Moss as an indicator of transboundary atmospheric nitrogen pollution in an alpine ecosystem

Increased atmospheric nitrogen (N) pollution affects N deposition in alpine ecosystems. Moss often dominates these ecosystems and their N content (%N) and stable isotope ratio (delta N-15) are used as indicators of atmospheric N deposition. Here, we used a single moss species (Hylocomiwn splendens) as a bioindicator of atmospheric N deposition in the mountainous areas of Japan. The moss was collected from 38 sites on all slope aspects (east, west, south, and north) ranging from 1800 to 2800 m in altitude. The correlation between moss N variables (%N and delta N-15) and environmental conditions (altitude and slope aspects) was analyzed using linear models based on a hierarchical Bayesian framework. In the constructed models, %N showed a significant or weak negative correlation with altitude on all slope aspects. In contrast, delta N-15 was significantly and positively correlated with altitude, with high values on the western alpine slopes. When using delta N-15 as a marker of N sources, N in alpine moss on the west slope seemed to be influenced by transboundary N pollutants through particulate matter (PM), which is brought to Japan from mainland Asia by prevailing westerly winds and northwest monsoons. However, as moss N is also affected by other factors, such as different N chemical forms, fixed N from cyanobacteria, and seasonal differences in atmospheric N, further research should directly compare moss N with atmospheric N deposition to elucidate the influence of transboundary N pollutants on moss N.