UC Merced

Soil erosion can alter the mechanisms of organic matter (OM) storage and persistence in soil, including aggregation, burial, and organo-mineral associations. I studied how extended transport of topsoil and associated OM alters OM stabilization mechanisms by comparing soil from different landform positions with sediment exported from eight, low-order watersheds in the Sierra Nevada, California. To assess the relative importance of different stabilization mechanisms, I separated free, unprotected OM fractions from OM that was physically protected inside aggregates and OM that was chemically associated with mineral surfaces. Annual sediment mass, carbon (C), and nitrogen (N) fluxes were positively and strongly correlated to stream flows. Sediment yield, composition, and stabilization mechanisms showed high interannual variation, with higher C and N concentrations in sediment collected in drier years. My results suggest that C and N is less protected in material exported from watersheds with coarse-textured soils than in watersheds with finer textured soils.