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 (NO
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). Perhaps the clearest demonstrations of how
forest disturbance influences terrestrial nutrient mobilization have used
experimental harvests in small catchments to document changes in stream
chemistry relative to undisturbed controls (Likens et al., 1970; Swank and
Vose, 1997). 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
Whereas several recent studies have addressed the removal of inorganic N within river networks (Helton et al., 2011; Wollheim et al., 2006; Worrall et al., 2012; Alexander et al., 2009), little has been done to investigate these processes in boreal landscapes subject to widespread and active forest management. A clearer understanding of how the enrichment of headwater environments through forestry is expressed at larger spatial scales (Futter et al., 2010) is important if policy makers are to consider the broader biogeochemical implications of forest management.
The degree to which surplus NO
Where forest harvests extend to channel margins, or when retention of
NO
In this paper we explore the potential for fluvial networks to remove
NO
The Balsjö
paired catchment experiment including the catchments RS-3, CC-4, NO-5, and
NR-7, as well as the two downstream sites BA-2 and BA-1 that integrate the
larger 22.9 km
This study was performed in the Balsjö paired catchment experiment
located in the boreal forest of northern Sweden (64
The Balsjö catchment is underlain by highly compacted till layers that have generally low hydraulic conductivities. Runoff generation is thus primarily from shallow saturated soil water entering streams laterally (Bishop et al., 2004; Schelker et al., 2013a). Thus, and in contrast to other stream systems, contributions from deep groundwater sources are thought to be minor at the spatial scale of this third-order stream network (Schelker et al., 2014).
Concentrations of NO
We used a mixing model to represent the landscape mass balance for
NO
The concentration at the downstream locations BA-1 and BA-2
(
A 100 % harvested catchment did not exist in Balsjö and N leakage
into first-order streams following clear-cutting may vary depending on local
factors, such as the presence of riparian forest buffers (Laurén et al.,
2005), and was also observed to differ between the two harvested sites in
Balsjö (Löfgren et al., 2009). Thus, we calculated
Stream discharge (
The definitions of
Comparison of modeled and measured Cl and Si concentrations for
BA-1
Inorganic nitrogen removal efficiency (
Catchment characteristics of the six nested Balsjö catchments.
Annual export of DIN and NO
To evaluate whether in-stream processes could be responsible for the modeled
removal of N in the landscape, we calculated net areal uptakes rates (
Statistical analysis of differences in measured concentrations before and
after clear-cutting in the same stream, as well as between sampling sites
were performed as two sample Student's
Forest harvesting increased DIN mobilization into first-order streams.
Average concentrations of NO
Measured and modeled annual DIN loads per unit catchment area from
all six Balsjö catchments during 2008–2011. The percentage of
NO
At the BA-1 downstream site, NO
When modeled concentrations of DON and DIN at BA-1 and BA-2 were compared to
the measured concentrations, distinct patterns emerged. First, modeled and
measured DON concentrations correlated well (relationships:
Modeled DIN removal efficiency calculated as the fraction of DIN that was
retained in the system showed a strong seasonal signal (Fig. 5a).
Increases in DIN export in response to forest harvesting are well documented
(Jerabkova et al., 2011) and illustrate how terrestrial ecosystem disturbance
can control N mobilization and delivery to small streams. In this study,
increases in stream water NO
At both downstream sites, and the CC-4 clear-cut catchment, concentrations of
NO
Results of the mass-balance modeling approach for DON (left) and DIN (right) for the downstream site BA-1.
Temporal variation in NO
We found a close correspondence between modeled and measured DON concentrations, similar to relationships previously observed for dissolved organic carbon (Schelker et al., 2014), as well as the two conservative tracers, dissolved silica and chloride (Fig. 2). This close relationship between observed and predicted concentrations is indicative of an approximately conservative downstream transport of DON in the network. These patterns provide additional support for the applicability of our mixing model in this landscape, and they are consistent with the idea that bulk DON is composed primarily of organic compounds of low bioavailability that are exported from landscapes without strong biotic controls (Hedin et al., 1995). For this reason, DON also often represents the major loss vector for N in catchments that are not subject to large anthropogenic inputs of DIN (Perakis, 2002; Kortelainen et al., 1997). Importantly, DON exports at CC-4 also increased following harvesting (Fig. 3d), a response that has been reported elsewhere in Scandinavia (Smolander et al., 2001). While this response was more subtle than that observed for DIN, the conservative behavior of DON in the stream network suggests that it likely represents an important and largely unappreciated source of terrestrially derived N to downstream receiving systems (Rosén et al., 1996).
In contrast to DON, we observed generally poor relationships between measured
and modeled DIN concentrations at BA-1 and BA-2 (Fig. 4; data for BA-2 not
shown). This mismatch most likely results from seasonal NO
Boxplot of the seasonal differences in net NO
Interestingly,
Our estimates of net DIN removal within this stream network suggest that,
during most periods, reasonable levels of in-stream activity (i.e., net
uptake) could account for the discrepancy between measured and modeled fluxes
at downstream stations. Assuming that all DIN retention was occurring within
the stream channels, median values and interquartile ranges (25th to 75th
percentile) of
As with
Important mechanisms that control DIN removal from stream water during the
growing season are biological uptake by riparian vegetation (Sabater et al.,
2000) and immobilization by in-stream autotrophs and heterotrophs. These
in-stream sinks may also change in response to forest harvesting, for
example, if elevated light conditions foster increased photoautotrophic
production (Bernhardt and Likens, 2004). Indication that such increased
in-stream DIN demand during the growing season may also be present in the
Balsjö stream network is given by
An additional process that may account for the permanent removal of
NO
Transferring this well-established process knowledge from the reach scale to
the network scale suggests that NO
In summary our work agrees with earlier studies in that terrestrial ecosystem
disturbance enhances DIN mobilization into first-order streams (Likens et
al., 1970) and that such increased NO
Funding for this work was provided by the Swedish Environmental Protection Agency, EU Life (Forest for Water), CMF, Future Forests, and the Formas (ForWater). We thank Peder Blomkvist, Viktor Sjöblom, and Ida Taberman for help in the field and the laboratory. Edited by: T. J. Battin