Biochar incorporation increased nitrogen and carbon retention in a waste-derived soil.

The synthesis of manufactured soils converts waste materials to value-added products, alleviating pressures on both waste disposal infrastructure and topsoils. For manufactured soils to be effective media for plant growth, they must retain and store plant-available nutrients, including nitrogen. In this study, biochar applications were tested for their ability to retain nitrogen in a soil manufactured from waste materials. A biochar, produced from horticultural green waste, was added to a manufactured soil at 2.5 and 10% (by weight), then maintained at 15 degrees C and irrigated with water (0.84 mL m(-2) d(-1)) over 6 weeks. Total dissolved nitrogen concentrations in soil leachate decreased by 25.2, 30.6 and 44.0 % at biochar concentrations of 2, 5 and 10 %, respectively. Biochar also changed the proportions of each nitrogen-fraction in collected samples. Three mechanisms for biochar-induced nitrogen retention were possible: i) increased cation and anion exchange capacity of the substrate; ii) retention of molecules within the biochar pore spaces; iii) immobilisation of nitrogen through microbial utilisation of labile carbon further supported by increased soil moisture content, surface area, and pH. Dissolved organic carbon concentrations in leachate were reduced (-34.7 %, -28.9%, and -16.7 %) in the substrate with 2, 5 and 10% biochar additions, respectively. Fluorescein diacetate hydrolysis data showed increased microbial metabolic activity with biochar application (14.7 +/- 0.5, 25.4 +/- 53, 27.0 +/- 0.1, 46.1 +/- 6.1 mu g FL g(-1) h(-1) for applications at 0, 2, 5, and 10%, respectively), linking biochar addition to enhanced microbial activity. These data highlight the potential for biochar to suppress the long-term turnover of SOM and promote carbon sequestration, and a long-term sustainable growth substrate provided by the reuse of waste materials diverted from landfill. (C) 2019 Elsevier B.V. All rights reserved.