Addition of Organic Matter to Agricultural Ditch Soils to be Used as Constructed Wetlands for Nitrate Treatment

A wetland mesocosm experiment was conducted in eastern N.C. to determine if organic matter (OM) addition to soils existing in ditches that are to be retrofitted into constructed wetlands would improve NO₃^–-N treatment. Not all soils are suitable for wetland substrate, so OM addition can provide a carbon and nutrient source to the wetland early in its development to enhance denitrification and biomass growth. Batch studies, with various initial NO₃^–-N concentrations were conducted in surface-flow wetland mesocosms. The results indicated that increasing the organic matter content of a Cape Fear loam soil from 50 g kg^–1 (5% d.w.) to 110 g kg^–1 (11% d.w.) enhanced NO₃^–-N wetland treatment efficiency in spring and summer batch studies, but increases to 160 g kg^–1 (16% d.w.) OM did not. Increased OM addition and biosolids to the soil significantly increased biomass growth in the second growing season, when compared to no OM addition. Based on the first-order kinetic constants for nitrate reduction calculated from these mesocosm studies, increased OM in the substrate will reduce the area required for wetlands constructed in ditches to treat nitrate-laden drainage water.