The contribution of the respiration of plant roots and microbes on CO_2 efflux at the surface of soil was determined during a period of wheat growing in a field of brown lowland soils at Ishikari town in Hokkaido. Calculation of CO_2 efflux at the surface soil was based on Fick's law with independent measurement of the soil-gas diffusion coefficients and the CO_2 concentration gradients within the soil. The CO_2 produced by wheat roots and microbes in the topsoil was estimated by using a mass-balance equation. Root respiration was estimated by subtracting microbial respiration, which was determined by the flask incubation method, from the total CO_2 production in the top layer. 1)The rate of CO_2 efflux at the surface of soil ranged from 0.11 to 0.20 mg CO_2 m^<-2>s^<-1>. It increased with an increase of soil water suction of the topsoil. 2)The rate of CO_2 production in the top 0.25 m layer ranged from 0.10 to 0.25 mg CO_2 m^<-2>s^<-1>. It increased from the heading stage to the ripening stage when the water suction in the topsoil increased and the soil temperature increased. 3)The microbial respiration rate in the topsoil was almost constant at 0.6 mg CO_2 m^<-2>s^<-1>. The variation in the CO_2 production rate was therefore ascribed to the variation of the rate of plant root respiration. Regression analysis indicated that the variation of the root respiration rate was explained by the soil temperature at a depth of 0.05 m. 4)Total CO_2 evolved from the soil surface for 45 d in the wheat growth period was 0.19 kg Cm^<-2> ; this accounted for 44% of the amount of carbon fixed by wheat within the period. Root respiration accounted for 63% of total CO_2 evolution and microbial respiration was 32%, while only 5% was supplied from subsurface soil. These facts suggest that root respiration strongly affected soil respiration and plant growth.
