The erosion and storage of soil organic carbon in upland hillslope ecosystems.

摘要

I quantified the soil organic carbon (SOC) erosion losses and depositional gains and investigated how soil erosion affects the temporal and spatial dynamics of the SOC storage within two grass-covered hillslopes in central California. Landscape surveys and measurements of plant C inputs and SOC contents were combined with soil transport models. The SOC erosion losses on convex slopes were 1.4--2.7 g C m-2 yr-1 at Tennessee Valley (TV) and 5--8 g C m-2 yr -1 at Black Diamond (BD). In convergent slopes, 15--24 and 15 kg C m-2 of SOC accumulated via long term rates of ∼1.9 and 1.7--2.8 g C m-2 yr-1 at TV and BD, respectively. Most depositional SOC is likely oxidized, and most of the C in depositional soils appeared to be from in-situ plant production. These hillslopes are not at C steady state, and are C sinks with global significance. The balance between soil erosion and production was found to be the primary determinant of hillslope SOC storage through their control on soil thickness. Using the observed relationship between SOC storage and curvature, SOC storage maps of the two field areas were created. These maps revealed that ∼70% of the SOC resides in depositional soils where sediment accumulation and evacuation repeat over the time scales of 103 to 104 years. I found that BD, which is the driest of the two sites, had a lower C % than TV, but stored more SOC because its higher soil production rate produced thicker soils. This suggested that SOC models should include both soil erosion and production in order to quantify hillslope SOC storage at regional to global scales. Given the pocket gopher's role in soil erosion on many California hillslopes, a model simulation investigated the energy that they expend in soil transport. At TV, the gopher mediated sediment transport was found to be sustained by ∼0.001% of photosynthesized energy. I also explored the effect of spatially variable gopher population densities on hillslope soil thickness. Gopher density and hillslope soil thickness co-developed with characteristic spatial and temporal variations, suggesting the co-evolution of hillslope soils and biological agents.