Long-range pollutants that are transported across national borders are termed ‘transboundary pollutants.’ These pollutants are transported long distances from their emission sources and threaten many of Earth’s ecosystems. In particular, transboundary pollutants can become trapped in high mountains owing to low temperature that support dry deposition, high precipitation and fogs that increase wet deposition, and high organic content in soil that can retain large amounts of pollutants (Belis et al. 2009 ; Brahney et al 2015; Daly and Wania 2005; Camarero 2017). Atmospheric pollutants impact tropic status and species interactions, and cause loss of biodiversity in mountain ecosystems. For example, increased inputs of atmospheric phosphorous affect tropic status in alpine lakes as nutrient availability may be tightly coupled with the deposition of these pollutants (Brahney et al 2015). Nitrogen pollutants impact species interactions by providing a competitive advantage to nitrophytic species over nitrogen sensitive species (Sala et al 2000; Bobbink et al 2010). The influence of nitrogen pollutants is particularly severe in nitrogen-depleted ecosystems such as alpine ecosystems (Bobbink et al 2010). Hazardous pollutants transported to mountains are bio-magnified through food chains (Mazzoni et al 2020), being a driver of ecosystem degradation.
The impacts of transboundary pollutants on mountain ecosystems are of great concern in East Asia because of the rapidly increasing emissions of atmospheric pollutants (Wang et al. 2014). Such pollutants are transported to Japan from mainland Asia via westerly winds (Var et al. 2000; Aikawa et al. 2010). The mechanism of these pollutants reaching mountainous areas has been examined based on analyses of aerosols (Suzuki et al. 2008), rain and fog (Uehara et al. 2015), snow (Yamamoto et al. 2018), lake sediments (Hosono et al. 2016), and alpine vegetation (Oishi 2018, 2019). Notably, transboundary pollutants are significantly associated with forest decline in Japan, having harmful effects on trees by injuring plant tissues, inhibiting the growth, and accelerating senescence (Takahashi et al. 2020). Despite these risks, pollutant monitoring in Japan’s mountainous areas remains limited owing to the difficulties of installing measuring equipment in mountainous terrane. Therefore, simple monitoring techniques are urgently required.
Content of trace metals in plants are indicative for evaluation of levels of atmospheric pollution. Several types of plant groups have been used for biomonitoring, among which, mosses have been identified as good indicators for atmospheric pollutants including transboundary sources (Oishi 2018). Biomonitoring using moss (hereafter ‘moss biomonitoring’) is based on the recognition that mosses efficiently take up pollutants from atmospheric deposition and, thus, the pollutant content of moss reflects atmospheric deposition (Harmens et al. 2015). Thus, moss biomonitoring can be utilized as an effective indicator for ecosystem health given that mosses play important ecological roles and are sensitive to environmental changes (Frego 2007). Because of these characteristics, moss biomonitoring has been increasingly applied in Japan’s mountainous areas in recent years, although the detection of transboundary pollutants varies according to pollutant concentrations and the efficiency of their uptake by mosses (Oishi 2019; Oishi et al. 2021). Therefore, further investigation of the application of moss biomonitoring for studying the influence of transboundary pollutants in this region is required.
Several indices, including Pb isotope ratio, Pb to Zn (Pb/Zn) ratios, and As to V (As/V) ratios (Hioki et al. 2008; Mukai et al. 1994; Yonemochi et al. 2018), have been applied for monitoring transboundary pollutants in Japan. Lead has four stable isotopes (204Pb, 206Pb, 207Pb, and 208Pb), the ratios of which are specific to source regions and do not change during industrial and environmental processes (Cheng and Hu 2010). Taking advantage of these characteristics, Pb isotope ratios have been successfully applied to detect transboundary pollutants in Japan (Mukai et al. 1999; Hosono et al. 2016). In comparison, Pb/Zn and As/V ratios are strongly linked to coal combustion, which is a major energy source in northern East Asia (Hayakawa 2009; Zhang et al. 2017). For example, concentrations of chalcophile elements, such as Pb and As, are elevated in the fly ash produced during coal combustion (Coles et al. 1979). Consequently, air masses transported from mainland Asia exhibit higher Pb/Zn and As/V ratios relative to those from within Japan (Mukai et al. 1994; Hioki et al. 2008; Yonemochi et al. 2018).
Based on these differences, trace metal indices (Pb isotope ratios and Pb/Zn and As/V ratios) offer great potential for moss biomonitoring of transboundary pollutants; however, the applicability of Pb/Zn and As/V ratios has not yet been tested in this context. Furthermore, existing research indicates that moss biomonitoring based on Pb isotope ratios may not be able to identify transboundary pollutants in Japan’s mountains (Oishi et al. 2021), despite its usefulness having been reported elsewhere (Farmer et al. 2002; Xiang et al. 2017; Schnyder et al. 2018, Sucharová et al. 2014).
This study examines the usefulness and limitations of various trace metal indices for moss biomonitoring of transboundary pollutants in Japan’s mountainous regions. Given that the concentrations of transboundary pollutants decrease with distance from mainland Asia (Toyama et al. 2013), a general hypothesis was applied assuming that the trace metal indices of moss will also change with distance. By exploring these relationships, this work contributes to the broader understanding of the applicability of moss biomonitoring in mountain ecosystems and supports the more widespread use of moss biomonitoring in East Asia as a relatively simple and useful technique.