Soil CO₂ efflux of a larch forest in northern Japan

Abstract. We had continuously measured soil CO₂ efflux (R_s) in a larch forest in northern Japan at hourly intervals for the snow-free period in 2003 with an automated chamber system and partitioned R_s into heterotrophic respiration (R_h) and autotrophic respiration (R_r) by using the trench method. In addition, we applied the soil CO₂ concentration gradients method to continuously measure soil CO₂ profiles under snowpack in the snowy period and to partition R_s into topsoil (O_a and A horizons) CO₂ efflux (F_t) with a depth of 0.13 m and sub-soil (C horizon) CO₂ efflux (F_c). We found that soil CO₂ effluxes were strongly affected by the seasonal variation of soil temperature but weakly correlated with soil moisture, probably because the volumetric soil moisture (30–40% at 95% confidence interval) was within a plateau region for root and microbial activities. The soil CO₂ effluxes changed seasonally in parallel with soil temperature in topsoil with the peak in late summer. On the other hand, the contribution of R_r to R_s was the largest at about 50% in early summer, when canopy photosynthesis and plant growth were more active. The temperature sensitivity (Q₁₀) of R_r peaked in June. Under snowpack, R_s was stable until mid-March and then gradually increased with snow melting. R_s summed up to 79 gC m⁻² during the snowy season for 4 months. The annual R_s was determined at 934 gC m⁻² y⁻¹ in 2003, which accounted for 63% of ecosystem respiration. The annual contributions of R_h and R_s to R_s were 57% and 43%, respectively. Based on the gradient approach, R_s was partitioned vertically into litter (O_i and O_e horizons) with a depth of 0.01–0.02 m, topsoil and sub-soil respirations with proportions of 6, 72 and 22%, respectively, on an annual basis. The vertical distribution of CO₂ efflux was consistent with those of soil carbon and root biomass.