Biochar Reduces CO2 Emissions Compared to Sugarcane Straw but Induces Short-Term Priming in Tropical Soil

Biochar is recognised as a feasible carbon dioxide (CO₂) removal technology for achieving net-zero carbon (C) targets to address climate change; however, once applied to the soil, biochar may behave differently compared to fresh biomass. Our study aimed to evaluate the effects of biochar and sugarcane straw on soil CO₂ emission dynamics in a weathered tropical soil. A 56-day incubation experiment was conducted following a completely randomised design, with four replicates and three treatments: soil, soil + straw, and soil + biochar. Gas samples were collected weekly until day 28 to assess the isotope signature of the CO₂ emitted, and until day 56 to determine the total CO₂ emission. Our results demonstrated that biochar was more effective in reducing CO₂ losses as it increased mineralisation rates by 19%, whereas sugarcane straw increased by 126%. Both organic amendments initially induced a positive priming effect; however, for biochar, this response was short-lived. Cumulative, priming effect, and amendment-derived CO₂ emissions had a positive correlation with labile C, oxygen, nitrogen, and CO, which were mainly seen in sugarcane straw. We conclude that, while biochar may be considered an effective tool for reducing CO₂ losses due to its chemical stability, its initial positive priming effect should be taken into account in future studies assessing its C sequestration potential in tropical environments.