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

J Schelker,H Laudon,S Löfgren,E Ring,L Högbom,R Sponseller

doi:10.5194/bg-13-1-2016

Abstract

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.

Highlights

Decades of research have shown that disturbance of forest ecosystems can lead to increased losses of nitrogen (N), especially as inorganic N from land. (Vitousek et al, 1979; Likens and Bormann, 1995; Aber et al, 2002; Houlton et al, 2003), with potentially negative consequences for water quality in streams and rivers (Martin et al, 2000)
Concentrations of NH+4 and dissolved organic nitrogen (DON) increased in the CC-4 catchment following harvesting (Fig. 3c and d) from 14.7 (±6.4; n = 30) μg N L−1 to 61.8 (±79.9; n = 151) μg N L−1 and from 324 (± 108; n = 30) μg N L−1 to 484 (±239; n = 151) μg N L−1 for NH+4 and DON, respectively
Despite obvious effects of forest harvesting on dissolved inorganic nitrogen (DIN) concentrations in first-order streams, only very subtle responses could be detected for the third-order streams within this same network, suggesting that significant DIN retention occurred between the harvested areas in the landscape and downstream monitoring sites

Summary

Introduction

Decades of research have shown that disturbance of forest ecosystems can lead to increased losses of nitrogen (N), especially as inorganic N from land. (Vitousek et al, 1979; Likens and Bormann, 1995; Aber et al, 2002; Houlton et al, 2003), with potentially negative consequences for water quality in streams and rivers (Martin et al, 2000). While the magnitude and duration of response to harvest varies among studies (Binkley and Brown, 1993; Kreutzweiser et al, 2008), most have documented increases in stream-water nitrate (NO−3 ) concentrations. Such responses reflect the loss of plant nutrient demand (Boring et al, 1981), accelerated rates of soil N mineralization and Published by Copernicus Publications on behalf of the European Geosciences Union. The majority of this research has addressed responses to forest disturbance at small spatial scales (e.g., catchments of first-order streams) and few studies have explored how localized increases in nutrient concentration are translated downstream within fluvial networks (Bernhardt et al, 2003)

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Conclusion