Effects of Rhizophora mangle leaf litter and seedlings on carbon and nitrogen cycling in salt marshes - potential consequences of climate-induced mangrove migration

Background and aims Due to the production of large amounts of tannins and phenolics by Rhizophora mangle, it was hypothesized that the invasion of this mangrove species in salt marshes due to global warming will result in changes in the cycling of carbon and nitrogen. Methods Leaf litter and/or seedlings of R. mangle were placed into 1-m2 experimental plots in a Distichlis spicata-dominated salt marsh on the Atlantic Coast of central Florida (USA). An additional litter decomposition experiment was conducted in all plots by adding litter bags containing 10 g of dried D. spicata shoot litter. Seedling growth was measured yearly. One and four years after the start of the experiment, soil samples were collected to determine physical and chemical soil conditions, potential nitrification and denitrification activities and abundances of genes that are related to microbial processes in the nitrogen cycle. Results Growth of R. mangle seedlings was stimulated in the presence of R. mangle litter, while decomposition rates of D. spicata litter were lower in plots with R. mangle litter and seedlings. The presence of R. mangle litter and/or seedlings had no significant effect on potential nitrification and denitrification activities and on the abundances of genes. Conclusion The colonization of R. mangle into D. spicata-dominated salt marshes will affect the carbon cycle, but not necessarily the nitrogen cycle, which is likely due to the pre-existing nitrogen-limited conditions in the salt marsh.