Organic matter controls aggregate stability in loam soils. Intensive farming can lead to a decrease in soil organic matter content. In areas where livestock have disappeared, the recycling of composted urban organic wastes on agricultural soils may represent a valuable source of organic matter for restoring soil organic matter content. The effects on the aggregate stability of a silt loam soil of three urban composts (a municipal solid waste compost, a co‐compost of sewage sludge and green waste, and a biowaste compost) sampled at two different stages of maturity (immature and mature composts) were studied during laboratory incubations. The results were related to (i) compost organic matter biodegradability, biochemical fractions, and humic substance content, (ii) microbial activity evaluated through organic C mineralization and microbial and fungal biomass evolution, (iii) hot‐water‐extractable polysaccharides, and (iv) aggregate hydrophobicity as revealed by the water drop penetration time test. Both immature and mature composts increased aggregate stability via different mechanisms. After immature compost addition, the enhanced microbial activity mainly improved aggregate stability by increasing water repellency. The fungal biomass was particularly involved in aggregate stabilization. The municipal solid waste compost was more efficient at improving resistance to slaking, probably because of its larger labile organic pool that enhanced microbial activity. The addition of mature composts immediately improved aggregate stability with similar efficiency for all composts but to a lesser extent than improvements from immature composts. The observed increase of interparticular cohesion could be due to the inward diffusion of binding organic substances within the aggregates.

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