Soil water and inorganic nitrogen contents drive soil microbial carbon fixation during wetland reclamation and restoration.

Wetlands are critical terrestrial carbon sinks, playing a vital role in mitigating global warming by fixing substantial atmospheric carbon dioxide. To explore how wetland reclamation and restoration affect soil microbial carbon fixation, we sampled soils from five land-use types in western Jilin Province, China: undisturbed natural reed wetland, reclaimed rice paddy and upland field, restored agricultural drainage wetland and naturally restored wetland after farmland abandonment. We analyzed the effects of wetland reclamation and restoration on microbial communities, metabolic pathways, and carbon fixation potential. Results showed Proteobacteria (31.81 %), Actinobacteria (27.08 %), and Acidobacteria (15.12 %) dominated carbon-fixing microbes, with the reductive tricarboxylic acid cycle and dicarboxylate/4-hydroxybutyrate cycle as major carbon-fixing pathways. The natural wetland had the highest carbon fixation potential, with a mean value of 1.58 mg·kg-1 across the 0-15 cm topsoil layer and 15-30 cm subsoil layer, which was 1.30 to 4.02 times that of the reclaimed wetlands and restored wetlands. Among reclaimed sites, the rice paddy retained soil microbial community similarity to the natural wetland with complex, stable microbial networks. Compared to the naturally restored wetland, the agricultural drainage-restored wetland showed superior restoration outcomes, including microbial communities more similar to the natural wetland, higher network stability, and greater carbon fixation potential. Soil water and inorganic nitrogen contents were core drivers regulating carbon fixation via RubisCO activity and microbial metabolic pathways. This result highlights that the key to wetland restoration lies in prioritising hydrological regulation and nitrogen management, thereby enhancing microbial carbon fixation potential.