Biochar use as a climate-smart strategy: Implications for the mineralization of distinct carbon pools in tropical systems

Biochar is regarded 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. Due to the C content of fresh biomass, such as straw, soil microorganisms may initially rapidly mineralize the material. Contrastingly, biochar – a carbonized material obtained by thermochemical transformations– may endure the enzymatic metabolization by the exoenzymes produced by soil decomposers. This recalcitrance is mainly attributed to the high temperatures applied during its formation, which increases the amount of condensed and aromatic structures in its composition that are more resistant to degradation. However, few studies around biochar’s mineralization dynamics were conducted in tropical regions, which naturally present a more intense C turnover due to climatic conditions. Therefore, our study aimed to evaluate the effects of biochar and sugarcane straw on mineralization dynamics in a weathered tropical soil. A 56-day incubation experiment was conducted following a completely randomized 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 for total CO₂ emission. Our results demonstrated that biochar was more effective in reducing C losses as it increased mineralization rates by 19%, while sugarcane straw increased by 126%. Despite biochar exhibiting lower total CO₂ emissions than straw, it stimulated the decomposition of native soil organic C during the first month after its application. Moreover, both organic amendments initially induced a positive priming effect (PE) in the soils. However, the biochar positive-priming effect was short-lived, reaching values near zero 28 days after its application, with soils amended with sugarcane straw presented constantly increasing PE values. The CO₂ emissions from all C pools, except native SOC, positively correlated with labile- and -oxidized C forms, 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 effects on native SOC mineralization and positive PE should be considered in future studies assessing its C sequestration potential in tropical environments. This would ultimately enable more accurate forecasting of C accrual patterns under biochar-amended soils, contributing to achieving several Sustainable Development Goals, such as “Climate action” and “Life of land.”