Is Moss Still a Reliable Biomonitor of Nitrogen and Sulfur Deposition After Decades of Emissions Reductions?

Mosses are widely used as biomonitors of atmospheric nitrogen (N) and sulfur (S) deposition due to their broad distribution, ease of sampling, and capacity to trap and accumulate atmospheric particles. However, since 2000, S emissions have decreased by more than 80% across Europe, and N oxides by 40-50%. This study evaluated whether moss remains effective for monitoring atmospheric N and S deposition after decades of emission reductions. This assessment was conducted at 33 rural sites in Tuscany, Central Italy, a region characterized by relatively low levels of N and S deposition. The content of N and S in moss were compared with the air concentrations (gases and particles) and wet and dry deposition of N and S from the Cooperative Programme for Monitoring and Evaluation of Long-range Transmission of Air Pollutants in Europe (EMEP) model and an air pollution index derived from trace element concentrations. The average N content of moss (1.15 ± 0.42%) was an order of magnitude greater than that of S (0.11 ± 0.02%), reflecting the dominance of N deposition. Nevertheless, N and S in moss were strongly correlated (r_s = 0.55), suggesting shared sources. Further, N showed a strong correlation with the dry deposition of oxidized N (r_s = 0.53), while S was strongly correlated with the wet deposition of S oxides (r_s = 0.53) and magnetic susceptibility (r_s = 0.69). Overall, our findings confirm that mosses remain effective biomonitors of N and S deposition and can provide reliable spatial and temporal monitoring, especially as the traditional monitoring networks decline.