DEVELOPMENT OF THE SELECTED PROPERTIES OF ZINC-CONTAMINATED SOIL FOLLOWING AN ADDITION OF NEUTRALISING SUBSTANCES

1Abstract. Studies have been undertaken which assess the possibility of neutralising the effect of zinc on the contents of: organic carbon, total nitrogen, available forms of: phosphorus, potassium and magnesium in soil by applying neutralising substances – compost, bentonite and zeolite. In the series without an addition of neutralising substances, increasing zinc soil contamination resulted in an increase in the contents of total nitrogen, available forms of phosphorus, magnesium and potassium (especially), and a decrease in the organic carbon content, compared to the control object (without Zn). Application of bentonite and zeolite had an advantageous effect on the contents of carbon and available phosphorus and on the C:N ratio in soil, compared to the series without additions. Bentonite had a similar effect on the available potassium content in soil. It had a much greater effect on the organic carbon content in soil than zeolite. Not all of the neutralising substances added to soil had a significant effect on the contents of nitrogen and available magnesium in soil, compared to the series without neutralising substances.

Soil is an integral element of the land ecosystem (Haslmayr et al. 2016) and is a non-renewable natural resource (Lal et al. 2015).It also plays a very important role in the cycling of elements, including heavy metals, in the terrestrial environment (Gorlach, Gambuś 1991).Due to the constant contact with air and water (Zhang et al. 2015) soil is susceptible to excessive enrichment with trace elements as a consequence of human activity (Cabral-Oliveira et al. 2015; Puga et al. 2015;Rovira et al. 2015).It causes slow and practically irreversible soil destruction (Cabral-Oliveira et al. 2015, Lal et al. 2015).Another source of trace elements may be bedrock weathering (Temmerman et al. 2003).Some trace elements are essential for living organisms to function properly (often the necessary amounts are very small, trace amounts actually), but after exceeding a certain level they become toxic (Wyszkowski, Wyszkowska 2009;Borowik et al. 2014;Puga et al. 2015;Zhang et al. 2015).
Soil is able to bind heavy metals and to limit their solubility (Gorlach, Gambuś 1991).The elements availability then decreases, they can even become unavailable.Thus, they become less toxic to soil living organisms (Feng et al. 2007).Their availability to plants depends on their content and form and on the soil properties: pH, organic matter content, granulometric composition, sorption capacity, fertilisation, the species of the plant grown and on the soil microorganism activity (Gorlach, Gambuś 1991;Wołoszyk et al. 2009;Ociepa 2011;Skwaryło-Bednarz et al. 2011;Kargar et al. 2015).Soil sorption can be improved by introducing unconventional fertilisers, such as compost or brown coal, since it is an effective way of limiting heavy metal availability to plants (Ociepa 2011;Fijałkowski et al. 2012).The following minerals also have similar properties: loams, bentonite or zeolite and iron oxides (Feng et al. 2007).With increasing pH and organic matter and clay mineral contents, soil sorbs more heavy metals (Gorlach, Gambuś 1991;Ociepa 2011;Fijałkowski et al. 2012).
For this reason, studies have been undertaken which assess the possibility of neutralising the zinc effect on the contents of: organic carbon, total nitrogen, available forms of: phosphorus, potassium and magnesium in soil by applying neutralising substances -compost, bentonite and zeolite.

MATERIALS AND METHOD
Pot culture experiments were carried out in the vegetation hall of the University of Warmia and Mazury in Olsztyn.Acid soil formed from sand was used.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 exchange bases: Ca 2+ , Mg 2+ , K + and Na + (TEB) -42.1 mmol(+) kg -1 ; cation exchange capacity (CEC) -75.7 mmol(+) kg -1 ; the degree of 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 first order factor was zinc soil contamination in doses of: 0, 150, 300, 600 and 1,200 mg of Zn kg -1 , and the second order factor was adding the following substances to the soil: compost (3%), bentonite (2%) and zeolite (2% by weight of the soil).To the soil in all the pots macro-and microelements were introduced, 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.In the experiment yellow lupine (Lupinus luteus L.) of the Mistral variety was sown.
Soil samples for laboratory analysis were taken during the annual 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 determined: the content of total nitrogen with the Kjeldahl method (Lityński et al. 1976), the content of organic carbon (C org. ) -with the Tiurin method (Lityński et al. 1976), the content of available phosphorus and potassium -with the Egner-Riehm method (Lityński et al. 1976) and the content of available magnesium -with the Schachtschabel method (Lityński et al. 1976).The results were statistically analysed with the Statistica software package using a two-way ANOVA analysis of variance.To assess the effect of zinc and neutralising substances on soil properties, a principal component analysis (PCA) was used.The ratio of variance components was also calculated, with the eta-squared method, using a two-way ANOVA analysis of variance.

RESULTS AND DISCUSSION
The contents of organic carbon, total nitrogen, available phosphorus, potassium and magnesium in soil depended on zinc soil contamination and on the application of neutralising substances (Tables 1-2, Fig. 1).In the series without an addition of neutralising substances, increasing zinc soil contamination resulted in an increase in the contents of available potassium and magnesium by 112% and 47%, respectively, of nitrogen and available phosphorus by 36%, and a decrease in the organic carbon content by 22% compared to the control object (without Zn).There was also a trend of narrowing the C:N ratio in soil.
The results presented in Tables 1 and 2 indicate the positive effect of the bentonite and zeolite use on the contents of organic carbon and available phosphorus in soil, compared to the series without an addition of a zinc neutralising substance.The effect of bentonite on the organic carbon content in soil was much greater than the effect of zeolite.Bentonite caused an increase in the organic carbon content in soil by 34% on average, compared to a 17% increase for the series with zeolite.The greatest average contents of available phosphorus in soil were reported for the series with the additions of bentonite and zeolite.The contents were respectively -by 35% and by 33% -higher than for the series without the additions.Bentonite and zeolite promoted broadening the C:N ratio in soil.A positive effect of bentonite on the available potassium content in soil was also reported, but an increase in the available potassium content was only 9%, compared to the series without additions.Not all of the neutralising substances added to soil had a significant effect on the contents of total nitrogen and of available magnesium in soil, compared to the series without neutralising substances.Introducing compost into soil also had no significant effect on the contents of organic carbon and of available phosphorus and potassium.From the Fig. 1 it follows that in the studied soil, the first two principal components represent 78.24% of the primary variable variance.The first principal component accounts for 43.36% of the total variance and the second principal component accounts for 34.88%.The vectors representing the contents of organic carbon, available phosphorus, potassium and magnesium and the C:N ratio in soil are very well represented by the first two principal components which form a coordinate system.The total nitrogen content was more weakly represented.The total nitrogen vector is most strongly slanted towards the nearest first principal component, which proves its reaction to zinc soil contamination was similar and negative.Similar, but weaker relations were observed for available magnesium and potassium.The opposite relations were reported for the organic carbon content and for the C:N ratio in soil.The vectors representing the primary variables for available potassium and magnesium, organic carbon and the C:N ratio in soil are close to each other, which indicates the relations between them were positive.The vectors between available potassium and magnesium and the C:N ratio and organic carbon, and, particularly, between total nitrogen and the C:N ratio and organic carbon were, in turn, very far apart from each other, which represents the negative correlations between these elements.When the percentage of observed variance was assessed with the coefficient η2, using the ANOVA method, it was demonstrated that the total nitrogen content, and particularly the contents of available magnesium and potassium, were to the greatest extent determined by zinc soil contamination; 29.37%, 80.24% and 86.52%, of the variance could be explained by this variable, respectively (Table 3).Relatively low values were reported for the contents of: available phosphorus (19.43%), organic carbon (4.99%) and for the C:N ratio (13.46%) in soil.Application of neutralising substances to soil had the greatest effect on the contents of available phosphorus (66.12%) and organic carbon (59.63%).Its effect on the C:N ratio (45.30%) was weaker.The proportion of variance in the contents of total nitrogen and available potassium and magnesium which can be explained by this variable was very small, and it did not exceed 4%.
The results of the authors' own research have been partly confirmed by other authors' studies.The studies carried out by Feng et al. (2007) confirmed that heavy metals are bound as a result of introducing loams, bentonite, zeolite, iron oxides or phosphates to soil minerals.It is also reflected in the study by Sahraoui et al. (2015), where clay minerals and iron and manganese oxides added to soil caused an increase in the sorption of soil trace elements.In the studies by Wyszkowski and Sivitskaya (2012), bentonite caused an increase in the contents of available forms of phosphorus, potassium and magnesium in soil.Wyszkowski and Ziółkowska (2009) confirmed that bentonite had an effect on the contents of available potassium and magnesium in soil.Zeolite can cause an increase in the contents of organic carbon and magnesium and caused a decrease in the phosphorus content in soil (Wyszkowski, Radziemska 2012).
According to Delgado and Follett (2002) and Wright et al. (2007), organic matter is an excellent source of nutrients for plants, particularly of organic carbon.Many authors reported an increase in the organic carbon content (Wyszkowski, Ziółkowska 2009, Wyszkowski, Radziemska 2012) and in the contents of available phosphorus (Wyszkowski, Ziółkowska 2008), potassium (Wyszkowski, Ziółkowska 2009) and magnesium (Wyszkowski, Radziemska 2012) in soil following the application of compost.Temmerman et al. (2003) indicated that prolonged application of organic fertilisers (manure) and mineral fertilisers to soil resulted in an increase in the total nitrogen content in soil.Delgado and Follett (2002) found that organic substance applied to soil enriches the soil in nitrogen, phosphorus and potassium.In the studies by Wright et al. (2007), adding compost resulted in an increase in the contents of phosphorus, potassium and magnesium in soil.However, it should be noted that compost, like other organic fertilisers, decomposes relatively slowly and the above elements become available to plants at a later time (Wright et al. 2007).Moreover, Temmerman et al. (2003) indicated that an application of manure to soil may result in additional heavy metal soil contamination.According to Fijałkowski et al. (2012), adding organic substance to soil may result in retention or release of heavy metals and other elements.

CONCLUSIONS
In the series without an addition of neutralising substances, increasing zinc soil contamination resulted in an increase in the contents of total nitrogen, available forms of phosphorus, magnesium and potassium (especially), and a decrease in the organic carbon content, compared to the control object (without Zn).
Application of bentonite and zeolite had an advantageous effect on the contents of carbon and available phosphorus and on the C:N ratio in soil, compared to the series without additions.Bentonite had a similar effect on the available potassium content in soil.Bentonite had a much greater effect on the organic carbon content in soil than zeolite.
Not all of the neutralising substances added to soil had a significant effect on the contents of nitrogen and available magnesium in soil, compared to the series without neutralising substances.

TABLE 3 .
PER CENT CONTRIBUTION OF VARIABLE FACTORS ACCORDING TO THE CONTENT OF ELEMENTS IN SOILS