DENITRIFICATION, IMMOBILIZATION, AND MINERALIZATION IN NITRATE LIMITED AND NONLIMITED RESIDUE-AMENDED SOIL

Although the use of cover crops and intercrops has been Increasing, little information exists on effects these plant residues may have on soil microbial processes. Our objectives were to determine effects of several crop residues representing a range of C/N ratios (11.3-65.5) on mineralization, immobilization, denitrification, and dissimilatory NO3- reduction in Brookston clay loam with limiting and nonlimiting NO3- concentrations. Residues were obtained from two legumes, hairy vetch (HV; Vicia villosa L.) and red clover (RC; Trifolium pratense L.); two grasses, annual ryegrass (ARG; Lolium temulentum L.,) and reed canarygrass (RCG; Phalaris arundinacea L.); and corn (Zea mays L.). Nitrate consumption and NH4+, NO2-, NO, and N2O production were measured in soil with 10 g residue kg(-1) in a gas how system. In soil without added NO3-, during a 5-d aerobic incubation, 86% of the existing NO3- was immobilized with RC, 66% with corn, 44% with HV and ARG, and 41% with RCG. Intense immobilization in the RC and corn treatments limited the amount of NO3- available for denitrification, whereas during the subsequent anaerobic phase, only HV, ARG, and RCG stimulated denitrification. The enhanced net immobilization with RC was surprising because the C/N ratio for this residue was relatively low (15:1). Ammonium production was greatest with the HV residue. Nitrate addition to HV-amended soil further increased NH4+ production with 91 mg N kg(-1) being produced during a 5-d aerobic, 6-d anaerobic incubation. The nature of the residue-derived organic material is more important than simply the C/N ratio in regulating these microbial processes in these short-term incubations.