PARENT SESSION
Oral Session #68: Nutrient Cycling. Presiding: D. Johnson.
Thursday, August 9, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Nutrient dynamics in subsurface flowpaths of an arid watershed.
Lewis, David Bruce1,2, Schade, John2, Kramer Huth, Anne3, Hamblen, Jennifer3, Conklin, Martha3, Grimm, Nancy1,2, 1 2 3
ABSTRACT- In watershed ecosystems, riparian zones can influence the chemistry of ground and surface waters. Nutrient transformations and changes in nutrient concentrations often result when subsurface water flows across the interface between the riparian zone and the stream channel or the uplands. We identified flowpaths of hydrologic exchange among these subsystems and tested the hypothesis that the riparian zone was a hotspot for inorganic nitrogen retention and transformation. In southern Arizona, we studied the San Pedro River, which is perennial and not impounded. Groundwater wells were placed along transects encompassing the active channel, parafluvial gravel bars, the riparian/parafluvial edge, the riparian zone, and upland areas. Nitrate was higher in gravel bars than in either the stream or the riparian areas. This pattern may result from nitrification occurring in the gravel bars, followed by denitrification (conversion to N2) or dissimilarity nitrate reduction (conversion to NH4) at the interface between the parafluvial and riparian zones. Ammonium, by contrast, was often higher in the riparian zone than in the stream or gravel bars. Several mechanisms might produce this pattern. First, ammonium may be generated by dissimilatory nitrate reduction. Second, some ammonium may be transported through the gravel bars without being nitrified. Third, ammonium may be produced within riparian soils via mineralization of organic N. The relative importance of these mechanisms will be determined with tracer experiments that follow hydrologic and nitrogen flowpaths.
KEY WORDS: nitrogen, riparian, nutrient, hydrology