Variability in soil respiration across riparian-hillslope transitions

The spatial and temporal controls on soil CO2 production and surface CO2 efflux have been identified as outstanding gaps in our understanding of carbon cycling. We investigated both across two riparian-hillslope transitions in a subalpine catchment, northern Rocky Mountains, Montana. Riparian-hillslope transitions provide ideal locations for investigating the controls on soil CO2 dynamics due to strong, natural gradients in the factors driving respiration, including soil water content (SWC) and soil temperature. We measured soil air CO2 concentrations (20 and 50 cm), surface CO2 efflux, soil temperature, and SWC at eight locations. We investigated (1) how soil CO2 concentrations differed within and between landscape positions; (2) how the timing of peak soil CO2 concentrations varied across riparian and hillslope zones; and (3) whether higher soil CO2 concentrations necessarily resulted in higher efflux (i.e. did surface CO2 efflux follow patterns of subsurface CO2)? Soil CO2 concentrations were significantly higher in the riparian zones, likely due to higher SWC. The timing of peak soil CO2 concentrations also differed between riparian and hillslope zones, with highest hillslope concentrations near peak snowmelt and highest riparian concentrations during the late summer and early fall. Surface CO2 efflux was relatively homogeneous at monthly timescales as a result of different combinations of soil CO2 production and transport, which led to equifinality in efflux across the transects. However, efflux was 57% higher in the riparian zones when integrated to cumulative growing season efflux, and suggests higher riparian soil CO2 production.