Contrasting spatial patterns of mercury and nitrogen in Mediterranean moss biomonitors: a multi-proxy survey across Tuscany (Italy)

Terrestrial mosses are widely used to monitor atmospheric deposition. However, concurrent assessment of mercury (Hg) and nitrogen (N), two pollutants characterized by different spatial scales of influence, from global to local, remains limited. To address this gap, we assessed Hg and stable isotopes (δ15N, δ13C) in pleurocarpous moss at 41 sites in Tuscany, central Italy. Concentrations of Hg ranged from 32 to 236 ng g−1 (median 64 ng g−1) with a regional upper baseline of 95 ng g−1 (median + 2×MAD). Concentrations of Hg tended to decrease with distance from the Mt. Amiata cinnabar district (ρ = −0.674, p < 0.001) but showed no correlation with elevation, moss N content, or δ15N. In contrast, moss δ15N (−4.16 ± 1.17‰) decreased significantly with altitude (ρ = −0.369, p = 0.017), consistent with attenuation of the lowland agricultural N-related signature at higher elevations. δ13C values (−30.77 ± 1.20‰) increased with elevation (ρ = +0.359, p = 0.021), suggesting that C isotope discrimination in mosses may provide complementary information on site water-stress conditions. MixSIAR modelling indicated that atmospherically processed N (traffic NOx and background wet deposition, considered jointly) accounted for almost the entire moss N budget, whereas the modelled contribution of direct agricultural NH3 was negligible (<1%). The δ15N signatures of these two processed pools are too similar to allow a reliable separation, so they are reported only as a combined term; this overall pattern remained stable across multiple sensitivity scenarios. Overall, the results of our study highlight the limitations of single-pollutant surveys in areas characterized by complex patterns of emissions and support the use of multiple proxies in biomonitoring.