Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions

Constructed wetlands are among the eutrophication abatement measures for which farmers may receive public subsidies in Finland. To assess the performance of constructed wetlands under boreal conditions, we monitored the retention of total suspended solids (TSS), total P (TP), dissolved reactive phosphorus (DRP), total N (TN), nitrate+nitrate-N (NO_x-N) and ammonium-N (NH₄-N) for 15–26 months in three constructed wetlands located in southern Finland. Annual retentions were −5–72% for TSS, −6–67% for TP, −33–33% for DRP, −7–40% for TN, −8–38% for NO_x-N and −50–57% for NH₄-N. The constructed wetland with the longest water residence time (WRT) showed the best performance, retaining annually about 25 kg of TP and 300 kg of TN per hectare. In contrast, the constructed wetland with the shortest WRT functioned only occasionally and was a net source for DRP and NO_x-N. In addition to the WRT, high P sorption capacity of constructed wetland soil and high input concentrations appeared to promote retention. Vegetation had a limited effect, input load being transported mostly outside the growing season. When carefully designed and located, constructed wetlands may efficiently reduce nutrient loading from agriculture.