Contrasting mechanisms of soil carbon sequestration in upland soils: Fungal-driven physical protection with straw versus bacterial-driven mineral stabilization with biochar.

Understanding the distinct mechanisms by which straw and biochar regulate soil organic carbon (SOC) sequestration is essential for optimizing agricultural practices. To investigate the physical-microbial coupling mechanisms of SOC sequestration, we amended three upland soils collected from Yuzhong (YZ), Yangling (YL), and Changwu (CW) of the Loess Plateau with 13C-labeled straw and biochar. Straw promoted the formation and carbon (C) enrichment of macroaggregates (>0.25 mm), increasing macroaggregate-associated SOC by 40-110% in YZ and YL soils. Conversely, biochar increased the proportion of silt-clay fractions (<0.053 mm) and elevated SOC therein, with a 257% increase observed in CW soil. Straw stimulated certain r-strategist bacteria such as Proteobacteria and Bacteroidota, whereas biochar favored K-strategists such as Actinobacteriota, leading to more complex bacterial co-occurrence networks and greater niche differentiation. Path modeling and correlation analyses revealed two distinct sequestration pathways: a fungal-driven physical protection pathway under straw, wherein fungi promoted macroaggregate formation that physically protected particulate organic C (POC); and a bacterial-driven mineral stabilization pathway under biochar, wherein its chemical stability and porous structure enhanced organic matter adsorption and bacterial activity, promoting the formation of stable mineral-associated organic C (MOC). Specifically, straw was better suited for coarse-textured or low-SOC soils with unsaturated aggregate formation, whereas biochar was more effective in fine-textured or high-SOC soils with abundant mineral surfaces for MOC stabilization. Matching amendment type to soil conditions could therefore maximize C sequestration potential in dryland farming systems.