DENITRIFICATION IN A NITROGEN-LIMITED STREAM ECOSYSTEM

Denitrification was measured in hyporheic, parafluvial, and bank sedim ents of Sycamore Creek, Arizona, a nitrogen-limited Sonoran Desert str eam. We used three variations of the acetylene block technique to esti mate denitrification rates, and compared these estimates to rates of n itrate production through nitrification. Subsurface sediments of Sycam ore Creek are typically well-oxygenated, relatively low in nitrate, an d low in organic carbon, and therefore are seemingly unlikely sites of denitrification. However, we found that denitrification potential (C & N amended, anaerobic incubations) was substantial, and even by our c onservative estimates (unamended, oxic incubations and field chamber n itrous oxide accumulation), denitrification consumed 5-40% of nitrate produced by nitrification. We expected that denitrification would incr ease along hyporheic and parafluvial flowpaths as dissolved oxygen dec lined and nitrate increased. To the contrary, we found that denitrific ation was generally highest at the upstream ends of subsurface flowpat hs where surface water had just entered the subsurface zone. This sugg ests that denitrifiers may be dependent on the import of surface-deriv ed organic matter, resulting in highest denitrification rate at locati ons of surface-subsurface hydrologic exchange. Laboratory experiments showed that denitrification in Sycamore Creek sediments was primarily nitrogen limited and secondarily carbon limited, and was temperature d ependent. Overall, the quantity of nitrate removed from the Sycamore C reek ecosystem via denitrification is significant given the nitrogen-l imited status of this stream.