GROUNDWATER NITRATE DYNAMICS IN GRASS AND POPLAR VEGETATED RIPARIAN BUFFER STRIPS DURING THE WINTER

Nitate retention in riparian buffer strips is well documented in summer periods, but the potential of winter retention within these zones is poorly documented. Two sites, grass (Lolium perenne L.), and poplar (Populus italica)-vegetated riparian strips, were investigated in southern England (River Leach). Groundwater flow was via subsurface pathways within the sites, NO3- concentration gradients and loading rates were calculated over the winter period. Nitrate retention was found to be linearly dependent on load rate. Nitrate retention occurred at the edge of the riparian zone. This was most obvious in the poplar site where all hillslope-derived NO3- was absorbed within the first 5 m of flow within the riparian strip. When loading rates into the sites increased, NO3- absorption migrated upslope from the riparian site. The poplar-vegetated riparian zone was found to be more resilient (99% retention of NO3-) than the grass-vegetated riparian zone (84% retention of NO3-) in the winter months. It is postulated that although vegetation has no active role in retaining NO3- in the winter, above-ground vegetative biomass does contribute C to the soil microbacterial biomass that is engaged in NO3- reduction in the winter months, this accounted for the greater efficiency of the poplar vegetated site.