Phosphorus and nitrogen pollution of freshwaters: new implications for managing nutrients in agricultural systems

摘要

Water quality legislation such as the European Water Framework Directive sets standards based on ecological quality, with targets allowing only minor deviations from the biota of undisturbed water-bodies. For freshwaters, this goal is incompatible with eutrophication which fundamentally alters lake biota. Phosphorus (P) rather than nitrogen (N) has been considered to be the limiting nutrient in freshwaters and, as eutrophication is widespread in agricultural regions, this implies that measures to limit P losses from agricultural land are required to return lakes to close to their reference state. Two recent developments as to how eutrophication and nitrate controls are being implemented within the European Union are reviewed. Firstly the exclusive role of P in freshwater eutrophication has been challenged, so that control measures under the European Nitrates Directive are now required in the catchments of freshwaters impacted by eutrophication. Secondly mandatory nitrate action programmes now require long closed periods of up to seven months for the application of manures during the winter. While these may not be necessary for controlling nitrate losses, they should be of benefit in lowering P losses given the dynamics of P loss via surface runoff from manure applications. Finally losses of P associated with a build up of soil P will be significantly curtailed by the setting of manure limit of 170 kg N ha~(-1) which is a requirement the Nitrates Directive especially when combined to lower the P content of animal feedstuffs.