Pools of selenium in some Indian soils at field capacity and submerged moisture regimes

Abstract

Pools of soil selenium (Se) as influenced by moisture regimes were investigated in some normal and seleniferous (which produce vegetation containing >5 mg Se/kg—the maximum permissible level for animal consumption) soils of India. Soil samples were equilibrated with ⁷⁵Se by subjecting to alternate wetting and drying cycles and incubated at field capacity and submerged moisture regimes. Each soil sample was subjected to either single extraction with 0.25 M KCl, 0.1 M KH₂PO₄, and 4 M HCl in a sequence or 7 extractions with KCl followed by 7 extractions with KH₂PO₄ and 2 with HCl. Amount of ⁷⁵Se extracted from different soils incubated at field capacity through single extraction with 0.25 M KCl varied from 8.4 to 30.5% of applied ⁷⁵Se for alkaline and 2.6 to 10.4% for acidic soils. Among alkaline soils, a greater amount of ⁷⁵Se was extracted from highly calcareous non-seleniferous soils than seleniferous soils. The amount of selenium extracted through 7 sequential extractions with 0.25 M KCl was 1.5–7.0 times greater than that extracted through single extraction. A significant coefficient of correlation between ⁷⁵Se extracted through repeated extractions with 0.25 M KCl and pH of the soils (r = 0.93, P ≤ 0.01) suggested greater availability of Se in alkaline than acidic soils. Specifically sorbed soil Se extracted with 0.1 M KH₂PO₄ through 7 sequential extractions ranged from 15.3 to 24.6% for alkaline and 24.4 to 40.1% for acidic soils. The amount of ⁷⁵Se desorbed through multiple extractions with 4 M HCl was negatively correlated with pH (r = –0.95, P ≤ 0.01) and electrical conductivity (r = –0.85, P ≤ 0.05) of the soils. After subjecting to single extraction, residual ⁷⁵Se left in soils incubated at field capacity moisture varied from 11.7 to 59.7%. It was negatively correlated with pH (r = –0.85, P ≤ 0.05) and CaCO₃ (r = –0.79, P ≤ 0.10) and positively correlated with ammonium oxalate extractable iron content (r = 0.76, P ≤ 0.10) of soils. Compared with the single extraction procedure, only a small amount (9–36%) of ⁷⁵Se was left as residual ⁷⁵Se after multiple extractions. Relatively less ⁷⁵Se was released when the alkaline or acidic soils incubated under submergence were subjected to single or multiple extraction procedures. The multiple extraction procedure could better assess different soil Se pools than the single extraction procedure. At field capacity moisture, large amounts of Se occur in easily available forms. Under submerged conditions, substantial amounts of Se are maintained in the soil solution when Se is removed from the solution through plant uptake or leaching.