Patchiness in groundwater nitrate removal in a riparian forest

Our ability to identify and manage riparian sites for groundwater nitrate (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 in the subsoil of riparian forests, even in situations,where groundwater 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 miss 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 saturated 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 and Br-. Changes in the NO3--N/Br- ratios were used to calculate groundwater NO3-N removal rates. The PD treatment demonstrated substantial groundwater NO3--N removal rates. The PD mesocosms contained patches of dark-stained material that often surrounded roots in various stages of 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 exhibited 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 removal.