Effect of free-air CO2 enrichment (FACE) on microbial biomass in paddy field soil

Abstract

To determine the effect of free-air CO2 enrichment (FACE)on microbial biomass in paddy soil, a field experiment was conducted in an Andosol field in Shizukuishi (Iwate Prefecture, Japan). Rice plants were grown under ambient CO2 or FACE (ambient +200 ppmv CO2 ) during the growing season in 1998 and 1999. Soil samples were collected at 5 different times from the upper (0–1 cm) and lower (1–10 cm) layers and analyzed for the amounts of microbial biomass C (B c) and N (B N) and mineralizable N (Min. N). The amount of chlorophylltype compounds (ChIs), an index of algal growth, in the upper soil layers was also measured. During the first growing season (1998), FACE treatment did not cause any significant difference in the above-measured parameters except for B C at harvest, compared to the ambient CO2 treatment. However in 1999, FACE caused a significant increase in the amounts of B N, Min. N, and ChIs in the upper soil layers at harvest, compared to ambient CO2. Furthermore, FACE significantly increased the amount of Be in both soil layers, from mid-plant growth until harvest. From early plant growth until harvest, the upper soil layers subjected to both FACE and ambient CO2 treatments exhibited significantly higher values for B C, B N and Min. N than those in the lower soil layers. The amount of ChIs in the FACE treatment increased compared to that in the ambient CO2 treatment, especially at harvest. These findings suggest that FACE exerted a positively significant effect on the amounts of B C and B N, Min. N, and ChIs in paddy soil, particularly in the surface soil layers.