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.

For economic, agricultural, and environmental reasons, composting is frequently used for organic waste recycling. One approach to limiting the potential risk from bacterial food-borne illnesses is to ensure that soil amendments and organic fertilizers are disinfected. However, more knowledge concerning the microbiological safety of composted substrates other than sludge and manure is necessary. Experimental in-vessel biowaste composts were used to study the survival of seeded Listeria monocytogenes, Salmonella enterica subsp. enterica serotype Enteritidis, and Escherichia coli. Four organic waste mixtures, containing various proportions of paper and cardboard, fruits and vegetables, and green waste, were composted in laboratory reactors with forced aeration. The physicochemical and microbiological parameters were monitored for 12 weeks during composting. The survival of bacteria over a 3-month period at 25°C was assessed with samples collected after different experimental composting times. Strain survival was also monitored in mature sterilized composts. Nonsterile composts did not support pathogen growth, but survival of seeded pathogens was observed. Salmonella serovar Enteritidis survived in all composts, and longer survival (3 months) was observed in mature composts (8 and 12 weeks of composting). Mature biowaste composts may support long-term survival of Salmonella serovar Enteritidis during storage at room temperature. E. coli and L. monocytogenes survival was observed only in 4-week-old composts and never in older composts. Proper composting may prevent long-term survival of E. coli and L. monocytogenes. These results suggest that like composted sewage sludge or manure, domestic waste composts may support pathogen survival. Survival was not related to the physicochemical characteristics of the composts.The presence of pathogens in food and outbreaks of foodassociated diseases can be linked to the poor microbial quality of raw agricultural products (5, 9). Furthermore, the transfer of microbial pathogens, such as Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella enterica serovar Typhimurium, from amended soil to fresh agricultural products has been described (3,24,37). Composting is a time-honored biological treatment used for organic wastes; the resulting composts are useful in agriculture and horticulture for improving many soil attributes. However, microbial safety of organic amendments and fertilizers used in agriculture is required to prevent colonization by food-borne illness pathogens, like E. coli O157:H7 and Salmonella spp. (9). In order to reduce the likelihood of pathogen transfer from waste to fresh produce, several international standards for meeting pathogen limits through composting are available (14, 39). According to the U.S. Environmental Protection Agency (US-EPA) standard, class A composts must not exceed maximal limits for either Salmonella spp. (less than 3 most probable numbers [MPN]/4 g) or thermotolerant coliforms (less than 1,000 MPN/g) (39). A draft of a forthcomin...

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