Ground Water Nitrate Removal in Subsoil of Forested and Mowed Riparian Buffer Zones

We studied two similar riparian sites in southern New England and examined ground water nitrate (NO−3‐N) removal in the subsurface of mowed (i.e., herbaceous) vs. forested (i.e., woody) vegetation. Each site consisted of poorly drained, fine to medium sands and contained adjacent areas of mowed and forested vegetation. We dosed mesocosms with bromide and 15N labeled NO−3‐N amended ground water to simulate the shallow ground water NO−3‐N dynamics of riparian buffer zones. Mesocosms were composed of undisturbed, horizontal soil cores (40 cm long, 15 cm diam.) extracted from seasonally saturated subsoil We observed substantial ground water NO−3‐N removal and denitrification at all locations. Ground water NO−3‐N removal rates were significantly correlated with carbon‐enriched patches of organic matter. This correlation supports previous work that patches function as hotspots of microbial activity in the subsoil. Within each site, we found no significant difference in ground water NO−3‐N removal rates in the subsoil of forested and mowed areas and we noted tree roots throughout the subsoil of the mowed areas. We found that ground water NO3‐N removal rates differed significantly between similar sites. We caution against ascribing specific ground water NO−3‐N removal rates to different riparian aboveground vegetation types without recognizing the importance of site differences, e.g., water table dynamics, land use legacy and adjacent vegetation. Riparian zones composed of a mix of forested and mowed vegetation, common in agroforestry and suburban land uses, may remove substantial amounts of ground water NO−3‐N.