Aggregation and availability of phosphorus in a Technosol constructed from urban wastes

To preserve natural soil resources and in order to create fertile constructed Technosols for plant cultivation, wastes and by-product mixtures were studied in relation to their pedogenic properties and especially soil organic matter contents. We assessed interactions between aggregation and nutrient availability, focusing on phosphorus (P) transfer in the soil-water-plant system. Four typical urban wastes, dried and sieved to pass 2 mm, were mixed selectively to mimic a fertile topsoil material: excavated subsoil AE, compost from sludge and green wastes CO, green wastes GW, and bricks BR. After characterization of the wastes for physico-chemical and toxicological parameters, we focused on four mixtures: AE/CO, AE/GW, BR/CO, and BR/GW. The mixtures were tested in a 55-day long pot experiment under controlled conditions in a climate chamber. Pots were bare and planted with Lolium perenne (ryegrass) and Brassica napus (rape). The two plant species were selected for contrasting root activities and architectures and phosphorus (P) acquisition strategies. The aggregate formation was tested using the mean weight diameter method at the end of the experiment. We have measured intense aggregation in mixture AE/GW, low aggregation in AE/CO, and no aggregation in BR/CO and BR/GW. After 55 days, neither Technosol aggregation nor aggregate stability was significantly affected by plant development. Available phosphorus (P-Olsen) content was sufficient for plant development in all the mixtures (from 0.28 to 0.58 g kg(-1)). The P-Olsen/P-total ratio was higher in mixtures with GW, even if the mixtures with compost (AE/CO and BR/CO) induced the highest biomass production for ryegrass and rape. The nutrient availability in constructed Technosols and the transfer of P to plant were highly dependent on organic matter type, with high or low delivery of P-Olsen linked to the mineralization potential and the size and distribution of aggregates. Therefore, pedological engineering processes can be improved by the selection of adapted constitutive wastes and by-products to create a fertile substrate allowing high biomass production.