Olive Pomace-Derived Biochar as a Composting Additive: Interactive Effects of Pyrolysis Temperature and Application Rate on Compost Quality

The sustainable management of olive pomace (OP) residues remains a major environmental challenge, requiring integrated strategies that enhance resource recovery while minimizing environmental impacts. This study evaluated the effects of OP-derived biochar (BC) produced at two pyrolysis temperatures (300 and 600 °C) and applied at two rates (5 and 10%, w/w) on composting performance and the physicochemical, biological, and nutrient characteristics of OP compost. Composting was initiated in a pilot-scale forced-aeration bioreactor to accelerate the decomposition of the composting mixture, after which the materials were transferred to indoor piles to complete the composting and maturation process. Two-way ANOVA showed that both BC pyrolysis temperature and application rate significantly affected several compost quality attributes, although their relative influence varied among the measured properties. The observed responses were consistent with differences in the physicochemical characteristics of the OP-derived BC resulting from the pyrolysis conditions. Compared with BC produced at 300 °C, BC produced at 600 °C significantly reduced moisture content (MC) from 17.81 to 11.44%, decreased bulk density (BD) from 0.69 to 0.52 g cm−3, increased organic matter (OM) from 70.61 to 82.78%, and raised compost pH from 7.67 to 8.43 (p < 0.001), and reduced microbial respiration from 2.00 to 1.60 CO2 g−1 OM day−1 (p = 0.0270). Increasing the BC application rate from 5 to 10% significantly enhanced nitrogen (N) (1.64–1.79%), phosphorus (P) (0.57–0.67%), ammonium (NH4+) (162–214 mg kg−1), nitrate (NO3−) (42.98–52.42 mg kg−1), sulfur (S) (0.35–0.39%), and compost stability index (SI). Overall, the higher BC application rate, particularly when combined with BC produced at 600 °C, generally resulted in the most favourable compost characteristics, although several responses were statistically comparable with those obtained using BC produced at 300 °C and applied at 10%. Germination indices of 97.5–135.8% indicated that the final composts exhibited low phytotoxicity and generally mature. These findings indicate that integrating OP-derived BC into OP composting can improve the quality and stability of the resulting compost while providing an integrated strategy for the valorization of within circular bioeconomy approach.