Fractionation of applied zinc in rice soils at two moisture regimes and levels of organic matter

We investigated in the laboratory the distribution of applied Zn at two levels (5 and 10 ppm) in two soils (Alfisol and Ultisol) under two different moisture regimes, viz., continuously submerged and alternately submerged, with two levels of organic matter (starch, 0 and 0.5% of soil weight). The soils were sequentially extracted at different periods after incubation with (1) 1.0 M NH4OAc (pH 7.0), (2) 0.05 M Cu(OAc)2, (3) 0.2 M (NH4)2C2O4 (pH 3.0), and (4) CBD solution to extract water-soluble plus exchangeable, organic complexed, amorphous sesquioxide-bound, and crystalline sesquioxide-bound zinc, respectively. Zinc in the extracts was estimated using atomic absorption spectrophotometry and expressed as the percentage of the applied amount after deducting the corresponding control values (0 level).The results showed a greater amount of applied Zn distributed to NH4OAc-and Cu(OAc)2-extractable fractions in the Alfisol than in the Ultisol soil, while the reverse was true for the (NH4)2C2O4-and CBD-extractable fractions; this has been attributed to the lower CEC higher sesquioxide, and lower organic matter content in the latter soil. A larger amount of distribution of applied Zn to (NH4)2C2O4-and Cu(OAc)2-extractable fractions occurred under the continuously than under the alternately submerged moisture regime, and the reverse occurred in respect of the other two Zn fractions. Organic matter application encouraged distribution of the applied Zn to the Cu(OAc)2-and NH4OAc-extractable fractions, though it discouraged such distribution to the (NH4)2C2O4-and CBD-extractable fractions, particularly during the later period of incubation. This has been attributed to the dissolution of sesquioxides under the reduced conditions developed in soil. These transformation processes of applied Zn are discussed in relation to Zn nutrition of wetland rice.