Short-Term Response of Soil Respiration to Rain Events in Temperate Forests

Rain-induced soil CO2 pulses have been observed by many field studies. However, effects of rainfall on soil respiration via rapid change in soil moisture are not well understood as a result of both difficulties in measuring during rain and episodic sampling strategies by most field studies. Better understanding of response dynamics of soil respiration during rain events is important to accurately estimate net ecosystem production (NEP) and to predict impacts on carbon cycling from a projected increase in precipitation variability due to global climate change.

Rain simulation field experiments were conducted in two temperate mixed hardwood forests in New England: Great Mountain Forest, CT, in 2002; Harvard Forest, MA, in 2004 and 2005. (1) Soil CO2 flux increased rapidly in plots with O horizon right after the onset of rain simulation and dropped back gradually to the pre-rain rate after rain stopped. The rapid and immediate response might well be overlooked if measurements were not made during rainfall. The pulse-like CO2 flux during rainfall is likely to result from the enhanced decomposition of carbon compounds in soil organic matter. (2) Spatial variation in CO2 flux enhancement showed a negative correlation to mean soil moisture of the plots across the two forests. Temporal variation in soil CO2 flux enhancement showed a strong negative correlation to soil moisture of O horizon at Harvard Forest in 2005, indicating that water was not limiting at Harvard Forest during the growing season in 2005. But no such correlation was observed in 2004 or at Great Mountain Forest. For plots with O horizon removed, flux enhancement showed a clear trend to decrease with increasing soil moisture at 15 cm depth at Harvard Forest. (3) Spatial variation in flux contribution of O horizon to total soil respiration showed a negative correlation to seasonal mean soil moisture of the plots across the two forests. Temporal variation in flux contribution of O horizon showed a strong positive correlation to pre-rain soil moisture at the Great Mountain site, but a negative correlation to that at Harvard Forest in 2004. (4) Repeated wetting at Harvard Forest did not show significant impacts on soil respiration.