Nitrogen and phosphorus retention in Danish restored wetlands.

Wetland restoration is considered an effective mitigation method for decreasing nitrogen (N) losses from agricultural land. However, when former cropland becomes rewetted, there is a risk that phosphorus (P) accumulated in soils will be released downstream. Here, we evaluate N and P retention in eight restored wetlands in Denmark monitored for 1 year using a mass balance approach. The wetlands represented different types, for instance, lakes and wet meadows, and ages (3-13 years). We also show the results from a long-term monitoring station established in 1973, located downstream a lake that was re-established in 2006. All restored wetlands removed total N (42-305 kg N ha-1 year-1), while some wetlands acted as source of total P and others as a sink (- 2.8 to 10 kg P ha-1 year-1). Our study confirms that restored wetlands are effective at removing N, whereas P can be released for several years after restoration.