Does replacing chemical fertilizer with ryegrass mitigate CH.sub.4 and N.sub.2O emissions and reduce global warming potential from paddy soil?

Purpose The incorporation of ryegrass (Lolium multiflorum Lam.; RG), a winter manure, could partly replace chemical N and reduce N loss during the succeeding rice seasons, but little is known about its impact on greenhouse gas emission. This study investigated the effect of different RG-urea substitution ratios (0% RG and 100% urea, 25% RG and 75% urea, 50% RG and 50% urea, 75% RG and 25% urea, 100% RG and 0% urea,) on net C and N mineralization, CH.sub.4 and N.sub.2O emissions in a paddy soil. Methods Gas samples for CH.sub.4 and N.sub.2O fluxes were collected by using a closed chamber and determined by chromatograph method. Net C and N mineralization from the incorporated RG residue were tested by a mesh bag method. Results Net C and N mineralization from RG followed a single exponential decay model, with 95.5%-97.8% of the original C and 98.7%-99.3% of N released during 192 days. The RG-urea substitution ratio increased CH.sub.4 emission, but was negatively correlated with N.sub.2O emission. In comparison with 0% substitution, global warming potential (GWP) and greenhouse gas intensity (GHGI) were not significantly different for the 25% and 50% RG substitutions, but were significantly higher for the 75% and 100% substitutions (P < 0.05). Soil redox, C and N remaining in litter residue were key characteristics explaining CH.sub.4 emission, while NH.sub.4.sup.+-N and NO.sub.3.sup.--N were correlated with N.sub.2O emission. Conclusion The increased GWP by CH.sub.4 emission after RG incorporation could be offset by N.sub.2O reduction when RG-urea substitution ratio was 50% or less.