Erosional Removal and Redistribution of Soil Organic Carbon in Upland Ecosystems

Rivers and streams perpetually dissect the land, creating hillslopes across California and the globe. These soil-covered hillslopes distinguish themselves among terrestrial ecosystems by experiencing active soil erosion and deposition. Nonetheless, hillslope sediment transport has not been rigorously linked to the cycle and storage of soil organic carbon (SOC), a crucial step to gauge the role of hilly uplands in the global C cycle. To tackle this problem, we combined landscape surveys and measurements of plant C inputs and SOC contents with sediment transport models. Field sites included Tennessee Valley (TV) and Black Diamond (BD) in central California. TV is cooler, wetter, and less erodable than BD. Both sites were chosen because the sediment production and transport had been well quantified. We list the major results as follows. First, erosion removes C on convex slopes at rates of 1.4-2.7 g C m-2 yr-1 and 5-8 g C m-2 yr-1 at TV and BD, respectively. Second, depositional slopes have accumulated 15-24 and 15 kg C m-2 of SOC with long-term rates of ~1.9 and 1.7-2.8 g C m-2 yr-1 at TV and BD, respectively. The C accumulation makes the studied hillslopes C sinks with a potential global significance. Third, we conclude that most depositional SOC is oxidized, and that the in-situ plant is the primary C source for the SOC in depositional slopes. Fourth, the soil thickness, a balance of soil erosion and production primarily determines topographic pattern of hillslope SOC storage. In conclusion, the mechanical integration of sediment transport and the soil C cycle provides a novel insight to the spatial and temporal dynamics of hillslope SOC storage. Furthermore, this study showed that factoring sediment transport in SOC cycle is essential in understanding the soil-atmosphere C exchange, a forefront topic of current global climate change