Physical protection of soil organic S studied using acetylacetone extraction at various intensities of ultrasonic dispersion

Soil structure can protect soil organic matter against microbial attack by physical protection within stable soil aggregates. We have evaluated a technique for extracting organic matter from aggregates with different stabilities. Organic bonded-S, organic bonded-P and some polyvalent metals (Mg, Mn, Ca, Fe, Al, Zn and Cu) were measured in aqueous acetylacetone extracts of a native and a cultivated soil subjected to different intensities of ultrasonic dispersion during extraction. Increasing ultrasonic energy input resulted in increased extraction of organic S, organic P and polyvalent metals, until full dispersion was achieved. Dispersion of the cultivated soil required less energy input than dispersion of the native, uncultivated soil. This is consistent with a lower organic matter content resulting in lower aggregate stability. Polyvalent metals are believed to be responsible for the stabilization of aggregates by forming clay-polyvalent metal-organic matter complexes. In our experiment Mn, Ca, Zn and Cu were only related to stabilization of sand-size aggregates, whereas Al, Fe and Mg were also related to the stabilization of microaggregates. Extraction of S was increased 2.0 and 1.3 times with dispersion of the native and the cultivated soils, respectively, showing that a major part of soil organic S is intimately associated with microaggregates. For both soils, a residual S pool of insoluble organic matter comprised about 50% of total organic S. It is suggested that this extraction technique effectively divides soil organic S into non-protected, protected and insoluble organic S.