Landfill depths alter microbial community structure and ecological assembly by affecting δ¹³C and organic matter

Microorganisms are crucial for the degradation of organic matter during landfill. However, the processes of microbial community assembly and ecological niche in landfill are poorly understood. Here, the mechanisms underlying microbial assembly in landfill were investigated based on neutral theory, niche distribution and network analysis. The results showed that moisture and potassium in the landfilled wastes increased with depth, while organic matter and δ¹³C were significantly higher in the middle layer than in the surface and bottom layers (P < 0.05). The richness and diversity of bacteria were significantly greater in the surface layer compared to the middle and bottom layers (P < 0.05), with moisture content, organic matter, total nitrogen and δ¹⁵N significantly influencing bacterial community composition. Deterministic processes over stochastic processes were pronounced in the surface layer, with the latter accounting only for 3.8 %. As landfill depth increased, variations in organic matter composition led to a greater influence of stochastic processes (52.7 %), while deterministic processes accounted for only 5.8 %. The niche breadth of abundant taxa was narrower than conditionally rare or abundant taxa, with their distribution primarily regulated by waste δ¹³C (P < 0.05), indicating greater environmental sensitivity. The niche overlap of microbial communities was lower in the surface layer, with the proportion of groups with high niche overlap being 2.69 times and 1.69 times higher in the middle and bottom layers. This study provided the first analysis of microbial niche dynamics across landfill depths, revealing critical interactions between δ¹³C driven organic matter availability and stochastic assembly processes.