ACID DEPOSITION AND FOREST SOILS

Editors V.V. Nikonov ⁽¹⁾ and G.N. Koptsik ⁽²⁾

Annotation:

Acid rains is an important regional problem of environment. Most scientists believe that acid rains have significantly contributed to the deforestation of Western, Central and Northern Europe and the east coast of the USA. In addition to the direct effect of acidic solutions on tree crowns, resulting in tissue damage and leaching of metabolites, the acid impact modify soil properties, which is believed to be the most important in forest degradation. These changes are: increase of soil acidity, decrease of mineral nutrition reserves, mobilization of toxic elements. All these aspects of soil transformation has not been yet adequately studied in Eastern Europe. Present book is aimed at making up this deficiency.

The effects of atmospheric deposition of sulphur and heavy metals from nickel-processing industry on sandy alfehumic podzols under pine and birch boreal forests in the Kola Peninsula have been considered in this monograph. We investigated the atmospheric and soil migration of elements, the organic matter dynamics, soil acidity and cation exchange properties, element uptake by plants in background, defoliating, sparse forest ecosystems and barren industrial lands. Migration and accumulation of elements in the podzols are caused by interaction of atmospheric deposition, leaching from the canopy, organic matter decomposition, displacement of base cations from the cation exchange sites by protons and heavy metal cations. The organic horizon of podzols appeared to be the important accumulator of heavy metals and the barrier against transport of pollution to underlying mineral horizons. In polluted areas the dominant forest plants accumulated large amounts of S, Ni, Cu, Fe, N, P, K, whereas the concentrations of Ca, Mg, Mn, Zn decreased.

The results of the experiment simulating the five-year impact of acid rains on different forest soils (acid automorphic Podzol and Cambisol and neutral hydromorphic Histosol) of Eastern Lithuania (Lithuanian National Park) are represented. Acid impact resulted in essential changes of the soil chemical properties and the composition of the effluent from soils. The contribution of cation exchange and dissolution of Al hydroxides in the process of soil acidification was caused by soil properties, the level of acid impact and its duration. Automorphic Podzol and Cambisol were more sensitive to acid impact than hydromorphic Histosol.

The buffering mechanism of Podzol at a high acid load was closely related to the functioning of Al buffering zone. At the first stage of acidification with pH 3.0 Al accounted for 30% of the mobilized cations, by the end of the experiment its level became as high as 65%. In the treatment with pH 2.5 mobilized Al accounted for 44% at the beginning and 91% at the end of the experiment.

In the Cambisol the mobility of Al increased only at the acid load of pH 2.5, while at pH 3.0 the cation exchange buffering zone provided for complete neutralization of excessive acidity (Al accounted for not more than 7 to 8% of the mobilized cations). The increase of acid load at pH 2.5 activated Al buffering zone after 2 years of simulated rainfall. In the neutral Histosol excessive acidity was neutralized within the cation exchange buffering zone in all treatments.

The process of leaching can affect nonexchangeable forms of elements, or be limited to the loss of exchangeable forms only. The probability of leaching of securely fixed elements from the soil is controlled by the properties of the soil itself, and by the intensity and duration of the acid load. The most intensive leaching of nonexchangeable Ca and Mg was characteristic of mineral horizons of Podzol.

The rate of exchangeable cations leaching is not the same in different soil types. The order of the removal rate (by percent of elements washed out in relation to their initial content) can also depend on the amount of the acid load. In particular Mg, which is considered a "critical" element for acid forest soils, changes its position in this series.

The treatment of soils with acidic solutions and leaching of base cations leads to increase the actual and potential soil acidity. The content of exchangeable Al increased and Ca²⁺:Al³⁺ ratio decreased in forest litter of Podzol and in A horizon of Cambisol.

Treatment of the soils with solutions whose acidity exceeded the natural acidity of the soil caused the decrease of organic matter and iron mobility. The concentrations of Al may be reduced when the acidification of the soil solution was moderate and may increased as the acid load rose. Al and Fe concentrations in soil solution decreased to the same extent as the organic carbon apart from the most acidic treatment, were Al concentration increased considerably. In the uppermost horizons of forest soils Al is mainly present in the form of organo-mineral complexes. Solubility of the Al, when soil exposed to an acid load, is therefore considerably controlled by the behavior of these compounds. The decomposition of organo-aluminium complexes and Al release require rather low pH values, so it is thus highly unlikely that Al3+ ions will be present in sufficiently high concentrations to exert a toxic effect on root systems in the uppermost soil horizons.

Nonexchangeable mobile Al (CH₃COONH₄-soluble) increased in all soils including Histosol. At the same time in the illuvial horizon of Podzol mobile Al decreased after treatment with extremely acidic solution (pH 2.5), as a result of intensive Al leaching. Maximum changes in the Al state was observed in Podzol. The general trend was the conversion of less mobile compounds into more mobile forms (for example, silicates into nonsilicates, crystals into amorphous). Fe compounds showed a similar transformation pattern. Difference was that Fe was not mobilized to soil solution even under pH 2.5.

It was identified that three different processes contributed to Al and Fe state in soil profiles:

1. The decrease in the mobility of water-soluble Al-Fe-humus complexes in soil, subjected to acidification. It may cause accumulation of mobile forms of metals in upper horizons;
2. Increase in the inorganic Al mobility in the upper horizons under strong acidification and in the mineral horizons under moderate one. It may accelerate Al removal and amorphous Al reduction in OE horizon of Podzol and in A horizon of Cambisol;
3. However balance calculations show that first and second factors account for only 30-50% of changes in Al and Fe states. Thus these changes also controlled by the transformation of metal compounds in situ.

Mn was very sensitive to acidification. As Mn complexes with organic substances are less stable in a rather wide pH interval in the acid area compared with Al complexes, it can be easily leached with percolating water. Its leaching can be comparable with that of Mg and Ca, or even Mn leaching to be of paramount importance. Acid solutions caused intensive leaching of Zn from the soils. Co, Ni, and Cd were also actively mobilized by acidity. The increase of theirs mobility was characteristic of only acid automorphic soils and does not take place in the neutral Histosol within the range of pH levels from 4.5 to 2.5. Cu, Pb, and Cr call for a much greater acid load to be mobilized.

Intensive leaching of Mn and Zn was accompanied by the changes in the state of these elements in soils. These changes differed from those for Ca, Mg, K, Fe, and Al. The transformation of Mn and Zn states were characterized by a combination of leaching (as for Ca, Mg, K within the all range of pH levels studied in the experiment and Al under strong acid load) and transformation from less into more mobile forms (as for Fe and Al). The intensity of these processes depends on both the acid load and initial status of the elements in the soil horizons. Thus the presence of Mn and Zn in the forest litter mainly as mobile compounds was the reason of the decrease of their content up to 90% of total amount. In the mineral horizons mobile Mn and Zn compounds accounted for a small percentage of their total content, therefore processes of transformation and leaching of the less mobile compounds from upper horizons and theirs accumulation in lower ones were going.

Sulphate adsorption in the mineral soils occurred only at the initial stage of the experiment (from 6 to 24 simulated months in Podzol and from 6 to 12 months in Cambisol). Sulphate adsorption capacity of the Histosol has not been fully depleted by the end of the experiment. Absolute amount of adsorbed S increased with the increase of it concentration in the solution. SO₄^2- was tightly fixed in the soils. Exchangeable SO₄^2- accounted for only a small proportion of the total adsorbed amount. Maximum of adsorption was observed in horizons reach by organic matter. At the same time the horizons with maximum accumulation of S were characterized by different status of Al and Fe compounds. Thus OE horizon of Podzol contained relatively low amounts of amorphous Al and Fe; A horizon of Cambisol on the contrary was high in these compounds; while the Histosol was noted for a very high nonsilicate Fe but not Al.

Decomposition rate of the main fractions of conifer litter and fluxes of nutrients through litterfall and decomposition were determined in pine, spruce and birch forests of the eastern Lithuania. The pine forest was characterised by the low input of nitrogen and ash elements with litterfall (90 kg/ha), low decomposition rate (k=0.2-0.6 yr^-1), insignificant annual release of this nutrients from decaying litter and their accumulation in the forest floor (5500 kg/ha). Increased litterfall nutrient transfers and decomposition rate in spruce and birch forests have determined the comparatively high soil nutrient availability. Acid treatment appeared to disturb the plant litter decomposition and soil biological activity both in field and laboratory experiments.

The book includes an overview of international and national approaches to monitoring of ecosystems and soils and its methodology, particularly of principles and methods of International Co-operative Programmes (ICP's) and Pilot Programme of Integrated Monitoring (IMP) started within UN/ECE Convention on Long-range Transboundary Air Pollution with the purpose of monitoring and assessing effects from air pollutants in the environment. The main aim of integrated monitoring in the terrestrial environment is to determine and predict the state of ecosystems (or catchments) and their changes in a long-term perspective, with respect to the regional variation and impact of air pollutants, especially nitrogen and sulphur, and including effects on biota. Integrated monitoring of ecosystems means physical, chemical and biological measurements over time of different ecosystem compartments simultaneously at the same location. In practice, monitoring is divided into a number of compartmental subprogrammes which are linked by the use of same parameters and/or same/close stations. One of the central IM-approaches is to monitor the mass balance of major chemical components within the site. In soil chemistry subprogramme emphasis is placed on acid-base relationships and levels of important nutrients. Soil water chemistry is one of the most essential subprogrammes for understanding geohydrochemical interaction with biological effects.

Soil response to acid deposition depends on it's buffering capacity. Study of the nature and magnitude of buffer capacity in various soils, information on mechanisms of buffer reactions is required to predict the rates of further acidification and to estimate the critical loads. The book presents acceptable information on estimation of soil sensitivity to acidification on the base of internal soil properties like acid-buffering capacity, acidity, content of exchangeable cations, etc., as well as on the base of soil type classification. The classification of forest soils on their vulnerability to acid deposition was worked out for the north-western part of European Russia.

The book includes the critical analysis of the approaches to setting critical loads for the acidifying components. The critical load concept utilizes the converse of the scenario assessment concept within the framework of a dose-response relationship. The critical load is calculated as the maximum pollutant deposition which will not cause damage to the ecosystem, or component of the ecosystem. Simple mass balance approaches and steady-state models are sufficient for determining acidification sensitivity of forest soils, lakes, streams and groundwater. The detailed analyses of these methods is undertaken in connection with their wide using for assessment and mapping critical loads in Europe. Possible changes in forest soils under the influence of acid precipitation are estimated with a steady-state mass balance PROFILE model. The model calculations demonstrate that sandy podzols prevailing in Eastern Lithuania seem to be highly sensitive to acid deposition.

We assessed critical loads of acid deposition and their exceedance for forest soils in the European part of the former Soviet Union using a simple balance method and mapped them within 1.0°x0.5° longitude per latitude grid cells. Present level of acid deposition in the main part of Russia doesn't lead to soil acidification including even most sensitive sandy podzols. But heavy acid loads in the Kola Peninsula, in the West region of Russia, in some parts of Byelorussia, Ukraine and Baltic countries exceed the area's critical loads and increase the risk of damage to forest ecosystems. The uncertainty in the estimated critical load values can be rather large due to uncertainty in critical chemical values for the soil as for a receptor, assessment method and data. Both limited number of experimental data and their spatial variability determines uncertainties in data. Obtained results are certainly preliminary and represent the current state of critical load assessment problem in Russia. These maps can be revised as new experimental input data become available and as assessment method will be improved.

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