Influence of base flow stream bank seepage on riparian zone nitrogen biogeochemistry

We examined the effect of sustained stream bank seepage during base flow conditions on the pore water nitrogen biogeochemistry of two riparian zones in lowland agricultural areas in southern Ontario, Canada. Nitrate, ammonium and dissolved oxygen concentrations in riparian subsurface water over a two-year period showed well-organized spatial patterns along stream bank seepage flow paths that extended seasonally up to 25 m inland. High levels of dissolved oxygen and NO3- in stream inflow were depleted rapidly at the stream bank interface suggesting the occurrence of aerobic microbial respiration followed by denitrification. A zone of NH4+ accumulation persisted in more anaerobic sediments inland from the bank margin, although the magnitude and intensity of the pattern varied seasonally. A bromide tracer and NO3- co-injection at the stream bank interface indicated that bank seepage occurred along preferential flow paths in a poorly sorted gravel layer in the two riparian zones. Depletion of NO3- in relation to co-injected bromide confirmed that the bank margin was a hot spot of biogeochemical activity within the riparian zone. Conceptual models of humid temperate riparian zones have focused on nitrogen biogeochemistry in relation to hillslope to stream hydrologic flow paths. However, our results suggest that sustained stream bank inflow during low flow conditions can exert a dominant control on riparian nitrogen cycling in lowland landscapes where level riparian zones bounded by perennial streams receive limited subsurface inflows from adjacent slopes.