The Effects of Vegetation on the Nitrate Removal
Potential of Riparian Zones in Southern Rhode Island

Miriam Rotkin-Ellman

In order to meet water quality goals through land management, an understanding of the landscape characteristics that affect nitrate removal potential in riparian zones is central. This study explored the relationship between the dominant overstory vegetation and the presence of patches of organic material in the subsurface soils of riparian zones. Poorly drained riparian soils from four sites in Southern Rhode Island, two dominated by red maple (Acer rubrum) and two dominated by white pine (Pinus strobus ) were analyzed for concentrations of patches of organic material and root biomass. Denitrifiation Enzyme Activity (DEA) and carbon content was determined for the patch material and compared between sites and between patches with different visual characteristics. An ex-situ incubation experiment was conducted to better understand the role of root decomposition in the formation of patch material. These incubations tested the effects of water content, root species and origin of material on relative rates of decomposition. CO2 and N2O evolution from the incubations was measured for eight weeks.
Sites dominated by red maple had greater concentrations of patch material than sites dominated by white pine (.84% versus .09% respectively). DEA and percent carbon did not vary significantly among sites or patches. At three out of four sites there was significant correlation between root biomass and concentrations of patch material. Over the course of the incubation white pine roots decomposed at a greater rate than red maple (4.05 versus 1.51 ug g-1 h-1 under unsaturated conditions). In contrast, the origin of the matrix material was found to be insignificant to the root decomposition rate. All roots decomposed more slowly under saturated conditions versus unsaturated. The larger concentrations of patch material observed at the red maple sites versus the white pine sites, coupled with the greater decomposition rates of white pine roots suggest that faster decomposition rates could result in lower concentrations of patches of organic material in subsurface soils. These results strongly suggest that red maple dominated sites may have a greater potential for removing nitrate from groundwater.