Modeling and simulation of sulfur availability in paddy soils under reducing environment considering heavy metal content.

Insufficient sulfur (S) can reduce rice yield under certain soil conditions. The availability of S in paddy soils is determined by the concentrations of inorganic S and some heavy metals that react with sulfide ions to form sparingly soluble sulfides. In order to evaluate the S fertility of paddy soils, a predictive model was formulated to simulate the formation of sulfides and the effect of heavy metal content on the concentration of sulfate ions (SO₄^2-) in the soil solution under reducing conditions. The basic computational algorithm of the model was an iterative method incorporating the equilibria of major aqueous species, sparingly soluble salts, and soil adsorption. A field survey was also conducted to evaluate S availability in actual rice paddy soils. SO₄^2- and heavy metal contents were measured in 250 paddy fields located in central Japan. The model calculation using the field survey results showed that as the redox potential (Eh) decreases from −80 to −170 mV, sulfide-forming metals (Ni, Cu, Zn, Cd, and Pb) take precedence over iron (Fe) in sulfide formation. The major sulfides present were chalcocite (Cu₂S), greenockite (CdS), sphalerite (ZnS), galena (PbS), chalcopyrite (CuFeS) and α-NiS, Due to their predominance in paddy soil, zinc (Zn) and copper (Cu) have a strong influence on the reduction of SO₄^2- concentration. The formation of metal sulfides increases the Eh value corresponding to the critical SO₄^2- concentration and decreases the S availability under the individual Eh condition. The model calculation also showed that an increase in sulfide-forming metal content leads to S deficiency, even if the SO₄^2- content is not exceeded. Model calculations for each geographical division against the field survey results suggest that S deficiency is more likely to occur in lowland paddy fields formed by river sediments than in Andisols on terraces. [ABSTRACT FROM AUTHOR]