Uptake and trophic transfer of nitrogen and carbon in a temperate forested headwater stream

In temperate headwater streams, riparian forests hinder the development of algae by reducing light availability and generate large inputs of detritus. Microbial assemblages associated with this detritus are expected to strongly influence in-stream elemental cycling. However, most research has focused on quantifying nitrogen (N) cycling while we know little about the coupling of N and carbon (C) cycling. We conducted a simultaneous whole-reach tracer addition of 15N-ammonium and 13C-acetate in a forested headwater stream to examine the importance of different primary uptake compartments (e.g. epilithic biofilm, leaves, small wood) on N and C uptake, storage, and transfer to consumers. We predicted high whole-reach uptake of N and C from the water column to satisfy requirements of microbial decomposers. We also predicted a dominant role of the abundant detrital compartments, especially leaf litter, in the uptake, storage and trophic transfer of these labile forms of N and C. Our results show efficient immobilization of both ammonium and acetate along the study reach. Leaf litter showed the highest percentage of contribution among all compartments to whole-reach ammonium and acetate uptake. We also found evidence of rapid transfer of N and C to higher trophic levels, thereby extending the retention time of these elements within the ecosystem. Overall, our study provides relevant insights into the influence of detritus on N and C cycling in headwater streams