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

Abstract

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 (δ15N) are used as indicators of atmospheric N deposition. Here, we used a single moss species (Hylocomium 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 δ15N) 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, δ15N was significantly and positively correlated with altitude, with high values on the western alpine slopes. When using δ15N 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.