Microbial carbon fixation pathways shifts during artificial Haloxylon ammodendron restoration with clay sand barriers in arid deserts: a metagenomic analysis.

Soil microbial carbon fixation is influenced by the combined effects of microbial community composition, functional gene distribution, and environmental factors, and is closely associated with vegetation restoration processes. However, the soil carbon fixation process and its coupling mechanisms mediated by microorganisms at different vegetation restoration stages in arid regions remain unclear. In this study, we applied metagenomic sequencing to investigate soil from a clay sand barrier Haloxylon ammodendron sand-fixing restoration area at the southeastern edge of the Badain Jaran Desert, spanning a 60-year vegetation restoration time sequence (1, 5, 10, 20, 40, and 60 years) and shifting sand as a control. We explored the impacts of vegetation restoration and its long-term sequence on soil properties, microbial community structure, carbon fixation genes, and carbon fixation pathways. The results showed that vegetation restoration improved regional soil nutrient levels and organic carbon accumulation, with these positive effects progressively amplified over the restoration time sequence. Additionally, vegetation restoration not only reshaped microbial community composition but also induced changes in carbon fixation-related genes and pathways. A 10-year restoration period served as a critical time point, with microbial community diversity and carbon fixation gene abundance exhibiting pronounced fluctuations during the first 10 years, followed by relative stabilization thereafter. This threshold likely reflects a transition from intense plant-microbe competition to a more balanced coexistence as vegetation succession progresses and soil conditions stabilize. Among the six major microbial carbon fixation pathways, the rTCA cycle had the highest relative gene abundance, making it the dominant carbon fixation pathway in the region. Soil properties, particularly soil water content (SWC) and total phosphorus (TP), were identified as critical factors influencing both microbial community composition and carbon fixation-related genes. These findings suggest that clay sand barrier Haloxylon restoration not only fulfills its role in sand stabilization but also alters the soil environment, driving a functional shift in the microbial community from autotrophic to heterotrophic processes. This study deepens our understanding of soil carbon fixation processes in arid desert ecosystems and provides theoretical guidance for carbon management in similar arid regions.