Combined effects of elevated CO2 and warming threaten soil microbial diversity and network stability over a six-year chronosequence

Soil microbiomes are fundamental to carbon cycling, organic matter decomposition, and greenhouse gas regulation, making them essential for maintaining ecosystem stability. In coastal wetlands, which store large amounts of carbon and act as major sources of methane emissions, these microbial communities play an especially crucial role. This study examines the impacts of elevated CO2 and warming on soil microbial communities over a six-year chronosquence in a C3 plant-dominated salt marsh. Results showed that bacterial community structure was primarily influenced by seasonal variability, with distinct clustering patterns driven by temporal shifts rather than treatment effects. In contrast, bacterial diversity and network characteristics responded strongly to climate factors. Elevated CO2 alone increased bacterial diversity, while warming alone caused a reduction. However, their combined effects led to a synergistic decline in bacterial diversity, reducing it to 80% of ambient conditions by year six. Network analysis further revealed that the combined treatment caused substantial disruptions to microbial networks, including reduced size, connectivity, and clustering, along with increased modularity. These findings highlight the vulnerability of soil microbiomes to the compounded effects of climate change factors, with potential consequences for the stability and functionality of coastal wetland ecosystems. Incorporating these interactive effects into predictive models is essential for accurately forecasting future carbon cycling dynamics and for guiding the effective management of coastal wetland ecosystems under future climate scenarios.