Biochar particle size controls carbon stability and fire vulnerability in Cerrado Oxisols

Highly reactive, fine-particle biochars are particularly effective in recarbonizing degraded soils, but they can be more vulnerable to fire degradation due to higher surface area. How biochar particle size influences its persistence in the environment remains poorly understood, particularly in regions affected by fire events such as the Cerrado. In this study, we investigated the stability of micrometric (200 µm) and nanometric (<50 nm) açaí seed biochar incubated in an Oxisol for 180 days, followed by simulated burning using a propane–butane torch. Biochar amended soil had higher total C and nitrogen (N) contents, with a 43% increase for microbiochar and 88% for nanobiochar in the mineral-associated organic matter (MAOM) fraction, indicating incorporation into stable pools. Burning reduced C associated with the particulate organic matter (POM) more when amended with microbiochar than with nanobiochar, demonstrating their vulnerability fire. An increase, up to fivefold, in dissolved organic carbon (DOC) and nitrogen (DN) contents occurred after burning. XRD data showed no mineralogical differences among samples, while FTIR indicated loss of polysaccharides after burning. TGA revealed that microbiochar was more thermally stable under unburned conditions but lost stability after burning. In contrast, nanobiochar was less stable in unburned soil but became more stable following burning, resulting in lower mass loss. The reduced mass loss in the burned control is attributed to its ~40% lower soil C content. Biochar’s particle size governs carbon stability, fire vulnerability, and post-burn soil resilience, thereby using nanobiochar for fire-prone areas act as strategy improving the management of agricultural soils.