Many catchments in high N-deposition areas have low N retention. It has been hypothesised that under such conditions P addition may increase N retention in both terrestrial and aquatic ecosystems. Fertiliser trials in N-saturated terrestrial ecosystems show that P addition can cause increased plant uptake of N. Few studies, however have focused on N leaching to surface waters. Theoretical considerations and limited empirical data suggest that P addition to acidic, oligotrophic lakes will stimulate NO3 assimilation and thereby increase alkalinity and N retention within the lake. Near-shore coastal waters may benefit from reduced riverine N loading during summer in terms of reduced algal biomass and reduced probability of development of toxic dinoflagellate blooms. P treatment may be suitable to neutralise moderately acidified lakes and streams (pH>5.2). P addition will also increase biodiversity, stimulate aquatic production and improve the resource base for fish production. In more acidic lakes and streams, a combination of liming and moderate P addition may be an optimal management tool. We conclude that a large-scale experiment is necessary to test if P addition (both terrestrial and aquatic) might provide a mitigation technique for areas experiencing high NO3 levels in surface waters. Possible technological solutions on how to use P to increase pH and stimulate biological production in acidified areas have been evaluated. Instead of adding P as PO4 3 salts, P-containing wastewaters can be utilised at relatively low cost, after removing particulate matter, BOD, and N, but not P.HYDRO AS
