A few years ago the study of the effects of atmosphericdeposition on forest ecosystems reached beyond the scientific sphereand the term “acid rain” was coined. This problem, which ignoresfrontiers, happens because, due to the burning of fossil fuels, theamount of sulphur and nitrogen oxides in the atmosphere is greater thanthat derived from natural processes. These oxides, in the presence ofwater vapour and under the oxido-reduction conditions present in theatmosphere, produce acids that are deposited, amongst other places, onthe forest biomass. Also, intensification in the cattle sector,together with stabling and grouping together of herds, have given riseto the concentration of ammonia emissions in certain zones. Thiscompound, deposited close to the sources of emission, is able to reactwith the acidic ions deposited at the same time. Subsequently, certainbacteria are capable of oxidising the compound, thus forming nitrateand liberating protons that acidify the soil. Amongst the effects ofthe deposition of these compounds on the forest mass are the well-knownnutrition disorders of the same. One classic effect is that of cationdeficit (particularly magnesium) due to the washing both of the forestcanopy and the soil produced by these together with anions (sulphatesand nitrates). This problem is not very common in forest ecosystemsclose to the sea given that, in these conditions, the uptake ofmagnesium with precipitation is high. Another consequence is what isknown as the eutrofization of terrestrial ecosystems due to theincrease in nitrogen availability (saturation) in systems wherehistorically this element has been the limiting factor in productivity.
Inthis research, the recycling of nutrients was studied in two, five-yearperiod 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 theforest canopy), and litterfall (vegetable material fallen from trees:leaves, twigs, fruit, and so on), the first 25 cm of mineral soil andgreen foliage were taken and analysed chemically, according to standardprotocols. The location of parcels was carried out as a function oftheir distance from different foci of emission of contaminants. A flowequilibrium model for the canopy was drawn up together with ageneralised micrometeorological model in order to estimate the totaldeposition of atmospheric constituents. Also, a model for foliar growthand abscision was designed using proportions of the various cohorts ofthe samples of green branches and litterfall.
The total nitrogendeposition was greater that that deemed to be the admissible criticalload in European forest ecosystems so that the nitrogen saturation ofthe ecosystems studied is, or shortly will be, a fact. The canopy ofthe forests was able to neutralise the atmospheric deposition in aneffective manner although the potential acidity was greater in thoseareas near emission foci. The uptake of acidifying ions and nitrogencaused an acceleration of the return of nutrients (amount of nutrientsthat the vegetation gives back to the soil together with the litterfalland foliar excretion) and a drop in their retranslocation (reabsorptionof nutritive elements). Thus, the efficiency in the use of cations wasaffected by the atmospheric deposition of contaminants. Magnesiumdeficiency was observed in all the adult formations studied. Theacceleration of the return of nutrients and the drop in the efficiencyof their use is proposed as an explanation of this disorder.
Thanksto this investigation, it was concluded that the atmospheric depositionof contaminants produces damage in the forest masses of Bizkaia and maybe aggravated in the future.
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