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Ayten Karaca 1, Rıdvan Kızılkaya 2, Koray Haktanır 1 1 Soil Science Department, Faculty of Agriculture Ankara University, Ankara 2 Soil Science Department, Faculty of Agriculture, Ondokuz Mayıs University, Samsun ABSTRACT In this research, the effects of added different lead (Pb) and cadmium (Cd) compounds on the catalase enzyme activity in the coarse-textured soil, fine-textured soil and organic soil were investigated. Cd and Pb were applied in the forms of cadmium nitrate (Cd(NO3)2), cadmium chloride (CdCl2), cadmium acetate (Cd(CH3COO)2), and cadmium carbonate (CdCO3), lead nitrate (Pb(NO3)2, lead chloride (PbCl2), lead acetate (Pb(CH3COO)2) and lead carbonate (PbCO3). The samples were kept at 70% field capacity during an incubation period of 60 days at 28 0C. The effects of Cd and Pb compounds, applied to three different soils, on the catalase activity was found to be significant (P<0.01) depending on time and doses during 60 incubation days. While different Cd components applied to soil decreased the catalase activity, Pb compounds, with the exception of PbCl2, increased the catalase activity. The highest inhibation effect of Cd compounds on the catalase activity was found in the coarse-textured soil. However, the lowest effects of both Cd and Pb compounds on the catalase activity was found in the organic soil. INTRODUCTION In recent years several reports have been documented the harmful effects of long-term heavy metal contamination of agricultural soils, due to sewage sludge (Karaca et al., 1999, Moreno et al, 1998, 1999), phosphate fertilizer (Williams and David, 1975; Karaca and Haktanır, 1997a) rubbish disposal (Ranby et al., 1978) and fly ash (Natusch, 1978, Karaca and Haktanır, 1997b) application on soil microbial activity at several sites. In soils, metals may (i) occur as microbiologically inactive forms such as insoluble precipitates, (ii) be adsorbed by clay-sized minerals and (iii) occur as humic metal solid organic complexes (Bruce et al., 1983). Among the various elements of inorganic origin that are responsible for soil pollution, heavy metals such as Cd and Pb are by far the most important. Once these metals enter the soil, they remain there for long periods of time without being destroyed by the soil microorganisms, whereas molecules of organic origin can be microbially degraded (Blum, 1989). The effect of heavy metals on soil biological activity has been studied by a number of authors. Doelmann and Haanstra (1984) showed that microbial respiration was reduced in soil contaminated with heavy metals, and Chaney et al. (1978) reported that Cd and Zn lead to a reduction in soil respiration. The inhibiting action of heavy metals on phosphatase activity has been demonstrated (Tyler, 1974; Math and Kovacs, 1980; Doelmann and Haanstra, 1989; Marzadori et al., 1996) and a similar inhibiting action on arylsulfatase has also been reported by Al-Khafaji and Tabatabai, 1979. The influence of various trace metals on soil urease activity has been studied (Tabatabai, 1977; Doelmann and Haanstra, 1986). Other parameters, such as the ratio between the C of the biomass and the organic C in the soil due to the presence of heavy metals (Chander and Brookes, 1991, 1992). A quick way to obtained information on the biological activity of soil microorganisms can be by the measure of soil catalase and dehydrogenase activities. They are very sensitive to heavy metals (Naplekova and Bulavko, 1983; Perez and Gonzalez, 1987; Wilke, 1991) and can be used as a simple toxicity test (Rogers and Li, 1985). The objective of this study was to investigate the effects of various compounds of Pb and Cd on soil catalase activity in different soil type. MATERIALS and METHODS Soil and Experimental Procedure : The fine-textured soil was a clay loam and the coarse-textured soil was a silty loam, taken from the experiment field of agriculture faculty of Ankara University. The organic soil was a peat and taken from Bolu-Yenicag peat soil. The soils were collected from the top 20 cm of soil. Soil samples were air dried, sieved (2 mm), and stored. It were obtained pH of soil at 1:2.5 soil water suspension according to Richards (1954), organic material by using modified Walkley-Black Method (Jackson, 1962), grain size distribution by Bouyoucos (1951), CEC was determined according to methods given in Black (1965), total nitrogen was assessed using the Kjeldahl method, as specified by Bremner (1965). Soil samples were extracted for available Pb and Cd in DTPA solution (0.005M DTPA+0.005M CaCl2+0.1M TEA (triethanolamine) pH 7.3), (Lindsay and Norvell, 1978). The soil samples were digested in aqua regia ( Loon and Lichwa, 1973) to determine the total Pb, and Cd. All the solutions of Pb and Cd were analyzed by atomic absorption spectrophotometer (AAS) with flame or graphite furnace when required. Incubation experiment : Cd and Pb nitrate, chloride, acetate, and carbonate were added to the 250 g soil samples at concentrations of 0, 250, 750, and 1500 mgkg-1 and mixed. The mixtures, three replicates of each treatment, were placed in pots and kept at temperature of 280C. The water content of the soil was adjusted to 70 % of field capacity. Throughout the two months, water losses exceeding 10% of the initial values were compensated for by addition of distilled water. RESULTS and DISCUSSION Physical and Chemical Characteristics of Soils : The soils were divided into three groups, based on soil texture. Some important pysico-chemical characteristics of soil are shown in Table 1.  Effects of Various Cd Compounds on Soil Catalase Activity : The effects of four different Cd compounds, applied in concentrations of 0, 250, 750, and 1500 mgkg-1 to the three different soil types, on soil catalase activity are showed in Table 2. During incubation, addition of Cd in the presence of four different compounds had different effects on soil catalase activity in all soils (P<0.01). In the presence of Cd, catalase activity was most elevated in the fine-textured soil and at its lowest value in the organic soil. It was discovered that all the different Cd compounds caused significant reduction in catalase activity in the fine-textured soil with incubation periods. When the 60 day incubation period was taken as the base time, the degree of inhibition of the catalase activity of the various Cd compounds was found to be; Cd nitrate (74.2 %) > Cd carbonate (53.8 %) > Cd acetate (40.2 %) > Cd chloride (27.9 %) for the fine-textured soil, while the order for the coarse-textured soil was Cd chloride (74.2 %) > Cd carbonate (72.6 %) > Cd acetate (53.6%) > Cd nitrate (51.6 %). Added Cd chloride had a greater effect on the catalase activity in the coarse-textured soil than in the fine-textured soil, due to the difference in soil type, variations in the amount of organic matter and CEC. In organic soils, it was discovered that the effects of the various Cd compounds were not as clear as in the other soil types, and that Cd nitrate, Cd chloride , and Cd acetate inhibited the catalase activity in the ratio 9.6%, 11.6%, and 6.5%, respectively. Because of the buffering properties of the organic soil, it can be assumed that the toxic effect of different amounts of the Cd compounds was less evident than in the fine and the coarse-textured soils. Linear regression analysis showed that the effects of each of the parameters (incubation period, soil type, doses, and compound) were significant in their influence on the inhibiting of the catalase activity. Further, it was found that the order of significance was incubation period**>soil type**>compounds**>dosesns (*=P<0.01, ns= not significant). When the effects of the different Cd compounds were compared it was found that they differed significantly. As an average of all soils, the effects of Cd compounds in inhibiting the catalase activity varied in the order; Cd chloride > Cd nitrate > Cd acetate > Cd carbonate. It was found that Cd chloride was the most effective in inhibiting the catalase activity.  As a result of the path analysis, it was determined that the significant factors were acting independently of each other. Incubation period : -0.3147 path coefficient, 100% Soil Type : 0.0560 path coefficient, 100% Application doses : -0.3655 path coefficient, 100% Cd compounds : -0.2194 path coefficient, 100%. Our data suggest that the reaction of the catalase activity to application of various Cd compounds depends on the soil type and on the particular compound and amount used. Cd has been referred to as an essential element for some soil microorganisms (e.g.Psalliota) and trace amounts stimulate microbial growth (Wollum, 1973). Perez and Gonzalez (1987) found that the increase of Cd did not affect the catalase activity significantly. They said that the concentration of Cd in their study was high and besides, active catalase can persist in extracellular state adsorbed on soil colloids. Bond et al. (1976) showed that the addition of 10ppm Cd stimulated soil respiration initially and at lower level (0.01ppm) the period of stimulation was of longer duration. However, Bitton et al. (1986) have shown that 1.8 ppm Cd inhibited the dehydrogenase activity of 50% of the bacterial population. Effects of Various Pb Compounds on Soil Catalase Activity : The effects of four different Pb compounds, applied in concentrations of 0, 250, 750, and 1500 mgkg-1 to the three different soil types, on soil catalase activity are shown in Table 2. In each soil, the catalase activity showed different sensitivity to the Pb additions in different compounds (P<0.01). In the presence of Pb, the catalase activity was most elevated in the fine-textured soil and at its lowest value in the organic soil. In the fine-textured soil, the catalase activity increased by the addition of Pb acetate and Pb nitrate during the incubation period (P<0.01). However, Pb chloride inhibited the catalase activity by 27.7% for all incubation periods, while Pb carbonate inhibited the catalase activity at the beginning of the incubation period. It increased the activity at the end of the incubation time (P<0.01). In the coarse-textured soil, Pb in the presence of Pb carbonate, Pb acetate, and Pb nitrate increased the catalase activity during the incubation period (P<0.01). However, Pb chloride inhibited the catalase activity by 69% during the incubation period (P<0.01). In the organic soil, which has a high CEC, addition on Pb in the presence of four different compounds did not effect on the catalase activity significantly. Because of the organic matter fraction of the soil has a strong capacity for Pb ions (Hildebrand and Blum, 1974), it can be assumed that the toxic effect of different amounts of the lead compounds in the organic soil was less evident than in the fine and the coarse-textured soils. Linear regression analysis showed that the effects of each of the parameters (incubation period, soil type, doses, and compound) were significant in their influence on the inhibiting of the catalase activity. Further, it was found that the order of significance was incubation period**>soil type**>compounds**>dosesns (*=P<0.01, ns= not significant). As a result of the path analysis, the same results were determined as Cd . When the effects of the different Pb compounds were compared it was found that Pb nitrate and Pb acetate increased the catalase activity. Since lead acetate is readily soluble, its effect can be equated with that of free Pb ions. However, acetate and nitrate effects on the catalase activity were the least toxic, this is probably explained by the anionic effect of the C and N source for some of the microorganisms. These proposals are supported by the finding of Perez and Gonzalez (1987), who found similar effect of Pb acetate on the catalase activity. Naplekova and Bulavko (1983) also found that the addition of Pb acetate increased the catalase activity in the Chernozem soil and inhibited it in the Sod-Podzolic soil. Pb chloride decreased the activity. Added Pb chloride had a greater effect on the catalase activity in the coarse-textured soil than in the fine-textured soil, due to the difference in soil type, variations in the amount of organic matter and CEC. The concentration of free Pb ions is considerably decreased by clay minerals such as montmorillonite, illite and kaolinite. The adsortion of Pb to these minerals is 2-3 times as strong as that of other divalent cations such as Ca and Cd (Bittell and Miller, 1974). Pb chloride has no inhibitory effect on the catalase activity in the organic soil and that inhibition may be caused by more soluble forms of Pb, which may be hard to define since after addition Pb chloride is immediately bound (adsorbed and chelated) or converted to compounds such as Pb phosphate, Pb carbonate or Pb oxide (Ter Haar, 1971). This results are supported by finding Doelmann and Haanstra, (1979) who reported that Pb chloride seriously inhibited the respiration in sandy soil, but it's effect was only small in clay soil and did not effect in peat soil. CONCLUSIONS The type of inhibition or activation of the soil catalase activity by heavy metals (Cd and Pb) was studied. In each soil, the catalase activity showed different sensitivity to the Cd and Pb additions in different compounds. While different Cd components applied to soil decreased the catalase activity, Pb compounds, with the exception of PbCl2, increased the catalase activity. 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