Articles | Volume 13, issue 1
Biogeosciences, 13, 1–12, 2016
Biogeosciences, 13, 1–12, 2016

Research article 14 Jan 2016

Research article | 14 Jan 2016

Nitrogen export from a boreal stream network following forest harvesting: seasonal nitrate removal and conservative export of organic forms

J. Schelker1,2, R. Sponseller3, E. Ring4, L. Högbom4, S. Löfgren5, and H. Laudon2 J. Schelker et al.
  • 1Department of Limnology and Bio-Oceanography, University of Vienna, Vienna, Austria
  • 2Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
  • 3Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden
  • 4The Forestry Research Institute of Sweden (Skogforsk), Uppsala, Sweden
  • 5Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden

Abstract. Clear-cutting is today the primary driver of large-scale forest disturbance in boreal regions of Fennoscandia. Among the major environmental concerns of this practice for surface waters is the increased mobilization of nutrients, such as dissolved inorganic nitrogen (DIN) into streams. But while DIN loading to first-order streams following forest harvest has been previously described, the downstream fate and impact of these inputs is not well understood. We evaluated the downstream fate of DIN and dissolved organic nitrogen (DON) inputs in a boreal landscape that has been altered by forest harvests over a 10-year period. The small first-order streams indicated substantial leaching of DIN, primarily as nitrate (NO3) in response to harvests with NO3 concentrations increasing by  ∼  15-fold. NO3 concentrations at two sampling stations further downstream in the network were strongly seasonal and increased significantly in response to harvesting at the mid-sized stream, but not at the larger stream. DIN removal efficiency, Er, calculated as the percentage of "forestry derived" DIN that was retained within the stream network based on a mass-balance model was highest during the snowmelt season followed by the growing season, but declined continuously throughout the dormant season. In contrast, export of DON from the landscape indicated little removal and was essentially conservative. Overall, net removal of DIN between 2008 and 2011 accounted for  ∼  65 % of the total DIN mass exported from harvested patches distributed across the landscape. These results highlight the capacity of nitrogen-limited boreal stream networks to buffer DIN mobilization that arises from multiple clear-cuts within this landscape. Further, these findings shed light on the potential impact of anticipated measures to increase forest yields of boreal forests, such as increased fertilization and shorter forest rotations, which may increase the pressure on boreal surface waters in the future.

Short summary
The scientific question that is addressed in this study is how forest disturbance affects organic and inorganic nitrogen export from a boreal landscape. The key findings are that the mobilization of inorganic nitrogen from the terrestrial environment to streams increased strongly as a response to harvesting, but the stream network removed a major fraction of this load before it reached the outlet, while organic nitrogen was not removed and transported downstream.
Final-revised paper