PRINCIPLES AND METHODS OF ASSESSMENT OF SOIL SUSTAINABILITY TO ATMOSPHERIC ACID DEPOSITION

Koptsik, G.N., Makarov, M.I., Kiseleva, V.V.

Annotation:

The textbook concerns an important problem of forest soil acidification by polluted precipitation. In summarizes both data and theoretical considerations from different publications and original results, obtained by the authors in field and laboratory experiments.

In many countries serious efforts were undertaken to reduce pollutant emission to the atmosphere. However, in Russia no decrease in atmospheric pollution is expected. Pollutant pressure to natural ecosystems is especially high near big point sources of emission - cities and industrial complexes. The most noticeable effect of atmospheric pollutants is forest decline. Soils play an important part in ecosystem resistance to atmospheric pollution due to its buffer capacity. So, the processes of pollutant transformation in soils under different conditions are the main topic of the book. The influence of sulphur and nitrogen - most common components of polluted precipitation, - is examined from the point of view of possible soil acidification.

Studies, conducted in numerous countries during many years, demonstrated the acid precipitation leads to multiple consequences for soil chemical properties. They are: pH decrease, increased acidity, the mobilization of Fe, Al and heavy metals with following plant intoxication, cation leaching, decrease of base saturation, decreased organic matter destruction and mineralization, suppression of microbiological activity.

Nitrogen, being the most important element of plant nutrition, may cause substantial negative changes in element cycles, if its input exceeds the demands of an ecosystem. The first stage is nitrogen saturation of forest ecosystems, which are usually nitrogen-defficient. Its consequences are an increased N content in plant tissues and the leaching of excess amounts of the elements with soil solution and groundwaters. The acidification occurs when excess N is nitrified within upper soil horizons. The nitrification rates can be high in acid forest soils due to the activity of soil geterotrophic microorganisms. Thus, even if the nitrogen-containing precipitation is not acid itself, it can cause soil acidification during the processes of N transformation in soil profile. The nitrification proceeds with the release of 2 equivalents of protons per one equivalent of nitrogen. As a result of the mechanisms of cation exchange and charge balance regulation, base cations are extracted from soil exchange complex and leached out.

A marked proton production and acidification of soils and soil waters was observed by the authors near the fertilizer-producing industrial complex in Novgorod district. Calculated annual proton production was equal to 4.1 keq/ha. During the last 10 years the decrease in pH by 0.2 units was observed in A horizon. The depletion of exchange cation pool was found, Ca losses were especially high.

Other consequences of increased nitrogen input are disturbances in the uptake of other nutrients, decreased resistance of forests to natural and anthropogenic stresses, etc.

Sulphur enters the soil directly as sulphuric acid or sulphates. We studied the effects of sulphuric acidification in a model experiment with different types of forest soils, simulating 5-year influence of artificial rain with different acidity (pH 4.5; 4.0; 3.5; 3.0 and 2.5). The regularities in transformation of chemical properties of both acid automorphic (Podzol and Cambisol) and neutral alluvial (Histosol) were examined. Soil acidity, exchangeable cations, sulphate sorption capacity, iron, aluminium and manganese compounds transformation as well organic substances and heavy metals mobilizations were studied.

The simulated rain at pH 4.5 didn't alter chemical properties of soils and soil solutions. The experiment demonstrated a drastic decrease of soil and solution pH in its extreme variant ("rain" pH equal to 2.5). A serious depletion of exchangeable cations took place in the litter and upper mineral horizons. The leaching of Ca and Mg was the most significant. It was demonstrated, that besides exchangeable forms, other forms of elements were involved in dissolution and leaching.

The neutralization of acidic solutions has been accounted for by the processes of cation exchange and aluminium hydroxides dissolution. The relative significance of each of the processes mentioned depends upon the intensity and length of acid load. In Histosol saturated with bases cation exchange was the main factor controlling the reaction of neutralization during the whole period of the experiment. In Cambisol cation exchange was of primary importance at low and moderate acid load. At more intensive acid load reactions of aluminium buffer zone took place in Cambisol and Podzol.

The general trend of iron and aluminium compounds transformation consists in a transfer of less mobile compounds into more mobile ones (crystallized into amorphous, silicate into non-silicate), it is accompanied by accumulation of exchangeable aluminium in the upper horizons of acid automorphic soils. Organic compounds and iron migration capacity decreased in cases when acidity of simulated precipitation was higher than that of soil solution. Aluminium migration capacity may either decrease with low and moderate acidity, or increase with strong acidification.

The transformation of Mn and Zn compounds and mobilization of Cu, Ni, Cr, Co, Pb, Cd have been found to higher extent in acid automorphic soils and to less extent in alluvial soil. The loss of Mn leached from the surface layer may be as high as 60%.

The biggest changes took place in Podzol, which was considered to be the less resistant to acidification. At the same time, Podzol demonstrated the highest capacity to sulphate adsorption.

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 the acid-buffering properties of both organic and mineral soil horizons. The main attention is paid to the studies on buffering of soil organic horizons which first of all horizons receive acid rainwater. Simultaneously organic horizons contain a large body of fine roots and thus determine forest vital functions and productivity. The buffering by the soil organic horizons exceeds that by the mineral horizons by almost one order of magnitude. The organic horizons consume protons through rapid cation exchange, and through non-exchangeable protonation of soluble and insoluble organic substances. The additional H⁺-consuming process is a slow reductive formation of Fe²⁺ and Mn²⁺. Organic horizons can completely neutralize the present acid load, protecting mineral horizons against acidification. The predominant reactions which are considered to be responsible for buffer capacity to acids in mineral horizons are likely the protonation and dissolution of Fe and Al hydroxides and Al hydroxisulphates which are followed by Al appearance in solution.

The problem of estimation of soil sustainability to acid deposition is considered. The book includes the critical analysis of the approaches to setting critical loads for the acidifying components. Several mathematical models, describing changes in soils and waters under the influence of acid precipitation, are regarded.

We assessed critical loads of acid deposition and their exceedance for forest soils in the European part of Russia, Byelorussia, Ukraine and Baltic countries 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 main parts of the Kola Peninsula receive high loads of sulphur, a primary component of acid deposition. The boreal forests of the Kola region are among the northernmost coniferous forests of the world. Under the prevailing extreme growth conditions, when trees and vegetation are under strong natural stresses, it can be presumed that even minor loads of air pollutants may have severe effects upon forest vitality. Critical loads of acidity vary from 200 to 800 eq/ha/y with the type of soil, parent rock, vegetation and climatic conditions. The critical loads' values are almost all related to sulphur only. Present sulphur depositions are higher than critical loads in the main part of the Kola Peninsula (40% of total area). The greatest excesses (1200-1800 eq/ha/y) occur in North-Western and Western parts, especially in surroundings of nickel smelter in Nikel.

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. Complex process-oriented multi-layer models seem to be reasonable to apply for acidification assessment and prediction as it is done in many European countries.

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