Carbon dioxide and methane fluxes by a forest soil under laboratory-controlled moisture and temperature conditions

Carbon dioxide and methane are important greenhouse gases whose exchange rates between soils and the atmosphere are controlled strongly by soil temperature and moisture. We made a laboratory investigation to quantify the relative importance of soil moisture and temperature on fluxes of CO2 and CH4 between forest soils and the atmosphere. Forest floor and mineral soil material were collected from a mixed hardwood forest at the Harvard Forest Long-Term Ecological Research Site (MA) and were incubated in the laboratory under a range of moisture (air-dry to nearly saturated) and temperature conditions (5–25°C). Carbon dioxide emissions increased exponentially with increasing temperature in forest floor material, with emissions reduced at the lowest and highest soil moisture contents. The forest floor Q10 of 2.03 (from 15–25°C) suggests that CO2 emissions were controlled primarily by soil biological activity. Forest floor CO2 emissions were predicted with a multiple polynomial regression model (r2=0.88) of temperature and moisture, but the fit predicting mineral soil respiration was weaker (r2=0.59). Methane uptake was controlled strongly by soil moisture, with reduced fluxes under conditions of very low or very high soil moisture contents. A multiple polynomial model accurately described CH4 uptake by mineral soil material (r2=0.81), but only weakly (r2=0.45) predicted uptake by forest floor material. The mineral soil Q10 of 1.11 for CH4 uptake indicates that methane uptake is controlled primarily by physical processes. Our work suggests that inclusion of both moisture and temperature can improve predictions of soil CO2 and CH4 exchanges between soils and the atmosphere. Additionally, global change models need to consider interactions of temperature and moisture in evaluating effects of global climate change on trace gas fluxes.