Enhancing soil carbon storage through carbon-iron coupling in small macro-aggregates during cut slope restorations

The corrosion of metal mesh and the adsorption of plastic mesh affect the biogeochemical cycles of soil carbon and iron during cut slopes restoration formed by road construction in mountainous areas. However, the carbon-iron coupling and its contribution to soil organic carbon (SOC) stocks during cut slopes restoration remains unclear. Herein, this study selected four types of restored cut slopes (cut slope with natural restoration (CSN), cut slope with plastic mesh restoration (CSP), cut slope with galvanized wire mesh restoration (CSG), and natural slopes (NS)) as research objects, and analyzed the effects of iron oxide fractions and bacterial communities in different aggregate sizes on iron-bound organic carbon (OC-Fe) and SOC stocks. The results showed that the content of iron oxide fractions was highest in CSG, and the difference was significant compared with that in CSN and CSP (P < 0.05). Although the restoration process explained the majority of variations in iron oxide fractions and Free, activation, and complexing indexes for Fe, aggregate size and their interactions still significantly (P < 0.05) affected iron oxides distribution. The network formed by bacterial communities and iron oxide fractions in silt + clay was more complex than that in large macro-aggregates, small macro-aggregates and micro-aggregates. Complex iron oxide (Fep), free iron oxide (Fed), comp1 and comp2 (the first two major components of Partial least squares discriminant analysis based on Bray-Curtis distance) contributed significantly (P < 0.05) to the difference of OC-Fe and SOC stocks in each aggregate size. Bacterial community composition, diversity and iron oxide fractions explained more than 90 % of the variation in OC-Fe and SOC stocks. Additionally, comp1, comp2 and Fep had the greatest contribution to OC-Fe and SOC stocks in each aggregate size. Our research showed that the carbon-iron coupling in small macro-aggregates was more conducive to OC-Fe sequestration and SOC stocks in cut slopes restoration.