SOIL & WATER MANAGEMENT & CONSERVATION Soil Aggregate Stability Improvement with Urban Composts of Different Maturities

oil structure is an important feature of agrosystem sustain-ability because of its involvement in many biological and physical soil processes. Good structure favors gas and water transfer in soils, germination, and crop rooting, while reducing susceptibility to erosion.Aggregate stability is highly dependent on organic mat-ter contents and dynamics in loam soils, since these soils are poor in mineral-stabilizing agents such as clays, oxides, and hydroxides. Different organic fractions contribute to aggregate stabilization, including microbial biomass in particular fungi (Degens, 1997), microbial-derived polysaccharides (Haynes and Francis, 1993), humic substances (Piccolo and Mbagwu, 1999), and lipids (Dinel et al., 1991). Two major mechanisms of aggregate stabilization are considered for soil organic matter: (i) the increase in aggregate cohesion through the binding of mineral particles by organic substances or physical enmeshment of particles by fungi or root fi laments; and (ii) the decrease of wettability, slowing down the water penetration rate and the slaking risk during aggregate wetting (Chenu et al., 2000).In France, 13% of the cultivated soils are loam soils with low organic matter contents due to intensive soil cultivation. These soils are at high risk for water erosion (Le Villio et al., 2001). In these loam soils, aggregate breakdown by water is mainly related to slaking after compression of entrapped air during wetting and mechanical breakdown from raindrop impact (Le Bissonnais, 1996).The addition of exogenous organic matter has been shown to improve soil aggregate stability, with farmyard manure being a traditional source. In many cropping areas where livestock production has disappeared (such as in the Paris Basin), how-ever, there is a shortage of these manures. On the other hand, in nearby suburban areas, the composting of organic municipal waste including sewage sludge, green waste from private and public gardens, and municipal solid waste may produce alter-native sources of exogenous organic matter that could be used as soil amendments to increase soil organic matter contents and improve related physical properties including soil aggre-gate stability.