Multi-scale evidence for declining microbial carbon fixation along forest succession gradients.

Although soil carbon accumulates during subtropical forest succession, changes in microbial communities and their carbon fixation capacity remain unclear. Using an integrative approach that combines field experimentation, extensive global metagenomic data, and isotope labelling, we analysed 84 soil microbiomes from a long-term successional site and 755 global metagenomes to investigate microbial community dynamics and their role in carbon fixation. Based on field data, bacteria, fungi, and protists had synchronous succession with vegetation; however, the relative abundance of carbon fixation genes declined significantly in later successional stages. To further investigate this outcome, we analysed global data from planted and mature natural forests and found significantly higher carbon fixation potential in planted forests, predominantly driven by Pseudomonadota and Actinomycota members. Field-based 13C labelling results further confirmed a significant decline in microbial CO₂ fixation rates with forest succession. These findings underscore the ecological importance of microbial carbon fixation in early forest succession, emphasizing its foundational role in initiating soil carbon accumulation and shaping long-term carbon cycling trajectories.