Predicting the effect of wetland restoration on phosphorus retention, Northwest Minnesota
Water quality in the Red River Valley and downstream in Lake Winnipeg has been diminishing due to excess nutrients from agriculture. One method to reduce nutrient loading to surface water is to create or restore wetlands in agricultural areas that drain cropped fields. It is commonly believed that wetlands improve water quality, but research has shown that restoring wetlands can actually release phosphorus (P) into solution. Reducing conditions caused by soil flooding can release Fe-bound P, since ferrous Fe is more soluble than ferric Fe. Little is known concerning how the soils in the Red River Valley will be affected by wetland restoration. This research at the Judicial Ditch 66 watershed in northwestern Minnesota measured soil properties to predict the effect wetland restoration will have on P retention. Surface water and groundwater samples were collected to determine P mobility and indicated very little soluble P is transported either within or out of the watershed. Soil analyses indicated that the site contains mineral soils that are Ca-rich. A phosphorus sorption index (PSI) was used to determine the degree of P soil saturation. The mean PSI was 24.7, indicating that the soils at the site have a moderately high degree of P saturation. Spearman Rank correlation coefficients show soil organic matter and exchangeable calcium correlate highest with PSI. A three-part sequential extraction was performed to determine the dominant form of soil P. Organic P comprised nearly 80% of soil P with the remaining being nearly all Ca-bound P. Throughout the study area P is associated with compounds that are not redox sensitive; therefore, it is predicted that wetland restoration will not release significant P into solution. In addition, wetlands store P by organic matter accumulation and the settling of particulate P. Wetland restoration at this site and at similar sites can be a long-term solution to improving water quality throughout the Red River Valley and in Lake Winnipeg.
Wetland restoration--Minnesota; Phosphorus--Environmental aspects--Minnesota, Geology
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