Nitrogen dynamics in paddy field under free-air CO2 enrichment (FACE) examined by the 15N tracer technique

"As part of the FACE (free-air CO2 enrichment) experiment in a rice paddy field in Shizukuishi (Iwate Prefecture, Japan), a microplot study was conducted to determine the effects of elevated CO2 on N dynamics. Rice plants were grown under ambient CO2 (A subCO2) or ambient + 200ppmv CO2 (E subCO2) conditions throughout the growing season in an Andosol soil. On transplanting day, rice hills with standard growth were selected randomly and enclosed with plow layer soil by stainless steel frames (30.0*17.5*18.0cm; L*B*W) designated as microplots. sup15 N fertilizer was applied in the microplots uniformly at three different growth stages of rice. Plant and soil (0-lcm upper and ca. l-13cm lower soil depths) samplings were done destructively 4 different growth stages of rice during the growing season and analyzed for plant N and sup15 N content, soil microbial biomass N and sup15 N (B subN and B sub15N) in addition to soil NH4**+ -N and sup15 NH4**+ -N. Compared to A subCO2, E subCO2 significantly increased total plant N and sup15 N (from fertilizer) uptake in the vegetative stage of rice plants. In addition to this, the significant increase of sup15 N uptake by rice plants was also observed at ripening under E subC02. The percentage of fertilizer sup15 N in the rice plants increased with the plant growth and it was 58.06 and 61.20% at harvest under E subC02 and A subC02, respectively. Soil microbial biomass N was higher in the upper soil layer than the lower soil layers throughout the growing season. Elevated CO2 significantly increased microbial biomass N (B subN) in the upper soil layer at harvest compared to A subCO2. On the other hand, B sub15N and sup15 NH4**+ -N recoveries were consistently higher in the lower soil layer than the upper soil layer throughout the growing season because the fertilizer was applied in the lower soil layer. These results indicate that elevated CO2 had positive influence with time and space on N dynamics in a paddy field."