comparisons should be valid. Soil N2O fluxes were near background levels during May with the exception of AA, which showed a few peaks starting on 21 May 2010 (Fig. 2). It is known that N from nitrification can contribute to N2O emissions (Panek et al., 2000). Regardless of treatment, starting at about the middle of July until the end of the growing season, fluxes were very low at approximately background levels. The highest soil N2O fluxes occurred most often after single or 2- to 3-d combined rain events totaling approximately >20 mm during June and July. Although during large spikes in emissions the control treatment remained relatively low, these plots emitted as much as 27 g N2O–N ha-1 d-1 on 6 July 2009, 56 to 79 g N2O–N ha-1 d-1 between 17 June and 6 July 2010, and 41 and 105 g N2O–N ha-1 d-1 on 6 and 16 June 2011, respectively. These data illustrate that a large amount of N can be mineralized and potentially released to the atmosphere as N2O from the organic matter naturally present in these prairie soils. Soil water levels have been reported to be one of the controlling factors of N2O emissions (Bao et al., 2012). The rapid increase in N2O emissions after rain events and soon after N fertilization also agrees with previous findings ( Jacinthe and Dick, 1997; Omonode et al., 2011). Although in general during high-flux periods in 2009 and 2010 peaks for ESN were at least numerically lower than for AA and urea, we did not observe a substantial delay in soil N2O fluxes from ESN relative to the other N sources. We also observed no delays in 418
emissions with ESN relative to the other N sources other than one small-magnitude event at the end of July in 2011 (Fig. 2). This finding contrasts the delay in emissions with ESN reported by Halvorson et al. (2011, 2010a) on an irrigated clay loam Alfisols in Colorado or reported during 1 of 2 yr by Sistani et al. (2011) on a silt loam Alfisols in Kentucky. Although we are unable to explain the contrasting results, it is possible that lower soil organic matter content, greater drainage, and greater rainfall (or irrigation in the case of Colorado) compared with our study site may have caused the difference.
Cumulative N2O Fluxes from Soil
Cumulative daily soil N2O fluxes for the entire growing season show rapid accumulation during June and July consistently for all 3 yr (Fig. 3) and correspond to the large emission peaks during periods of greater soil water content (Fig. 2). In 2009, the most striking difference was for AA, which showed a rapid rise in cumulative daily flux levels, whereas urea and ESN remained low and similar to the unfertilized check (Fig. 3). In 2010, AA showed earlier higher cumulative daily flux levels than urea and ESN, likely reflecting the fact that AA was applied 20 d before the other treatments. There was no delay in emissions from ESN relative to urea in 2010, but in 2011 a modest delay was observed. Cumulative daily flux levels in 2011 for urea and ESN were similar during June, when there was adequate moisture. However, the dry conditions during most of July and August likely slowed the release of the remaining encapsulated urea in ESN and caused ESN to continue to emit slightly more N2O than urea later in the season. Cumulative total N2O emissions for the 2009 and 2011 growing seasons were substantially lower than for the 2010 growing season (Table 1). Although in 2009 we applied 70 kg N ha-1 less than in 2010 and 2011 (and the lower N fertilizer rate could result in reduced emissions), the reason for the low Journal of Environmental Quality
Fig. 2. Soil surface fluxes of N2O–N in the check (no N added) and in plots receiving anhydrous ammonia (AA), polymer-coated urea (ESN), and urea. Error bars indicate SE. The y axis scale is different among years.
cumulative levels in 2009 was most likely an underestimation in flux due to a delay in sampling after fertilization. Furthermore, in 2009 the initial AA flux measurement was 3.8 times greater than ESN and 4.6 times greater than urea. Likely, unaccounted AA fluxes before the first sampling were also elevated above the other treatments, which could result in greater underestimation of the cumulative N2O emissions of AA relative to the other treatments that had initial fluxes comparable to the check. Finally, although the largest peaks in emissions occurred during the month of June (Fig. 2), temperature and precipitation levels, in relative terms, were less conducive to biologically mediated N transformations (nitrification and denitrification) in the soil in 2009 and 2011 than in 2010. During June, there were 13 rain events totaling 108 mm in 2009 and 11 rain events totaling 107 mm in 2011, whereas in 2010 there were 15 rain events totaling 198 mm, and mean monthly air temperature for June 2010 was 23.8 (1°C higher than in 2009 and 2011). These
Fig. 3. Cumulative daily N2O–N emissions for the 2009 to 2011 growing seasons for various N sources: check (no N added), anhydrous ammonia (AA), polymer-coated urea (ESN), and urea. The y axis scale is different among years.
conditions likely resulted in lower N2O emissions in 2009 and 2011 compared with 2010. Nitrogen source–induced differences in growing season total N2O emissions were only observed in 2010 (Table 1). In 2010, the unfertilized check emitted the lowest amount, followed by ESN (9.77 kg N2O–N ha-1). Urea produced 44% more and AA produced 73% more N2O emissions than ESN. Reduced N2O emissions with ESN relative to urea have been observed for irrigated systems in Colorado with strip-till (Halvorson et al., 2011) and in continuous corn under no-till but not under conventional tillage (Halvorson et al., 2010a, 2010b). Sistani et al. (2011) observed similar findings in 1 yr of a 2-yr study. In our study, averaged across years, results for N source were similar to those observed for 2010, except that emission values were not statistically different between urea and ESN (Table 1). Over a 2-yr period, Nash et al.
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Table 1. Cumulative N2O–N emissions and fertilizer-induced N2O–N emissions as a percentage of the applied N fertilizer (check subtracted from treatments) for different nitrogen sources for the 2009, 2010, and 2011 cropping seasons and the combined 3-yr mean. N treatment†
Check AA ESN Urea Mean
2009
Cumulative N2O–N emissions 2010 2011
Mean
N2O–N emissions as % of applied N 2010 2011
2009
———————————— kg N ha−1 ———————————— 1.20 2.21c‡ 1.06 1.49c 3.55 16.89a 3.99 8.14a 1.23 9.77b 3.84 4.95b 0.97 14.07a 3.63 6.22ab 1.74B§ 10.73A 3.13B
Mean
———————————— % ———————————— 2.14 0.03 0.00 0.72B
8.15a 4.20b 6.59ab 6.31A
1.63 1.55 1.43 1.54B
3.97a 1.93b 2.67ab
† AA, anhydrous ammonia; Check, no N added; ESN, polymer-coated urea. ‡ Within a column, values followed by different lowercase letter are significantly different (P < 0.1). § Within a row, mean values followed by different uppercase letter are significantly different (P < 0.1).
(2012) also observed no N2O emission differences between urea and ESN on a poorly drained clay-pan soil in Missouri. Similarly, Venterea et al. (2011) found no difference in total cumulative N2O emissions between ESN and urea in a silt loam Mollisols in Minnesota. Our results indicate that under conditions of high N loss potential, ESN can reduce N2O emissions relative to urea and AA, but in years with less N loss potential, the benefits of ESN to reduce N2O emissions might not be fully realized, especially compared with urea. To our knowledge, this is the first study comparing N2O emissions from AA and ESN for corn production. A study done by Burton et al. (2008) in Manitoba, Canada, compared N2O emissions from a polymer-coated urea and AA but for wheat (Triticum aestivum L.) under much drier and cooler conditions (514 mm annual precipitation and mean annual temperature of 2.6°C). They found no differences in N2O emissions between the two N sources. On the other hand, several studies have compared emissions from urea and AA. Although our study showed a constant trend for greater N2O emissions with AA than urea, the lack of statistical difference contrasts the increase in N2O emissions with AA compared with urea reported for corn production under varied soil characteristics in different states (Iowa, Minnesota, and Tennessee) (Breitenbeck and Bremner, 1986; Thornton et al., 1996; Venterea et al., 2005, 2010; Fujinuma et al., 2011). This contrast likely highlights the complex relationships present in N2O generation and emissions in agricultural systems. Finally, the 3-yr mean clearly illustrates that N applications can substantially increase N2O emissions. Averaged across N sources, we observed more than a 4-fold increase in N2O emissions with N applications compared with the unfertilized
check. This increase in N2O emissions with N fertilization has been well documented by others (Halvorson et al., 2008; Hoben et al., 2011; Smith et al., 2011). Nitrous oxide emissions calculated as percent of applied N showed differences due to N treatment only in 2010, where the highest losses for the 3 yr were also observed (Table 1). Anhydrous ammonia losses were 1.9 times greater than ESN, whereas urea produced intermediate values. Similar results for the different N sources were observed when the data were averaged across all 3 yr. These data indicate that there is substantial potential for reduction of N2O emissions with the use of ESN relative to AA. Across years and N sources, we estimated that 2.85% of the applied N is lost as N2O (Table 1). The Intergovernmental Panel on Climate Change (2007) reported a much smaller value of 1.25%. However, another study conducted in Indiana (approximately 100 km eastnortheast from our study) under similar soils and climate estimated that 2.27% of the applied N was lost as N2O (Smith et al., 2011). Other studies have reported as much as 2.6% loss of the applied N (Sistani et al., 2011), 3.77% loss for urea and 7.33% loss for AA (Thornton et al., 1996), and 3.8% loss for surface-broadcast N fertilizers in a no-till field (Nash et al., 2012). All these values are higher than the value reported by the Intergovernmental Panel on Climate Change (2007) and highlight the fact that N2O emissions can vary substantially under different environments and management conditions. Although yield was always higher for corn receiving N compared with the unfertilized check, grain yield differences in response to N source did not show a consistent pattern from year to year and resulted in no differences due to N source when averaged across years (Table 2). Nonetheless, for the two
Table 2. Corn grain yield and N2O–N emissions per unit of corn yield produced for different nitrogen sources for the 2009, 2010, and 2011 cropping seasons and the combined 3-yr mean. N treatment†
Check AA ESN Urea Mean
2009
Corn grain yield 2010 2011
Mean
———————————— Mg ha−1 ———————————— 3.31c‡ 3.72c 3.79c 3.61b 7.86ab 9.98ab 8.52b 8.78a 7.28b 10.63a 10.00a 9.30a 8.45a 9.17b 9.49ab 9.04a 6.72B§ 8.37A 7.95A
2009
N2O–N emissions per unit of corn yield 2010 2011
Mean
————————— kg N2O–N Mg yield−1 ————————— 0.37 0.64b 0.28 0.43b 0.46 1.73a 0.48 0.89a 0.17 0.93b 0.38 0.49b 0.11 1.56a 0.39 0.69ab 0.28B 1.21A 0.38B
† AA, anhydrous ammonia; Check, no N added; ESN, polymer-coated urea. ‡ Within column, values followed by different lowercase letter are significantly different (P < 0.1). § Within row, mean values followed by different uppercase letter are significantly different (P < 0.1). 420
Journal of Environmental Quality
overall highest-yielding years, ESN produced 16% greater yield than urea in 2010 and 17% greater yield than AA in 2011. The improved yield with ESN over urea in 2010 may be the result of better protection against loss of the applied N with ESN during June, which had high potential for N loss with precipitation 93 mm above the 30-yr normal and mean monthly air temperature 1.5°C above the 30-yr normal. The fact that ESN did not compete as well against urea in 2009 is likely due to a late application on 1 June. Urea provided a readily available source of N, whereas ESN likely did not release N quickly enough to supply the needs of the rapidly developing corn crop. Our grain yield data may indicate that ESN applications are better suited for early-season pre-plant applications when N loss potential of readily available sources may be high, but ESN applications closer to the time of rapid uptake may risk not having an adequate N supply. As with N2O emissions as percent of applied N (Table 1), we observed an overall reduction of N2O emissions per unit of corn yield produced with ESN relative to AA and urea in 2010, the year with the most favorable conditions for N2O loss in our study (Table 2). In 2010, ESN produced 2.9 times more grain yield than the unfertilized check but with similar N2O emissions per unit of grain produced as the check. Averaged across years, ESN produced low N2O emissions per unit of grain, similar to the check and substantially lower than AA, whereas urea produced intermediate values. These data highlight the potential to increase production while minimizing N2O emissions when ESN is used. Others have reported similar results when using this and other enhancedefficiency fertilizers (Halvorson et al., 2010b; Drury et al., 2012; Sistani et al., 2011).
Conclusion Although N2O emissions varied substantially by year, the temporal patterns for N sources were consistent across years, with most emissions occurring during June and July. Across years and N sources, 2.85% of the applied N is lost as N2O. Although cumulative N2O emissions were 4.3 times greater with N fertilization, the unfertilized check emitted 1.49 kg N2O–N ha-1, illustrating the natural potential of these soils to emit N2O. Only in 2010, the year with greater N2O production due to greater rainfalls and warmer temperatures, did we observe differences due to N source where cumulative N2O emissions and emissions based on unit of corn yield produced followed the order ESN < urea = AA. In 2010, soil N2O emissions as percent of applied N showed that AA losses were 1.9 times greater than ESN. Across years, compared with AA, ESN reduced N2O emissions, but the differences compared with urea were not large enough to assign statistical differences due to treatment. Our study shows that, under conditions of high N loss potential, ESN could reduce N2O emissions per unit of applied N and per unit of corn yield compared with the traditional N fertilizers urea and AA. The fact that N2O emissions were different between urea and ESN in 2010 but not across years highlights the need for further investigation on mitigating N2O emissions with different N sources under different environments and growing season conditions.
Acknowledgments The authors thank Agrium Inc., Calgary, Alberta, for supporting this study; the Brigham Young University Soil and Plant Analysis Laboratory for conducting gas analysis; Kristin Greer at the University of Illinois for directing many hours of field work; and Justin Babbel and David Shurtz at Brigham Young University for performing gas chromatography analysis. This work was presented at a conference supported by NSF Research Coordination Network award DEB1049744 and by the Soil Science Society of America, the American Geophysical Union, The International Plant Nutrition Institute, The Fertilizer Institute, and the International Nitrogen Initiative.
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