Contrasting spatial patterns of mercury and nitrogen in Mediterranean moss biomonitors: a multi-proxy survey across...

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 NO_x 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.