Physical protection of soil organic S studied by extraction and fractionation of soil organic matter

Physical protection of soil organic S may, in part, be responsible for the constraints on the ability of soils to supply S to plants. We investigated the short-term cycling of S into soil organic matter fractions protected by soil structure or by the nature of the organic compounds. Organic S fractions were labelled by incubation with carrier-free 35S, and then extracted by aqueous acetylacetone, with or without ultrasonic dispersion of the soil. The extracted organic S was fractionated according to molecular weight (MW) using gel permeation chromatography. It was found that soil dispersion increased the amount of extractable S by 1.6 – 3.3 times, suggesting a large part of organic S was physically protected inside aggregates. Turnover of S in the protected fraction was slow, as illustrated by low incorporation of 35S during incubation. The main part of the soil organic S was present in either very large organic matter molecules (MW > 100,000 Da) or in small molecules (700–5000 Da). During the 8-wk incubation, 35S was initially incorporated into the MW < 700 Da fraction and then recycled into the 700–5000 Da fraction. Little S-cycling was evident in fractions >5000 Da, and it is concluded that physical protection of organic S was caused by both the nature of the organic compounds and by association with clay in soil aggregates.