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Funda CIMEN , Sonay SOZUDOGRU OK Soil Science Department, Faculty of Agriculture, Ankara University, Ankara Abstract The adsorption of linuron was studied on clay textured soils which varied in their organic matter content, belonging to the Inceptisol (Beyazbayır) and the Entisol (Kepir) ordos, collected from Polatlı, Ankara. The adsorption of linuron on the soils after removal of organic matter fractions and calcium carbonate was compared. The adsorption of linuron by natural soil samples, alkali extracted fractions, H2O2 oxidized fractions and HCl treated fractions was determined through the batch-equilibrium technique using analytical grade linuron. The Freundlich equation was used to characterize the adsorption isotherms and Freundlich coefficients (K and 1/n) were obtained. Adsorption isotherms for soil-Beyazbayır indicated that adsorption of linuron by natural soil samples was higher as compared to the alkali extracted fraction and H2O2 oxidized fraction. Also, similar results were obtained for soil-Kepir. Adsorptions for both soil-Beyazbayır and soil-Kepir free from calcium carbonate, were the highest. According to 1/n values, for all soil samples belonging to soil-Beyazbayır and soil-Kepir, L-type adsorption isotherms were obtained. Introduction The behavior of herbicides in the soil depends on factors such as the physicochemical characteristics of the herbicide, the active surface of the soil mineral and organic components, and the amount of the herbicide applied (Govi et al., 1996). Adsorption of pesticides by soils has frequently been found to be correlated with organic matter and clay contents. It is generally accepted that this effect is due to the high adsorptive capacity of these soil constituents for herbicides. Adsorption of herbicides, therefore, is basic to understanding the behavior of herbicides in soil. Most studies on the adsorption of pesticides by soil organic matter were based on extracted soil organic matter fractions which behave differently with regard to herbicide sorption. But extraction of these fractions is time consuming and tedious. Therefore, instead of extraction of these fractions, sub-sequential removal of fractions may be the easiest way for organic matter studies. The objective of this study was to determine the role of organic matter fractions in the adsorption of linuron. Materials and Methods Soils Two surface soil samples of Beyazbayır and Kepir soil series having similar properties except organic matter contents were used as research materials. Some properties of the soils are given in Table 1.  Fractionation of soil samples Soil samples were treated as described by Kozak et al., (1983). a) Natural soil sample: Soil samples were passed through a 1 mm sieve, washed with hot distilled water and then dried at 60ºC. b) HCl treatment (CaCO3 removed soil): After washing with hot distilled water, soils were treated with 0.1 M HCl to remove calcium carbonate. When the reaction was completed, residues were washed with distilled water and dried at 60ºC. c) Alkali extraction (humin+mineral fraction): Soils were treated with alkali to remove humic acid (HA) and fulvic acid (FA). For the purpose, after washing with hot distilled water, soil samples were shaken overnight with 0.1 M Na-pyrophosphate (1:2 w/v). The supernatant was decanted and soil residues were shaken with 0.5 M NaOH overnight (1:2 w/v). Again supernatant was thoroughly decanted and residues were washed with distilled water and then dried at 60ºC. d) H202 oxidization (mineral fraction): Soils after removing HA and FA by alkali extraction, were oxidized by three additions of 30 % H202 to remove humin. After oxidation, residues were washed with distilled water and then dried at 60ºC. Adsorption Isotherms Adsorption isotherms were obtained using batch equilibrium technique. The adsorptive properties of the materials were determined by shaking a series of 1 g samples for 24 h with 10 ml of aqueous linuron (analytical grade) solutions of different concentrations (0, 5, 10, 15, 20, 25 mg ml-1). After equilibrium, the concentration of the unadsorbed linuron was determined spectrophotometrically by measuring the adsorbance at 248 nm (Khan and Mazurkewich, 1974). All treatments carried out in triplicate. The analytical data obtained from the adsorption experiments was revealed by using the logarithmic form of Freundlich adsorption equation. Logarithmic form of the Freundlich equation is: Results Adsorption of linuron by two soils and their fractions is presented as Freundlich adsorption isotherms in Fig 1 and 2. When concentration of adding linuron increased, adsorption of linuron by all of the samples also increased. This increase is clearer for natural soil samples and HCl treated fractions than the H202 oxidized fractions. Adsorption isotherms indicate that adsorption of linuron by both soils decreased when the organic matter fractions were removed. Adsorption of linuron by natural soil samples and alkali extracted fractions for both soils were similar to each other at low concentrations but at high concentrations, natural soil sample adsorbed more linuron than alkali-extracted fraction. It means that after removal of HA and FA, linuron adsorption was decreased. In addition, with the removal of humin, adsorption of linuron drastically decreased. Hance (1974) indicated that removing organic matter fraction that is soluble in alkali, decreased adsorption of linuron and also peroxidation of soil constituted a material that has very small adsorption capacity. Kozak et al. (1983) obtained similar results for prometryn and metolachlor. After removing organic matter fractions, decrease in adsorption capacity of soil samples can be explained by decrease in soil organic matter contents (Table 2). Only organic matter contents of HCl treated fractions were higher than natural soils, as removal of CaCO3 caused an increase in organic matter content of soils. After removal of HA and FA (alkali extraction) organic matter content of natural Beyazbayır decreased from 1.68 % to 1.21 %. After removing humin (H2O2 oxidation), organic matter could not have been determined in soil samples (Table 2). The values indicated that 72.02 % of organic matter was consisted of humin fraction and 27.98 % of organic matter was consisted of HA and FA. Organic matter content of the natural soil sample of Kepir decreased from 3.02 % to 1.68 % after extraction of HA and FA. Finally after removal of humin, organic matter could not have been determined in the samples (Table 2). These values indicated that 56.63 % of organic matter is consisted of humin fraction and 44.37 % of organic matter is consisted of HA and FA. Due to comparison of linuron adsorption of soil samples, concentration of 15 mg ml-1 for applying rate in practice was selected. The calculated values are given in table 3. For Beyazbayır, after removing humin with H2O2 oxidization, decrease of adsorption was the highest. Difference of adsorption ratios between natural soil and alkali extracted fraction indicated that adsorption by HA and FA fractions was very low for Beyazbayır. For Kepir adsorption ratios provided by HA, FA and provided by humin are too close to each other. This may be attributed to their similar organic matter contents.   Adsorption of linuron by HCl treated (calcium carbonate removed) fractions of Beyazbayır and Kepir was higher when compared with natural soil samples, alkali extracted and H2O2 oxidized fractions. Removing calcium carbonate caused an increase in organic matter per unit soil sample since adsorption of linuron increased relatively. Adsorption isotherms indicated that besides organic matter, mineral part of soils was also involved in adsorption of linuron. Table 3 showed that mineral fraction of Beyazbayır soil adsorbed 9.93 % of initial linuron, and mineral fraction of Kepir soil adsorbed 23.32 % of initial linuron. It is thought that adsorption of linuron by mineral fractions of clay textured soil is related with clay contents of soils. Stevenson (1976) pointed out that the amount of organic matter required to coat the clay will depend on the soil type, the kind and amount of clay that is present. Although both soils have similar clay mineral types adsorption of linuron values were differed each other, due to different organic matter and, HA, FA and humin contents. Entisol Kepir contains more organic matter than Beyazbayır, as can be seen in Table 3 that the adsorption of linuron on Kepir was higher than Beyazbayır. Many scientists found that the herbicidal activity of phenylureas is related to the organic matter content of the soils. (Weber et al., 1974, Carringer et al., 1975, Grover 1975). K and 1/n constants belonging to Beyazbayır and Kepir soil samples were calculated from logarithmic form of Freundlich equations and are presented in Table 4. In the study Freundlich coefficients (K) ranged from 1.013 (Beyazbayır- Natural soil) to 1.613 (Beyazbayır-CaCO3 removed fraction). It is expected that adsorption coefficients (K) decrease with removal of organic matter fractions. But in this study conversely, adsorption coefficients (K) increased with removal of organic matter fractions. Grover (1975), pointed out in his study with urea herbicides, that comparing K coefficients of soils is valid when Ce=1 and this relationship could be different for other concentrations. This finding of Grover is consistent with our findings. For instance, expected results would be obtained using 15 mg ml-1 linuron concentration, the application rate in practice.  Conclusion The results of the study showed that either organic matter or mineral part of soil is involved in adsorption of linuron. Increase in organic matter content of the soil caused an increase in adsorption of linuron. On the other hand it is thought that adsorption of linuron is related with rates of clay type rather than clay content. Therefore organic matter content, clay type and contents of the soils should be considered when linuron is applied in practice. Adsorbed linuron by organic matter may desorb afterwards and has a phytotoxic effect on crops. On the other hand adsorption of linuron by organic matter can prevent it from leaching thorough soil profile and accumulating in groundwater. And this characteristic of linuron is in demand like all pesticides to prevent environmental pollution. References . Carringer, R.D, Weber, J.B. and Monaco, T.H. 1975. J.Agric. Food Chem. 23: 568-572. In Khan, S.U. 1980. Pesticides in the soil environment fundamental aspects of pollution control and environmental science 5. Elsevier Scientific Publishing Company Amsterdam. . Govi, M., Sarti, A., Di Martino, E., Ciavatta, C. and Rossi, N. 1996. Sorption and desorption of herbicides by soil humic acid fractions. Soil Sci., 161: 5 265-269. . Grover, R. 1975. Adsorption and desorption of urea herbicides on soils. Can. J. Soil Sci., 55:127-135. . Hance, R. J. 1974. Soil organic matter and the adsorption of the herbicides atrazine and linuron. Soil Biol. Biochem., 6 : 39-42. . Khan, S.U. and Mazurkewich, R. 1974. Adsorption of linuron on humic acid. Soil Sci., 118: 5 339-342 . Kozak, J., Weber, J.B. and Sheets T.J. 1983. Adsorption of prometryn and metolachlor by selected soil organic matter fractions. Soil Sci., 136: 94-101. . Osgerby, J.M. 1970. Sorption of un-ionized pesticides by soils. In Sorption and Transport Processes in Soils. SCI Monograph, 37: 63-78. . Singh, R., Gerritse, R.G. and Aylmore, L.A.G, 1989. Adsorption-desorption behaviour of selected pesticides in some western Australian soils. Aust. J. Soil Res., 28, 227-43. . Stevenson, F.J. 1976. In: D.D. Kaufman, G.G. Still, G.D. Paulson and S.K. Bandal (Editors): Bound and conjugated pesticide residues, ACS Smyp. Ser., 29: 180-207. . Weber, J.B., Weed. S.B. and Waldrep, T.W. 1974. Weed Sci., 22: 454-459. In Khan, S.U. 1980. Pesticides in the soil environment fundamental aspects of pollution control and environmental science 5. Elsevier Scientific Publishing Company, Amsterdam. |