Measurement and prediction of nitrogen loss by simulated erosion events on cultivated forest soils of contrasting structure

The objectives of this study were to determine nitrogen (N) loss associated with erosion of forest soils and to explore the role of soil structure and other factors governing N enrichment of sediment to aid prediction of N loss. We measured erosion, size distribution of aggregates in the sediment and N distribution in various aggregate fractions using simulated rainfall on samples of three cultivated forest soils of contrasting structure (repacked in trays) exposed to four erosion conditions. Both sediment loss and N loss increased with slope and kinetic energy of rainfall suggesting greater dependency of N loss on sediment loss than on N concentration in the sediment. Irrespective of erosion treatments and soil type, the bulk sediment and its size fractions were mostly richer in N than those of the uneroded soils. The enrichment ratio (ER) and concentration ratio (CR) of N for sediment cast some doubt on the application of raindrop stripping as a mechanism of N enrichment of sediment for soils of widely differing characteristics. Previously published models of N enrichment of sediment did not predict ER for our soils satisfactorily. However, an empirical model using published data on erosion and N loss agreed well with the results. This predictive method only requires information on sediment loss that can be easily obtained from an erosion model.