Nitrogen dynamics in paddy field as influenced by free-air CO2 enrichment (FACE) at three levels of nitrogen fertilization

As part of a FACE (free-air CO2 enrichment) experiment in a rice paddy field in Shizukuishi (Iwate Prefecture, Japan), studies were conducted to determine the effects of elevated CO2 on N dynamics at three levels of N application. Rice plants were grown under ambient CO2 or ambient + 200 ppmV CO2 conditions throughout the growing season in an Andosol soil with each treatment having 4 replicated plots. Three levels of N fertilizer (high, standard and low; 15, 9 and 4 gN m(-2), respectively) were applied to examine different N availability under both CO2 conditions. Soil samples were collected at 4 different times from upper and lower soil layers (0-1 cm and 1-10 cm soil depths, respectively) and analyzed for microbial biomass N (B-N), mineralizable N (Min. N) and NH4+-N in soil. Plant sampling was also done at 3 different times during the growing season to determine the N uptake by plant. Elevated CO2 significantly increased B-N and Min. N in the upper soil layer at harvest by 25-42% and 18-24%, respectively, compared to ambient CO2, regardless of N application rate. In low N soil, these significant increases were also observed at the ripening stage. In addition, elevated CO2 only significantly increased the NH4+-N in the upper soil layer at harvest in low N soil compared to ambient CO2. The N uptake was not significantly affected by CO2 treatment. These results indicate that elevated CO2 had significant positive influence on B-N and Min. N in the upper soil layer in paddy soil at the later period of the cropping season at all levels of N application rates, but only at low levels of application rate on NH4+-N.