Dissolved Organic Carbon Enrichment Alters Nitrogen Dynamics in a Forest Stream

Dissolved organic carbon, DOC, is an important source of carbon for stream heterotrophs. In many systems, bacteria and fungi are carbon limited. Thus, carbon availability may control both the availability and the dynamics of other nutrients (N or P) in heterotrophic systems. To test the interdependence of C and N cycles in forested streams, we continuously added DOC, as potassium acetate, to the stream draining Watershed six at the Hubbard Brook Experimental Forest, New Hampshire, for six weeks during summer 1999. For two months prior to and throughout the addition, we monitored streamwater NO3− and NH4+ concentrations throughout the 70-m upstream reference and the 70-m downstream treatment reaches. We also estimated uptake lengths and uptake rates of NO3− and NH4+ using back-to-back, short-term enrichments in both the treatment and reference reaches. The change in NO3− concentration during the NH4+ addition was used to estimate whole-stream nitrification. Once before the DOC addition and once during the DOC addition we measured denitrification rates and whole-stream respiration in both reaches. In the treatment reach, the increase in labile DOC caused nitrate concentrations to decrease from pretreatment concentrations during the DOC addition. Metabolism was higher in the treatment than in the reference reach during that addition, while metabolism had been similar in the two reaches before the DOC addition. Prior to that addition, uptake lengths for both NO3− and NH4+ were longer in the treatment reach than in the reference reach. During the DOC addition, the uptake lengths for both forms of nitrogen were much shorter in the treatment than in the reference reach. Contrary to expectations, the DOC enrichment did not lead to increased rates of denitrification. The principal effect of DOC enrichment was to stimulate bacterial growth, leading to high respiration and a corresponding increase in the assimilative demand for nitrogen. Results from this experiment demonstrate tight linkages between DOC and nitrogen availability in headwater streams.