A transition from arbuscular to ectomycorrhizal forests halts soil carbon sequestration during subtropical forest rewilding

Abstract：Ecological succession and restoration rapidly promote multiple dimensions of ecosystem functions and mitigate global climate change. However, the factors governing the limited capacity of forests to sequester soil carbon (C) in old ecosystems are poorly understood. Ecological theory predicts that plants and microorganisms jointly evolve into a more mutualistic relationship to accelerate detritus decomposition and nutrient regeneration in old than young ecosystems, likely explaining the changes in C sinks across forest succession or rewilding. To test this hypothesis, we conducted a field experiment of root-mycorrhizal exclusion in successional subtropical forests to investigate plant-decomposer interactions and their effects on soil C sequestration. Our results showed that soil C accrual rate at the 0-10 cm soil layer was higher in early-successional arbuscular mycorrhizal (AM) forests with an average value of 1.26 mg g−1 yr−1 than that in the late-successional ectomycorrhizal (EcM) forests with nonsignificant change. A transition from early-successional AM to late-successional EcM forests increase fungal diversity, especially EcM fungi. In the late-successional forests, the presence of ectomycorrhizal hyphae promotes soil C decomposition and nutrient cycle by increasing soil nitrogen and phosphorus degrading enzyme activity as well as saprotrophic microbial richness. Across early- to late-successional forests, mycorrhizal priming effects on soil C decomposition explain a slow-down in the capacity of older forests to sequester soil C. Our findings suggest that a transition from AM to EcM forests supporting greater C decomposition can halt the capacity of forests to provide nature-based global climate change solutions.