Mitigation Measures to Reduce Losses of Phosphorus during the Non-Cropping Period - a Northern European Perspective.

Degradation of natural waters by phosphorus (P) due to agricultural activities has been a problem in several countries for many years. Accordingly, mitigation measures to minimize this issue have been developed and used with varying success. Non-point source P from agricultural fields is one of the most important types of losses to reduce as this often contributes more than 50% of total anthropogenic P loads to surface and ground waters. In Sweden, a large phosphorus program involving 24 different projects was started in 2008, with focus on testing different best management practices (BMPs) to reduce P losses from agricultural soils. Precipitation surplus and snowmelt periods during winter result in these losses occurring mainly during the non-cropping season. Some of the BMPs tested were: application schemes for animal manure, structure liming in clay soils, catch crops and constructed wetlands. These trials were carried out in tile-drained field plots and lysimeters. Application of pig slurry to the soil surface under wet conditions in late autumn increased P leaching in a clay soil. In another study, it was shown that incorporation of the slurry in a clay soil reduced total P (TP) leaching by 50% compared to surface application. It should be noted that application of manure on frozen soil is not allowed in Sweden. Structure liming reduced TP loads by up to 55%, whereas P leaching was not reduced by catch crops. In contrast, leaching increased 7-fold in a clay soil when catch crops were used, due to plant destruction during freezing/thawing events. A constructed wetland reduced TP loads by 36% over a year, but high-flow periods during winter could be a problem due to reduced sedimentation. The many BMPs tested in this program showed that P losses can be reduced substantially during the non-cropping season, but some have their limitations.