ZINC EXCHANGEABILITY AS A FUNCTION OF PH IN CITRIC ACID-AMENDED SOILS


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Abstract

Organic C additions and changes in soil chemical properties caused by waste disposal, root exudation, and organic matter (OM) decomposition processes affect heavy metal solubility, mobility, and bioavailability. The solubility and exchangeability of zinc (Zn) were determined in two contrasting Maryland A horizon soils to evaluate the influence of an organic C amendment (citric acid) on Zn retention and cation exchange capacity as a function of pH, two properties that interact to control Zn speciation in soils. Water soluble Zn in the A horizons of Galestown loamy sand (Psammentic Hapludults) and Jackland silt loam (Typic Hapludalfs) soils, with and without citric acid addition, decreased with increasing pH from 4.0 to 7.0 and with increasing cation exchange capacity (CEC) from 4.7 to 96.7 mmolc/kg soil. At pH values < 5.0, the water soluble Zn fraction dominated and became exchangeable Zn as pH increased from 5.0 to 7.0 in the Galestown soil, whereas the exchangeable Zn fraction dominated from pH 4.0 to pH 6.0 and became nonexchangeable Zn as pH increased to 7.0 in the Jackland soil. Citric acid addition, compared with no citric acid addition, lowered (P < 0.05) the solubility of Zn and favored the exchangeability of Zn at pH < 5.0, and this effect of C addition on the solubility of Zn was more pronounced in the coarse-textured Galestown sandy soil than it was in the finer textured Jackland soil. The addition of citric acid at a Zn/citric acid molar ratio of 1:1 enhanced nonexchangeable Zn through chemisorption at pH > 6.0, whereas the effect of citric acid on levels of nonexchangeable form of Zn retention was not significant at pH < 6.0. The citric acid addition caused an abrupt transition from water soluble and exchangeable to nonexchangeable forms of retention and reduced the potential availability of Zn in the Jackland soil as soil pH approached 6.0. The results indicate that the pH influence on CEC was the dominant factor influencing the effect of citric acid on the redistribution of water soluble, exchangeable, and nonexchangeable Zn, especially in the finer textured soil. These results have implications for controlling soil pH to minimize Zn activity and bioavailability in soils used for organic waste disposal.

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