Accumulation of Atmospheric Metals and Nitrogen Deposition in Mosses: Temporal Development between 1990 and 2020, Comparison with Emission Data and Tree Canopy Drip Effects

Mosses are suitable for recording the bioaccumulation of atmospheric deposition over large areas at many sites. In Europe, such monitoring has been carried out every five years since 1990. Mosses have been collected and chemically analysed for metals (since 1990), nitrogen (since 2005), persistent organic pollutants (since 2010) and microplastics (2020). The aims of this study were the following: (1) to analyse the temporal trends of metal and nitrogen accumulation in mosses between 1990 or 2005, respectively, and 2020 in Germany; (2) to compare the accumulation trends with emission data; and (3) to determine the effect of tree canopy drip on metal and nitrogen accumulation in mosses. For the temporal trend analysis, the minimum sample number required for a reliable estimation of arithmetic mean values and statistical parameters based on it was calculated. It was only achieved for nitrogen, but not for metals. Therefore, the temporal trends of the bioaccumulation of metals and nitrogen were calculated on the basis of median values. For the analysis of tree canopy effects on element accumulation in mosses, 14 vegetation structure measures were used, which together with 80 other descriptors characterise each moss collection site and its environment. The comparison of the data obtained during the first monitoring campaign with those of the 2020 survey showed a significant decrease in metal bioaccumulation. However, in contrast to the emission data, an increase in the accumulation of some metals was observed between 2000 and 2005 and of all metals from 2015 to 2020. Trends in Germany-wide nitrogen medians over the last three campaigns (2005, 2015 and 2020) show that nitrogen medians decreased by −2% between 2005 and 2015 and increased by +8% between 2015 and 2020. These differences are not significant and do not match the emission trends. Inferential statistics confirmed significantly higher metals and nitrogen accumulation in mosses collected under tree canopies compared to adjacent open areas. Measured concentrations of metals and nitrogen were significantly higher under tree canopies than outside of them, by 18–150%.