The influence of rainstorm on soil respiration of a mixed forest in southern New England, USA was investigated with eddy covariance, rain simulation and laboratory incubation. Soil respiration is shown to respond rapidly and instantaneously to the onset of rain and return to the prerain rate shortly after the rain stops. The pulse-like flux, most likely caused by the decomposition of active carbon compounds in the litter layer, can amount to a loss of 0.18 t C ha À1 to the atmosphere in a single intensive storm, or 5-10% of the annual net ecosystem production of midlatitude forests. If precipitation becomes more variable in a future warmer world, the rain pulse should play an important part in the transient response of the ecosystem carbon balance to climate, particularly for ecosystems on ridge-tops with rapid water drainage.

To gain new insights into the underlying mechanisms responsible for wetting-induced soil respiration, rain simulation field experiments were carried out in two temperate mixed-hardwood forests in New England (Great Mountain Forest and Harvard Forest). The rain-induced CO 2 pulses were observed in both xeric and mesic soils. The pulse intensity was negatively correlated with the site moisture level and the pre-rain soil CO 2 flux. At both forests, plots without O horizon responded to wetting with limited or even negative enhancement, confirming previous finding that the rain pulse was likely due to enhanced microbial consumption on substrates mainly of microbial origin. Our results show that the flux rain pulse was a reproducible phenomenon not limited to dry soils.