Broadleaf litter addition effects on Nitrogen Dynamics in a Swedish Boreal headwater stream

Sammanfattning

Broadleaf riparian vegetation is increasingly promoted in boreal forestry management to improve stream ecosystem function, yet evidence for its biogeochemical effects in conifer-dominated headwaters remains limited. This study examined how experimental broadleaf litter additions influenced ecosystem metabolism, inorganic nitrogen (NO₃⁻, NH₄⁺ and DIN) dynamics, and nitrification rates in a boreal headwater stream in northern Sweden. Whole-stream metabolism was estimated using a two-station diel oxygen approach, reach-scale nitrogen uptake was assessed through longitudinal mass balance analyses, and a laboratory nitrification assay was used to isolate effects of litter from environmental variability. Litter addition produced a detectable but transient increase in ecosystem respiration, consistent with short-term stimulation of heterotrophic activity. Despite this, no clear treatment effect on reach-scale nitrogen uptake was detected in the field, with elevated autumn discharge identified as the primary confounding factor. The laboratory assay demonstrated significant suppression of nitrification with litter and labile carbon additions, suggesting heterotrophic microbes outcompete nitrifiers for ammonium under increased carbon supply. These results indicate that broadleaf litter can alter nitrogen cycling at the microbial scale, but that translating such effects to detectable reach-scale change in boreal headwaters is constrained by hydrological variability coinciding with peak litter inputs. Management expectations regarding the nutrient processing benefits of broadleaf riparian buffers should therefore be interpreted cautiously, particularly in catchments where recalcitrant dissolved organic carbon may already limit heterotrophic nitrogen immobilisation.