Soil-Specific Biochar effects on Carbon and Nitrogen Stabilization through Aggregate-Scale Structural Reorganization and Enzymatic Responses

Biochar amendments are increasingly used in farming systems to improve soil health, but their field effects remain inconsistent due to limited understanding of impacts at aggregate scale. In this study, we conducted a two-year field study using two biochar types in two different soils linking aggregate level measurements to bulk soil carbon and nitrogen accumulation, extracellular enzyme activities, and respiration. Sugarcane bagasse biochar (SB) enhanced macroaggregate formation and increased geometric mean diameter (GMD) in loess-derived Gigger soil, promoting organic carbon and nitrogen co-localization within 2–0.25 mm and >2 mm fractions, whereas wood biochar (WB) instead drove aggregate dispersion, leading to redistributed carbon toward microaggregate and mineral-associated fractions. Extracellular enzyme activities (β-glucosidase and N-acetyl-β-glucosaminidase) increased in alluvial Cancienne soil, where aggregate reorganization improved substrate accessibility, while activities remained suppressed in the Gigger soil where stronger macro-aggregation limited physical access to substrates. Principal component analysis revealed two distinct biochar-induced soil functional states: biologically driven carbon turnover in the less structured, lower carbon Cancienne soil, and aggregate mediated carbon protection in the more stable, higher carbon Gigger soil. Together, these findings indicate that biochar regulates soil function primarily through reorganizing aggregate structure, and that effective carbon and nutrient stabilization depends on how well biochar properties match the existing structural capacity of the soil. The latter is important for soil health improvement and farming system crop production sustainability.