Non-steady State Soil Organic Carbon Storage in Undisturbed Watersheds Due to Diffusive Sediment Transport

Most soil C models assume that plant C inputs are matched by C loss through heterotrophic respiration. While these models are applicable for level terrain, on soil mantled uplands in hilly to mountainous regions, persistent soil mass transport represents a potentially large, but unstudied, flux of soil C. In this research we quantify the soil C erosional fluxes and non-steady state soil C storage within two undisturbed grass-covered hillslopes in Coastal California: Tennessee Valley (TV) (coastal Marin County) and Black Diamond (BD) (interior Contra Costa County). At both sites, previous geomorphic studies have quantified both the sediment transport processes (TV= gopher driven sediment transport; BD= abiotic soil shrink/swell) and their rates. Hillslope patterns of soil C storage were examined in relation to slope position with a hillslope sediment transport model. The average C erosion rates from convex slopes are between 1.4 and 2.7 g C m ^-2 yr^-1 at TV and approximately 8 g C m^-2 yr^-1 at BD. The C erosional flux is locally as high as 14% of above ground net primary productivity (NPP) at TV and 8% at BD. The convex slopes are net C sinks because NPP likely exceeds respiration by a value equaling the size of C erosion. Eroded soils ultimately accumulate in depositional settings which have residence times on the order of 13kyrs at TV and 5.3kyrs at BD. At TV hollow, 15-24 kg C m^-2 of soil C has accumulated at a long-term rate of 1.6-1.9 g C m^-2 yr^-1 . The present rates of C accumulation were calculated to be 0.3 g C m^-2 yr^-1 at TV and 0.6 g C m^-2 yr^-1 at BD. During the hollow infilling, the depositional C inputs have been greater than C accumulation rates, meaning that much of the incoming eroded C is ultimately oxidized to CO₂. At both sites, a fraction of the eroded C is exported from the watershed (C of 0.1-0.5 g C m^-2 yr^-1 at TV and 2 g C m^-2 yr^-1 at BD). When all hillslope components are integrated, these watersheds are continuous atmospheric C sinks at rates of up to 0.3 and 2.4 g C m^-2 yr^-1 . We suggest that the upland soil C cycle may significantly affect the global C balance if scaled to continental levels.