Excess nitrogen leaching and C/N decline in the Tillingbourne catchment, southern England : INCA process modelling for current and historic time series

Measurements of nitrate deposition and streamwater chemistry in the Tillingbourne Catchment, in Southern England, made in 1979-1982 and 1999-2001 show a 216% increase in Nitrogen leaching despite a reduction in N inputs. Both the historical and current data sets have been modelled using the Integrated Nitrogen Model in Catchments (INCA). The process based model is shown to reproduce the historical patterns of N release from the catchment. However, modelling the increased leaching of N during recent years required an increase of the mineralization control parameter in the model, suggesting enhanced mineralization rates. Comparing historic and current soils data for C/N ratios shows that there has been a reduction in C/N from 38 to 26 in the humus layer and a reduction from 33 to 26 in the mineral soil horizon. This significant fall in C/N is consistent with the increase in N saturation in the H and Ah horizons of the major catchment soil.