Physical controls of nitrogen supply in early successional riparian ecosystems along the Tanana River, Alaska

Abstract

Accumulation of nitrogen in the early successional vegetation on the Tanana River floodplain cannot be explained by conventional measurements of soil nitrogen dynamics. This study examined these riparian areas and the effect on nitrogen cycling created by interactions among the aquatic, terrestrial, and atmospheric environments. Two hypotheses were tested; one centered on hydrologic, and one on atmospheric, mechanisms of nitrogen supply. We experimentally tested contributions of nitrogen from the hyporheic zone by measuring nitrogen-flux in the presence and absence of moving water. We then characterized hydrologic properties and ground-water chemistry along three ground-water well transects. In addition, we conducted a study to determine ambient levels of ammonia gas in the local atmosphere and to identify plants potentially utilizing atmospheric nitrogen based on stable isotope natural abundances. Static acid traps were used to calculate atmospheric levels of ammonia and mean levels ranged as high as 14 μg m⁻³, and there was isotopic evidence that the Salix alaxensis communities are utilizing atmospheric ammonia. Both hydrologic and atmospheric sources of nitrogen potentially provide greater amounts of nitrogen when compared to the in-situ processes of nitrogen fixation, nitrogen mineralization, and nitrogen deposition during this critical transition from bare silt to productive floodplain forests.