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

Abstract

Response of N2 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 N2 fixation. In this study, we assessed the impacts of elevated CO2 and temperature interactions on nitrogenase activities, readily mineralizable C (RMC), readily available N (NRN) contents in an alluvial and a laterite rice soil of tropical origin. Soil samples were incubated at ambient (370 μmol mol-1) and elevated (600 μmol mol-1) CO2 concentration at 25oC, 35oC, and 45oC 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 25oC, 35oC and 45oC, respectively. Increases in RMC contents were 7.7%, 10.0%, and 10.6% at 25°C, 35°C and 45°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 N2 fixation and thereby higher available N.