Ammonia Volatilization from Rice Paddy Soils Fertilized with 15 N-Urea Under Elevated CO 2 and Temperature

Abstract

It has widely been observed that the effect of elevating atmospheric CO2 concentrations on rice productivity depends largely on soil N availabilities. However, the responses of ammonia volatilization from flooded paddy soil that is an important pathway of N loss and thus affecting fertilizer N availability to concomitant increases in atmospheric CO2 and temperature has rarely been studied. In this paper, we first report the interactive effect of elevated CO2 and temperature on ammonia volatilization from rice paddy soils applied with urea. Urea labeled with 15N was used to quantitatively estimate the contribution of applied urea-N to total ammonia volatilization. This study was conducted using Temperature Gradient Chambers (TGCs) with two CO2 levels [ambient CO2 (AC), 383 ppmv and elevated CO2 (EC), 645 ppmv] as whole-plot treatment (main treatment) and two temperature levels [ambient temperature (AT), 25.7℃ and elevated temperature (ET), 27.8℃] as split-plot treatments (sub-treatment) with triplicates. Elevated temperature increased ammonia volatilization probably due to a shift of chemical equilibrium toward NH3 production via enhanced hydrolysis of urea to NH3 of which rate is dependent on temperature. Meanwhile, elevated CO2 decreased ammonia volatilization and that could be attributed to increased rhizosphere biomass that assimilates NH4+ otherwise being lost via volatilization. Such opposite effects of elevated temperature and CO2 resulted in the accumulated amount of ammonia volatilization in the order of ACET>ACAT>ECET>ECAT. The pattern of ammonia volatilization from applied urea-15N as affected by treatments was very similar to that of total ammonia volatilization. Our results suggest that elevated CO2 has the potential to decrease ammonia volatilization from paddy soils applied with urea, but the effect could partially be offset when air temperature rises concomitantly.