title: Long-term declines in stream and river inorganic nitrogen (N) export correspond to forest change.
abstract: Human activities have exerted a powerful influence on the biogeochemical cycles of nitrogen (N) and carbon (C) and drive changes that can be a challenge to predict given the influence of multiple environmental stressors. This study focused on understanding how land management and climate change have together influenced terrestrial N storage and watershed inorganic N export across boreal and sub-arctic landscapes in northern Sweden. Using long-term discharge and nutrient concentration data that have been collected continuously for over three decades, we calculated the hydrologic inorganic N export from nine watersheds in this region. We found a consistent decline in inorganic N export from 1985 to 2011 over the entire region from both small and large watersheds, despite the absence of any long-term trend in river discharge during this period. The steepest declines in inorganic N export were observed during the growing season, consistent with the hypothesis that observed changes are biologically mediated and are not the result of changes in long-term hydrology. Concurrent with the decrease in inorganic N export, we report sustained increases in terrestrial N accumulation in forest biomass and soils across northern Sweden. Given the close communication of nutrient and energy stores between plants, soils, and waters, our results indicate a regional tightening of the N cycle in an already N-limited environment as a result of changes in forest management and climate-mediated growth increases. Our results are consistent with declining inorganic N efflux previously reported from small headwater streams in other ecosystems and shed new light on the mechanisms controlling these patterns by identifying corresponding shifts in the terrestrial N balance, which have been altered by a combination of management activities and climate change.
authors.forename: Richard W
authors.initials: RW
authors.lastname: Lucas
authors.forename: Ryan A
authors.initials: RA
authors.lastname: Sponseller
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gundale
authors.forename: Johan
authors.initials: J
authors.lastname: Stendahl
authors.forename: Jonas
authors.initials: J
authors.lastname: Fridman
authors.forename: Peter
authors.initials: P
authors.lastname: Högberg
authors.forename: Hjalmar
authors.initials: H
authors.lastname: Laudon
mesh_terms: D065928:Forests; D009584:Nitrogen / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D013548:Sweden; D013997:Time Factors; D014872:Water Movements; D014874:Water Pollutants, Chemical / Q000737:chemistry
chemical_list: D014874:Water Pollutants, Chemical; D009584:Nitrogen