Distributions of Organic Carbon and Nitrogen in Aggregates of Saline‐Alkaline Soils in the Yellow River Delta

ABSTRACT Soil salinity alters aggregate structure, organic matter dynamics, and carbon stabilization; however, its mechanistic effects on carbon‐nitrogen distribution and soil organic matter (SOM) stability remain inconclusive. We hypothesize that salinity‐induced shifts toward microaggregate dominance weaken the physical protection of SOM, increasing SOC lability and promoting C  N decoupling. Soil samples from low‐ and high‐salinity sites were fractionated into aggregates of < 0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm, and > 2 mm, and analyzed for SOC, total nitrogen (TN), soil inorganic carbon (SIC), and water‐extractable fractions of organic carbon (WEOC), organic nitrogen (WEON), and inorganic carbon (WEIC). High salinity shifted aggregate distribution toward microaggregates, with the < 0.25 mm fraction comprising 75%–84% of mass compared with 25%–30% under low salinity, indicating dispersion and reduced structural stability. This breakdown caused depletion of stabilized C and N pools, as SOC declined to 6.7 g kg −1 under high salinity but remained higher, 13.2 g kg −1 in low salinity soils, while TN ranged from 0.5 to 0.6 g kg −1 versus 1.4 g kg −1 , indicating weaker organic matter retention. SIC was enriched in low salinity soils in the 1–0.5 and 0.5–0.25 mm fractions but reduced under high salinity, suggesting diminished carbonate‐mediated stabilization. Elevated WEOC:SOC and WEON:TN ratios in high salinity soils indicated greater SOM solubilization and mobility. The negative relationship between WEOC:SOC and SIC demonstrated that carbonate minerals constrain organic matter desorption. Salinity therefore destabilizes SOM through coupled physical and geochemical pathways, decreasing C  N retention and increasing lability. These mechanisms apply broadly to salt‐affected soils and highlight the need for salinity management to preserve structure and carbon retention.