Summary:

Salt marshes, highly productive and intertidal ecosystems, are dominated by the cordgrass Spartina alterniflora on the US Atlantic and Gulf coasts. Salt marshes provide crucial ecosystem services to coastal communities. Although soil microorganisms are well known to mediate important biogeochemical cycles in marshes, little is known about the role of plant microbiomes in supporting primary productivity and ecosystem functioning. This study uses the gradient in S. alterniflora productivity that occurs along tidal creeks as a natural laboratory to explore plant-microbe interactions. The abundance, diversity, and community composition of microbiomes is investigated in the bulk sediment, rhizosphere and endosphere of S. alterniflora in parallel with sediment biogeochemistry. Our results suggest that environmental filtering is the main assembly processes in the studied system. However, it is relaxed at closer distances to the root due to increased competition and/or stochastic processes such as dispersal, ecological drift, and priority effect due to historical contingency.