Agricultural nutrient inputs to rivers and groundwaters in the UK: policy, environmental management and research needs.

Losses of nitrogen (N) and phosphorus (P) in land run-off and drainage from agricultural land can impair river water quality and may pose a potential health hazard. Losses of P are up to an order of magnitude smaller than those of N, but may be more significant with respect to freshwater eutrophication. At the field scale, research suggests that rates of nutrient loss are sensitive to both nutrient and land management, in particular, where nutrient inputs continuously exceed production requirements and where farming methods increase land vulnerability to run-off and erosion. A clear distinction can be made between N and P in the timescales over which inputs of these nutrients are buffered by terrestrial ecosystems against loss, which has implications for control strategies. At the river basin scale, any targets for reducing nutrient loss are best guided by site-specific information on their likely ecological impact, but this information rarely exists for UK rivers affected by eutrophication, and only general guidelines are available. True management of the environment requires integrated approaches which include both N and P taking account of differences in their source areas and delivery mechanisms, the vulnerability of land use and adoption of safe management options in relation to landscape characteristics and the sensitivity of the watercourse along its reach. For P, the identification of vulnerable zones represents a step forward to the management of the river basin in smaller definable units, which can provide a focus for safe management practices. This requires a better understanding of the linkages between nutrient sources, transport and impacts and is considered an urgent research priority.