The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no‐till (NT) cropping systems study initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of CO2, CH4, and N2O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional‐till continuous corn (CT‐CC) and NT continuous corn (NT‐CC) plots and in NT corn–soybean rotation (NT‐CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kg N ha−1 Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N2O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net CO2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.

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ABSTRACT

Nitrous oxide (N2O) is often the largest single component of the greenhouse‐gas budget of individual cropping systems, as well as for the US agricultural sector as a whole. Here, we highlight the factors that make mitigating N2O emissions from fertilized agroecosystems such a difficult challenge, and discuss how these factors limit the effectiveness of existing practices and therefore require new technologies and fresh ideas. Modification of the rate, source, placement, and/or timing of nitrogen fertilizer application has in some cases been an effective way to reduce N2O emissions. However, the efficacy of existing approaches to reducing N2O emissions while maintaining crop yields across locations and growing seasons is uncertain because of the interaction of multiple factors that regulate several different N2O‐producing processes in soil. Although these processes have been well studied, our understanding of key aspects and our ability to manage them to mitigate N2O emissions remain limited.

When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the 'greenhouse' effect, the entire suite of biogenic greenhouse gases -carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) -needs to be considered. Storage of atmospheric CO 2 into stable organic carbon pools in the soil can sequester CO 2 while common crop production practices can produce CO 2 , generate N 2 O, and decrease the soil sink for atmospheric CH 4 . The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40-44% of the GWP from both rain-fed sites and contributed 16-33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO 2 , i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO 2 equivalents ha À1 y À1 . Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N 2 O emission rates decline or increase with time after conversion to NT.

With increased cropping intensity, one would expect that more crop residue and C would be added to the Soil C sequestration can improve soil quality and reduce agriculsoil than with a crop-fallow system (Campbell et al., ture's contribution to CO 2 emissions. The long-term (12 yr) effects of tillage system and N fertilization on crop residue production and 1995, 2000b; Janzen et al., 1998a; Peterson et al., 1998). soil organic C (SOC) sequestration in two dryland cropping systems As the amount of crop residue returned to the soil is in North Dakota on a loam soil were evaluated. An annual cropping increased, SOC sequestration is expected to increase if (AC) rotation [spring wheat (SW) (Triticum aestivum L.)-winter the residue C is not lost as CO 2 to the atmosphere wheat (WW)-sunflower (SF) (Helianthus annuus L.)] and a spring because of tillage induced decomposition (Larney et al., wheat-fallow (SW-F) rotation were studied. Tillage systems included 1997; Reicosky, 1997a,b). Research in the Great Plains conventional-till (CT), minimum-till (MT), and no-till (NT). Nitrogen has shown that SOC sequestration is enhanced by N rates were 34, 67, and 101 kg N ha Ϫ1 for the AC system and 0, 22, fertilization (Campbell and Zentner, 1993; Campbell et and 45 kg N ha Ϫ1 for the SW-F system. Total crop residue returned

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Nitrogen fertilization is essential for optimizing crop yields; however, it may potentially increase nitrous oxide (N2O) emissions. The study objective was to assess the ability of commercially available enhanced-efficiency N fertilizers to reduce N2O emissions following their application in comparison with conventional dry granular urea and liquid urea-ammonium nitrate (UAN) fertilizers in an irrigated no-till (NT) corn (Zea mays L.) production system. Four enhanced-efficiency fertilizers were evaluated: two polymer-coated urea products (ESN and Duration III) and two fertilizers containing nitrification and urease inhibitors (SuperU and UAN+AgrotainPlus). Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. Enhanced-efficiency fertilizers significantly reduced growing-season N2O-N emissions in comparison with urea, including UAN. SuperU and UAN+AgrotainPlus had significantly lower N2O-N emissions than UAN. Compared with urea, SuperU reduced N2O-N emissions 48%, ESN 34%, Duration III 31%, UAN 27%, and UAN+AgrotainPlus 53% averaged over 2 yr. Compared with UAN, UAN+AgrotainPlus reduced N2O emissions 35% and SuperU 29% averaged over 2 yr. The N2O-N loss as a percentage of N applied was 0.3% for urea, with all other N sources having significantly lower losses. Grain production was not reduced by the use of alternative N sources. This work shows that enhanced-efficiency N fertilizers can potentially reduce N2O-N emissions without affecting yields from irrigated NT corn systems in the semiarid central Great Plains.