Foliar stable isotope ratios of carbon and nitrogen in boreal forest plants exposed to long-term pollution from the nickel-copper smelter at Monchegorsk, Russia.

Long-term exposure to primary air pollutants, such as sulphur dioxide (SO 2 ) and nitrogen oxides (NO x ), 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 δ 13 C and δ 15 N (i.e. the ratio of stable isotopes 13 C to 12 C 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 δ 13 C and δ 15 N 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 SO 2 , 250,000 t of metals, primarily nickel and copper, and 140,000 t of NO x . The δ 13 C value in evergreen plants and the δ 15 N value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar δ 13 C values of evergreen species mainly to direct effects of SO 2 on stomatal conductance, in combination with pollution-related water stress, which jointly override the potential opposite effect of increasing ambient CO 2 concentration on δ 13 C values. Stomatal uptake of NO x and root uptake of 15 N-enriched organic N compounds and NH 4 + may explain the increased foliar δ 15 N 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 SO 2 and heavy metal emissions on plant biomass. We conclude that, despite the uncertainties in interpreting δ 13 C and δ 15 N 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.