The Michigan Department of Natural Resources (DNR) manages numerous wetland areas for the primary purpose of bird habitat and waterfowl hunting opportunities. These managed wetland units fall into three broad types: (1) marshes, which have wetland vegetation, are inundated year-round, and typically have a less intensively managed hydroperiod, (2) moist soil units, which are partially cultivated but also support wet-meadow vegetation and have managed inundation from mid-fall through spring; and (3) flooded agricultural units, which are actively cultivated for row crops (corn, soybean, buckwheat) during the growing season and also have managed inundation from mid-fall through spring.
Our aim was to examine what services (nutrient retention) and potential disservices (greenhouse gas emissions) these wetland management regimes provide in addition to their primary purpose of supporting bird habitat. To that end, we collected soil cores from three units if each type in spring, summer and fall of 2024 and 2025. We measured soil nitrogen and phosphorus, net mineralization, and potential denitrification rates. In addition, we quantified limiting substrates of denitrification using factorial additions of NO3- and carbon. Greenhouse gas emissions were quantified using static chambers deployed during the same sampling periods as soil core collection.
Preliminary results suggest that moist soil and flooded agricultural units had higher soil nitrogen and phosphorus concentrations than marshes., reflecting the addition of fertilizers for growing crops. Denitrification potential (denitrification rates when both NO3- and carbon are provided) did not differ among units, but moist soil and flooded agricultural units were carbon limited, while marsh denitrification was nitrogen limited, suggesting more capacity for NO3- removal in marsh units. Additionally, moist soil and flooded agricultural units emitted more CO2 and N2O gases than marshes, but marshes had high CH4 emissions and thus higher overall greenhouse gas production (as a CO2-equivalent).
Overall, these preliminary results suggest nutrient cycling and retention differs among unit types and there may be tradeoffs in management strategies (e.g. more potential NO3- removal but higher greenhouse gas emissions in marshes). Continued work will provide more actionable information that can help the Michigan DNR balance these tradeoffs to maximize the services these systems provide.