IMPACT OF STREAM WATER LEVEL ON WATER TABLE FLUCTUATION AND NITROGEN EVOLUTION IN A GRASSED RIPARIAN ZONE

Recent research has shown that riparian water table fluctuations can be influenced by stream water level fluctuations in riparian sites with a flat topography and moderate groundwater inputs. Nevertheless, the impact of stream water level changes on riparian water table fluctuations and nitrate fate in riparian zones is poorly understood. We monitored the impact of stream water level dynamics on water table fluctuations, soil leaching and the evolution of the soil inorganic nitrogen pool in a riparian zone in Brittany, France. The magnitude of water table fluctuations was 1.5 to 3 times larger 5 m from the stream than 15 m into the riparian zone and soil leaching rates were 1.4 times higher at 5 m than at 15 m. Dissolved oxygen concentrations were higher near the stream than at depth further into the riparian zone. A net gain in nitrate 5 m from the stream was observed while a net nitrate loss was observed 15 m from the stream where water table fluctuations were limited. Biogeochemical conditions suggest that denitrification was the main nitrate removal process in areas almost permanently saturated, whereas nitrification dominated in areas where large water table fluctuations or large water table drawdown were observed. Data suggest that the riparian site studied is a net source of nitrate to the stream and that stream water level fluctuations can create conditions favorable to nitrification near the stream in riparian sites with a flat topography and moderate groundwater inputs from the upland.