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

Although the use of cover crops and intercrops has been Increasing, li ttle information exists on effects these plant residues may have on so il microbial processes. Our objectives were to determine effects of se veral 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 canaryg rass (RCG; Phalaris arundinacea L.); and corn (Zea mays L.). Nitrate c onsumption and NH4+, NO2-, NO, and N2O production were measured in soi l 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 im mobilized with RC, 66% with corn, 44% with HV and ARG, and 41% with RC G. Intense immobilization in the RC and corn treatments limited the am ount of NO3- available for denitrification, whereas during the subsequ ent 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 productio n was greatest with the HV residue. Nitrate addition to HV-amended soi l 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 res idue-derived organic material is more important than simply the C/N ra tio in regulating these microbial processes in these short-term incuba tions.