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 inv estigated the short-term cycling of S into soil organic matter fractio ns protected by soil structure or by the nature of the organic compoun ds. Organic S fractions were labelled by incubation with carrier-free S-35, and then extracted by aqueous acetylacetone, with or without ult rasonic dispersion of the soil. The extracted organic S was fractionat ed according to molecular weight (MW) using gel permeation chromatogra phy. It was found that soil dispersion increased the amount of extract able S by 1.6-3.3 times, suggesting a large part of organic S was phys ically protected inside aggregates. Turnover of S in the protected fra ction was slow, as illustrated by low incorporation of S-35 during inc ubation. The main part of the soil organic S was present in either ver y large organic matter molecules (MW > 100,000 Dal or in small molecul es (700-5000 Da). During the 8-wk incubation, S-35 was initially incor porated into the MW < 700 Da fraction and then recycled into the 700-5 000 Da fraction. Little S-cycling was evident in fractions > 5000 Da, and it is concluded that physical protection of organic S was caused b y both the nature of the organic compounds and by association with cla y in soil aggregates.