The long-term effects of elevated atmospheric CO2 and warming on soil microbial communities in a tidal marsh ecosystem

Tidal marshes are an important transition zone connecting marine, freshwater, and terrestrial ecosystems. Tidal marshes are known to be sensitive to global climate change such as elevated CO₂ and warming. In particular, it is yet to be revealed whether the such change may increase or decrease carbon stock in tidal marshes. Although soil microorganisms play an important role in carbon storage in tidal marshes by determining carbon mineralization, little is known about the interactive effects of elevated CO₂ and warming on soil microbial communities in field conditions. Here, we elucidate the effects of 6-year experiment of climate manipulation on soil microbial communities in a tidal marsh. The manipulation experiment included 4 treatments of 1) elevated atmospheric CO₂ concentrations (750 ppm) only, 2) warming (+5.1 °C) only, 3) both elevated CO₂ and warming, and 4) ambient conditions. Elevated CO2 significantly changed the structure of the RNA-derived (active) and DNA-derived (total) soil microbial community, but warming did not affect either. The relative abundances of Acidobacteria, Actinobacteria, Chloroflexi, and Planctomycetes were higher in the DNA-derived soil microbial communities than in the RNA-derived soil microbial communities, whereas those of Campilobacterota, Desulfobacteria, Gammaproteobacteria, Myxococcota, and Spirochaetota were higher in the RNA-derived soil microbial communities. In addition, elevated CO₂ changed the microbial communities from r- to K-strategists in both RNA and DNA-derived communities, suggesting that this may offset additional C input by roots in a future elevated atmospheric CO₂ environment. This study provides a better understanding of microbial response to the combined effects of elevated atmospheric CO₂ concentrations and global warming in the tidal marsh ecosystems.