This problem, which ignores frontiers, happens because, due to the burning of fossil fuels, the amount of sulphur and nitrogen oxides in the atmosphere is greater than that derived from natural processes.

These oxides, in the presence of water vapour and under the oxido-reduction conditions present in the atmosphere, produce acids that are deposited, amongst other places, on the forest biomass.

Also, intensification in the cattle sector, together with stabling and grouping together of herds, have given rise to the concentration of ammonia emissions in certain zones. This compound, deposited close to the sources of emission, is able to react with the acidic ions deposited at the same time. Subsequently, certain bacteria are capable of oxidising the compound, thus forming nitrate and liberating protons that acidify the soil.

Amongst the effects of the deposition of these compounds on the forest mass are the well-known nutrition disorders of the same. One classic effect is that of cation deficit (particularly magnesium) due to the washing both of the forest canopy and the soil produced by these together with anions (sulphates and nitrates).

This problem is not very common in forest ecosystems close to the sea given that, in these conditions, the uptake of magnesium with precipitation is high. Another consequence is what is known as the eutrofization of terrestrial ecosystems due to the increase in nitrogen availability (saturation) in systems where historically this element has been the limiting factor in productivity.

In this research, the recycling of nutrients was studied in two, five-year period stages and in two young radiata pine forests (the first stage) and in two oak woods (the second).

To this end, weekly samples of rain, transcolation (fraction of the precipitation that passes through the forest canopy), and litterfall (vegetable material fallen from trees: leaves, twigs, fruit, and so on), the first 25 cm of mineral soil and green foliage were taken and analysed chemically, according to standard protocols. The location of parcels was carried out as a function of their distance from different foci of emission of contaminants.

A flow equilibrium model for the canopy was drawn up together with a generalised micrometeorological model in order to estimate the total deposition of atmospheric constituents. Also, a model for foliar growth and abscision was designed using proportions of the various cohorts of the samples of green branches and litterfall.

The total nitrogen deposition was greater that that deemed to be the admissible critical load in European forest ecosystems so that the nitrogen saturation of the ecosystems studied is, or shortly will be, a fact. The canopy of the forests was able to neutralise the atmospheric deposition in an effective manner although the potential acidity was greater in those areas near emission foci.

The uptake of acidifying ions and nitrogen caused an acceleration of the return of nutrients (amount of nutrients that the vegetation gives back to the soil together with the litterfall and foliar excretion) and a drop in their retranslocation (reabsorption of nutritive elements).

Thus, the efficiency in the use of cations was affected by the atmospheric deposition of contaminants. Magnesium deficiency was observed in all the adult formations studied. The acceleration of the return of nutrients and the drop in the efficiency of their use is proposed as an explanation of this disorder.

Thanks to this investigation, it was concluded that the atmospheric deposition of contaminants produces damage in the forest masses of Bizkaia and may be aggravated in the future.

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