Influence of soil properties on Zn solubility dynamics under different redox conditions in non-calcareous soils

Zinc deficiency in paddy soils is often a problem for rice production. Flooding can decrease availability of metals in non-calcareous soils through different mechanisms associated with the redox status of the soil. Laboratory experiments were combined with equilibrium modeling to explain the processes that governed the dynamics of Zn in non-calcareous paddy soils at varying redox potentials (Eh). Air-dried soil samples were submerged and incubated in a reaction cell under N2 for 4 weeks with continuous stirring, and then purged with compressed air for another week to reoxidize the system. The Eh of the four contrasting soils started at 120 to 300 mV, decreased to -220 to -300 mV after 100 to 250 hours of reduction, and was maintained at this low plateau for about two weeks before increasing again upon reoxidation. Zn solubility showed contrasting patterns in the four soils, with two of the soils showing a decrease in soluble Zn as the Eh became low, probably due to ZnS precipitation. In contrast, the other two soils showed that Zn solubility was maintained during the reduced phase which could be due to competition with Fe for precipitation with sulfide and Zn binding to Mn oxides. In this study, important soil parameters, primarily total S, SO42--S, and Fe- and Mn-(hydr)oxides, have been identified to explain the decrease of Zn availability after flooding.