Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N2O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24 degrees C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization immobilization and N2O emission. Residues were added at the rate of 3 t C ha(-1) to soil with, and without, 150 kg urea N ha(-1). The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P<0.05) lower NO3--N, which reached minimum values of 2.8 mg N kg(-1) for sugarcane (at day 28), 10.3 mg N kg(-1) for maize (day 7), and 5.9 mg N kg(-1) for sorghum (day 7), compared to 22.7 mg N kg(-1) for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues+N treatments were decreased by 45 mg N kg(-1) in sugarcane, 34 mg kg(-1) in maize, 29 mg kg(-1) in sorghum, and 16 mg kg(-1) in cotton amended soil compared to soil+N fertilizer, although soil NO3--N increased by 7 mg kg(-1) in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N2O from crop residue amended soils occurred in the first 4-5 days of incubation. Overall, after 84 days of incubation, the cumulative N2O emission was 25% lower with cotton+N fertilizer, compared to soil+N fertilizer. The cumulative N2O emission was significantly and positively correlated with NO3--N (r=0.92, P < 0.01) and total mineral N (r=0.93, P<0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO2 in the first 3 and 5 days of incubation (r=0.59, P< 0.05).