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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 (CO2), methane (CH4), and nitrous oxide (N2O) – needs to be considered. Storage of atmospheric CO2 into stable organic carbon pools in the soil can sequester CO2 while common crop production practices can produce CO2, generate N2O, and decrease the soil sink for atmospheric CH4. 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 CO2, 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 CO2 equivalents ha−1 y−1. 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 N2O emission rates decline or increase with time after conversion to NT.

We present a new soil respiration model, describe a formal model testing procedure, and compare our model with five alternative models using an extensive data set of observed soil respiration. Gas flux data from rangeland soils that included a large number of measurements at low temperatures were used to model soil CO2 emissions as a function of soil temperature and water content. Our arctangent temperature function predicts that Q10 values vary inversely with temperature and that CO2 fluxes are significant below 0 °C. Independent data representing a broad range of ecosystems and temperature values were used for model testing. The effects of plant phenology, differences in substrate availability among sites, and water limitation were accounted for so that the temperature equations could be fairly evaluated. Four of the six tested models did equally well at simulating the observed soil CO2 respiration rates. However, the arctangent variable Q10 model agreed closely with observed Q10 values over a wide range of temperatures (r2 = 0.94) and was superior to published variable Q10 equations using the Akaike information criterion (AIC). The arctangent temperature equation explained 16–85% of the observed intra-site variability in CO2 flux rates. Including a water stress factor yielded a stronger correlation than temperature alone only in the dryland soils. The observed change in Q10 with increasing temperature was the same for data sets that included only heterotrophic respiration and data sets that included both heterotrophic and autotrophic respiration.

A 2-year study was conducted to investigate the potential of no-till cropping systems to reduce N2O and NO emissions under different N application rates in an irrigated corn field in northeastern Colorado. Flux measurements were begun in the spring of 2003, using vented (N2O) and dynamic (NO) chambers, one to three times per week, year round, within plots that were cropped continuously to corn (Zea mays L.) under conventional-till (CT) and no-till (NT). Plots were fertilized at planting in late April with rates of 0, 134 and 224 kg N ha-1 and corn was harvested in late October or early November each year. N2O and NO fluxes increased linearly with N application rate in both years. Compared with CT, NT did not significantly affect the emission of N2O but resulted in much lower emission of NO. In 2003 and 2004 corn growing seasons, the increase in N2O-N emitted per kg ha-1 of fertilizer N added was 14.5 and 4.1 g ha-1 for CT, and 11.2 and 5.5 g ha-1 for NT, respectively. However, the increase in NO-N emitted per kg ha-1 of fertilizer N added was only 3.6 and 7.4 g ha-1 for CT and 1.6 and 2.0 g ha-1 for NT in 2003 and 2004, respectively. In the fallow season (November 2003 to April 2004), much greater N2O (2.0-3.1 times) and NO (13.1-16.8 times) were emitted from CT than from NT although previous N application did not show obvious carry-over effect on both gas emissions. Results from this study reveal that NT has potential to reduce NO emission without an obvious change in N2O emission under continuous irrigated corn cropping compared to CT.

In late March 1997, an open-top-chamber (OTC) CO^sub 2^ enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO^sub 2^ (720 μmol mol^sup -1^) on plant production, photosynthesis, and water use of this mixed C^sub 3^/C^sub 4^ plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO^sub 2^ on trace gas exchange. From this study, we report here our weekly measurements of CO^sub 2^, CH^sub 4^, NO^sub x^ and N^sub 2^O fluxes within control (unchambered), ambient CO^sub 2^ and elevated CO^sub 2^ OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C^sub 3^ and C^sub 4^ plant biomass increased under elevated CO^sub 2^ and soil moisture content was typically higher than under ambient CO^sub 2^ conditions, none of the trace gas fluxes were significantly altered by CO^sub 2^ enrichment. Over the 43 month period of observation NO^sub x^ and N^sub 2^O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 μg N m^sup -2^ hr ^sup -1^ in elevated CO^sub 2^ OTCs, respectively. NO^sub x^ flux was negatively correlated to plant biomass production. Methane oxidation rates averaged -31 and -34 μg C m^sup -2^ hr^sup -1^ and ecosystem respiration averaged 43 and 44 mg C m^sup -2^ hr^sup -1^ under ambient and elevated CO^sub 2^, respectively, over the same time period.[PUBLICATION ABSTRACT]

In this paper we discuss three topics concerning N₂O emissions from agricultural systems. First, we present an appraisal of N₂O emissions from agricultural soils (Assessment). Secondly, we discuss some recent efforts to improve N₂O flux estimates in agricultural fields (Measurement), and finally, we relate recent studies which use nitrification inhibitors to decrease N₂O emissions from N-fertilized fields (Mitigation). To assess the global emission of N₂O from agricultural soils, the total flux should represent N₂O from all possible sources; native soil N, N from recent atmospheric deposition, past years fertilization, N from crop residues, N₂O from subsurface aquifers below the study area, and current N fertilization. Of these N sources only synthetic fertilizer and animal manures and the area of fields cropped with legumes have sufficient global data to estimate their input for N₂O production. The assessment of direct and indirect N₂O emissions we present was made by multiplying the amount of fertilizer N applied to agricultural lands by 2% and the area of land cropped to legumes by 4 kg N₂O-N ha⁻¹. No regard to method of N application, type of N, crop, climate or soil was given in these calculations, because the data are not available to include these variables in large scale assessments. Improved assessments should include these variables and should be used to drive process models for field, area, region and global scales. Several N₂O flux measurement techniques have been used in recent field studies which utilize small and ultralarge chambers and micrometeorological along with new analytical techniques to measure N₂O fluxes. These studies reveal that it is not the measurement technique that is providing much of the uncertainty in N₂O flux values found in the literature but rather the diverse combinations of physical and biological factors which control gas fluxes. A careful comparison of published literature narrows the range of observed fluxes as noted in the section on assessment. An array of careful field studies which compare a series of crops, fertilizer sources, and management techniques in controlled parallel experiments throughout the calendar year are needed to improve flux estimates and decrease uncertainty in prediction capability. There are a variety of management techniques which should conserve N and decrease the amount of N application needed to grow crops and to limit N₂O emissions. Using nitrification inhibitors is an option for decreasing fertilizer N use and additionally directly mitigating N₂O emissions. Case studies are presented which demonstrate the potential for using nitrification inhibitors to limit N₂O emissions from agricultural soils. Inhibitors may be selected for climatic conditions and type of cropping system as well as the type of nitrogen (solid mineral N, mineral N in solution, or organic waste materials) and applied with the fertilizers.

Denitrification of nitrate in the soil can be a mechanism of significant loss of fertilizer and soil nitrogen, but it can also serve to remove excess NO 3 that is leached below the root zone. Inappropriate management of irrigation water and fertilizer N in irrigated corn has resulted in leaching of excess N from the rooting zone and contamination of groundwater and also has contributed to the increasing concentration of N 2 O in the atmosphere. Denitrification can be both microbial and chemical, but the microbial process dominates in most soils through a stepwise reduction of NO 3 to N 2 . Soil atmosphere O 2 concentration, which is regulated by soil water content interactively with soil texture and microbial respiration, is the main controller of the process. The oxygen consumption rate depends on the amount of easily degradable organic C compounds and the interplay of water and carbon in developing in the soil reduced oxic conditions, which regulate not only the amount of total denitrification but also the ratio of N 2 O to N 2 produced. Appropriate management of nutrient input, relative to crop demand and soil water status, can limit nitrogen loss from denitrification. This paper describes the role of denitrification in the nitrogen economy of crop production and the environment, describes the process involved, and presents suggestions for limiting N loss caused by denitrification.

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 C(O)2, CH4, and N(2)O 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 N(2)O 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 C(O)2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.

Nitrous oxide (N2O) flux simulations by four models were compared with year-round field measurements from five temperate agricultural sites in three countries. The field sites included an unfertilized, semi-arid rangeland with low N2O fluxes in eastern Colorado, USA; two fertilizer treatments (urea and nitrate) on a fertilized grass ley cut for silage in Scotland; and two fertilized, cultivated crop fields in Germany where N2O loss during the winter was quite high. The models used were daily trace gas versions of the CENTURY model, DNDC, ExpertN, and the NASA-Ames version of the CASA model. These models included similar components (soil physics, decomposition, plant growth, and nitrogen transformations), but in some cases used very different algorithms for these processes. All models generated similar results for the general cycling of nitrogen through the agro-ecosystems, but simulated nitrogen trace gas fluxes were quite different. In most cases the simulated N2O fluxes were within a factor of about 2 of the observed annual fluxes, but even when models produced similar N2O fluxes they often produced very different estimates of gaseous N loss as nitric oxide (NO), dinitrogen (N2), and ammonia (NH3). Accurate simulation of soil moisture appears to be a key requirement for reliable simulation of N2O emissions. All models simulated the general pattern of low background fluxes with high fluxes following fertilization at the Scottish sites, but they could not (or were not designed to) accurately capture the observed effects of different fertilizer types on N2O flux. None of the models were able to reliably generate large pulses of N2O during brief winter thaws that were observed at the two German sites. All models except DNDC simulated very low N2O fluxes for the dry site in Colorado. The US Trace Gas Network (TRAGNET) has provided a mechanism for this model and site intercomparison. Additional intercomparisons are needed with these and other models and additional data sets; these should include both tropical agro-ecosystems and new agricultural management techniques designed for sustainability.

Two field experiments were conducted in a rice–fallow–rice cropping sequence during consecutive dry and wet seasons of 1997 on a Fluvic Tropaquept to determine the fate and efficiency of broadcast urea in combination with three residue management practices (no residue, burned residue and untreated rice crop residue). Ammonia volatilization losses from urea (70 kg N ha−1) broadcast into floodwater shortly after transplanting for 11 d were 7, 12 and 8% of the applied N from no residue, burned residue and residue treated plots, respectively. During that time, the cumulative percent of N2 + N2O emission due to urea addition corresponded to 10, 4.3 and nil, respectively. The 15N balance study showed that at maturity of the dry season crop, fertilizer N recovery by the grain was low, only 9 to 11% of the N applied. Fifty to 53% of the applied 15N remained in the soil after rice harvest, mainly in the upper 0–5 cm layer. The unaccounted for 15N ranged from 27 to 33% of the applied N and was unaffected by residue treatments. Only 4 to 5% of the initial 15N-labeled urea applied to the dry season rice crop was taken up by the succeeding rice crop, to which no additional N fertilizer was applied. Grain yield and N uptake were significantly increased (P=0.05) by N application in the dry season, but not significantly affected by residue treatments in either season.