Microbial nitrate processing in shallow groundwater in a riparian forest

We measured denitrification, immobilization, and respiration in microcosms that simulated groundwater conditions in a riparian forest in Rhode Island. Measured rates were compared with rates of NO3- removal measured in a companion study using a groundwater monitoring well network and 10-mo injection of NO3- and a bromide tracer to groundwater in the same riparian forest. Limiting factors for denitrification were assessed, and microbial and root biomass and potential net N mineralization and nitrification were compared in aquifer material exposed to the 10 mo of NO3- dosing and unexposed control material. While there was significant variation In water table levels, dissolved oxygen, dissolved organic C, and total C within the aquifer in the riparian forest, there was little spatial variation in denitrification or respiration rates. Rates were higher in summer than in winter and were nearly always limited by a lack of organic C. Limited evidence suggested that immobilization was not a significant sink for NO3-. Measured groundwater denitrification rates were high enough to eliminate 27% of an incoming NO3- concentration of 10 mg N L(-1) over a riparian zone width of 60 m, with a travel time of 1600 d, and equaled 6.0 kg N ha(-1) yr(-1). The measured denitrification rates were much lower than rates of groundwater NO3- removal directly measured in the companion hydrologic study at the same time (120 kg N ha(-1) yr(-1)), suggesting that there is still considerable uncertainty about the mechanisms of NO3- removal from groundwater in riparian forests.