Hydrology masks warming effects on microbial communities in salt marsh soils.

Soil microbial communities play a pivotal role in salt marsh ecosystem functioning, driving processes such as organic matter decomposition and greenhouse gas cycling. Despite their importance, it remains unclear how climate warming will affect the diversity and activity of salt marsh soil microbial communities, limiting our ability to predict the fate of the vast stores of soil organic carbon in these so-called blue carbon ecosystems. Here, we leveraged the Marsh Ecosystem Response to Increased Temperature (MERIT) experiment to investigate the effects of sustained warming on the structure and function of the putatively active microbial community, as assessed by rRNA transcripts, alongside measurements of exo-enzymatic activities involved in carbon and nitrogen acquisition. Our results reveal that, after 5 years of experimental warming by +1.5°C and +3.0°C, the overall structure of the active microbial community remains remarkably stable, suggesting a high degree of resilience to elevated temperatures in this dynamic environment. However, warming selectively promoted drought-tolerant phyla, particularly Actinobacteriota and Firmicutes, which are known for their ability to degrade complex organic compounds and withstand desiccation. These findings suggest that while the active microbial community is broadly resistant to warming, subtle compositional shifts may enhance decomposition of recalcitrant soil carbon.