Spatial patterns of soil organic carbon on hillslopes: Integrating geomorphic processes and the biological C cycle

Significant portions of the global soil organic carbon (SOC) pool must reside on sloping terrains where the spatial distribution of SOC reflects the combined effects of geomorphic processes and biological C cycling. Using a newly developed soil C mass balance model that explicitly includes soil production and sediment transport, we investigated the relative roles of sediment production/transport vs. biological C cycling in creating the observed spatial patterns of SOC storage within two grass-covered hillslopes in California. The study sites differed in bedrock geology, climate, and erosion rates. Measurements of SOC, soil texture, plant C inputs, and soil thickness were combined with topographic surveys and published soil erosion and production rates in the analysis. Soil thickness was found to be the key control on SOC storage, and soil thickness is balance between soil production and curvature-dependent erosional losses. Additionally, topographically varying rates of plant C inputs, decomposition rates, and SOC erosional losses or depositional inputs were found to only partially explain the observed SOC storage patterns. We used the measured relationships between SOC storage, soil thickness, and topographic curvature to create SOC storage maps of the two watersheds. At both sites, about 70% of the hillslope SOC is stored in depositional areas that are susceptible to episodic mass wasting. At the drier site, there was a larger SOC storage despite the lower soil C % because the clay-rich bedrock resulted in the development of relatively thick soils for a given slope curvature. We conclude that the geomorphic processes driving soil thickness provide fundamental mechanisms that control the spatial SOC patterns on vegetated hillslopes.