Early structural stability of fine dam sediment in soil construction

Fine sediments accumulate upstream of hydroelectric dams. To ensure that dams can operate properly, most sediments are returned downstream reaches, but a portion of them have to be dredged and land managed. In parallel, using topsoil from agricultural parcels for urban greening is currently controversial because arable surface areas are decreasing. An alternative idea for protecting these natural resources consists in reusing fine dredged sediment to construct functional soils. The agronomical use of fine dredged sediment raises the question of its ability to provide acceptable physical properties for plant growth. Six dredged sediments with contrasted initial agronomical properties were mixed or not with green waste compost and submitted or not to drying-wetting cycles in a 105-day-long experiment under greenhouse conditions. The sediments were analyzed for their capacity to generate stable aggregates. We measured the proportion of macro aggregates > 2 mm with dry-sieved aggregate (DSA) fraction and the > 0.25 mm water-stable aggregate (WSA) fraction for each treatment over time. We also assessed the main sediment physicochemical properties, microbial biomass carbon, and carbon mineralization potential. Compost addition (40% in volume) enhanced the WSA fraction of the low-organic-matter-content sediments (< 30 g kg(-1)), whereas it had a non-significant effect in the other sediments. Multiple linear regressions highlighted a determining role of organic matter in the WSA fraction. At this scale of aggregation, organic matter seemed to mainly interact with clay in an acidic context. Compost addition had either no significant effect or a negative effect on the DSA fraction. Multiple linear regressions suggested that organic matter was not clearly involved in DSA formation in the sediment, whereas macro-porosity was. The first drying-wetting cycles enhanced the DSA proportion of the sediments, but the following cycles had a negative effect. We suggest that disaggregating mechanisms gradually predominated over aggregating mechanisms over time. All sediments showed acceptable physicochemical properties for plant growth. WSA formation in low-organic-matter-content sediments was improved by adding compost. Drying-wetting cycles had a controversial effect on DSA formation, and only the very first cycles had a beneficial effect on macro-aggregation. The mechanisms involved in aggregation and aggregate stability in soil constructed from sediments seem to be similar to those involved in natural soils. That is why we propose that the soil hierarchical aggregation model initially developed for natural and cultivated soils could be adapted to sediments.