Elevated delta N-15 values in aquatic plants from cleared catchments: why?

delta N-15 values of plants and animals are being increasingly used to iden tify the flow of nitrogen through aquatic ecosystems. The delta N-15 values of crops, riparian trees, emergent and submerged aquatic vegetation in str eams from both cleared (agricultural) and forested (rainforest) catchments were sampled. Riparian and aquatic plants had similar delta N-15 values in forested streams, suggesting a similar source of inorganic nitrogen. In cle ared catchments, however, aquatic plants had delta N-15 values 4-8 parts pe r thousand higher than adjacent riparian vegetation and aquatic plants from streams in forested catchments. The elevated delta N-15 values of aquatic vegetation in streams with cleared catchments suggest that these plants eit her have access to a different source of N than those in undisturbed catchm ents or that high rates of microbial decomposition and nutrient cycling in the cleared catchments influence the delta N-15 value of available N. This also suggests that the aquatic plants in disturbed catchments are incorpora ting a different source of nitrogen than the adjacent riparian vegetation. This supports the proposal that in-stream processing of N affects the delta N-15 value of available N in streams with cleared catchments. These result s begin to identify potential pathways for nitrogen transport to streams an d the effect that internal cycling may have on a stream's nitrogen load.