Organic Selenium Distribution in Selected California Soils

Abstract The distribution of organic Se was determined in seven soils from central California. Six of these soils (one Typic Pelloxerert, three Typic Torriorthents, one Vertic Torriorthent, and one Entic Chromoxerert) were in a toposequence from an area thought to be relatively high (>1000 µg total Se kg −1 ) in native Se. The seventh soil, Yolo (a Typic Xerorthent), thought to be low in native Se, was included for comparison. Three of the soils in the toposequence had total Se levels less than 1000 µg kg −1 ; the basin soil (Tachi; Typic Pelloxerert) had less than the Yolo soil. A solution of 0.1 M NaOH/0.1 M Na 4 P 2 O 7 (alkaline pyrophosphate) extracted an average of 12% of the total Se, but this ranged from 5 to 32%. Organic Se accounted for 50% or more of the NaOH‐Na 4 P 2 O 7 extractable Se in all but the poorly drained Ciervo clay (Typic Torriorthent), which had about 40% in an organic form. Extracted organic Se compounds were fractionated into humates and hydrophobic and hydrophilic fulvates. Selenium associated with hydrophobic fulvates was high in the soils of intermediate toposequence elevation. Selenomethionine was identified by gas chromatography/mass spectrometry in the hydrophilic fulvate fraction of the Yolo soil and in three soils from the toposequence. The high percentage of extractable Se associated with organic matter in these soils represents a pool of potentially plant‐available Se.