A multi-scale mechanistic framework for biochar in soil: quantifying carbon stabilization and biotic responses in long-term field and mesocosm studies.

Soil Organic Carbon (SOC) dynamics are central to global terrestrial cycling. Biochar represents a long-term carbon store due to its persistence compared with fresh organic matter, offering potential to stock carbon out from the atmosphere. Although biochar seems promising for soil carbon sequestration, many unanswered questions remain to estimate its effect on SOC balance. Understanding biochar's role requires elucidating its partitioning, stabilization, and interactions with soil ecosystems and microbial communities, the primary drivers of SOC transformation and persistence. This research applied a multi-scale framework: 1- SOC fractionation showed biochar incorporation significantly increases Particulate Organic Carbon (POC), while Mineral-Associated Organic Carbon (MAOC) effects vary without consistent change. Pyrolysis temperature and feedstock type influence outcomes; high-temperature biochars (>700 °C) and crop residue-derived ones promote POC accumulation. Pedo-climatic factors like climate and soil pH regulate impacts; arid climates favor POC enhancement, subtropical ones show positive POC but negative MAOC effects. 2- Across Italian vineyard experiments, biochar raised Light Particulate Organic Carbon (LPOC) at all sites and Heavy Particulate Organic Carbon (HPOC) in Merano and Tebano soils. Merano soil alone showed positive priming effect via glucose-induced respiration. 3- Mesocosm study revealed biochar alters carbon stocks and fluxes. Biochar-treated soils exhibited increased soil respiration linked to enhanced microbial degradation of MAOC, with specific bacterial groups decomposing complex organics. Biochar carbon was rapidly detected across soil fractions, including MAOC, and some carbon loss through leaching indicated initial mobilization prior to stabilization. Biochar improved soil hydraulic properties by enhancing pore structure and water retention, thereby affecting microbial habitats and dissolved organic carbon transport. Microbial community shifts were evident under biochar treated soil, with biochar enriching beneficial taxa and saprotrophic fungi while reducing potential pathogens. Overall, biochar drives fundamental changes in soil carbon stocks and microbially mediated fluxes, yet uncertainties remain regarding long-term biochar and native carbon interactions.