PATCHINESS IN GROUNDWATER NITRATE REMOVAL IN A RIPARIAN FOREST

Our ability to identify and manage riparian sites for groundwater nitr ate (NO3-) removal is limited by uncertainty surrounding the relative importance of plant uptake vs, microbially mediated removal processes. Microcosm studies often demonstrate negligible transformation rates i n the subsoil of riparian forests, even in situations,where groundwate r well networks showed substantial groundwater NO3- removal during the winter and a decline in dissolved oxygen (DO) in ambient groundwater mowing through the site. We hypothesize that microcosm studies may mis s groundwater transformations that occur within microsites, that is, ' 'hotspots'' of riparian subsoils. We created mesocosms of large (15 cm diam. x 40 cm length), undisturbed cores from the seasonally saturate d zone of poorly drained (PD) and moderately well drained (MWD) sandy soils from a forested riparian area in southern New England. We dosed the mesocosms for 130 d with ambient groundwater amended with NO3--N a nd Br-. Changes in the NO3--N/Br- ratios were used to calculate ground water NO3-N removal rates. The PD treatment demonstrated substantial g roundwater NO3--N removal rates. The PD mesocosms contained patches of dark-stained material that often surrounded roots in various stages o f decay, The dry mass of patches in the PD treatment ranged from 0.07 to 1.4% of the mesocosms. The MWD treatment contained no patches and e xhibited no groundwater NO3--N removal. Further investigations on the relationships between the extent of subsurface patchiness, water table dynamics and plant characteristics might yield fruitful insights into the management of vegetated riparian zones for groundwater NO3--N rem oval.