Long-term exposure to primary air pollutants, such as sulphur dioxide (SO2) and nitrogen oxides (NOx), alters the structure and functions of forest ecosystems. Many biochemical and biogeochemical processes discriminate against the heavier isotopes in a mixture; thus, the values of delta C-13 and delta N-15 (i.e. the ratio of stable isotopes C-13 to C-12 and that of (15) N to (14) N, respectively) may give insights into changes in ecosystem processes and identify the immediate drivers of these changes. We studied sources of variation in the delta C-13 and delta N-15 values in the foliage of eight boreal forest C3 plants at 10 sites located at the distance of 1-40 km from the Monchegorsk nickel-copper smelter in Russia. From 1939-2019, this smelter emitted over 14,000,000 metric tons (t) of SO2, 250,000 t of metals, primarily nickel and copper, and 140,000 t of NOx. The delta C-13 value in evergreen plants and the delta N-15 value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar delta C-13 values of evergreen species mainly to direct effects of SO2 on stomatal conductance, in combination with pollution-related water stress, which jointly override the potential opposite effect of increasing ambient CO2 concentration on delta C-13 values. Stomatal uptake of NOx and root uptake of N-15-enriched organic N compounds and NH4+ may explain the increased foliar delta N-15 values and elevated foliar N concentrations, especially in the evergreen trees (Pinus sylvestris), close to Monchegorsk, where the soil inorganic N supply is reduced due to the impact of long-term SO2 and heavy metal emissions on plant biomass. We conclude that, despite the uncertainties in interpreting delta C-13 and delta N-15 responses to pollution, the Monchegorsk smelter has imposed and still imposes a great impact on C and N cycling in the surrounding N-limited subarctic forest ecosystems.

Background The present-day demand for digital availability of distributional data in biodiversity studies requires a special effort in assembling and editing the data otherwise scattered in paper literature and herbarium collections, which can be poorly accessible or little understood to present-day users and especially automatic data processors. Our project on developing the information resource for the vascular plant flora of Murmansk Region, Russia, includes processing and making digitally available all the data on the taxonomy and distribution of this flora. So far, published distribution maps are limited to the old set in the Flora of Murmansk Region (published in 1953-1966) and the Red Data Book of Murmansk Region (ed. 2, published in 2014). These publications did not take into account the main part of the herbarium collections kept at the Kandalaksha Strict Nature Reserve, which are the basis for numerous local publications that appear scattered and, therefore, little accessible nowadays. New information We present a complete dataset of all holdings of vascular plants in the Herbarium of the Kandalaksha Strict Nature Reserve, totalling 10,218 specimens collected during 1947-2019, which are referable to 764 species and 19 subspecies. All specimens were georeferenced with the utmost precision available. This dataset offers a complete and dense coverage of the Nature Reserve's territory (islands and adjacent mainland coastal areas of the Barents and White Seas, Murmansk Region and Republic of Karelia, Russia); these data are little represented in herbarium collections elsewhere.