Decoupling soil organic carbon fractions and microbial community composition across aggregate size classes driven by salinity in arid loess

Saline‐alkali soils are globally widespread and represent a significant reservoir of soil organic carbon (SOC), yet SOC stability under saline conditions remains poorly understood. This study investigated changes in SOC fractions and microbial community composition across aggregate size classes (>2, 0.25–2, 0.053–0.25, and <0.053 mm) in saline‐alkali loess from central Gansu Province, Northwest China. Results showed that the 0.25–2 mm aggregate fraction dominated soil mass (44.53%–47.81%). With increasing salinity, macro‐aggregate (>0.25 mm) proportion decreased from 70.33% to 64.81%, while micro‐aggregate (<0.25 mm) increased from 30.82% to 33.80%; accordingly, aggregate stability reduced. High salinity significantly reduced total organic carbon (TOC), particulate organic carbon (POC), and mineral‐associated organic carbon (MOC) contents across all aggregate fractions, with reductions of 14.69%–43.95%, 5.96%–26.61%, and 18.64%–51.68%, respectively. The highest TOC content (6.90–10.84 mg/g) was found in 0.053–0.25 mm fraction, while the highest POC/TOC ratio occurred in the >2 mm fraction. By contrast, both the MOC/TOC and MOC/POC ratios peaked in the 0.053–0.25 mm fraction, suggesting that the persistent SOC components are largely stored in micro‐aggregates. Correspondingly, bacterial and fungal community diversity and abundance were higher in the 0.053–0.25 mm and 0.25–2 mm fractions, and approximately 67.71% of microbial necromass carbon (MNC) was retained in micro‐aggregates. Partial Least Squares Path Modeling further revealed that fungal communities and their derived MNC play a more significant role in maintaining aggregate stability and preserving persistent SOC fractions. These findings provide new insights into aggregate protection of SOC and microbial community dynamics in arid saline‐alkali loess.