Nitrogen export from a boreal stream network following forestharvesting: seasonal nitrate removal and conservative export of organicforms

[摘要] Clear-cutting is today the primary driver of large-scale forest disturbancein boreal regions of Fennoscandia. Among the major environmental concerns ofthis practice for surface waters is the increased mobilization of nutrients,such as dissolved inorganic nitrogen (DIN) into streams. But while DIN loading tofirst-order streams following forest harvest has been previously described,the downstream fate and impact of these inputs is not well understood. Weevaluated the downstream fate of DIN and dissolved organic nitrogen (DON)inputs in a boreal landscape that has been altered by forest harvests over a10-year period. The small first-order streams indicated substantial leachingof DIN, primarily as nitrate (NO₃⁻) in response to harvests withNO₃⁻ concentrations increasing by  ∼  15-fold. NO₃⁻concentrations at two sampling stations further downstream in the networkwere strongly seasonal and increased significantly in response to harvestingat the mid-sized stream, but not at the larger stream. DIN removal efficiency,E_r, calculated as the percentage of "forestry derived" DIN that was retained within the stream network based on a mass-balance model washighest during the snowmelt season followed by the growing season, butdeclined continuously throughout the dormant season. In contrast, export ofDON from the landscape indicated little removal and was essentiallyconservative. Overall, net removal of DIN between 2008 and 2011 accounted for ∼  65 % of the total DIN mass exported from harvested patchesdistributed across the landscape. These results highlight the capacity ofnitrogen-limited boreal stream networks to buffer DIN mobilization that arises from multiple clear-cuts within this landscape. Further, these findings shed lighton the potential impact of anticipated measures to increase forest yields ofboreal forests, such as increased fertilization and shorter forest rotations,which may increase the pressure on boreal surface waters in the future.

[学科分类] 地球化学与岩石