Damage to forest stands and their decline in the Czech Republic evoked a necessity to elucidate the function of soil in these processes. Main factors affecting acidification of forest soils are assessed in the paper. On the one hand, it refers to natural factors such as properties of soil-forming substrates and species composition of forest stands, on the other hand, to the effects of anthropogenic character such as influences of man-made spruce monocultures and effects of acid depositions. Special attention is paid to the development of forest soils in the Krušné hory Mts., where the effects of acid depositions are highest. 15ty of forest soils in the territory of the CR. Generally, we can state that substrates of acid character predominate. It applies particularly to crystalline rocks such as granite, gneiss, mica schist, Cretaceous sandstones and also sandstones of Carpathian flysch. On these rocks under the interaction of climate and plant cover, soils developed being of acid reaction such as Podzols and acid Cambisols. Soils of slightly acid reaction such as Luvisols were formed on loess sediments and soils saturated with basic elements on igneous basic rocks of the České Středohoří Mts. Of special character are the soils that developed on alluvia of the Labe, Odra, Morava and Dyje rivers which can be primarily of neutral to slightly alkaline reaction. Soils developed on limestone karst rocks are of neutral acidity. MAŘAN (1944MAŘAN ( , 1948) also attributed the importance of natural soil-forming factors for the condition of soil acidity according to particular soil types but the character of surface humus was considered to be the principal factor. The fact was then generalized by MAŘAN (1948) as follows: (1) humus of closed conifer stands is much more acid than that of closed broadleaved stands of the same region, (2) forest floor layers are more acid than layers of mineral soil.Similarly ZLATNÍK et al. (1938) studying natural forests of Trans-Carpathian Ukraine described sites where soil acidity of upper horizons reached 4.5-4.8 pH and in some cases even 3.8, 3.6 and 3.4 pH.Therefore, in studying anthropogenic impacts it is necessary to take into consideration this natural condition affecting also the sensitivity of soil to acidification by spruce monocultures and particularly by acid depositions.In assessing the sensitivity of soils to acidification by air pollutants, it is possible to take into account the opinion of WIKLANDER (1978) that very acid soils are far less sensitive to further acidification. They are relatively stable, their pH being usually lower than pH of precipitation. Particularly podzol soils and acid brown forest soils rank among these soils. In coniferous forests, WIKLANDER (1978) considers decomposition processes of organic matter to be the main source of acidification.A special case can occur if E horizon is completely impoverished of soil colloidal fractions by podzolization processes and only an inert fraction of siliceous sand remains in it as shown by the profile of m...

Dynamics of oxidation of inorganic sulphur compounds to sulphate by the soil of spruce forests was investigated. Sulphide, sulphite and thiosulphate are oxidized to sulphate at a maximal rate at the beginning of the reaction, oxidation of elemental sulphur exhibits a lag phase. Linear relationships between the amounts of the produced sulphate and concentrations of substrates in the soil could be detected. On the basis of ;this finding a method for comparison of the oxidative activity of various soils was proposed.

Some factors influencing the oxidative activity of upper horizons of spruce forest soils (a mixture of fermentative and humus layers) toward intermediates of the oxidative part of the sulphur cycle were investigated. Preincubation of the soil with added cysteine, sulphide, elemental sulphur or thiosulphate was found to stimulate enzyme systems oxidating any of these compounds. Sulphite and sulphate were ineffective in this respect. The oxidation of elemental sulphur was stimulated by CaCO3, technical urea and high doses of superphosphate and potassium sulphate. It was inhibited by KH2PO4, pure urea, 40 % potassium salt, ammonium nitrate with calcium carbonate and the fertilizer NPK I. It proceeded at the highest rate at approximately 60 % capillary capacity (61 % of mass water content). Oxidation of thiosulphate was stimulated by KH2PO4, pure urea, superphosphate, potassium sulphate and only slightly by the fertilizer NPK I. It was inhibited by CaCO3, 40 % potassium salt and only slightly by ammonium nitrate with calcium carbonate. Potassium chloride, glucose and technical urea were without effect. The oxidation proceeded at the highest rate at 35 % maximal capillary capacity (48 % mass water content).

  In 1986 (1987) research plots were established in a forest stands on the south-western slope of Šerlich Mt., Orlické hory Mts. (Kristina Colloredo-Mansfeld – Forest Administration Opočno), at the altitude of 950 to 970 m, to study deposition, chemistry of precipitation and soil water and development of soil chemistry. The plots were established on a clear-cut area, in a young stand and a mature stand of spruce, in a mature beech stand, and in an advanced growth of spruce and European mountain ash. The content of solutes in creek water was studied at the same time. Since 1993 the concentration of substances in precipitation water intercepted in the summit part of Šerlich Mt. has been measured. Research on water chemistry in the stands terminated in 1997. Soil analyses were done in 1986 (1987), 1993 and 1999. The load of acid air pollutants in these forest ecosystems was high in the eighties. After 1991 the deposition of H⁺, S/SO₄^2–, N/NO₃^–+ NH₄⁺, Mn, Zn, Al decreased. Similarly, an increase in pH was observed in soil water, and the concentrations of SO₄^2–, and N, Al compounds decreased. But in 1993 the concentrations of SO₄^2– and Al increased again under the spruce stand for several months. The concentrations of NO₃^–, Mn, Zn and Al in the stream water also gradually decreased in the nineties. On the contrary, the average values of S-ions increased compared to those of 1987 to 1991. Strongly acid soil reaction developed in deeper layers until 1993. In the second half of the nineties the pH/H₂O value somewhat increased again, however the reserve of K, Mg, Ca available cations in the mineral soil constantly decreased. The saturation of sorption complex by basic cations in the lower layer of rhizosphere did not reach even 10% in 1999. The forest ecosystems of Šerlich Mt. were also loaded by a high fall-out of Pb, and increased fall-out of Cu. The lack of balance of N-compound transformations and consumption in the soil and increased leaching of N in the form of nitrates contribute to soil acidification on the investigated plots.

Effect of industrial emissions with high sulphur dioxide content on the upper horizons of spruce forest soils in NW Bohemia was investigated. The content of sulphates, oxidative activity towards sulphide, elemental sulphur, thiosulphate and sulphite, concentration and species representation of thiobacilli in horizons F, H and A in regions highly affected by emissions (two localities) and in regions relatively less influenced (three localities) were followed. In the affected areas the sulphur content in the soil was higher, the species representation of thiobacilli was similar and their concentration was higher, the ability of the soil to oxidize thiosulphate was inhibited and oxidation of elemental sulphur was stimulated. The oxidation of sulphide and sulphite was not significantly affected by the emissions. Changes caused by emissions could be observed only in horizons F and H and did not involve horizons A.