Microbial and mineral drivers of organic carbon sequestration in soilless revegetated zinc smelting slag site: Insights from a 12-year field study

Soilless revegetation is a cost-effective and sustainable strategy for restoring metal-contaminated smelting slags. Organic carbon (OC) accumulation plays a crucial role in transforming these barren substrates into functional, eco-engineered soils. However, the distribution of OC fractions and the mechanisms driving their sequestration under long-term soilless revegetation remain poorly understood. This study investigated the dynamics and drivers of OC sequestration in a zinc smelting slag site after 12 years of field-based revegetation. Revegetation markedly increased the contents of various OC fractions, including dissolved organic carbon, microbial biomass carbon, light-fraction, particulate, and mineral-associated OC, as well as microbial living (phospholipid fatty acid) and necromass carbon. It also enhanced carbon-degrading enzyme activities and enriched functional microbial communities and associated carbon-cycling genes. Mechanistically, OC stabilization was primarily governed by the accumulation and protection of mineral-associated OC and microbial necromass carbon, regulated by interactions among Fe/Al oxides, enzymes, and microbial carbon metabolism. Functional groups such as aromatics, phenols, and polysaccharides further contributed to carbon persistence through chemical recalcitrance and mineral association. Overall, the findings provide new insights into the coupled microbial-mineral mechanisms of OC stabilization in pyrometallurgical slags, highlighting their potential to evolve into carbon-retaining, soil-like systems under long-term revegetation and offering a valuable model for understanding incipient soil formation on anthropogenic substrates.