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

Abstract

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.