Vegetated ditch habitats provide net nitrogen sink and phosphorus storage capacity in agricultural drainage networks despite senescent plant leaching

Authors: Taylor, Jason; Moore, Matthew; SPEIR, SHANNON - University Of Notre Dame; Testa Iii, Sam

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2020
Publication Date: 3/20/2020
Citation: Taylor, J.M., Moore, M.T., Speir, S.L., Testa III, S. 2020. Vegetated ditch habitats provide net nitrogen sink and phosphorus storage capacity in agricultural drainage networks despite senescent plant leaching. Water. 12(3)875. https://doi.org/10.3390/w12030875.
DOI: https://doi.org/10.3390/w12030875

Interpretive Summary: Excess nutrients from wastewater and agricultural runoff globally threaten freshwater and marine ecosystems. Widespread recognition of the negative impacts of phosphorus and nitrogen enrichment has led to considerable efforts to develop and implement best management practices that reduce nutrient impacts to critical water resources. One approach to reducing nutrient impacts is to establish wetland vegetation in novel areas such as agricultural ditches. Vegetation enhances removal of nitrogen by plants and bacteria. A potential drawback to this practice is that vegetation may release nutrients back into water bodies during the winter as vegetation decays. This study uses a thorough experiment combined with a simple model to demonstrate that vegetation permanently removes excess nitrogen from agricultural runoff by stimulating microbial transformations of dissolved nitrogen to gaseous nitrogen. Results also showed a higher than expected retention of excess phosphorus after accounting for losses due to decay. These results provide stronger support for establishing vegetation in agricultural drainage ditches to reduce nutrient loss to downstream ecosystems like the Gulf of Mexico.

Technical Abstract: The utility of vegetated ditch environments as nutrient sinks in agricultural watersheds is dependent on biogeochemical transformations that control uptake and release during plant decomposition. We investigated nitrogen (N) and phosphorus (P) uptake and release across four P enrichment treatments in ditch mesocosms planted with rice cutgrass (Leersia oryzoides) during summer growing and winter decomposition seasons. Measured N retention and modeled denitrification rates did not vary, but P retention significantly increased with P enrichment. At the end of the growing season root biomass stored significantly more N and P than aboveground stem or leaf biomass. Decomposition rates were low (< 10% organic matter loss) and not affected by P enrichment. Nitrogen and P export during winter did not vary across the P enrichment gradient. Export accounted for < 10% of observed summer N uptake (1363 mg m2) with denitrification potentially accounting for at least 40% of retained N. In contrast, net P retention was dependent on enrichment with balanced uptake and release (only 25% net retention) in unenriched mesocosms whereas net retention increased from 77 to 88% with increasing enrichment. Our results indicate that vegetated ditch environments have significant potential to serve as denitrification sinks, while also storing excess P in agricultural watersheds.