Long-term recovery of aggregate-associated organic carbon in reclaimed mine soil

Abstract

The restoration of agricultural soil quality and soil carbon stocks in compacted reclaimed mine soil (RMS) relies on the recovery of soil aggregates. This study explored the factors and mechanisms influencing aggregate-associated organic carbon (AOC) and other soil properties in RMS. A total of 28 soil samples were collected from post-reclamation farmland at various stages (ranging from 0 to 22 years post-reclamation) at a depth of 0–20 cm. Complex network theory and structural equation modelling (SEM) were used to analyse complex network relationships and pathway connections. The results indicate that mechanical compaction during landform reshaping disrupted the structure, stability, and carbon storage capacity of soil aggregates, leading to enhanced mineralization of soil organic carbon (SOC) and nitrogen, nutrient loss, and reduced microbial activity. After re-cultivation, continuous agricultural management interventions—such as fertilization, straw return, and crop cultivation—significantly improved soil structure and carbon storage. For instance, compared to samples collected in the first year post-reclamation, samples gathered 22 years post-reclamation exhibited significant increases in small macroaggregates (+25.9 %), mean weight diameter (+34.4 %), AOC in large macroaggregates (+121.0 %), and AOC contribution of small macroaggregates (+35.6 %) (p < 0.05). The variation of SOC in RMS is primarily driven by AOC associated with aggregate structure. During the geomorphic reshaping stage, the chemical protection of inorganic cementing substances played a significant role in the process of AOC storage. After re-cultivation, active organic carbon components and iron-aluminum oxides synergistically promote macroaggregate formation to enhance AOC storage. The enhancement of microbial activity is crucial for AOC storage. The microbial-mediated AOC storage process exhibits a positive response to improvements in soil moisture and nitrogen supply conditions. For reclaimed farmland, maintaining suitable moisture conditions, nitrogen levels, microbial activity, and active iron-aluminium oxide supply can effectively promote the formation of macroaggregates and their AOC storage after re-cultivation.