The effects of rainfall events on soil CO2 fluxes were examined in a cool temperate Quercus/Betula forest in Japan. The soil CO2 fluxes were measured using an open‐flow gas exchange system with an infrared gas analyzer in the snow‐free season from August 1999 to November 2000. Soil CO2 flux showed no significant diurnal trend on days without rain. In contrast, rainfall events caused a significant increase in soil CO2 flux. To determine the effect of rainfall events and to evaluate more precisely the daily and annual soil carbon flux, we constructed a multiple polynomial regression model that included two variables, soil temperature and soil water content, using the soil CO2 flux data recorded on sunny days. Daily soil carbon fluxes on sunny days calculated by the model were almost the same as those determined by the field measurements. On the contrary, the fluxes measured on rainy days were significantly higher than those calculated daily from the soil carbon fluxes by the model. Annual soil carbon fluxes in 1999 and 2000 were estimated using models that both do and do not take rainfall effects into consideration. The result indicates that post‐rainfall increases in soil CO2 flux represent approximately 16–21% of the annual soil carbon flux in this cool temperate deciduous forest.

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A trenching method was used to determine the contribution of root respiration to soil respiration. Soil respiration rates in a trenched plot (Rtrench) and in a control plot (Rcontroi) were measured from May 2000 to September 2001by using an open-flow gas exchange system with an infrared gas analyser. The decomposition rate of dead roots (Ro) was estimated by using a root-bag method to correct the soil respiration measured from the trenched plots for the additional decaying root biomass. The soil respiration rates in the control plot increased from May (240-320 mg C02 m-2 h-1 ) to August (840-1150 mg C02 m-2 h-i ) and then decreased during autumn (200-650 mg C02 m-2 h-1 ). The soil respiration rates in the trenched plot showed a similar pattern of seasonal change, but the rates were lower than in the control plot except during the 2 months following the trenching. Root respiration rate (Rr) and heterotrophic respiration rate (Rh) were estimated from Rcontrol, Rtrench, and Ro. We estimated that the contribution of Rr to total soil respiration in the growing season ranged from 27 to 71 %. There was a significant relationship between Rh and soil temperature, whereas Rr had no significant correlation with soil temperature. The results suggest that the factors controlling the seasonal change of respiration differ between the two components of soil respiration, Rr and Rh.Abbreviations: Ro -carbon emission due to the decomposition of residual roots; Rh -heterotrophic respiration rate; Rr -root respiration rate; NEP -net ecosystem production; NPP -net primary productivity

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Soil respiration was measured throughout the year (June 1992 to May 1993) in a mature, deciduous, broad-leaved forest and an adjacent, clear-felled stand which was made in November 1991, in Hiroshima Prefecture, west Japan. The same soil temperature and soil moisture content as those in the forest stand were maintained in two frame boxes covered with sheers of white netting in the dear-felled stand to observe soil respiration. A herbicide was applied to the cut end of all stumps in one of the two frame boxes in order to kill the root system. There was no significant difference in the aboveground biomass and soil environmental conditions between the forest and the frame boxes in the clear-felled stands. The difference in soil respiration rate between the forest and the frame box, in which the root system was killed by the herbicide, was considered to be due largely to the contribution of root respiration. Taking into consideration CO 2 evolution due to the decomposition of roots killed and the change in A o layer respiration rate after clear-felling, the proportion of root respiration to the total soil respiration before dear-felling was estimated to be 5 i% annually, which coincides closely with those values estimated previously in mature forests by other methods. The difference in the soil respiration rate between the two frame boxes (one with killed roots and the other with undisturbed roots) suggested that the annual root respiration rate just after clear-felling dropped to about two-thirds (70%) of that before clear-felling.