In many agricultural areas, hedgerows give rise to strong expectations of reducing the inputs of excess nitrate to the groundwater and rivers. This study aims to analyse the spatial and seasonal influences of a hedgerow on nitrate dynamics in the soil and groundwater. Nitrate (NO3-) and chloride (Cl-) concentrations were measured with spatially dense sampling in the unsaturated soil and in the groundwater along a transect intersecting a bottomland oak (Quercus rubor) hedgerow after the growing season and during the dormant season. We explain NO3- dynamics by using Cl- as an index of tree-root extension and water transfer. At the end of the growing season, NO3- is entirely absorbed by the trees over a large and deep volume corresponding to the rooting zone, where, in contrast Cl- is highly concentrated due to root exclusion. However, these observed patterns in the soil have no influence on the deep groundwater composition at this season. During the dormant season, water transfer processes feeding the shallow groundwater layer are different upslope and downslope from the hedgerow in relation to the thickness of the unsaturated zone. Upslope, the shallow groundwater is fed by rainwater infiltration through the soil which favours Cl- dilution. Right under the hedge and downslope, the rapid ascent of the groundwater near the ground surface prevents rainwater input and Cl- dilution. Under the hedgerow the highest concentrations of Cl- coincide with the absence of NO3- in the shallow groundwater layer and with high concentrations of dissolved organic carbon. The absence of NO3- during the dormant season seems to be due to denitrification in the hedgerow rooting zone when it is rapidly saturated by groundwater.
