Coupling mechanism of soil carbon and nitrogen cycling changed by remediation using Pongamia pinnata in the vanadium titanium magnetite tailings

The long-term exploitation of V–Ti magnetite mines has led to significant environmental issues, including ecological deterioration, degraded vegetation, soil contamination, and reduced fertility in mining regions. Phytoremediation using Pongamia pinnata has proven to be an effective method for restoring the ecological environment in the V–Ti magnetite mining area. However, the mechanisms underlying soil C and N interactions during this restoration process remain poorly understood. Therefore, based on the impact of P. pinnata restoration on the soil C cycle, further research was conducted on the soil N cycle and the coupling mechanism between the soil C and N cycles in the V–Ti magnetite mining area. The results revealed that soil quality improved with increases in the contents of soil organic N components (AAN, HUN, AMN, and ASN), inorganic N components (NH4+-N and NO3−-N), and N-related invertases (S-UE and S-NR) in the V–Ti magnetite tailings, following the restoration of P. pinnata. The restoration of P. pinnata reshaped the soil bacterial communities, increased the abundance of functional genes regulating soil N transformation, and modified the network relationships between soil microbial communities, N components, and N transformation-related functional genes. The structural equation modeling analysis revealed that the reshaped bacterial community altered the soil C/N ratio by directly or indirectly regulating the functional genes involved in C/N cycling, which, in turn, altered the activities of C- and N-related enzymes and C/N components. Notably, the functional genes involved in C cycling positively affected N components, whereas N-related genes negatively modulated C-related enzyme activities after the restoration of P. pinnata in V–Ti magnetite tailings. These results provide a theoretical basis for P. pinnata-based remediation, which strengthened soil C and N cycling and coupling functions to improve the barren soil ecological environment in the V–Ti magnetite mining area.