Subsurface denitrification in a forest riparian zone: Interactions betweenhydrology and supplies of nitrate and organic carbon

The influence of hydrology and patterns of supply of electron donors and ac ceptors on subsurface denitrification was studied in a forest riparian zone along the Boyne River in southern Ontario that received high nitrogen inpu ts from a sand aquifer. Two hypotheses were tested: (1) subsurface denitrif ication is restricted to localized zones of high activity; (2) denitrificat ion zones occur at sites where groundwater flow paths transport NO3- to sup plies of available organic carbon. A plume of nitrate-rich groundwater with concentrations of 10-30 mg N L-1 flowed laterally at depths of 1.5-5 m in sands beneath peat for a horizontal distance of 100-140 m across the ripari an zone to within 30-50 m of the river. In situ acetylene injections to pie zometers revealed that significant denitrification was restricted to a narr ow zone of steep NO3- and N2O decline at the plume margins. The location of these denitrification sites in areas with steep gradients of groundwater D OC increase supported hypothesis 2. Many of these denitrification "hotspots '' occurred near interfaces between sands and either peats or buried river channel deposits. Field experiments involving in situ additions of either g lucose or NO3- to piezometers indicated that denitrification was C-limited in a large subsurface area of the riparian zone, and became N-limited beyon d the narrow zone of NO3- consumption. These data suggest that denitrificat ion may not effectively remove NO3- from groundwater transported at depth t hrough permeable riparian sediments unless interaction occurs with localize d supplies of organic matter.