Denitrification Potential, Root Biomass, and Organic Matter in Degraded and Restored Urban Riparian Zones

Hydrologic changes associated with urbanization often lead to lower water tables and drier, more aerobic soils in riparian zones. These changes reduce the potential for denitrification, an anaerobic microbial process that converts nitrate, a common water pollutant, into nitrogen gas. In addition to oxygen, denitrification is controlled by soil organic matter and nitrate. Geomorphic stream restorations are common in urban areas, but their effects on riparian soil conditions and denitrification have not been evaluated. We measured root biomass, soil organic matter, and denitrification potential (anaerobic slurry assay) at four depths in duplicate degraded, restored, and reference riparian zones in the Baltimore, Maryland, U.S.A., metropolitan area. There were three main findings in this study. First, although reference sites were wet and had high soil organic matter, they had low levels of nitrate relative to degraded and restored sites and therefore there were few differences in denitrification potential among sites. Evaluations of riparian restorations that have nitrate removal by denitrification as a goal should consider the complex controls of this process and how they vary between sites. Second, all variables declined markedly with depth in the soil. Restorations that increase riparian water tables will thus foster interaction of groundwater nitrate with near-surface soils with higher denitrification potential. Third, we observed strong positive relationships between root biomass and soil organic matter and between soil organic matter and denitrification potential, which suggest that establishment of deep-rooted vegetation may be particularly important for increasing the depth of the active denitrification zone in restored riparian zones.