Dynamics of soil organic carbon fractions, microbial communities, and carbon cycle functional genes during vegetation restoration in zinc smelting slag sites.

Vegetation restoration has emerged as an effective strategy to enhance soil organic carbon (SOC) pools in degraded mining areas. However, the dynamics of SOC fractions and the underlying microbial mechanisms during vegetation restoration remain poorly understood. This study examined the dynamics of SOC fractions, microbial community structure, and carbon cycle functional genes in the rhizosphere soils of Cryptomeria fortunei (C. fortunei) and Trifolium repens (T. repens), established at zinc smelting slag sites for 4, 6, and 12 years. Results indicated that SOC fractions, including dissolved organic carbon (DOC), microbial biomass carbon (MBC), particle organic carbon (POC), and mineral-associated organic carbon (MAOC), significantly increased with restoration age and were predominantly distributed within micro-aggregates. Bacterial and fungal abundance and diversity were significantly higher in revegetated soils compared to control (CK) soils. Both microbial phospholipid fatty acids (PLFAs) and microbial residue carbon in bulk soils and aggregates significantly increased over time, with bacterial PLFAs exceeding fungal PLFAs, while fungal residue carbon contributed more than bacterial residue carbon to microbial carbon. Total PLFAs were more abundant in 2-0.25 mm aggregates, while total microbial residue carbon was concentrated in <0.25 mm aggregates. In bulk soils, key carbon fixation genes (e.g., korA, gltA, sdhA, mcmA) and degradation genes associated with hemicellulose (e.g., rfbB, xylF, xylH) and starch (e.g., SGA1, malQ) were identified. The abundance of carbon cycling genes initially increased and then decreased in C. fortunei-revegetated soils, whereas a continuous decline was observed in T. repens-revegetated soils. Partial least squares pathway modeling (PLS-PM) revealed that restoration years directly influenced SOC, enzyme activity, and microbial communities, while indirectly affecting carbon cycling genes through changes in microbial communities. Notably, the effects varied between the two plant species. These findings provide valuable insights into SOC dynamics and microbial ecological processes during vegetation restoration in zinc smelting slag sites.