Midwestern wetlands can provide a variety of ecosystem services to the surrounding landscape including nutrient retention. Although wetlands are often times sinks for phosphorus (P), accumulation of P in wetland soils, referred to as legacy phosphorus, can affect water quality when flushed out of a system. The prevention, management, or removal of stored soil P within treatment wetlands can be challenging for land managers, but P may be stored within plant biomass and removed by harvesting. In Minnesota, a small-scale edge-of-farm treatment wetland (Granada treatment wetland), a micro-scale experiment of 30 mesocosm wetland systems (University of Minnesota, St. Paul campus), and a large-scale flood water storage impoundment (North Ottawa Impoundment) were assessed. In the small scale Granada treatment wetland and the St. Paul mesocosm experiments, harvested wetland vegetation was compared in autumn for P retention within the above ground biomass. A wet prairie vegetation mix from the Granada treatment wetland was sampled from each cell within the wetland and various native Minnesota plants were tested in the mesocosms at different times of the year. The monoculture species in the mesocosm experiments ultimately removed more P per biomass than the wet prairie mixes at the Granada treatment wetland. Biomass was not witnessed to be a direct indicator of P removal per the species studied in our experiments. The time of season of harvest and correlated phosphorus content was found to be an indicative factor for phosphorus removal potential. The Granada treatment wet prairie vegetation mix removed phosphorus each season through harvesting in the fall with approximately 2.3 kg/ha removed by vegetation in 2017 and 3.2 kg/ha removed in 2018. From the 2017-2018 mesocosm experiments, both Schoenoplectus tabernaemontani and Scirpus fluviatilis removed approximately 1.6 g of P per tank or up to approximately 12 kg/ha of P. Both bulrush species removed more P than Calamagrostis canadensis, Spartina pectinata, and Carex stricta. In the large scale North Ottawa Impoundment (NOI), a 2014 Typha x glauca harvest was analyzed for P removal potential in which results indicated up to 2,564 kg of P removal from biomass harvesting, or approximately 3.11 kg/ha. Each site was also monitored for soil legacy phosphorus reductions. In the 2018 St. Paul mesocosm soil analysis, reductions of soil P after plant harvest significantly exceeded loading of P. This may indicate potential for legacy phosphorus reduction by removing vegetation. Harvesting vegetation in treatment wetlands based on phosphorus content within the shoots of selected species can be a successful management practice to reduce phosphorus accumulation over time. Currently, treatment wetland vegetation harvesting is not widely practiced in Midwestern agricultural watersheds; if treatment wetland design and harvests were cost-effective and compatible with surrounding farm systems, there may be potential for widespread application of harvesting vegetation for P removal. Harvesting treatment wetland vegetation annually may aid in reducing legacy phosphorus content within soil and may further prevent water quality degradation within agricultural watersheds at different scales.
University of Minnesota M.S. thesis. December 2019. Major: Natural Resources Science and Management. Advisors: Chris Lenhart, Joe Magner. 1 computer file (PDF); vi, 90 pages.
Treatment Wetland Vegetation Harvesting for Phosphorus Removal in Upper Midwest Agricultural Watersheds.
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