Olive pomace-derived biochar mitigates gaseous emissions and enhances compost quality during thermophilic phase of composting

Introduction The application of biochar (BC) has gained increasing attention as an effective strategy for reducing gaseous emissions and improving compost quality. This study evaluated the effects of olive pomace (OP)-derived BC produced at two pyrolysis temperatures (300 °C and 600 °C) and applied at two rates (5% and 10%) on gaseous emissions and compost physicochemical properties during the thermophilic phase of OP composting. Methods Five treatments were evaluated, including a control without BC and four BC-amended treatments, each conducted in triplicate using three pilot-scale bioreactors operated sequentially under identical conditions. Emissions of ammonia (NH 3 ), nitrous oxide (N 2 O), methane (CH 4 ), and carbon dioxide (CO 2 ) were continuously monitored over the 9-day thermophilic phase. Data were analyzed using two-way repeated-measures ANOVA to assess the effects of treatment, sampling day, and their interaction, followed by the LSD test at P ≤ 0.05. Results BC addition significantly reduced cumulative NH 3 , N 2 O, CH 4 , and CO 2 emissions by 56-78%, 62-93%, 43-55%, and 79-85%, respectively, compared with the control. The greatest mitigation was achieved with BC produced at 600 °C and applied at 10%. These reductions were associated with improved aeration and enhanced retention of nitrogen and labile carbon. BC also improved compost quality by increasing total nitrogen (up to 1.41%), phosphorus, potassium, and micronutrient contents, while decreasing ammonium concentration, electrical conductivity, bulk density, and the C ratio (18.2), indicating enhanced compost maturity. Discussion OP-derived BC, particularly when produced at 600 °C and applied at 10%, effectively mitigated greenhouse gas and NH 3 while improving compost physicochemical properties and maturity during the thermophilic phase. These findings support the use of BC as a sustainable amendment for enhancing compost quality and reducing the environmental impacts of organic waste management.