Nutrient Cycling in Midwestern Agricultural Wetlands in Response to Altered Hydrologic Regimes.

The export of nutrients (N and P in particular) from intensively managed croplands of the US Midwest has resulted in the eutrophication of surface waters and a host of related ecological problems. Past studies have shown that constructed wetlands are efficient N and P sinks and thus could provide a means of reducing these nutrient exports. With the understanding that the nutrient retention capacity of treatment wetlands depends on soil conditions, antecedent land-use and hydrologic regimes, results of these past studies may not be directly applicable to the Midwest where regional climate is projected to becoming more variable (frequent wet-dry cycles) and wetlands are established on nutrient-enriched soils resulting from several decades of mineral fertilizer application. A mesocosm study, using intact soil cores (L: 30 cm; diam: 10 cm) has been conducted in order to gain an understanding of the response of Midwestern wetlands to changing hydrologic conditions. Cores were taken from two operating wetlands and a poorly-drained cropland soon to be converted into a treatment wetland. Several hydrologic conditions (dry, moist, and wet) were imposed in the laboratory for four weeks (3 cores per site) followed by flooding of all cores. The fluxes of organic (DOC, DON, DOP) and mineral nutrients (NO₃^-, NH₄⁺, PO₄^-3) were measured periodically for 4 weeks. Results will be evaluated in conjunction with P distribution in the wetland soils (Fe, Al, and Ca-bound) and with temporal variation in N gas emission, phosphatase activity, redox status and dissolved Fe in the flooded cores.