Nutrient release from legume and cereal crop residue is important to N cycling and the success of conservation and sustainable farming systems. Residue type, placement, and degree of incorporation, and soil water regimes largely control availability and loss of soil N and were evaluated in the laboratory. Four residues, i.e., vetch (Vida villosa Roth.), soybean (Glycine max [L.] Meir.), corn (Zea mays L.), and wheat (Triticum aestimm L.) having C/N ratios ranging from 8 to 82 were applied on the soil surface or incorporated in repacked cores of a NicoIIet loam (fine-loamy, mixed, mesic Aquic Hapludoll) and incubated for 17 or 35 d at 60 and 90% water-filled pore space (WFPS) with enriched "N-NO, (76.7%). Denitrification losses from all treatments were negligible at 60% WFPS. At 90% WFPS, total denitrification losses from residue-incorporated soils represented 87 to 127% of the initial soil NO 3 level (80.5 mg N kg-'); losses increased with decreasing residue C/N ratio. Denitrification was greatest during the first 8 d, as was CO 2 evolution. Initial denitrification with surface-placed residues was less than with incorporated residues, but cumulative losses over 35 d did not differ significantly. Substantial N immobilization occurred with incorporated or surface-placed wheat, corn, and soybean residue with wide C/N ratios at 60% WFPS, whereas, with low-C/N-ratio vetch, significant mineralization occurred. After 35 d, 51 and 36% of N in incorporated and surface-placed vetch residue, respectively, was mineralized. Residue C/N ratio was inversely related to initial rates of residue decomposition, and effects of residue type and placement and soil water on denitrification and mineralization were most important during early (8-10 d) decomposition. I NCREASED COSTS and environmental concerns promote research on agricultural production systems that reduce soil erosion and balance synthetic chemical use with renewable biological resources. With conventional tillage, crop residues are incorporated into the soil; in conservation farming (reduced-or ridgetillage management systems), residues are left on the soil surface or are partially incorporated just before

Nitrous oxide (N2O) and methane (CH4) are greenhouse gases that are contributing to global warming potential. Nitrogen (N) fertilizer is one of the most important sources of anthropogenic N2O emissions. A field study was conducted to compare N‐use efficiency and effect on N2O and CH4 flux, of urea, urea plus the nitrification inhibitor dicyandiamide (U + DCD), and a control release fertilizer, polyolefin coated urea (POCU) in irrigated spring barley (Hordeum vulgare L.) in northeastern Colorado. Each treatment received 90 kg urea‐N ha−1 and microplots labeled with 15N‐fertilizer were established. Average N2O emissions were 4.5, 5.2, 6.9, and 8.2 g N ha−1 d−1 for control, U + DCD, POCU, and urea, respectively. During the initial 21 d after fertilization, N2O emissions were reduced by 82 and 71% in the U + DCD and POCU treatments, respectively, but continued release of N fertilizer from POCU maintained higher N2O emissions through the remainder of the growing season. No treatment effect on CH4 oxidation in soils was observed. Fertilizer 15N found 50 to 110 cm below the soil surface was lower in the POCU and U + DCD treatments. At harvest, recovery of 15N‐fertilizer in the plant‐soil system was 98, 90, and 85% from POCU, urea, and U + DCD, respectively. Grain yield was 2.2, 2.5, and 2.7 Mg ha−1 for POCU, urea, and U + DCD, respectively. Dicyandiamide and POCU showed the potential to be used as mitigation alternatives to decrease N2O emissions from N fertilizer and movement of N out of the root zone, but N release from POCU does need to be formulated to better match crop growth demands.

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Application of 56, 112, or 224 kg N ha−1 as ammonium nitrate (NH4NO3) (AN) to barley (Hordeum vulgare) plots in northeastern Colorado led to a marked, but short‐lived, increase in nitrous oxide (N2O) emissions that was linearly related to the amount of AN applied. The AN treatment effects became statistically insignificant after about 6 weeks. Compared with total N2O emissions of 0.54 kg N ha−1 from the control plots, totals from the AN‐treated plots ranged from 0.93 to 1.43 kg N ha−1, representing an average 0.5% of the fertilizer added. Nitrous oxide emissions from plots treated with 16.7 metric tons ha−1 dry anaerobically digested sewage sludge (SS) totaled 1.09 kg N ha−1 and exhibited temporal variability similar to that of AN‐treated plots, while the emissions from plots treated with 83.5 metric tons SS ha−1 were significantly larger than from the other treatments throughout the 155‐day study period and totaled 4.19 kg N ha−1. The increase in N2O emissions that results from application of organic or inorganic N amendments appears to be much smaller than assumed by models developed to describe the effect of agricultural fertilizers upon stratospheric ozone depletion.