A pilot study in Scandinavian countries had shown that mosses can potentially be used to monitor atmospheric nitrogen deposition. Therefore, in the European moss survey 2005/2006 the majority of participants determined the total nitrogen concentration in mosses in order to establish whether mosses can be used as biomonitors of atmospheric nitrogen pollution at the European scale. The lowest nitrogen concentrations in mosses were observed in northern Finland and northern parts of the UK, the highest concentrations were found in central and eastern Europe. The spatial distribution of total the nitrogen concentration in mosses was similar to that of the total nitrogen deposition modelled by EMEP for 2004, except that the nitrogen deposition tended to be relatively lower in Eastern Europe. The total nitrogen concentration in mosses is moderately correlated with modelled atmospheric deposition and concentrations in air or precipitation of various nitrogen forms. Multivariate analysis identified the concentration of reduced nitrogen in air to be statistically the most significant factor, followed by the concentration of oxidised nitrogen in air (see annual report 2008/9)
Mean total nitrogen concentration in mosses per 50 x 50 km2 EMEP grid square in 2005
However, the apparent asymptotic relationship between total nitrogen concentration in mosses and modelled total nitrogen deposition, based on mean values per 50 x 50 km2 EMEP grid square, showed considerable scatter. Some of this scatter can be explained by the fact that actual nitrogen deposition values vary considerably within each grid cell due to for example topography, vegetation, local pollution sources and climate. When the total nitrogen concentration in mosses was plotted against site-specific nitrogen deposition values in for example Switzerland, a strong positive linear relationship was observed. Therefore, the application of mosses as biomonitors of atmospheric nitrogen deposition at the European scale requires further investigation.
In a separate study, the ICP Vegetation Programme Centre investigated long-term temporal trends (ca. 1860-2000) in the nitrogen concentration in mosses by analysing herbarium material collected from selected European countries. In general, the total nitrogen concentration in mosses did not change before 1960, but increased after 1960, most likely due to the increased deposition of oxidised nitrogen
The Working Group on Effects has adopted the critical load methodology as an effects-based approach for developing pollutant abatement strategies to reduce emissions of air pollutants. The critical load for nitrogen is the highest nitrogen deposition load that will not cause changes leading to long-term harmful effects on ecosystem structure and function. Exceedance of the critical load is a measure of the potential risk of harmful effects occurring in the long-term. Based on experimental evidence of harmful effects of nitrogen on vegetation, empirical critical loads were defined for various habitats during a workshop in 2002.
In a preliminary exercise, the ICP Vegetation developed a likelihood-based approach for ‘Heathland’ and ‘Grasslands’ to assess empirical critical load exceedances across Europe, taking into account uncertainties in modelled nitrogen deposition values as well as uncertainties in empirical critical loads for these vegetation classes. Moist grasslands, the most dominant grassland type across Europe with the highest empirical critical load range, showed no exceedance. In contrast, ‘Alpine and sub-alpine grasslands’ and ‘Arctic, alpine and sub-alpine scrub habitats’, minor grassland types across Europe with the lowest minimum empirical critical load, had the greatest exceedance and are therefore most at risk from adverse affects of atmospheric nitrogen pollution. These sensitive habitats will benefit most from reductions in nitrogen emissions in the future.
The ICP Vegetation has started to develop a meta-database describing (inter)national or regional surveys on nitrogen impacts on vegetation So far these surveys indicate that impacts of nitrogen deposition on vegetation are often difficult to separate from other factors that might affect vegetation, for example changes in climate, land use and management. However, some surveys indicate increases in species with that like higher nitrogen supply or a reduction in species richness with an increase in nitrogen deposition. Future work should focus on further analysis of the existing meta-database, identification of additional field surveys, in particular in areas which are currently under-represented (e.g. Mediterranean) and linking more data on for example changes in species composition with measured or modelled nitrogen deposition data.
There are many groups within Europe studying the atmospheric nitrogen fluxes and its impact on vegetation (e.g. NinE, NitroEurope, COST 729). Within the ICP Vegetation, we synthesise the main results on impacts of nitrogen on vegetation for the benefit of the LRTAP Convention.
Examples of nitrogen effects on vegetation include (see annual report 2008/9):
(1) Lichens and mosses contain species that are among the most sensitive to elevated atmospheric nitrogen deposition. Therefore, critical levels of ammonia have recently been set at a lower concentration (1 μg m-3) for lichens and mosses (and ecosystems where lichens and mosses are a key part of ecosystem integrity) than for higher plants (3 μg m-3).
(2) Sensitive habitats with low empirical critical loads for nitrogen include raised and blanket bogs, nutrient poor mires, tundras, Racomitrium containing wet heathlands, and arctic, alpine and sub-alpine scrub habitats. Despite conservation efforts, many lowland heaths in Western Europe have become dominated by grass species over the past 20-50 years. For boreal forests it was recently recommended to reduce the empirical critical load to 5-10 kg N ha–1 y–1.
(3) The loss or decline in abundance of species with a high retention efficiency (so called nitrogen ‘filters’) such as mosses and lichens results in an increase in the amount of inorganic nitrogen available to higher plants and soil microbes. Elevated nitrogen availability favours faster growing, more nitrogen-loving species, leading to competitive exclusion of plants adopted to low nitrogen availability, ultimately resulting in a decrease in plant diversity. High nitrogen concentrations in soils might lead to leaching of nitrogen to ground and surface waters. Recovery from nitrogen enrichment can be a very slow process and it might already be later then we think.