Influence of Contrasting Soil Moisture Conditions on Carbon Dioxide and Nitrous Oxide Emissions from Terminated Green Manures

Carbon dioxide (CO2) and nitrous oxide (N2O) emissions from decomposing legume green manures largely depend on soil moisture. A potential management to mitigate N2O emissions could be to incor-porate legumes during dry periods based on the short-term rainfall forecast. The present mesocosm study was designed to examine the impact of soil moisture due to different timing of rainfall after incorporation of legume cover crops on CO2 and N2O emissions. Two timings of rainfall were simu-lated as early and late rainfall that received 80 mm deionized water at or 1 wk after incorporation of the legumes. An additional 20 mm water was added after 2 wk of the first simulated rainfalls. Gas fluxes of CO2 and N2O were measured using closed chamber method for 28 d incubation assay. Soil concentra-tions of NH4+ and NO3–, concentrations of N in undecomposed biomass, and abundances of denitri-fier bacterial genes (nirK, nirS, and nosZ) and arbuscular mycorrhiza fungi (AMF) were determined at weekly intervals. Carbon dioxide emissions increased immediately after the first simulated rainfall events and peaked around Day 2 to 3, whereas N2O emissions reached peak level around Day 8 to 10 from both legume treatments. After the first rainfall simulations, soil NH4+ and NO3– concentrations increased, whereas biomass N concentrations decreased rapidly. Abundance of nirK, nosZ, and AMF was positively correlated (P < 0.05) to N2O emissions. Dynamics and magnitude of emissions after first rainfall events remained similar irrespective of the timing of simulated rainfall. In conclusion, our results indicated that soil incorporation of legumes based on a short-term rainfall forecast may not be an effective tool to avoid large N2O emissions.