Variability in soil CO2 production and surface CO2 efflux across riparian-hillslope transitions
Pacific, Vincent Jerald
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The spatial and temporal controls on soil CO2 production and surface CO2 efflux have been identified as an outstanding gap in our understanding of carbon cycling. I investigated both the spatial and temporal variability of soil CO2 concentrations and surface CO2 efflux across eight topographically distinct riparian-hillslope transitions in the ~300 ha subalpine upper-Stringer Creek Watershed in the Little Belt Mountains, Montana. Riparian-hillslope transitions provide ideal locations for investigating the spatial and temporal controls on soil CO2 concentrations and surface CO2 efflux due to strong gradients in respiration driving factors, including soil water content, soil temperature, and soil organic matter. I collected high frequency measurements of soil temperature, soil water content, soil air CO2 concentrations (20 cm and 50 cm), surface CO2 efflux, and soil C and N concentrations (once) at 32 locations along four transects. Soil CO2 concentrations were more variable in riparian landscape positions, as compared to hillslope positions, as well as along transects with greater upslope accumulated area. This can be attributed to a greater range of soil water content and higher soil organic matter availability.Soil gas diffusion also differed between riparian and hillslope positions. Soil gas transport limited surface CO2 efflux in riparian landscape positions due to high soil water content (despite strong concentration gradients), while efflux was gradient (production) limited in hillslope positions. This led to spring-fall reversal of maximum riparian and hillslope soil CO2 concentrations, with highest hillslope concentrations near peak snowmelt and highest riparian concentrations during the late summer and early fall. Soil temperature was a dominant control on the overall temporal variability of soil CO2. However, soil water content controlled differences in the timing of soil CO2 concentration peaks within and between riparian and hillslope positions, as exemplified by those locations closest to Stringer Creek (wetter landscape positions) peaking up to three months later than those riparian locations near the riparian-hillslope transition. This work suggests that one control on the spatial and temporal variability of watershed soil CO2 concentrations and surface CO2 efflux is a soil water content mediated tradeoff between CO2 production and transport.