Ten years of elevated atmospheric CO2 doesn't alter soil nitrogen availability in a rice paddy

Elevated atmospheric carbon dioxide (CO2) concentration can affect soil nitrogen (N) cycling in natural and seminatural ecosystems, but the response of soil N cycling to elevated atmospheric CO2 in intensively managed agricultural ecosystems characterized by large N fertilizer inputs remains poorly understood. Here, we investigated the effects of seven and 10 years of elevated CO2 levels on soil gross N transformation rates using the 15N dilution technique at the Rice Free Air CO2 Enrichment (Rice-FACE) experiment in China. Our results show that under aerobic incubation conditions after the first seven years of CO2 enrichment, gross rates of N mineralization, NH4+ immobilization, nitrification, and NO3− immobilization were not significantly different between N application rates or between CO2 treatments. None of the four rates were affected by elevated CO2 levels under both aerobic and water-logged incubation for a further three years of CO2 enrichment. As a result, elevated CO2 levels did not result in changes in available N for plants and soil microbes, and thus did not increase the potential risks of N losses through leaching and runoff. These results are probably associated with the lack of changes in soil organic C and N concentrations due to elevated CO2. In contrast, elevated CO2 levels significantly increased N2O emissions for both incubation conditions. In general, our results suggest that 10 years of elevated atmospheric CO2 concentration had negligible effects on soil N availability in a rice paddy field.