Contribution of soil moisture to seasonal and annual variations of soil CO₂ efflux in a humid cool-temperate oak-birch forest in central Japan

To quantify the contribution of soil moisture to seasonal and annual variations in soil CO₂ efflux in a cool-humid deciduous broadleaf forest, we measured soil CO₂ efflux during the snow-free seasons of 2005–2008 using an automated chamber technique. This worked much better than manual chambers employing the same steady-state through-flow method. Soil CO₂ efflux (g C m⁻²period⁻¹) during the snow-free season ranged from 979.8 ± 49.0 in 2005 to 1131.2 ± 56.6 in 2008 with a coefficient variation of 6.4 % among the 4 years. We established two-parameter (soil temperature and moisture) empirical models, finding that while soil temperature and moisture explained 69–86 % and 10–13 % of the temporal variability, respectively. Soil moisture had the effect of modifying the temporal variability of soil CO₂ efflux, particularly during summer and early fall after episodic rainfall events; greater soil moisture enhanced soil CO₂ efflux in the surface soil layers. High soil moisture conditions did not suppress soil CO₂ efflux, leading to a positive correlation between normalized soil CO₂ efflux (ratio of the measured to predicted efflux using a temperature-dependent Q₁₀ function) and soil moisture. Therefore, enhanced daily soil CO₂ efflux following heavy rainfall events could significantly reduce net ecosystem exchange (i.e. daily net ecosystem production) by 32 % on some days. Our results highlight the importance of precisely estimating the response of soil CO₂ efflux to changes in soil moisture following rainfall events when modeling seasonal carbon dynamics in response to climate change, even in humid monsoon regions.