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Distribution and bioavailability of selenium fractions in some seleniferous soils of Punjab, India

Soil-solid phase associations of Se in seleniferous soils of Punjab were investigated by following sequential extraction procedures involving multiple extractions with 0.2 M K2SO4 (2 times), 0.1 M Na2SeO3 (4 times), 0.05 M NH4OH (4 times), 6 M HCl (2 times) and 9 M HNO3 (2 times) vis-a-vis single extractions with 0.25 M KCl, 0.1 M KH2PO4, 4 M HCl and concentrated HCl. Soil samples were equilibrated with 75Se (as Na2SSeO3) @ 9.25 kBq g - 1 soil by incubating at field capacity moisture regime and subjecting to alternate wetting and drying cycles. Following multiple extraction procedure, out of total 75Se added, 8.8 - 26.1% was present in readily available form (0.2 M K2SO4 extractable); 27.6 - 49.0% as isotopically exchangeable (0.1M Na2SeO3 extractable) and 5.3 - 12.0% as organic Se (0.05 M NH4OH extractable). Selenium extractable in K2SO4 was significantly correlated with free iron (r = - 0.774, p < 0.05) and CaCO3 (r = 0.670, p <0.10) content of the soils. Negative relationship was observed between Se uptake by maize (Zea mays L.) and ammonium hydroxide extractable (r = - 0.752, p <0.05) as well as residual Se (r = - 0.726, p <0.05) in soils. Highly positive coefficients of correlation between isotopically exchangeable Se and Se content (r = 0.851, p <0.01) as well as its uptake by maize (r = 0.841, p <0.01) indicated that the isotopically exchangeable form of Se may be considered as an index of bioavailable Se in seleniferous soils of Punjab. None of the fractions defined by following single extraction procedure was correlated with either the soil characteristics or Se uptake by maize plants. Multiple extraction procedure could, thus, better explain the distribution of Se in different fractions and uptake by plants.