CROP RESIDUE TYPE AND PLACEMENT EFFECTS ON DENITRIFICATION AND MINERALIZATION

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 (Vicia villosa Roth.), soybean (Glycine max [L.] Merr.), corn (Zea mays L.), and wheat (Triticum aestivum L.) having C/N ratios ranging from 8 to 82 were applied on the soil surface or incorporated in repacked cores of a Nicollet 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-15-NO3 (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 NO3 level (80.5 mg N kg-1); losses increased with decreasing residue C/N ratio. Denitrification was greatest during the first 8 d, as was CO2 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.