Facing challenges in the monitoring of atmospheric nitrogen pollution: relevance of organic nitrogen deposition and cutting edge applications of mosses diagnostic bioindicators

Over the last century, the use of synthetic fertilizers and the fossil fuel combustion to satisfy the increasing demand for food and energy of a growing population has resulted in a massive alteration of the nitrogen (N) cycle. The main objective of this thesis was to investigate atmospheric N deposition through the use of different monitoring approaches, both physical and biomonitor-based ones, paying special attention to those issues that currently remain poorly understood. The first work was focused on the study of the nitrogenous organic fraction of rain samples. Both canopy throughfall and bulk deposition samples from four holm oak forests of the Iberian Peninsula were analyzed for their dissolved organic nitrogen (DON) content. The results showed that the contribution of the organic fraction to the total N budgets ranged in BD from 34% to 56%, and in TF from 38% to 72%. Data also revealed that agricultural activities and traffic-related pollutants generated in metropolitan areas may play an important role as potential sources of organic nitrogenous compounds. Finally, it was found that DON might be an additional nutrient for this forests type (canopy uptake). In the other chapters we studied N atmospheric pollution by using mosses as biomonitors. Firstly, we evaluated the feasibility of Pleurochaete squarrosa (Brid.) Lindb. (PS) as an alternative species for heavy metals and N deposition surveys in a Mediterranean area of southern Europe. To that end we compared the response of this species to that of Hypnum cupressiforme Hedw. (HC), a commonly used and validated species in biomonitoring surveys. Both bryophytes showed similar spatial distribution patterns for N. Similarly, the two species discriminated sites both highly and scarcely affected by trace element deposition. Regarding O15N, PS gave a more congruent picture with the location of the main sources of N emissions in the area. All these data suggested that PS may be a suitable biomonitor in Mediterranean areas. Besides, bryophytes are among the most vulnerable organisms to N pollution. Thus, in our third study we evaluated the physiological response of Hypnum cupressiforme to an NH3 concentration gradient from a comprehensive and temporal perspective, to better understand the mechanisms implied when mosses cope with this pollutant. We aimed at identifying which variables are the most responsive, and therefore, which ones are the most promising for the use of HC in ecosystem surveys as early warning indicators of NH3 toxicity. The results showed that N accumulation and NH3-induced oxidative stress were the most important drivers of the physiological functioning of HC along the gradient. Moreover, the sampling season was determinant in the responsiveness of certain physiological variables. Finally, in the last research work we investigated the use of isotopic ratios to provide supplementary information about the nature of the atmospheric pollutants in biomonitoring surveys carried out at the European scale. To that end, more than 1300 samples of mosses from 15 countries, all of them participants of the ICP-Vegetation programme 2005-2006, were analyzed for their C and N content and O15N and O13C. The preliminary evaluation of these data suggested that additional measurements of C and N stable isotopes in mosses might be a useful tool in European surveys, not only in providing valuable information for identifying likely pollution sources, but also as biological markers of key environmental processes.