Increasing atmospheric CO₂ concentration ([CO₂]) may affect future rice agriculture either directly through plant physiology or indirectly by changing interactions in the soil-plant-atmosphere continuum. While direct effects of elevated [CO₂] on rice plant have been well documented, indirect effects on soil biological activities and C pool are still limiting. Free air CO₂ enrichment (FACE) is a powerful approach to simulate likely future [CO₂] under in-situ conditions. Soil microbial biomass C (MBC) is an indicator for pool and flux of C in the soil. We therefore conducted a FACE experiment to determine the effects of [CO₂] on MBC and the translocation of plant-assimilated C to total and soluble organic C in a paddy soil.

     The FACE site was located at Shizukuishi, Iwate, Japan (39^o40’N, 141^o00’E). Six pots (2.3L) were filled with a soil of high δ¹³C (–15.6‰) collected from a C4 plant (sugar cane) cultivating field, and seedlings of rice (Oryza sativa L. cv. Akitakomachi) were transplanted to each pot. Then, the pots were installed into soils in the rings either under Ambient (control [CO₂]) or FACE (control + 200ppm) conditions, replicated for three blocks. MBC was determined by the chloroform fumigation-extraction method, and the contribution of plant-assimilated CO₂ to soil C was determined by the mass balance method using soil and plant δ¹³C after 4-month cultivation period.

     FACE significantly increased MBC (P = 0.005). While soil C amount did not respond to FACE, δ¹³C values were significantly depleted in FACE (–17.4 and –20.4‰ for total and soluble organic C, respectively) as compared with in Ambient (–17.0 and –18.7‰), indicating that fumigated CO₂ in FACE rings having ¹³C-depleted isotopic signature translocated to soils. The calculated contribution of plant-assimilated C to soil organic C did not change due to FACE treatment. Elevated [CO₂] enhanced MBC (a short-term pool) but the effects on soil organic C (a long-term pool) was not detected in this study.