Impact of elevated CO2, flooding, and temperature interaction on heterotrophic nitrogen fixation in tropical rice soils

Response of N-2 fixation to elevated CO2 would be modified by changes in temperature and soil moisture because CO2 and temperature or water availability has generally opposing effects on N-2 fixation. In this study, we assessed the impacts of elevated CO2 and temperature interactions on nitrogenase activities, readily mineralizable C (RMC), readily available N (N RN) contents in an alluvial and a laterite rice soil of tropical origin. Soil samples were incubated at ambient (370 mu mol mol(-1)) and elevated (600 mu mol mol(-1)) CO2 concentration at 25 degrees C, 35 degrees C, and 45 degrees C under non-flooded and flooded conditions for 60 days. Elevated CO2 significantly increased nitrogenase activities and readily mineralizable C in both alluvial and laterite soils. All these activities were further stimulated at higher temperatures. Increases in nitrogenase activity as a result of CO2 enrichment effect over control were 16.2%, 31.2%, and 66.4% and those of NRN content were 2.0%, 1.8%, and 0.5% at 25 degrees C, 35 degrees C and 45 degrees C, respectively. Increases in RMC contents were 7.7%, 10.0%, and 10.6% at 25 degrees C, 35 degrees C and 45 degrees C, respectively. Soil flooding resulted in a more clear impact of CO2 enrichment than the non-flooded soil. The results suggest that in tropical rice soils, elevated CO2 increased readily available C content in the soil, which probably stimulates growth of diazotrophic bacteria with enhanced N-2 fixation and thereby higher available N.