干旱区生态环境知识资源中心

We investigated nitrogen dynamics over a 15-mo period in the parafluvial zone (the part of the active channel without surface water) of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert stream. The parafluvial zone and surface stream are linked hydrologically; thus, nitrogen dynamics in the parafluvial zone potentially influence whole-system functioning. We identified discrete parafluvial flowpaths by following the movement of fluorescent dye through gravel bars over time, sampled subsurface water along these flowpaths, and collected parafluvial sediments for measurement of nitrification rate. Water samples were analyzed for nitrate-N, ammonium-N, dissolved oxygen, temperature, and conductivity. Nitrate-N concentration increased along parafluvial flowpaths, with the largest increases occurring in summer. Although ammonium-N concentration was low and did not vary with season or location on flowpath, dissolved oxygen declined as water moved through parafluvial gravel bars. Net nitrification rate was highest in the summer and at the heads of flowpaths where surface water entered the parafluvial zone, suggesting that nitrification may be dependent upon ammonium, dissolved organic nitrogen, or particulate organic nitrogen imported from the surface stream. Overall, the parafluvial zone of Sycamore Creek was a source of nitrate to the nitrogen-limited surface stream, and may play an important role in the productivity of the stream ecosystem.