Choice of pyrolysis parameters for urban wastes affects soil enzymes and plant germination in a Mediterranean soil

The production of organic waste has steadily increased in recent years, with subsequent impact on the environment. The European Union committed to diminish the volume of biodegradable municipal waste disposed of in landfills by 2016-2020. The synthesis of biochar from urban waste and its application to improve soil quality can constitute a novel route for valorization. The aim of this paper was to study the effect of three biochars originated from pyrolysis of the organic fraction of urban waste at two different temperatures (300°C and 500°C) and two residence times (1h and 5h) on the biochemical properties of an agricultural soil. Soil was amended with biochars at a rate of 8% and incubated for 74days. A phytotoxicity assay, using garden cress as the test species, was conducted. CO₂ emissions, microbial biomass C and the enzymes dehydrogenase, phosphomonoesterase and β-glucosidase were measured in tested soils. Biochars prepared at 300°C resulted in lower germination index values, which could partly be ascribed to a higher bioavailability of heavy metals and higher soluble organic matter, while the biochar prepared as 500°C exhibited a phytostimulant effect. Biochars produced at 300°C (B300-1h, B300-5h) augmented soil CO₂ emissions while there was no effect on microbial respiration in the soil amended with the biochar prepared at 500°C. Pyrolysis temperature and, for some enzymes, residence time, controlled soil enzymatic activity.