Scientific registration n o : 68 Symposium n o : 38 Presentation : poster Regulation of soil adsorbent properties in copper polluted acid soils Régulation des propriétés d'adsorption de sols acides pollués par du cuivre RAICHEV Toshko, ARSOVA Antoaneta N.Poushkarov Institute of Soil Science and Agroecology, 7 Shosse Bankja str., 1080 Sofia, Bulgaria Introduction Acid soils are very sensitive to heavy metal pollution (Cu, Zn, etc.). Clay minerals in these soils have a low sorption capacity that decreases the effectiveness of liming. Organic meliorants are used as additives in copper polluted acid soils for reducing the copper toxicity. The question, however, about the influence of the organic substances on the mobility of microelements is discussible (Alloway and Ayres, 1994; Cambier, 1994). Unsoluble humic acids could diminish the mobility and bioactivity of microelements. Soluble organic ligands could enhance their mobility by complex formation. In the same time the complex bound metals are weakly toxic among the soluble forms compared to the free metal cations. These relationships influence plant growth (Massheleyn et al., 1991; Merry et al., 1986) and provoke difficulties in the use of appropriate meliorative technology. In field experiments liming leads to significant losses of lime materials because of leaching of bicarbonates in vertical and lateral water flow. Additional losses are caused by accumulation of bases in plant roots and their association with negative charged humus anions as well. Humus anions are released in the breaking of bounds between anions and positive charged Fe and Al hydroxides (Ganev, 1990). The colloid character of components in the soil adsorbent complex gives a possibility to develop a model eliminating the difficulties above mentioned. The theoretical scheme is based on the ability of electrical double-layer to take on complex three-layer structure in over-adsorption process of counter-ions in the outer layer (Kokotkov et al., 1986). According to authors hypothesis an association of Cu 2+ ions in sandwich charged colloid structures will decrease copper toxicity in the presence of organic substances in advanced humification stage (coal dust). The association of Cu 2+ in complexes with ogranic compounds in initial stage of humification will not decrease copper toxicity in alkaline medium. In acid soils heavy metals are in the form of soluble hydroxides as well as in ionic form on the basal surfaces of clay minerals and their sorption capacity increases in relation to humus acids. The almost zero charge of humus colloids diminishes the electric factor role under these conditions. The expectable 1
meliorative effect is achieved by combined treatment of acid soils with compost of lime materials and coal dust. The compost activates the electro-chemical charge of humus substances. Their colloid-chemical reacting depends on the concentration, ph and ionic strenght in the medium (Arora et al., 1979; Schnitzer, 1991). The aim of the study is to confirm in a biological experiment the hypothesis that a combined melioration of copper polluted acid soils with lime materials and coal dust decreases copper mobility due to association of Cu 2+ ions in organomineral complexes. Criteria for evaluation the meliorative effect is plant yield and copper content in biomass. Material and methods A two-year-long pot experiment with alfalfa (1200g soil per pot in three replications) is carried out on copper polluted acid soils from the regions of Karlievo (Cu=151 mg/kg) and Chelopech (Cu=297 mg/kg). Physico-chemical characteristics of the soils determined by authors methods (Ganev and Arsova, 1980) are the following: Karlievo: ph(h 2 O)=4.7;CEC=29.7mequ/100g;exch.Al=1.6mequ/100g;exch.(Ca+Mg)=19.7 mequ/100g; humus=2.7%. Chelopech: ph(h 2 0)=4.7;CEC=18.1mequ/100g;exch.Al=1.2mequ/100g;exch.(Ca+Mg)=10.9 mequ/100g; humus=2.3%. Alfalfa is used as a test plant because of its sensitivity to soil acidity and to ionic forms of heavy metals in soils. Besides, the post-effect of the applied meliorants could be evaluated by the yield of cuts. Scheme of the experiment: Ca 0 - control variant (acid soil without melioration); Caopt. - lime material (waste carbonate deposits from a power station) added in amounts calculated by exch.al in the soil according to the optimal liming rate of acid soils (Ganev, 1987): Caopt.(Karlievo)=1.51g/kg and Caopt.(Chelopech)=1.16g/kg ; K 1 - compost of optimal lime dose (Caopt.) and coal dust (1:4) calculated on the base of the proportion 1g Ca - 10g coal dust in amounts (g/kg): K 1 (Karlievo)=6.56 and K 1 (Chelopech)=5.04 ; K 2 - compost (2K 1 ) in amounts (g/kg): K 2 (Karlievo)=13.1 and K 2 (Chelopech)=10.0 ; K 3 - compost (3K 1 ) in amounts (g/kg): K 3 (Karlievo)=19.7 and K 3 (Chelopech)=15.1. Mineral nutrition (1N:1P:0.5Ca:0.5K:0.2S:0.2Mg) is added as 1 g salts per kg soil. Soil moisture is maintained at 60% field capacity. Dry biomass from each cut is weighted and copper content (I and IV cut) is determined by AAS. Soil ph (H 2 O) is measured in all pot variants at the end of the experiment. Results and discussion The meliorative effect on alfalfa yield could be seen in Table 1. The meliorants applied in the soil from Karlievo characterized by a low copper content (Cu=151 mg/kg) form the following rows according to their positive effect on yield: 2
I cut: K 2 > K 1 > K 3 ~ Caopt. II cut: K 3 ~ K 2 ~ K 1 > Caopt. III cut: K 3 > K 2 > K 1 > Caopt. IV cut: K 3 ~ K 2 > Caopt. > K 1 V cut: K 3 ~ K 2 ~ Caopt. > K 1 VI cut: K 3 > K 2 ~ K 1 ~ Caopt. Compost of lime and coal dust used in the two higher doses (K 2 and K 3 ) has better expressed effect on yield than liming (Caopt.). The positive effect on yield at the soil from Chelopech (Cu=297 mg/kg) could be seen in the following rows of meliorants: I cut: K 2 > K 1 > K 3 ~ Caopt. II cut: K 2 > K 1 > K 3 > Caopt. III cut: K 2 > K 3 > K 1 > Caopt. IV cut: K 3 > K 2 > K 1 ~ Caopt. V cut: K 3 ~ K 2 > K 1 > Caopt. VI cut: K 3 ~ K 2 ~ K 1 > Caopt. The highest yield (I - III cut) is obtained at compost applied in dose K 2. The effect of maximal dose of compost (K 3 ) increases after III cut. Yield obtained at soil treatment with lime material (Caopt.) is lower than those at compost application (K 1 -K 3 ). Alfalfa yield depending on the applied meliorants in the two soils is shown in fig.1 (Karlievo) and fig.2 (Chelopech). The meliorative post-effect could be evaluated by the yield of cuts. The post-effect on yield of compost (K 1 -K 3 ) added in the soil from Karlievo is not very significant because of the lower copper pollution. Nevertheless it should be noticed the increase of biomass amount at soil melioration with the two higher doses compost (K 2 and K 3 ). The organo-mineral melioration of the soil from Chelopech (Cu=297 mg/kg) has better expressed posteffect than optimal liming (Caopt.) resulting in maximum yield obtained in variants with compost K 2 and K 3. The total yield increases significantly compared to those in the control (Ca 0 ). Obviously, the positive effect of copper immobilization on yield is much better manifested at organo-mineral melioration of acid soils with higher copper pollution. These results confirm previous authors investigations. The combined melioration (lime and coal dust) stabilizes soil ph in optimal ranges (ph 5.7-6.0) during the two-year-long pot experiment. In fig.3 the total yield depending on the applied meliorants in the soils could be seen. The meliorative effect on yield is weakly expressed at the soil from Karlievo because of the lower copper pollution although the total amount of biomass increases at soil treatment with the two higher doses compost (K 2 and K 3 ). The strongest effect is observed at melioration with compost (K 2 and K 3 ) of the soil from Chelopech characterized with higher copper pollution. The total yield increases significantly compared to those in the control (Ca 0 ). In Table 2 is presented copper content in biomass (I and IV cut) depending on the applied meliorants in the soils. Copper content (I cut) at the soil from Karlievo decreases weakly in variants with melioration (K 1 -K 3 and Caopt.) in relation to the acid control (Ca 0 ). This tendency is much better expressed at IV cut - the decrease of biomass copper in the variants with melioration is about 1.5-2.0 times compared to the control. Copper immobilization is very significant in the higher polluted soil 3
(Chelopech). The melioration of this soil with maximal dose compost (K 3 ) leads to more than twice decrease of copper content (I cut) compared to those in the control. Conclusions In a two-year-long pot experiment with alfalfa carried out on two copper polluted acid soils is established decrease of copper toxicity at melioration with compost of lime and coal dust in appropriate doses. An important requirement is the applied organic materials to be rich of humus substances with cyclic-nucleated structure. The increased amount of strongly bound calcium in the soil adsorbent raises the effectiveness of melioration. Compost maintains soil ph in optimal ranges during the whole experiment and has significant post-effect on yield in difference of liming. These positive effects of organo-mineral melioration are much better manifested at soils with higher copper pollution. References Alloway, J., D. Ayres, 1994. Chemical principles of environmental pollution. Blackie Academic & Professional, Glasgow, U.K., X, 291 p. Arora, S., N. Coleman, 1979. The influence of electrolite concentration on flocculation of clay suspensions, Soil Sci., 127, 134-139. Cambier, P., 1994. Contamination of soils by heavy metals and other trace elements: A chemical perspective. Analysis, 22(2), 21-24. Ganev, S., 1987. Strongly acid and weakly acid state of soil adsorbent and determination of liming rate of acid soils (in Bulg.). Forest Sci. 24(1), 19-25. Ganev, S., A. Arsova, 1980. Methods for determination the strongly acid and weakly acid cation exchange in soils (in Bulg.). Soil Sci. and Agrochemistry, 15(3), 22-33. Ganev, S., 1990. Contemporary Soil Chemistry (in Bulg.). Ed. Science and Art, Sofia, 356p. Kokotkov, A., P. Zolotarev, T. Elkin, 1986. Theoretical fundamentals if ion exchange (in Russ.), Ed. Chemistry, Leningrad, 280 p. Schnitzer, M., 1991. Soil Organic matter- the next 75 years. Soil Sci., 151(1), 41-49. Keywords : acid soils, copper pollution, humus substances, colloid-chemical melioration, alfalfa yield. Mots clés : sols acides, pollution par le cuivre, substances humiques, colloïdes, rendements en luzerne 4
Table 1. Yield of alfalfa depending on the applied meliorants in copper polluted acid soils from Karlievo and Chelopech and soil ph at the end of the experiment Variant soil ph I cut II cut III cut IV cut V cut VI cut (H 2 0) g / 10 plant (dry matter) K a r l i e v o Ca 0 4.4 1.02 1.23 1.04 0.83 0.66 0.86 5.64 Caopt. 5.6 1.18 1.28 0.86 1.66 1.87 1.28 8.13 K 1 5.6 1.08 1.40 1.26 1.41 1.32 1.53 8.00 K 2 5.8 1.46 1.42 1.65 2.08 1.75 1.82 10.18 K 3 6.1 1.18 1.47 1.95 2.10 1.78 2.42 9.90 GD 1% 0.228 0.079 0.104 0.117 0.186 0.571 0.1% 0.309 0.110 0.159 0.103 0.230 0.698 *** *** *** *** *** *** C h e l o p e c h Ca 0 4.5 0.85 0.38 0.25 0.26 0.48 0.78 3.00 Caopt. 5.5 1.32 2.27 1.91 2.97 2.18 2.62 13.27 K 1 5.7 3.18 3.86 2.59 3.47 3.02 4.32 20.40 K 2 5.8 3.81 4.35 4.82 5.23 4.43 4.47 27.11 K 3 6.0 1.96 3.46 4.41 6.16 4.88 4.93 25.80 GD 1% 0.609 0.327 0.239 0.604 0.791 0.402 0.1% 0.880 0.455 0.347 0.859 0.725 0.730 *** *** *** *** *** *** Table 2. Copper content in alfalfa (I and IV cut) depending on the applied meliorants in copper polluted acid soils from Karlievo and Chelopech Cu mg / kg Variant K a r l i e v o C h e l o p e c h I cut IV cut I cut IV cut Ca 0 17.4 13.0 41.2 19.0 Caopt. 16.8 10.0 39.0 17.0 K 1 16.8 8.0 28.0 16.0 K 2 16.8 6.0 24.8 14.0 K 3 16.0 6.0 18.0 13.0 5
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