Fractionation of Copper in Soils as Influenced by Waterlogging and Application of Crop Residues

Redox potential and pH are two soil conditions that exert major influence on the solubility and bioavailability of metals including copper (Cu). A study was conducted to investigate the effect of waterlogging on fractions of Cu in four soils with different pH values. Alfalfa powder, as an organic material, was also applied to induce more severe reducing conditions in the waterlogged soil samples. Bulk samples were collected from four cultivated areas in Iran: an acid soil from a tobacco field, a neutral soil from a rice paddy around Anzali wetland area, and two calcareous soils from wheat and alfalfa fields. The soil samples were treated with 0 and 2% alfalfa powder and then incubated for 12 weeks under two aerated (60% water holding capacity) and waterlogged (1:2.5, soil: water suspension) conditions at constant temperature (25oC). Subsamples were taken after 1and 12 weeks of incubation and various fractions of Cu were also extracted using a sequential extraction technique. The results in both conditions (aerated and saturated) showed that alfalfa powder application increased soluble and extractable Cu initially, but, during the incubation time, decreased them in almost all soils. However, in waterlogged condition, this treatment, increased Cu concentration in amorphous iron oxides (AFeO) fraction and decreased that in residual (RES) fraction in all soils, except the one taken from the rice fields. The small changes in the Cu fractionation in rice soil may be attributed to the neutral pH as well as the higher organic matter and the initial waterlogged condition as compared to the other soils. This treatment also increased Cu concentration bonded to carbonate (CAR) only in the two calcareous soils. The decrease in redox potential induced by waterlogging increased the mobility factor of Cu, indicating enhanced Cu availability.