Contribution of Root and Microbial Respiration to Soil CO2 Efflux and Their Environmental Controls in a Humid Temperate Grassland of Japan*1

Soil CO2 efflux, root mass, and root production were investigated in a humid temperate grassland of Japan over a growing season (Apr. to Sep.) of 2005 to reveal seasonal changes of soil CO2 efflux, to separate the respective contributions of root and microbial respiration to the total soil CO2 efflux, and to determine the environmental factors that control soil respiration. Minimal microbial respiration rate was estimated based on the linear regression equations between soil CO2 efflux and root mass at different experimental sites. Soil CO2 efflux, ranging from 4.99 to 16.29 μmol CO2 m−2 s−1, depended on the seasonal changes in soil temperature. The root mass at 0–10 cm soil depth was 0.82 and 1.27 kg m−2 in Apr. and Sep., respectively. The root mass at 0–10 cm soil depth comprised 60% of the total root mass at 0–50 cm soil depth. The root productivity at 0–30 cm depth varied from 8 to 180 g m−2 month−1. Microbial and root respiration rates ranged from 1.35 to 5.51 and 2.72 to 12.06 μmol CO2 m−2 s−1, respectively. The contribution of root respiration to the total soil CO2 efflux averaged 53%, ranging from 33% to 72%. The microbial respiration rate was exponentially related to soil temperature at 10 cm depth (R2 = 0.9400, P = 0.002, n = 6), and the root respiration rate was linearly related to the root production at 0–30 cm depth (R2 = 0.6561, P = 0.042, n = 6).