DENITRIFICATION AND MINERALIZATION IN SOIL AMENDED WITH LEGUME, GRASS, AND CORN RESIDUES

Since cover crops and intercrops are increasingly used to reduce soil erosion and N loss and provide efficient N utilization, it is important to evaluate the consequences of crop residue to N cycling processes. The objectives of this study were to determine effects of incorporating hairy vetch (HV, Vicia villosa Roth subsp. villosa), red clover (RC, Trifolium pratense L.), annual ryegrass (ARG, Lolium multiflorum Lam.), reed canarygrass (RCG, Phalaris arundinacea L.), and corn (Zea mays L.) residues on denitrification, dissimilatory NO3- reduction, and N mineralization-immobilization in a Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll). A gas now system was used with 5 or 10 g residue kg-1 amended soil. With only anaerobic incubation, all residues stimulated denitrification about equally with net NO and N2O production rates two to three times greater than in the control soil. Ammonium accumulation over the 48-h anaerobic period was 5 to 11 mg N kg-1. When a 5-d aerobic incubation preceeded the anaerobic phase, losses of NO + N2O amounted to 5 to 17 times that in the control soil during the anaerobic phase. These losses were: 59.4, 47.1, 25.1, 24.4, 17.6, and 3.5 mg N kg-1 for HV, RC, ARG, RCG, corn, and the control, respectively. Mineralization in the HV treatment occurred from the third to the fifth day of the aerobic incubation and NH4+ continued to increase during the subsequent 2-d anaerobic period, reaching 58.4 mg NH4+-N kg-1. Nitrite accumulated during the anaerobic phase in all treatments, with 46 and 49 mg N kg-1 for ARG and HV, respectively, during with the 2-d aerobic/2-d anaerobic incubation. With the 5-d aerobic/2-d anaerobic incubation, NO2- levels were lower in all but the ARG treatment.