The effect of long-term nitrogen (N) and sulfur (S) deposition on litter mass loss and changes in carbon (C), N, and S composition and enzyme activities during litter decomposition was investigated in a boreal forest. This study included four N × S treatments: control (CK), N application (30 kg N ha−1 yr−1), S application (30 kg S ha−1 yr−1), and N plus S application (both at 30 kg ha−1 yr−1). Two experiments were conducted for 22 months: 1) a common litter decomposition experiment with litter bags containing a common litter (same litter chemistry) and 2) an in-situ litter decomposition experiment with litter from each treatment plot (and thus having different litter chemistry). Litterbags were placed onto the four treatment plots to investigate the direct effect of N and S addition and the combined effect of N and/or S addition and litter chemistry on litter decomposition, respectively. Regardless of the source of litter, N and/or S addition affected C, N and S composition at a certain period of the experiment but did not affect litter mass loss and enzyme activity throughout the experiment, indicating that the N and S addition rates were below the critical level required to affect C and N cycling in the studied ecosystem. However, the greater change in N composition per unit of litter mass loss in the N addition treatment than in the other treatments in the common litter but not in the in-situ litter experiment, suggests that the effect of N addition on N loss and retention depends on the initial litter chemistry. We conclude that the studied N and S addition rates did not affect litter decomposition and elemental cycling in the studied forest ecosystem even though the N and S addition rates were much greater than their ambient deposition rates.

Global nitrogen (N) deposition has been enhanced with anthropogenic N emissions, and its impacts on mosses are receiving more and more attention. This study investigates how N deposition influence the biomass and stoichiometry of moss Rhytidium rugosum, using a 3-year N enrichment experiment with 0, 2, 5 and 10 g N m−2 yr−1 in a boreal forest in Northeast China. Low N additions caused an N redundancy and moderate to high N additions resulted in a biomass loss. N additions reduced biomass ratios of green to brown tissues and increased N and phosphorus (P) contents, suggesting changes in photosynthetic capacity and litter decomposition. Biomass N pools showed a unimodal response to the N additions, and P pools decreased under moderate and high N additions. Our findings indicate significant stoichiometric and biomass changes caused by N deposition may lead to a substantial carbon and nutrient loss in boreal moss carpets.

Nitrogen (N) deposition and biological N fixation (BNF) are main external N inputs into terrestrial ecosystems. However, few studies have simultaneously quantified the contribution of these two external N inputs to global NPP and consequent C sequestration. Based on literature analysis, we estimated new net primary production (NPP) due to external N inputs from BNF and N deposition and the consequent C sinks in global boreal, temperate and tropical forest biomes via a stoichiometric scaling approach. Nitrogen-induced new NPP is estimated to be 3.48 Pg C yr⁻¹ in global established forests and contributes to a C sink of 1.83 Pg C yr⁻¹. More specifically, the aboveground and belowground new NPP are estimated to be 2.36 and 1.12 Pg C yr⁻¹, while the external N-induced C sinks in wood and soil are estimated to be 1.51 and 0.32 Pg C yr⁻¹, respectively. BNF contributes to a major proportion of N-induced new NPP (3.07 Pg C yr⁻¹) in global forest, and accounts for a C sink of 1.58 Pg C yr⁻¹. Compared with BNF, N deposition only makes a minor contribution to new NPP (0.41 Pg C yr⁻¹) and C sinks (0.25 Pg C yr⁻¹) in global forests. At the biome scale, rates of N-induced new NPP and C sink show an increase from boreal forest towards tropical forest, as mainly driven by an increase of BNF. In contrast, N deposition leads to a larger C sink in temperate forest (0.11 Pg C yr⁻¹) than boreal (0.06 Pg C yr⁻¹) and tropical forest (0.08 Pg C yr⁻¹). Our estimate of total C sink due to N-induced new NPP approximately matches an independent assessment of total C sink in global established forests, suggesting that external N inputs by BNF and atmospheric deposition are key drivers of C sinks in global forests.

The production and use of pentachlorophenol (PCP) was recently prohibited/restricted by the Stockholm Convention on persistent organic pollutants (POPs), but environmental data are few and of varying quality. We here present the first extensive dataset of the continent-wide (Eurasia and Canada) occurrence of PCP and its methylation product pentachloroanisole (PCA) in the environment, specifically in pine needles. The highest concentrations of PCP were found close to expected point sources, while PCA chiefly shows a northern and/or coastal distribution not correlating with PCP distribution. Although long-range transport and environmental methylation of PCP or formation from other precursors cannot be excluded, the distribution patterns suggest that such processes may not be the only source of PCA to remote regions and unknown sources should be sought. We suggest that natural sources, e.g., chlorination of organic matter in Boreal forest soils enhanced by chloride deposition from marine sources, should be investigated as a possible partial explanation of the observed distributions. The results show that neither PCA nor total PCP (ΣPCP = PCP + PCA) should be used to approximate the concentrations of PCP; PCP and PCA must be determined and quantified separately to understand their occurrence and fate in the environment. The background work shows that the accumulation of airborne POPs in plants is a complex process. The variations in life cycles and physiological adaptations have to be taken into account when using plants to evaluate the concentrations of POPs in remote areas.

The interaction of atmospheric sulphur (S) was investigated within the canopies of two boreal forests in Québec, Canada. The net canopy exchange approach, i.e. the difference between S–SO₄ in throughfall and precipitation, suggests high proportion of dry deposition in winter (up to 53%) as compared to summer (1–9%). However, a 3.5‰ decrease in δ¹⁸O–SO₄ throughfall in summer compared to incident precipitation points towards a much larger proportion of dry deposition during the warm season. We suggest that a significant fraction of dry deposition (about 1.2 kg ha⁻¹ yr⁻¹, representing 30–40% of annual wet S deposition) which contributed to the decreased δ¹⁸O–SO₄ in throughfall was taken up by the canopy. Overall, these results showed that, contrary to what is commonly considered, S interchanges in the canopy could be important in boreal forests with low absolute atmospheric S depositions.

In the Athabasca oil sands region of northern Alberta, mining companies are evaluating reclamation using constructed wetlands for integration of tailings. From May to July 2008, reproductive performance of 40 breeding pairs of tree swallows (Tachycineta bicolor), plus growth and survival of nestlings, was measured on three reclaimed wetlands on two oil sands leases. A subset of nestlings was examined for i) feather corticosterone levels, ii) delayed-type hypersensitivity response, and iii) innate immune function. Nestlings on one of two wetlands created with oil sands process affected material (OSPM) were heavier and had greater wing-lengths, and mounted a stronger delayed-type hypersensitivity response compared those on the reference wetland. Corticosterone was significantly higher in male nestlings on one of two OSPM-containing wetland compared to the reference wetland. Body condition of 12-day-old female nestlings was inversely related to feather corticosterone. Under ideal weather conditions, reclaimed wetlands can support healthy populations of aerially-insectivorous birds. Under ideal weather conditions, tree swallow nestlings on reclaimed OSPM-affected wetlands are in good body condition and mount strong cell-mediated immune responses.

Elevated levels of selenium (Se) have been detected in wintering and spring-staging lesser scaup. Here, we compared spring scaup Se and mercury (Hg) levels to those of ring-necked ducks and white-winged scoters, species exhibiting increasing and decreasing boreal populations, respectively. Mercury concentrations were low in all three species. Geometric mean (95%CI) liver Se concentrations were 6.2 (5.5-7.0), 4.6 (4.0-5.4), and 32.6 (28.4-37.3) mg/kg dry weight (dw) in scaup, ringnecks and scoters, respectively. Only scoter livers (66%) were above 33 mg/kg dw Se. Scaup and ringneck Se levels were unrelated to breeding status or lipid and protein levels; breeding scoters and females with greater lipid mass had higher Se than non-breeders. Egg and follicle concentrations in scaup and scoters were normal (mean [95%CI] = 2.3 [1.9-2.6] and 2.4 [2.1-2.7] mg/kg dw, respectively). Overall, we found no support for a relationship between selenium and boreal scaup and scoter declines, and discuss current Se threshold concentrations. Selenium is not affecting boreal scaup, ring-necked duck, or scoter body condition or reproduction.

Polycyclic aromatic hydrocarbons (PAHs) are formed by the incomplete combustion of fossil fuels and forest or biomass burning. PAHs undergo long-range atmospheric transport, as evidenced by in situ observations across the Arctic. However, monitored atmospheric concentrations of PAHs indicate that ambient PAH levels in the Arctic do not follow the declining trend of worldwide anthropogenic PAH emissions since the 2000s, suggesting missing sources of PAHs in the Arctic or other places across the Northern Hemisphere. To trace origins and causes for the increasing trend of PAHs in the Arctic, the present study reconstructed PAH emissions from forest fires in the northern boreal forest derived by combining forest carbon stocks and MODIS burned area. We examined the statistical relationships of forest biomass, MODIS burned area, emission factors, and combustion efficiency with different PAH congeners. These relationships were then employed to construct PAH emission inventories from forest biomass burning. We show that for some PAH congeners, for example, benzo[a]pyrene (BaP)—the forest-fire-induced air emissions are almost one order of magnitude higher than previous emission inventories in the Arctic. A global-scale atmospheric chemistry model, GEOS-Chem, was used to simulate air concentrations of BaP, a representative PAH congener primarily emitted from biomass burning, and to quantify the response of BaP to wildfires in the northern boreal forest. The results showed that BaP emissions from wildfires across the northern boreal forest region played a significant role in the contamination and interannual fluctuations of BaP in Arctic air. A source-tagging technique was applied in tracking the origins of BaP pollution from different northern boreal forest regions. We also show that the response of BaP pollution at different Arctic monitoring sites depends on the intensity of human activities.

The boreal forest’s pulp and paper industry plays a major role in economic prosperity but, historically, caused an environmental burden. Remnants of discharges of contaminated suspended solids (fiberbanks) are continuously being discovered on the beds of shallow seas, rivers and lakes in the northern hemisphere. We investigated the dispersion of Cd, Cr, Cu, Hg, Ni, Pb and Zn from deeper to surficial layers in fiberbanks in a Swedish estuary and the larger-scale transport of the same metals to distal areas of sediment accumulation. We also tested the C:N ratio as a common denominator for these anthropogenic, cellulose-rich deposits. Sampling and analyses of three fiberbanks located in the inner part of the estuary and from sediment accumulation sites outside and along the estuary reveals that metal concentrations are regressing to background levels towards the surface at the accumulation sites. The fiberbanks show a higher degree of contamination and C:N ratios demonstrate inclusion of cellulose fibers. C:N ratios also indicate that there is currently no significant transport of fiberbank material into the distal areas. A ∼10 cm natural cap of recently settled fine-grained sediment covering one of the fiberbanks seems to prevent metals dispersing into overlying water whereas the other two fiberbanks show signs of metal enrichment and potential mercury methylation in surficial layers. Although the estuarine system seems to recover from the impact of industrial waste, there is no evidence that the fiberbanks will be remediated naturally but instead will continue to threaten the aquatic environment.