Modelling the impact of land-use change on water quality in lowland catchments: the role of dynamic nitrogen reservoirs

The consequences of a decline in soil nitrogen following ploughing of permanent pasture are explored in terms of nitrate export at the catchment scale. The release and build up of reserves of organic nitrogen developed in grassland soils is modelled as a first-order kinetic decay and shows supply-limited hysteresis. In comparison with observed data, overestimates are achieved at times of maximum release of nitrogen reserves, implying that new sinks of nitrogen or the form of nitrogen release needs to be explored.