Soil aggregate stability and its relationship with soil organic carbon fraction in slopes for ecological restoration of mining waste dumps

As an important part of carbon sequestration mechanism in ecological restoration of mining areas, soil aggregate protection is considered. Further exploration of the relationship between soil aggregate stability and its association with soil organic carbon (SOC) fractions helps enrich and improve the aggregate-mediated carbon sequestration mechanism in mining areas. For this purpose, investigations are carried out at the waste dump of Suhaitu Mining Area. The Yellow River sediment-based planting substrate slope (YR-PS), soil spraying slope (SS), soil spraying slope with internal gangue spontaneous combustion (SS-H), abandoned slag accumulation site (AS) and concurrent natural restoration site (NR) are selected as the research objects. Indices such as the stability of mechanically stable soil aggregates (M) and water-stable soil aggregates (W), as well as amino sugars, lignin phenols, and soil physical and chemical properties are analyzed. The results show that among the artificially ecologically restored slopes, the highest soil aggregate stability is observed in YR-PS. Contents of total nitrogen (TN), total phosphorus (TP), SOC, mineral-associated organic carbon (MAOC), particulate organic carbon (POC), microbial residue carbon and plant residue carbon are all the highest in YR-PS, which are 2.46, 2.77, 1.89, 1.78, 1.82, 4.71 and 24.40 times those of AS, respectively. The optimal restoration effect is therefore presented. This is closely related to the diversified carbon sequestration pathways induced by straw addition. Under the comprehensive impacts of gangue and its spontaneous combustion, vegetation degradation, relatively single sources of plant residue carbon and the highest oxidation degree of lignin phenols are found in SS-H. Furthermore, the results of Mantel test and random forest model consistently indicate that the mean weight diameter of water-stable aggregates, soil erodibility of water-stable aggregates, geometric mean diameter of water-stable aggregates, as well as TN, TP and MAOC are identified as the key influencing factors affecting SOC content, microbial and plant residue carbon contents and their contribution rates to SOC. The results of partial least squares structural equation model further reveal that the accumulation of microbial residue carbon is directly promoted by soil aggregate stability in mining areas, and the accumulation of plant residue carbon is indirectly facilitated through soil nutrients. Plant residue carbon is further converted into microbial residue carbon mediated by the microbial carbon pump, and the formation of MAOC is driven by microbial residue carbon via microbial metabolic activities. Eventually, MAOC serves as the core precursor substance to contribute to SOC accumulation, and a hierarchical impact mechanism is formed with microbial residue carbon as the key intermediate variable.