Co-occurring Mangroves and Salt Marshes Differ in Microbial Community Composition

Coastal ecosystems such as mangroves and salt marshes store large amounts of carbon due to high rates of plant productivity and low organic matter decomposition rates in anoxic soils. As woody mangroves continue to encroach into herbaceous salt marshes, it is important to understand how these wetland biomes differ in soil microbial communities. Mangroves and marsh plants have different rooting structures and chemical qualities and could generate different environments for soil microbes, thus leading to changes in soil carbon processing. In an ecotonal ecosystem in Florida, where mangroves are rapidly encroaching into salt marshes, we compared wetland soil microbial community composition and function in mangrove-dominated vs. salt marsh-dominated plots. Microbial community structure differed between mangrove-dominant and marsh-dominant plots. The top indicator genera in the marsh-dominated plots belonged to putatively anaerobic groups (Tepidibacter, Caldithrix, Desulfovibro, Fibrobacteres, Thiotrichaceae) while top indicator genera in mangrove-dominated plots had representatives within Acidobacteria, Nitrospirae, and Proteobacteria. In a substrate-induced respiration assay, samples from mangrove plots with the greatest root mass also had the highest rate of labile C substrate consumption. Our results suggest that mangroves and marsh plants have different sediment microbial communities and that future mangrove encroachment into salt marshes could alter soil microbial communities with potential implications for soil carbon storage.