Enhanced gross nitrogen transformation rates and nitrogen supply in paddy field under elevated atmospheric carbon dioxide and temperature

Climate change, particularly the combined effects of elevated CO2 and temperature, is likely to alter the internal nitrogen (N) cycle of agricultural ecosystems. However, little is known about such phenomena in paddy soils, which are expected to expand in the near future due to population increase. A 15N tracer study, with soil taken from field manipulation treatments, showed that elevated CO2, either alone or combined with elevated temperature, stimulated the mineralization of labile organic N 37-fold but decreased the mineralization of recalcitrant organic N. In contrast, elevated temperature alone accelerated the mineralization of recalcitrant organic N approximately 2-fold but had no effect on the mineralization of labile organic N. Ammonium immobilization increased under elevated CO2 and elevated temperature. Gross and net NO3− production decreased under elevated CO2 and the combined treatments, whereas elevated temperature caused an increase in both rates. Dissimilatory reduction of NO3− to NH4+ increased under elevated CO2 but decreased with elevated temperature. Our findings suggest that progressive N limitation can be alleviated by increasing gross N transformation rates under each climate change treatment and that counteraction will dominate the interactive responses of CO2 and temperature. Because we expect a concomitant increase in both CO2 and temperature, we only expect minor effects of these particular factors arising as a result of climate on soil N dynamics in paddy soils.