Abstract
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 δ13C and δ15N (i.e. the ratio of stable isotopes 13C to 12C 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 δ13C and δ15N 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 δ13C value in evergreen plants and the δ15N value in all plants increased near the smelter independently of the plant mycorrhizal type. We attribute the pollution-related increase in the foliar δ13C 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 δ13C values. Stomatal uptake of NOx and root uptake of 15N-enriched organic N compounds and NH4+ may explain the increased foliar δ15N 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 δ13C and δ15N 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.
Highlights
IntroductionThese observations justify the need for further studies on the effects of chronic exposure to air pollutants on ecosystem processes, on the biogeochemical cycles of carbon (C) and N in terrestrial ecosystems, and on the plant physiology
Decreases in the global emissions of sulphur dioxide (SO2) and nitrogen oxides (NOx) from energy production and traffic have occurred rather slowly (Huang et al 2017; Zhong et al 2020), and the dry and wet depositions of sulphur (S) and nitrogen (N) remain high over large areas (Tan et al.Communicated by Gangrong Shi.2018)
We found that the foliar N concentration increased with increasing pollution load in conifers, whereas it decreased especially in shrubs (Empetrum nigrum ssp. hermaphroditum (Hagerup) Böcher, Vaccinium myrtillus L., Vaccinium uliginosum L., Vaccinium vitis-idaea L.)
Summary
These observations justify the need for further studies on the effects of chronic exposure to air pollutants on ecosystem processes, on the biogeochemical cycles of carbon (C) and N in terrestrial ecosystems, and on the plant physiology. The intimate mechanisms behind pollution-induced changes in ecosystem processes can be deciphered by studying the values of δ13C and δ15N (i.e. the ratios of the stable isotopes 13C to 12C and 15 N to 14 N, respectively) in tree foliage, rings and roots (Gebauer and Schulze 1991; Korontzi et al 2000; Wagner and Wagner 2006; Savard 2010; Pardo et al 2006; Savard et al 2021). Polluted areas, which still persist in many countries, can be considered opportunistic macrocosms
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