Soil CO2 efflux of a larch forest in northern Japan

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 (Q(10)) 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-r 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.