Modeling soil carbon transported by water erosion processes

Long-term monitoring is needed for direct assessment of soil organic carbon (SOC), soil, and nutrient loss by water erosion on a watershed scale. Howe ver, labor and capital requirements preclude implementation of such monitor ing at many locations representing principal soils and ecoregions. These co nsiderations warrant the development of diagnostic models to assess erosion al SOC loss from more readily obtained data. The same factors affect transp ort of SOC and mineral soil fraction, suggesting that given the gain or los s of soil minerals, it may be possible to estimate the SOC flux from the da ta on erosion and deposition. One possible approach to parameterization is the use of the revised universal soil loss equation (RUSLE) to predict soil loss and this multiplied by the per cent of SOC in the near-surface soil a nd an enrichment factor to obtain SOC loss. The data obtained from two wate rsheds in Ohio indicate that a power law relationship between soil loss and SOC loss may be more appropriate. When measured SOC loss from individual e vents over a 12-year period was plotted against measured soil loss the data were logarithmically linear (R-2 = 0.75) with a slope (or exponent in the power law) slightly less than would be expected for a RUSLE type model. The stable aggregate size distribution in runoff from a plot scale may be used to estimate the fate of size pools of SOC by comparing size distributions in the runoff plot scale and river watershed scales. Based upon this compar ison, a minimum of 73 per cent of material from runoff plots is deposited o n the landscape and the most stable carbon pool is lost from watershed soil s to aquatic ecosystems and atmospheric carbon dioxide. Implicit in these m odels is the supposition that water stable soil aggregates and primary part icles can be viewed as a tracer for SOC, Copyright (C) 2000 John Wiley & So ns, Ltd.