The atmospheric deposition of nitrogen compounds in Europe is due, in roughly equal parts, to emissions of nitrogen oxides and ammonia. The problems are largely unrestricted by national borders, especially in the case of nitrogen oxides and their transformation products. Ammonia is generally not transported such long distances.
One way to define the critical load for nitrogen is to calculate the level at which nitrogen starts to leak from the system into the groundwater. This is done with the aid of mass balances. These look at the way that nitrogen is converted in the ecosystem — its uptake by vegetation, fixing in the soil, conversion by micro-organisms in the soil (nitrification and denitrification), its removal if biomass is harvested, etc.
Another way of determining the critical load for nitrogen is to study the deposition levels of nitrogen at which visible changes start to appear in ecosystems, e.g. changes in the composition of species. Knowledge in this area is however incomplete, since it is difficult to establish which changes are due to nitrogen deposition and which are caused by other changes, such as the way that land is used.
Most nitrogen reaches the sea as run-off from the surrounding land. Some of this is airborne nitrogen that is deposited on land and then leaches into surface water and is carried to the sea. Warmer climate leads to faster decomposition of organic matter, with a risk of increased leaching of nitrogen from the land into the sea.
It has been found that the amount of nitrogen leaking from the system falls considerably soon after the supply is stopped, even though the load has been high for a long time and a large reserve of nitrogen has therefore built up in the soil. The store of nitrogen in the soil does not decrease as quickly as the leaching out of nitrogen. Large amounts may be bound up in vegetation and humus layers, and in many cases nitrogen is conserved very effectively. Interference, such as forestry felling or liming of the soil can accelerate decomposition, however, and lead to the risk of increased leakage. This risk may remain for a long time.
Vegetation responds very slowly to reductions in the supply of nitrogen. In the trial areas that are covered by a roof, no change has yet been seen, even though almost ten years have passed since deposition was greatly reduced. This is probably because the soil and plants are still “charged" with nitrogen that is continuing to circulate within the system. Only when the supply of nitrogen starts running out will the original plants be able to compete again, but this recolonization may take a very long time.
Source: Air and the environment by P.Elvingson and C.Ågren
Guidance on GAINS application (pdf, 2.4 MB) model in the state environmental management system of the Russian Federation (Russian; English version is under development)
Guidance document on Critical Load assessments (in Russian) (pdf, 790.3 kB)