Temporal and vertical dynamics of microbial communities during open-pit coal mine topsoil stockpiling in a black soil (mollisol) region of Northeast China.

Topsoil stockpiling is a critical yet often overlooked stage in mine reclamation. This study investigated the effects of stockpiling duration (0-3 years) and depth (0-6 m) on soil properties and microbial communities in a black soil (Mollisol) stockpile from a coal mine in Northeast China. Results showed that compared to the undisturbed farmland soil, stockpiled soils exhibited a significantly higher pH but lower soil organic carbon (SOC), total nitrogen (TN), available phosphorus (AP), and enzyme activities (invertase, urease, acid phosphatase). Vertically, SOC and TN were better preserved whereas AP and enzyme activities were significantly lower in deeper layers. High-throughput sequencing and phospholipid fatty acid analysis revealed differential responses of bacteria and fungi. Bacterial community composition transitioned from a prevalence of copiotrophic (e.g., Pseudomonadota) toward a dominance of oligotrophic taxa (e.g., Acidobacteriota) with increasing time and depth, while fungal communities showed sharper decline in biomass and greater sensitivity in composition, marked by stronger vertical differentiation, reduced network complexity, and increased competitive interactions. Functional predictions indicated suppressed bacterial metabolic potential and a shift toward saprotrophic dominance in fungi. Partial least squares path modeling revealed that stockpiling-induced pH increases and nutrient losses was closely associated with microbial shifts, with bacterial community acting as a key biological mediator directly linked to the decline in soil enzymatic function. Collectively, these findings suggest that topsoil stockpiling can lead to a dynamic process of progressive soil impoverishment, mediated by distinct microbial responses. This study provides a scientific basis for optimizing stockpiling management-such as controlling pile height and storage duration-and highlights the potential for developing microbially informed strategies to preserve soil quality and reclamation potential in black soil regions.