The relative importance of different hydrological pathways (surface runoff, subsurface flow, preferential flow) for nitrogen (N) and phosphorus (P) delivery in agricultural catchments is evaluated. The assessment takes account of the different nutrient loads from agricultural land and the role of land use distribution and zonation (e.g. buffer zones) in modifying diffuse nutrient loads before they reach the drainage network. The paper draws on a number of research studies on nutrient transport and transformation in hydrological pathways in southwest England.
The geographical scale of these process studies ranges from experimental plot (0.5 m2) scale to hillslope scale. The focus is on grassland systems, in particular the impact of livestock grazing and different forms of organic fertilizers (manure, slurry, directly voided) in modifying the magnitude of nutrient export from the land and in controlling the form that N and P are transported in. The latter was evaluated by chemical fractionation of the total nutrient load. Results suggest that most of the phosphorus load is transported in the unreactive (particulate and organic) fraction via surface runoff. The speciation of the nitrogen load varies
with land use and grazing intensity. Ammonium-nitrogen is the main form of nitrogen export in surface runoff from heavily grazed land. Its mobilization appears to be linked with sediment transport.