Prediction of sediment-bound nutrient delivery from semi-arid California watersheds

Soil carbon (C), nitrogen (N), and phosphorus (P) are lost from hillslopes in particulate forms through soil erosion. The fate of the eroded C (e.g., sequestration or oxidation) may affect the global C budget, and delivery of N and P to waterbodies can lead to eutrophication. Whereas the magnitude of particulate nutrient losses may be similar to or greater than dissolved losses, it is rarely estimated. We couple a sediment delivery model with measurements of C, N, and P in soil to account explicitly for hillslope sediment transport processes that yield sediment-bound nutrients to fluvial networks. The model is applied to a site in California dominated by coastal sage scrub and gopher-rich grasslands. Although the magnitude of sediment delivery predicted by the model has been tested with reservoir sedimentation records, no data exist to test the predicted rates of nutrient delivery. Nevertheless, the model results are provocative; it predicts that losses of particulate C from sage covered hillslopes (23 kg/ha/yr) are nearly double that from grassland hillslopes (13 kg/ha/yr), despite a lower annual sediment yield from the sage hillslopes. The model predicts similar average annual N and P losses for sage and grasslands but dramatic differences in the frequency and magnitude of delivery events. Nutrient delivery from grasslands is chronic whereas delivery from the coastal sage is highly episodic, with large pulses driven by fire frequency. These results suggest that changes in the vegetation community can alter the delivery regime of sediment-bound C, N, and P.