Harvestable Nitrogen Accumulation for Five Storm Water Wetland Plant Species: Trigger for Storm Water Control Measure Maintenance?

As the use of constructed storm water wetlands to treat storm water runoff becomes more frequent, strategies for maintaining or increasing their pollutant removal over time must be examined. One potential strategy is plant harvesting at the water surface to remove nutrients that would otherwise be deposited back into the wetland during senescence. This technical note presents a first look at this strategy from a storm water management perspective. Vegetation was harvested from two storm water wetlands located in Smithfield and Pactolus, North Carolina, to evaluate the ability of five wetland plant species to sequester nitrogen. Biomass samples were collected from the following species of emergent vegetation: Pontederia cordata (Pickerelweed), Saururus cernuus (Lizard Tail), Scirpus cyperinus (Wool Grass), Sagittaria latifolia (Arrowhead), and Schoenoplectus tabernaemontani (Softstem Bulrush). Samples were collected immediately prior to senescence in September and October 2007 and analyzed for nitrogen content on a percent of biomass basis. At the Pactolus wetland the Pontederia cordata, Scirpus cyperinus, and Schoenoplectus tabernaemontani biomass retained significantly (p<0.05) more nitrogen than Sagittaria latifolia and Saururus cernuus. At the Smithfield wetland, Pontederia cordata and Sagittaria latifolia biomass retained significantly more nitrogen than Scirpus cyperinus and Saururus cernuus. Wetland maturity appears to have a direct relationship to harvestable nitrogen, which could explain why the relative amount of nitrogen sequestered in Scirpus cyperinus and Sagittaria latifolia varied between the two wetlands. A positive relationship existed between the density of harvested biomass and nitrogen removal for two species. To estimate the relative amount of harvestable nitrogen, a model for estimating total nitrogen loading in Coastal Plain watersheds in North Carolina was used to compare estimated inflow nitrogen mass to that which could theoretically be harvested. The harvestable N mass was greater than 20% of inflow nitrogen on an annual basis. Storm water wetland plant harvesting as a maintenance activity has potential to supplement wetland nitrogen removal.