Influence of Soil Properties on Zinc Solubility Dynamics Under Different Redox Conditions in Non-Calcareous Soils

Zinc (Zn) deficiency in paddy soils is often a problem for rice production. Flooding can decrease metal availability in some non-calcareous soils through different mechanisms associated with soil redox status. Laboratory experiments were performed in order to better understand the processes that governed the dynamics of Zn in non-calcareous paddy soils at varying redox potentials (Eh). Air-dried non-calcareous soil samples collected from four different paddy field sites in the Philippines were submerged and incubated in a reaction cell with continuous stirring and nitrogen purging for 4 weeks, and then purged with compressed air for another week to reoxidize the system. The Eh of the four soils started at 120 to 300 mV, decreased to −220 to −300 mV after 100 to 250 h of reduction, and was maintained at this low plateau for about 2 weeks before increasing again upon reoxidation. Zinc 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 zinc sulfide (ZnS) precipitation. In contrast, the other two soils showed that Zn solubility was maintained during the reduced phase which could be due to the competition with iron (Fe) for precipitation with sulfide. Differences in the relative amounts of S, Fe, and manganese (Mn) oxides in the four soils apparently influenced the pattern of Zn solubility after flooding.