Nitrogen removal and the fate of nitrate in riparian buffer zones
Riparian buffer zones, adjacent to waterways, may protect water quality by intercepting and removing nitrogen in runoff from agricultural land. This research comprised four parts: (1) a field study of nitrogen buffering by differently vegetated riparian zones in a United Kingdom (UK) sheep-grazed pastoral catchment; (2) a field study of surface and subsurface runoff hydrology, and nitrogen flux, in a UK riparian wetland; (3) a laboratory study ((^15)N tracer-isotope dilution) of microbial inorganic nitrogen production and removal processes in the UK riparian wetland soil; and (4) a laboratory microcosm study ((^15)tracer) of nitrate removal processes in bare and plant-inhabited (Glyceria declinata) New Zealand (NZ) riparian wetland soil. Dissolved organic nitrogen and ammonium were generally more important components of subsurface runoff than nitrate in the three UK riparian zones. All riparian zones were poor buffers having minimal effect on the nitrogen concentration of subsurface runoff. In the UK riparian wetland site subsurface (saturated zone) and surface 'preferential flow paths' typically conveyed large quantities of catchment runoff rapidly into, and across the site, and hindered nitrogen buffering. However, under low flow conditions, runoff-riparian soil contact increased and the wetland decreased the catchment nitrogen flux by 27%. In the UK riparian wetland soil most nitrate removal was attributable to denitrification (87- 100%) as opposed to dissimilatory nitrate reduction to ammonium (DNRA) (0-13%) and immobilisation (0-10%). Total ((^14)N+(^15)N) transformation rates for these processes were 1.3-47, 0.5-1.5 and 0.6-2.5 μg N g soil(^-1) hr(^-1) respectively. In the NZ riparian wetland soil Glyceria declinata assimilated 11-15% of nitrate but, more importantly, increased soil oxidation and altered the proportions of nitrate removal attributable to denitrification (from 29% to 61-63%) and DNRA (from 49 to <1%), but not immobilisation (22-26%). Denitrification and, thus, nitrogen buffering might be enhanced, in some riparian zones by increasing the extent of moderately anoxic soil with plants that release oxygen from their roots or with water table management.