TRANSFORMATION OF ZINC FRACTIONS IN RICE SOILS

Abstract

ABSTRACT We investigated in the laboratory the changes in the different forms of zinc in two soils (alluvial and laterite) under two different moisture regimes, viz., continuously submerged and alternately submerged, mith two levels of organic matter (starch application) (0 and 0.5% of soil). The soils were sequentially extracted at different periods after incubation with different extractants: (1) 1.0 M NH4OAc (pH 7.0), to extract water-soluble plus exchangeable Zn; (2) 0.05 M Cu(OAc)2, to extract organic complexed Zn; (3) 0.2 M acidified ammonium oxalate (pH 3.0), to extract amorphous sesquioxide-bound Zn; and (4) CBD solution, to extract crystalline sesquioxide-bound zinc. Zinc in the extract was estimated with the help of atomic absorptions spectrophotometry. The results showed that the NH4OAc-and the CBD-extractable zinc concentration in soils decreased below the initial contents; the magnitude of decrease was always higher under the continuously than under the alternately submerged moisture regime in both soils. Application of starch caused a further decrease in the concentrations of the two zinc fractions, particularly during the initial 15 d of incubation. The Cu(OAc)2-extractable fraction of zinc recorded a decrease during the initial period of incubation, the magnitude of decrease being higher under the continuously than under the alternately submerged moisture regime. Starch application further enhanced the decrease. The effect of continuous submergence and starch application, however, showed a reverse trend during the later period of incubation. The acidified ammonium-oxalate-extractable zinc fraction recorded a progressive increase with the period of incubation and attained a peak value after 15 d of incubation and thereafter remained almost unchanged. Continuous submergence and starch application, in general, favored the increase in Zn content. The decrease in the NH4OAc- and Cu(OAc)2-extractable zinc fractions has been attributed to their adsorption on the surface of amorphous sesquioxide in soil, and that in the CBD-extractable form has been sesquioxide unde the reduced condition developed in soil followed by subsequent adsorption on the amorphous sesquioxide which has a high scavenging power for zinc. The results further showed a net release from or transformation to some more resistant fractions of zinc as influenced by different soil environmental conditions. Such changes in different zinc fractions in soil have important implications for zinc nutrition of wetland rice.