Inubushi, Kazuyuki

Yabashi, Shingo

As part of the FACE (free-air CO_2 enrichment) experiment in a rice paddy field in Shizukuishi (Iwate Prefecture, Japan), a microplot study was conducted to determine the effects of elevated CO_2 on N dynamics. Rice plants were grown under ambient CO_2 (A_<CO2>) or ambient+200ppmv CO_2 (E_<CO2>) 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. ^<15>N fertilizer was applied in the microplots uniformly at three different growth stages of rice. Plant and soil (0-1cm upper and ca. 1-13cm lower soil depths) samplings were done destructively 4 different growth stages of rice during the growing season and analyzed for plant N and ^<15>N content, soil microbial biomass N and ^<15>N (B_N and B_<15N>) in addition to soil NH^+_4-N and ^<15>NH^+_4-N. Compared to A_<CO2>, E_<CO2> significantly increased total plant N and ^<15>N (from fertilizer) uptake in the vegetative stage of rice plants. In addition to this, the significant increase of ^<15>N uptake by rice plants was also observed at ripening under E_<CO2>. The percentage of fertilizer ^<15>N in the rice plants increased with the plant growth and it was 58.06 and 61.20-0x1.fa74p+0t harvest under E_<CO2> and A_<CO2>, respectively. Soil microbial biomass N was higher in the upper soil layer than the lower soil layers throughout the growing season. Elevated CO_2 significantly increased microbial biomass N (B_N) in the upper soil layer at harvest compared to A_<CO2>. On the other hand, B_<15N> and ^<15>NH^+_4-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 CO_2 had positive influence with time and space on N dynamics in a paddy field.