Decreased leaching and increased retention potential co-operative measures to reduce diffuse nitrogen load on a watershed level

Abstract

Governmental programmes and international agreements to counteract eutrophication have largely not attained agreed goals (e. g. reduction by half of the anthropogenic nitrogen load on Swedish coastal waters and to the Baltic Sea). One explanation to the fact that the goals are not achieved yet might be that the most effective building blocks to counteract diffuse pollution are far from being fully implemented. It is therefore important to stress the potential in some very effective measures and find ways to fully implement them in a strategic way at the watershed level. To avoid overdoses of fertilizers in crop production is very important since overdoses lead to an exponential growth in the leaching magnitude. Field experiments indicate that the use of winter crops or an undersown catch crop outside the main cropping season has cut back the nitrate losses with up to 75% in a single year and close to 50% during a longer period of successive years. In southern Sweden, the area of wetlands has been decreased considerably (more than 90%) by melioration activities. In a recent project, mass balances in full scale restored ponds, verified the importance of ponds and wetlands in nitrogen retention. Per unit area, increased nitrogen loading implied increased nitrogen retention, but often a decreased retention percentage. Sedimentation of organic material must be favoured in order to obtain adequate conditions for denitrification. To achieve the governmental goal in nitrogen load reduction, proper agricultural practices must be combined with restoration of ponds/wetlands.