ACIDITY AND SORPTION PROPERTIES OF ZINC-CONTAMINATED SOIL FOLLOWING THE APPLICATION OF NEUTRALISING SUBSTANCES

Soil plays the most important role in the cycling of elements in the terrestrial environment. Contamination of soil with heavy metals have effect on all soil organism and plants. For this reason, studies have been carried out to assess the possibility of neutralising the effect of zinc (0, 150, 300, 600 and 1200 mg Zn∙kg-1 of soil) on soil properties by applying neutralising substances (compost, bentonite and zeolite). Soil acidity and sorption properties depended on zinc contamination and on the type of neutralising substances added. In the series without an addition of neutralising substances, increasing zinc soil contamination resulted in a decrease in total exchangeable bases, in cation exchange capacity and in the saturation degree of the degree of base saturation, and an increase in soil hydrolytic acidity. An addition of bentonite to zinc-contaminated soil had the most advantageous effect of all the additions of neutralising substances on the tested soil properties. It resulted in an increase in soil pH, in total exchangeable bases, in cation exchange capacity, in the degree of base saturation and simultaneously decreased soil hydrolytic acidity compared to the series without additions. Adding zeolite and compost to soil had a small and usually insignificant effect on soil acidity and soil sorption properties.


INTRODUCTION
ly available, or even unavailable, and less toxic to plants and soil organisms [Feng et al. 2007]. Their mobility and availability depends on many factors, of which the most important are soil pH and soil type, the content and form of the metal in soil, organic matter content, cation exchange capacity, granulometric composition, fertilisation and the contents of macro-and microelements, the species and the variety of plants and, furthermore, soil microorganism activity [Feng et al. 2007, Fijałkowski et Wyszkowski, Wyszkowska 2009]. Zinc belongs to the group of heavy metals considered to be the most mobile [Fijałkowski et al. 2012] and most sensitive to pH changes [Sady, Smoleń 2004]. Zinc mobility increases with soil pH below 6-6.5 [Sady, Smoleń 2004]. Heavy metal mobility is greater in strongly acidified soils compared to soils with neutral and alkaline pH [Fijałkowski et al. 2012, Sady, Smoleń 2004. Soil pH, together with organic matter, have the greatest effect on the solubility, content and availability of trace elements to plants [Fijałkowski et al. 2012, Ociepa 2011, Sady, Smoleń 2004. Soil sorption complex sorbs exchangeable heavy metals with different force [Sady, Smoleń 2004] and also prevents soil pH fluctuations [Ociepa 2011]. In soils containing clay minerals, as well as in soils with high sorption capacity, trace elements are easily accumulated [Fijałkowski et al. 2012]. Adding to soil mineral and organic substances which allow for increasing sorption capacity, such as: zeolites, bentonites, diatomites, loams, iron oxides and compost or brown coal, contributes to limiting trace elements uptake by plants [Feng et al. 2007, Fijałkowski et al. 2012, Ociepa 2011.
For this reason, studies have been carried out to assess the possibility of neutralising the effect of zinc on soil properties by applying neutralising substances.

MATERIALS AND METHODS
A study was carried out in the vegetation hall of the University of Warmia and Mazury in Olsztyn (north-eastern Poland) using acidic soil formed from sand. The soil had the following properties: pH in a 1 mol KCl•dm -3 solution -5.32; hydrolytic acidity (HAC) -33.6 mmol(+) • kg -1 ; total exchangeable bases Ca ++ , Mg ++ , K + and Na + (TEB) -42.1 mmol(+)•kg -1 ; cation exchange capacity (CEC) -75.7 mmol(+)•kg -1 ; base saturation (BS) -55.6%, the organic-C content -13.8 g•kg -1 ; the contents of available forms of: phosphorus -40.3 mg•kg -1 ; potassium -11.9 mg•kg -1 and magnesium -33.6 mg•kg -1 of soil. The soil was contaminated with the following amounts of zinc: 0, 150, 300, 600 and 1200 mg Zn•kg -1 of soil. In order to limit zinc soil contamination, compost (3%), bentonite (2%) and zeolite (2% by weight of the soil) was used. Macro-and microelements were introduced to the soil in all pots, in the same amounts: 30 mg of N; 30 mg of P; 100 mg of K; 50 mg of Mg; 0.33 mg of B; 5 mg of Mn and 5 mg of Mo per kg of soil. Yellow lupine (Lupinus luteus L.) of the Mistral variety was then sown. Soil samples for laboratory analysis were taken during yellow lupine harvest, at the flowering stage.
The samples were then dried and sieved through a sieve. In the prepared soil, the following properties were analysed: pH -with potentiometric method, in a KCl water solution with a concentration of 1 mol·dm -3 . Hydrolytic acidity (HAC) and total exchangeable bases (TEB) were determined with the Kappen method and, on this basis, cation exchange capacity (CEC) was calculated according to the formula: CEC = TEB + HAC, and the degree of base saturation (BS) was calculated according to the formula: BS = (TEB : CEC) • 100 [Lityński et al. 1976]. The results were statistically analysed with the Statistica software package [StatSoft, Inc. 2014] using a two-way ANOVA analysis of variance. The effects of zinc and neutralising substances on the tested soil properties were assessed using a principal component analysis (PCA). The ratio of variance components was calculated using a two-way ANOVA analysis of variance, with the eta-squared method.

RESULTS AND DISCUSSION
Soil acidity and sorption properties depended on the zinc contamination and on the type of the neutralising substances added. The results presented in Table 1 indicate that when the dose of zinc was increased to 1,200 mg Zn•kg -1 of soil, soil pH decreased. In the series without an addition of neutralising substances, increasing zinc contamination resulted in a decrease in soil pH from 5.49 to 5.29 (r = -0.643) and in an increase in hydrolytic acidity from 16.80 to 22.35 mmol(+)·kg -1 (r = -0.933). This confirms the study by Guala et al. [2013] in which heavy metal accumulation in plant mass increased with increasing soil acidity and with increasing heavy metal content in soil. Similar conclusions were drawn by Sady and Smoleń [2004], who reported an increase in zinc mobility when soil pH decreased below 6-6.5. Takáč et al. [2009] showed that zinc availability decreases with increasing soil pH. Gorlach and Gambuś [1991] confirmed that with increasing soil pH, the phytotoxic effects of trace elements on plants decreased. With increasing zinc contamination total exchangeable bases (r = -0.901), soil cation exchange capacity (r = -0.891) and base saturation (r = -0.956) decreased significantly. Sady and Smoleń [2004] proved that the higher the cation exchange capacity of the soil is, the more trace elements soil can contain. In their study, the yield of carrot grown on soil with pH higher than 7.0 and the degree of base saturation higher than 98% had an acceptable cadmium content.
Among the applied substances, bentonite had the greatest and most advantageous effect on the tested soil properties (Tables 1, 2). Following its application, an increase in soil pH by over one unit was observed. Simultaneously, soil hydrolytic acidity decreased by 40% on average, compared to the series without additions. In addition, average increases in total exchangeable bases (by 140%), in cation exchange capacity (by 67%) and in the degree of base saturation (by 27%) were reported. A similar effect of bentonite on soil properties was reported by Wyszkowski They report that in soil with alkaline pH and with high organic matter content, the mobility of trace elements may increase by creating heavy metal complexes with organic matter. These complexes are both readily available and accessible to plants. Esmaeili et al. [2014] reported that all the heavy metals occurring in agricultural soils significantly correlate with pH value. In addition, Shaheen and Rinklebe [2014] showed that a high zinc content in soil was correlated with a low pH value, which was confirmed by the results of this study. The results of the author's own research indicate that adding zeolite and compost to soil had a small, often insignificant, effect on soil acidity and on soil sorption properties, compared to a series without neutralising substances. A similar relation was observed by Ociepa [2011], who found that adding to soil sewage sludges, a mixture of the sludges with brown coal and mineral fertilisation resulted in small changes in pH value and improved the soil sorption properties to varying degrees.
In a study by Ociepa [2011], sewage sludges and their mixture with brown coal had the most advantageous effect on an increase in total exchangeable bases, in cation exchange capacity and on the degree of base saturation. Sady and Smoleń [2004] also demonstrated that following an introduction of diatomite to soil there was  a decrease in cadmium accumulation in carrot roots, both for soil with natural cadmium content and for cadmium-contaminated soil. As well, a 25-year-long manure fertilisation study showed an increase in the degree of soil saturation with alkaline elements [Sady, Smoleń 2004], which greatly limits trace element accumulation in cultivated plants. Gorlach and Gambuś [1991] also indicated the strong participation of the organic part of the soil sorption complex in binding trace elements in the soil. In addition, heavy metal toxicity to maize and radish decreased with increasing soil cation exchange capacity.
In the analysed zinc-contaminated soil, the first two principal components represent 97.14% of primary variable variance. The first principal component accounts for 92.87% of the total variance and the second principal component for 4.27% (Figure 1). All vectors representing the tested soil properties are very well represented by the first two principal components, which form a coordinate system. Most of the vectors, particularly the vectors of total exchangeable bases and of the degree of base saturation, are very strongly slanted towards the nearest first principal component. This proves that their reaction to soil zinc contamination was similar and negative. The opposite relations were observed for soil hydrolytic acidity. The vectors representing the primary variables for the degree of base saturation and pH, for the degree of base saturation and total exchangeable bases, and for total exchangeable bases and soil cation exchange capacity are close to each other, which indicates the relations between them LSD (least squares deviation) for: a -zinc dose, b -kind of neutralizing substance, a · b -interaction; significant for: ** -P = 0.01, * -P = 0.05, n.s. non-significant; r -correlation coefficient.
were positive. The vectors between soil hydrolytic acidity and other soil properties, particularly soil pH, were, in turn, very far apart from each other, which represents the negative correlations between these elements. When the percentage of observed variance was calculated with the coefficient η2, using the ANOVA method, it was demonstrated that the tested properties of zinc-contaminated soil were to the greatest extent determined by an application of neutralising substances. This is particularly true for soil pH and for soil hydrolytic acidity: 85.86% and 73.76% of the variance can be explained by this variable, respectively (Table 3). Explanations: pHKCl -pH in 1M KCL, HAC -hydrolitic acidity, TEB -total exchange bases, CEC -cation exchange capacity, BS -base saturation. High values were also reported for the other properties, which ranged from 54.70% (the degree of base saturation) to 64.21% (total exchangeable bases). The percentage of the variance which could be explained by the second variable, i.e. by zinc soil contamination, was much lower and varied between 11.38% (pH KCl ) and 40.36% (the degree of base saturation). A comprehensive study of the mechanisms of bioavailability to plants of trace elements occurring in soil allows for rapid assessment of the risk of food chain contamination, and for choosing the right means to neutralise harmful substances, including heavy metals.