Microaggregation and organic matter storage at the silt-size scale

The relationship between soil aggregation at the silt-size scale, and soil organic C dynamics has scarcely been studied. In this work, we developed a fractionation protocol comprising complete dispersion of the fraction > 50 μm of the original soil sample, and subsequent size and density fraction of the < 50 μm fraction to isolate stable aggregates from the silt-size fraction of a silty cultivated temperate soil. This fraction also contained non-aggregated silt-size mineral particles and small amounts of non-occluded light organic particles. Slaking-resistant silt-size aggregates stored almost half of the total soil organic C (TOC) as organic matter bound to silt (8% TOC) and clay minerals (28% TOC) and as occluded organic particles (8% TOC). Clay mineralogy was related to aggregation at this scale, since aggregates were selectively formed from 2:1 clays, while quartz and kaolinite-like clays were preferentially found in the non-aggregated clay fraction. We conclude that stable microaggregates are formed within the silt-size fraction and store organic C both by adsorption and by entrapment of small organic particles. The turnover rate of organic carbon in such microstructures, their sensitivity to land use and cultivation practices, and their importance in different soil types remain to be determined.