Soil carbon dioxide efflux in forested ecosystems: The comparison of evergreen vs. deciduous watersheds

Soil CO₂ efflux is a key component of the global carbon cycle and is likely to increase as a result of increased soil temperatures due to global climate change. However, understanding the changes and interactions between soil temperature and soil moisture on soil biological processes are critical to predicting future changes in soil CO₂ efflux. We compare the soil CO₂ efflux at Coweeta Hydrologic Laboratory in the southern Appalachians in a watershed containing native deciduous forest (Quercus, Acer, Carya, and Liriodendron) with a watershed converted in 1957 to an evergreen forest (Pinus strobus). We used automated non-mobile chambers and portable chambers with infrared gas analyzers to measure soil CO₂ efflux across the watershed topographic gradient. Soil temperature was statistically similar between the watersheds, but volumetric soil moisture was significantly lower in the evergreen vs. the deciduous watershed (annual mean 15% vs. 24%, respectively). Monthly soil CO₂ efflux measured across the watershed topographic gradient showed significantly greater efflux from the deciduous vs. evergreen watersheds from June to October (mean 4.4 vs. 3.1 μmol m^-2s^-1), but no biologically significant difference during the remainder of the year. Automated soil CO₂ efflux systems were also running in each watershed to measure the differences in soil CO₂ efflux on a diurnal time scale. We hypothesize that results to date suggest that the increased soil CO₂ efflux from the deciduous vs. evergreen watershed may be due to greater water availability, nutrient availability, fine root biomass, and/or a greater labile soil carbon pool in the deciduous vs. evergreen watershed.