Inter-annual controls on nitrate export from an agricultural catchment — how much land-use change is safe?

This study presents a model for linking land-use change to nitrate export at the catchment scale. The model combines an export coefficient model with a description of the catchment buffering capacity and a dynamic description of the behaviour of soil nitrogen reserves built up in grassland soils. The soil reserves of nitrogen are described as a set of transfers: release from ploughed grassland and sequestration of nitrogen upon the development of either permanent or temporary grassland. Allowing for the effect of rainfall on these reservoirs and the export from the catchment, the model is compared with streamwater nitrate concentrations measured in the Slapton Wood catchment, Southwest England. The dominant control on nitrate export from the catchment is a runoff from the current land-use but changes in land-use show the potential to be both a sink and a source of nitrogen export to the surface waters in the catchment. An approach that balances these sinks and sources of nitrogen export at the catchment scale is outlined. This shows that it is possible to estimate the maximum amount of grassland that can be ploughed up in any one year while maintaining soil nitrogen reserves and minimising their contribution to nitrate export. Such equilibrium grassland rotation is shown to cut nitrate export levels by significant quantities and to provide an economic alternative to fertiliser management schemes.