Soil Carbon Transformation in Heterogeneous Landscapes

Understanding critical belowground carbon processes (e.g., soil carbon turnover, root and microbial dynamics, and greenhouse gas generation and flux) requires examination of coupled physical and biological processes. The spatial patterns of first-order controls such as soil water content, soil temperature, substrate, and vegetation cover has been shown to impose spatial and temporal organization of soil CO2 efflux to the atmosphere. We examined the spatial and temporal variability of soil CO2 ([CO2]) and soil CH4 ([CH4]) concentrations and flux, and the stable isotope composition of CO2 (δ13CCO2) across two watersheds of differing topographic complexity and vegetation cover, including both forested and harvested areas. Samples were collected at 5cm, 20cm, and 50cm at multiple sites (6-10) along seven transects in the Tenderfoot Creek Experimental Forest (TCEF), central Montana. Our results show that [CO2] increases with depth while [CH4] decreases with depth in all dry sites, meaning that dry sites simultaneously act as a source for CO2 and a sink for CH4. Wet sites, however, had pronounced differences in their [CO2] and [CH4] profiles, depending on soil water content and water table depth. Isotopically, deep soil layers had systematically more negative δ13CCO2 values, but the difference between shallow and deep δ13CCO2 values varied as a function of landscape position and vegetation cover. Our results suggest that belowground processes and rates of soil carbon transformation vary across the landscape as a function of environmental gradients.