A critical review of protocols for moss biomonitoring of atmospheric deposition: sampling and sample preparation.

Currently, the most important guideline for the application of the moss technique to monitor the atmospheric deposition of heavy metals is the "Heavy metals, nitrogen and POPs in European mosses: 2015 survey" published by the UNECE ICP Vegetation. Two main problems have been identified with this guideline: i) some of the recommendations regarding the methodological aspects involved in the application of the moss technique are not based on scientific criteria; and, ii) some recommendations in the manual are very vague and some aspects are even left out (e.g., elevation, distance to the coast). As a result there exists a high variability in the application of the protocol and many scientists adapt it to the specific conditions in the studied areas without evaluating how changes affect the results obtained. Therefore, in this article a total of 369 studies were reviewed including both methodological and application studies of the passive biomonitoring of the atmospheric deposition of heavy metals with terrestrial mosses. The results of this review have shown on the one hand, that none of the articles completely accomplished the ICP-Vegetation protocol suggestions, either because the information regarding some aspects was lacking or simply because the authors did not follow the manual suggestions. On the other hand, it was found that the results of methodological studies sometimes contradicted the ICP Vegetation manual recommendations. Thus, a new protocol in which each suggestion has been carefully and rigorously contrasted with the available literature has been proposed in this paper. In addition, practical and economic issues have also been considered and much more concise suggestions have been proposed which would facilitate its fulfilment in a more objective way.