Nitrogen Source Effects on Nitrous Oxide ... - ACS Publications

Chapter 2

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Nitrogen Source Effects on Nitrous Oxide Emissions from Irrigated Cropping Systems in Colorado A. D. Halvorson* and S. J. Del Grosso USDA, Agricultural Research Service, 2150 Centre Ave, Bldg. D, Ste. 100, Fort Collins, CO 80526 *E-mail: [email protected]

Nitrogen (N) fertilization is essential in most irrigated cropping systems to optimize crop yields and economic returns. Application of inorganic N fertilizers to these cropping systems generally results in increased nitrous oxide (N2O-N) emissions. Nitrous oxide emissions resulting from the application of commercially available enhanced-efficiency N fertilizers [ESN, Duration III, SuperU, and UAN with AgrotainPlus] were compared with emissions from commonly used urea and urea-ammonium nitrate (UAN) fertilizers under differing tillage (conventional-till and no-till) practices and crop rotations (continuous corn, corn-barley, corn-bean). Significant differences in the amount of N2O-N emitted among N sources were found. Some of the enhanced-efficiency N fertilizers reduced N2O-N emissions as much as 50% when compared to dry granular urea and 35% compared to liquid UAN fertilizers commonly used by farmers in this semi-arid region. Further work is required to quantify the effectiveness of enhanced-efficiency N fertilizers in reducing N2O-N emissions in other irrigated and non-irrigated systems, on different soil types, and in wetter climates.

Managing Nitrogen in Cropping Systems Nitrogen fertilization to optimize crop yields and economic returns from agricultural production systems is an essential basic management decision (2–4) in Not subject to U.S. Copyright. Published 2011 by American Chemical Society In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


the U.S. Central Great Plains. The application of inorganic N fertilizer generally results in an increase in N2O-N emissions (5–7), although increased emissions also occur with the application of organic sources (8, 9). Nitrous oxide is a potent greenhouse gas that contributes to climate change, since the global warming potential of N2O is ~296 times greater than CO2, the reference greenhouse gas. In the U.S., agriculture contributes about 67% of the N2O-N emissions (10), therefore, development of management practices to reduce N2O-N emissions from cropping systems is very important. Despite the essential role of agriculture and the development of enhanced-efficiency N fertilizers that can potentially decrease N2O-N losses from N fertilizer application, there is relatively little known about the effects of agricultural management on N2O-N emissions. An extensive literature review of greenhouse gas emissions from cropping systems by Snyder et al. (7) revealed that little information is available on N2O-N emissions from cropping systems where direct comparisons have been made among different fertilizer N sources. They reported few studies evaluating the effects of commercially available controlled-release and stabilized N fertilizers on N2O-N emissions. Based on data available, they reported that enhanced-efficiency N fertilizers (e.g. controlled-release, nitrification inhibitors, and urease inhibitors) might be management options to reduce N losses and thereby reduce indirect emissions of N2O-N. Recent research has shown that N fertilizer source can influence N2O-N emissions from cropping systems. Venterea et al. (11) observed greater N2O-N emissions from the application of anhydrous ammonia applied to corn in Minnesota than from urea-ammonium nitrate (UAN) and dry granular urea. They found no differences in N2O-N emissions between conventional till (CT) and no-till (NT) corn systems when UAN was applied, but significant differences between CT and NT when urea was surface broadcast, with NT having the higher level of emissions. Venterea et al. (12) also reported higher N2O-N emissions from anhydrous ammonia than from urea in another corn cropping system in Minnesota. Hyatt et al. (13) reported that a preplant application of polymer-coated urea fertilizers to potatoes grown on a loamy sand soil in Minnesota had yields equal to that from urea applied in multiple applications (5 to 6 times) during the potato growing season, with lower N2O-N emissions from the polymer-coated urea sources compared to normal dry granular urea. Greenhouse gas (GHG) emissions resulting from fossil fuel consumption by an applicator tractor during the 5 to 6 applications of urea fertilizer would add to the total GHG emissions compared to the single preplant application of polymer-coated urea fertilizers. Although polymer-coated N fertilizers have potential to decrease N2O-N emissions in relatively short-time periods (20-40 days) after application, their longer-term effects on growing season N2O-N emissions are relatively unknown. Halvorson et al. (6) observed that the application of polymer-coated urea in split applications with UAN or urea resulted in no immediate increase in N2O-N emissions following application of the polymer-coated urea (May), but increased N2O-N emissions later in the growing season (late June-August). The initial suppression of N2O-N emissions following application of the polymer-coated urea was in contrast to increased emissions within days of UAN or urea application. 16 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


To determine the effectiveness of commercially available controlled-release and stabilized N fertilizers to reduce N2O-N emissions under typical agricultural management practices in the semi-arid Central Great Plains, Halvorson et al. (14) initiated research to compare N2O-N emissions from the application of urea and the polymer-coated urea, ESN, to CT and NT irrigated continuous corn production systems on a Fort Collins clay loam soil. They also compared the application of dry granular urea and a stabilized urea (SuperU, containing urease and nitrification inhibitors) in NT, irrigated corn-barley and corn-dry bean rotations on the same soil type. Halvorson et al. (15) also compared N2O-N emissions from several inorganic N fertilizer sources (urea, UAN, SuperU, ESN, Duration III, and UAN+AgrotainPlus (1)) under NT, irrigated continuous corn production. The purpose of this chapter is to summarize and discuss the key results from the Halvorson et al. (14, 15) studies.

Methodology Fertilizer Sources Evaluated The dry granular urea (46% N) and UAN (32% N) are commercially available N fertilizers commonly used by farmers in the central Great Plains of Colorado. The enhanced-efficiency N fertilizer sources evaluated were ESN (44%N), Duration III (43% N), SuperU (46% N), and UAN+AgrotainPlus (32% N). The polymer-coated urea fertilizers (ESN and Duration III) are registered products of Agrium Advanced Technologies, Sylacauga, AL. SuperU and AgrotainPlus contain a urease inhibitor [N-(n-butyl)-thiophosphoric triamide (NBPT)] and a nitrification inhibitor [dicyandiamide (DCD)]. SuperU and AgrotainPlus are registered products of Agrotain International, LLC, St. Louis, MO. All N sources were surface banded near (0-10 cm) the corn row at crop emergence in mid-May in all studies, except for the NT barley crop where the fertilizers were broadcast applied at crop emergence, followed within 1 to 2 days with the application of 13+ mm of irrigation water. The N rates compared were 0 and 246 kg N ha-1 for corn, 0 and 156 kg N ha-1 for barley, and 0 and 56 kg N ha-1 for dry bean (14). The exception was for the 2008 N source study under NT continuous corn when 202 kg N ha-1 was applied to the corn (15). Cropping and Tillage Systems The conventional plow tillage (CT), continuous corn (CT-CC) cropping system used an intensive tillage system with disking and moldboard plow tillage after corn harvest in the fall followed by two roller harrow and land leveling operations in the spring followed by light cultivation if necessary to reduce soil erosion by wind. The no-till (NT) continuous corn (NT-CC) system was a plant, spray, and harvest system with no tillage operations. The other cropping systems used were a NT corn-barley rotation and a NT corn-dry bean rotation. Details of the tillage and crop production systems, as well as other field and laboratory procedures used to measure greenhouse gas (GHG) emissions are presented by Halvorson et al. (4–6, 14, 15). A linear-move, sprinkler irrigation system was 17 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

used to apply water to the crops. A randomized complete block design with three replications was used with two GHG measurements per replication (total of 6 GHG measurements per treatment) for each sampling date. Nitrous oxide measurements were made 1 to 3 times per week, immediately following crop planting until crop harvest (growing season). A static, vented chamber technique was used to collect the gas samples in the field. A gas chromatograph was used to determine N2O-N concentration in each gas sample [see (5, 14, 15) for more details on sampling protocol and methods].


Nitrous Oxide Emissions in Cropping System Studies Continuous Corn The N2O-N fluxes resulting from urea and ESN application to the CT-CC and NT-CC systems are shown in Figure 1 for 2008. Following N fertilizer application, a rapid rise in N2O-N fluxes occurred within several days after urea application in both tillage systems, with a small rise in N2O-N flux following ESN application, then a decline to near background levels, followed by a rise again in N2O-N flux from the ESN application from late-June through August in both tillage systems. The N2O fluxes from the NT-CC system were much lower (50%) than from the CT-CC system both years. The delayed N2c2O-N emissions with the ESN source points out the need to maintain an intensive or frequent GHG sampling schedule for the entire growing season when evaluating polymer-coated or slow-release N products. Failure to do so could result in an erroneous characterization of N2O-N emissions from such products. The cumulative N2O-N fluxes in 2007 for the CT-CC and NT-CC systems are illustrated in Figure 2, which clearly shows the rapid rise in N2O-N emissions following urea application in both tillage systems, with a slower release following ESN application. The check treatment (no N applied for 8 yr) had the lowest level of N2O-N emissions in both tillage systems. In both 2007 and 2008, N2O-N emissions from the CT system were greater than from the NT system (Figure 3) with no significant difference in total N2O-N emissions between urea and ESN in the CT system, but a significant reduction in N2O-N emissions with ESN compared to urea in the NT system. The lower N2O-N emissions in the NT system compared to the CT system were consistent with the observations reported by Halvorson et al. (6) who compared the effects of N rate (UAN) on N2O-N emissions in the CT-CC and NT-CC systems from 2005-2006. Corn-Barley and Corn-Bean The application of a stabilized urea (SuperU) in the NT corn-barley (NT-CB) and NT corn-dry bean (NT-CDb) cropping systems in 2007 and 2008 resulted in significantly reduced growing season N2O-N fluxes compared to dry granular urea. As was observed in the CT and NT continuous corn systems, there was a rapid rise in N2O-N flux (data not shown) within days of urea and SuperU application to both of the NT-CDb and NT-CB rotations; however, the rise was much smaller for SuperU which possibly shows the benefit of the urease and nitrification inhibitors 18 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


present in the SuperU fertilizer (14). The N2O-N fluxes remained high for the first 20 days with urea, then declined to near background levels by about 40 days after application. The check with no N applied for 8 yr had the lowest level of N2O-N emissions in both NT cropping systems. Total growing season N2ON emissions for these two NT cropping systems are shown in Figures 4 and 5. SuperU reduced N2O-N emissions significantly compared to urea both years in both cropping systems.

Figure 1. Daily N2O-N fluxes with standard error bars in the CT-CC and NT-CC systems in 2008. Reproduced from reference (14). 19 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 2. Cumulative daily N2O-N emissions for each N source during the 2007 growing season in the CT-CC and NT-CC cropping systems.

Figure 3. Average cumulative N2O-N emissions for the 2007 and 2008 growing seasons for each N treatment in the CT-CC and NT-CC cropping systems. Reproduced from reference (14). 20 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 4. Cumulative N2O-N emissions for the control, urea, and SuperU treatments during the 2007 (barley) and 2008 (corn) growing seasons in the NT-CB cropping system. Reproduced from reference (14).

Despite substantially different N application rates between barley and corn (157 and 246 kg N ha-1 applied, respectively) N2O-N emissions in 2007 during the barley growing season with urea were nearly the same as the N2O-N emissions during the corn growing season in 2008 (Figure 4). The growing season emissions from SuperU in 2007 were greater than in 2008. These results point out that cropping system, soil temperature and water content, and years have a significant effect on N2O-N emissions, as reported by Mosier et al. (5). In 2007 during the dry bean phase of the NT-CDb rotation which was fertilized with 56 kg N ha-1, the dry bean crop was damaged by a residual herbicide. Therefore, corn was replanted in the field in early July as a silage crop with no additional N applied. The growing season N2O-N emissions in 2007 were smaller than in 2008 with 246 kg N ha-1 applied (Figure 5) as would be expected due to the lower fertilizer N rate in 2007 (6). In the NT cropping systems, we found decreased N2O-N emissions from the application of a polymer-coated urea, ESN, and a stabilized urea, SuperU. The magnitude of reduction varied with NT cropping system and N rate applied. A significant reduction in N2O-N emissions with the application of ESN in the CT continuous corn system compared to urea was not observed.

21 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 5. Cumulative N2O-N emissions for the control, urea, and SuperU treatments during the 2007 (dry bean/corn) and 2008 (corn) phases of the NT-CDb cropping system. Reproduced from reference (14).

N Source Study In 2007 and 2008, another study (15) was conducted to compare the effects of six different fertilizer N sources on N2O-N emissions in an irrigated, NT continuous corn system. The fertilizers included urea, two polymer-coated ureas (ESN and Duration III), a stabilized urea (SuperU), UAN, and a stabilized UAN (UAN+AgrotainPlus). Following N fertilizer application in 2007 and 2008, cumulative N2O-N fluxes from urea and UAN increased within days of application as shown for 2008 in Figure 6. Super U and UAN+AgrotainPlus had smaller immediate increases in N2O-N fluxes following N application, with N2O-N emissions returning to near background levels in about 40 days (see (15)). The ESN and Duration III treatments showed very little increase in N2O-N emissions immediately after N application, but increases in N2O-N fluxes occurred in late June through August, mainly following irrigation and precipitation events. As stated previously, this is an important observation for those evaluating the impact of polymer-coated N fertilizers or other controlled release N fertilizers on N2O-N emissions. The frequency of GHG measurements need to be maintained at several times per week during the mid- to late-growing season to properly characterize the N2O-N fluxes from controlled release or slow-release N fertilizers, possibly including manures. The check treatment (no N applied) had the lowest total growing season N2O-N emissions. 22 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 6. Cumulative daily N2O-N emissions during the 2008 growing season for each N source in a NT-CC system. Reproduced from reference (15). The two year average growing season N2O-N emission for each N source evaluated is shown in Figure 7. Compared to urea, all other N sources had lower N2O-N emissions for the growing season, with no differences among UAN, Duration III, and ESN. SuperU and UAN+AgortainPlus had lower emissions than the other N sources, but significantly greater than the check treatment. Averaged over the two years, the UAN treatment had a lower corn grain yield (11.87 Mg ha-1) than urea (12.75 Mg ha-1), but greater than the check treatment (8.92 Mg ha-1) (15). The loss of N2O-N per unit of N applied for each N source is shown in Figure 8 for each year. The years are shown separately since a higher fertilizer N rate was used in 2007 than in 2008. The trends were similar both years with all sources showing a lower level of N2O-N emissions per unit of N applied in 2008, except for urea which had a higher level of N2O-N emissions in 2008 than in 2007. In general, urea had the highest level of N2O-N emissions per unit of N applied both years, with UAN, ESN, and Duration III having lower emissions than urea, and Super U and UAN+AgrotainPlus having the lowest level of emissions. All N2O-N emission levels were considerably lower than the default 1% from Tier I methodology of IPCC (16) used to estimate yearly N2O-N emissions resulting from N fertilizer application. The degree of N2O-N loss may vary strongly depending on cropping system, tillage management, and site-specific conditions as pointed out by Halvorson et al. (14). This indicates the need for source and site specific N2O-N emission data before sound crop management decisions or mitigation policies can be formulated for effectively reducing N2O-N emissions in a cropping region or area. 23 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 7. Two-year average total cumulative growing season N2O-N emissions for each N source and check treatment in a NT-CC system. Developed from reference (15).

The effectiveness of N sources in reducing N2O-N emissions in the NT-CC system compared to urea were in the order: UAN (27%), Duration III (31%), ESN (34%), SuperU (48%), and UAN+AgrotainPlus (53%). Compared to UAN, Duration III (6%) and ESN (9%) reduced N2O-N emissions only slightly (not significant), but SuperU (29%) and UAN+AgrotainPlus (35%) reduced N2O-N emissions significantly. The lower N2O-N emissions of UAN compared to urea probably resulted because 33% of UAN is in the NO3-N form and N2O-N emissions from this NT-CC cropping system primarily resulted from the nitrification process and not denitrification. These data show that the enhanced-efficiency fertilizers offer potential for reducing N2O-N emissions under irrigation in a semi-arid climate. The enhanced-efficiency N products need to be tested under other climatic conditions, soil types, and cropping systems to further evaluate their value in reducing N2O-N emissions across the U.S.



Figure 8. The N2O-N loss per unit of N fertilizer applied during the 2007 and 2008 growing seasons, with each year shown separately due to different N rates in a NT-CC system. Developed from reference (15).

Expressing N2O-N emissions as a function of grain yield is one way to account for variability in N2O-N emissions and grain yield for each N source. Nitrogen fertilization is essential in most production systems to optimize grain yields and economic returns. The relationship between N2O-N emissions and grain yields, averaged over 2 yr, is shown in Figure 9 for each N source evaluated in this study. Urea had the highest level of N2O-N emissions per unit of corn grain yield, followed by lower emissions from UAN, Duration III, ESN, and SuperU; SuperU had N2O-N emission levels no different than those from UAN+AgrotainPlus, which had the lowest emissions among the N sources, and the check had the lowest level of emissions per unit of yield. Despite low N2O-N emissions, the check did not have sustainable grain yields (2, 17). These studies show that the enhanced efficiency fertilizers have potential to reduce N2O-N emissions per unit of grain production in this semiarid, irrigated corn production area of the Central Great Plains.



Figure 9. Two-year average growing season N2O-N emissions per unit of grain yield for each N source in a NT-CC system. Developed from reference (15).

Summary Nitrogen fertilization is essential in most irrigated cropping systems to optimize crop yields and economic returns. Application of inorganic N fertilizers to these cropping systems generally results in increased N2O-N emissions. This chapter summarizes work (14, 15) conducted by USDA-ARS near Fort Collins, Colorado on the effects of inorganic N fertilizer sources on N2O-N emissions from irrigated cropping systems. The research shows that there are significant differences in the amount of N2O-N emitted among N sources, and that emission measurements including controlled-release or slow-release N sources should span the entire growing season with sampling more than once per week. Commercially available enhanced-efficiency N fertilizers reduced N2O-N emissions as much as 50% when compared to dry granular urea and 35% when compared to liquid UAN; fertilizers commonly used by farmers in this semi-arid region. Grain yields were not greatly affected by N source. Therefore, significant reductions in N2O-N emissions per unit of grain production were observed in this study with the use of enhanced-efficiency N fertilizers. Further work is required to quantify the effectiveness of enhanced-efficiency N fertilizers in reducing N2O-N emissions in other non-irrigated and irrigated systems, wetter climates, different soil types, and cropping systems. 26 In Understanding Greenhouse Gas Emissions from Agricultural Management; Guo, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

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Nitrogen Source Effects on Nitrous Oxide ... - ACS Publications

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