Assessment of Waste-Derived Biochars on the Health and Biological Activity of Soil

Biochar is the product obtained by heating biomass through pyrolysis at high temperatures and has emerged as a common soil amendment based on its potential to improve soil properties such as water retention, pH, and nutrient availability. However, biochar prepared from certain waste materials can exert negative ecotoxicological effects on soil biota. The overall aim of this study was to evaluate the impact of selected biochars derived from coffee grounds, spent hops, spruce wood, and cigarette butts on soil health, focusing on microbial activity, plant growth, and potential ecotoxicity. Biochar-amended soils were incubated for 15 days in total, and a series of assays were performed, including dehydrogenase activity, bacterial quantification, a phytotoxicity bioassay using seeds of Sinapis alba, and an ecotoxicity test using Enchytraeus albidus. Biochar amendments can alter soil pH and impact water retention. Cigarette-butt biochar showed the highest stimulation of bacterial colonies, with counts reaching up to 1.00 × 10⁶ CFU/mL compared to 8.47 × 10⁴ CFU/mL across control replicates, whereas coffee-grounds biochar enhanced dehydrogenase activity (3.53 × 10^-3 mU/g), implying higher microbial metabolic capacity. Phytotoxicity tests indicated that plant growth was enhanced by hops (162.96%) and spruce-wood biochars (206.66%), whereas higher application rates of cigarette-butt biochar suppressed seedling growth (62.36% inhibition). The ecotoxicity test with Enchytraeus albidus indicated that all biochars had a detrimental effect except the cigarette-butt biochar (33% stimulation). The findings of this study highlight that the effect of biochar on soil health is source-dependent, with important implications for soil management. The study reveals the need for biochar application adjustments based on concentration and type to maximize its benefits while minimizing potential hazards to soil systems.