Cover loss in a seagrass Posidonia oceanica meadow accelerates soil organic matter turnover and alters soil prokaryotic communities

Abstract

Carbon stocks in coastal vegetated ecosystems account for half of the carbon in marine sediments and soils. Disturbance in these ecosystems can release vast amounts of carbon through mineralization, depending on poorly understood factors, such as soil organic matter (SOM) quality and environmental factors. The meadows of the seagrass Posidonia oceanica, which is the species with the larger carbon stocks, are currently under threat. The aim of this study was to determine the impact of cover loss on the SOM and microbial community composition in a P. oceanica meadow. For this purpose, soil cores were taken in dead, degraded and healthy areas in the same meadow. Cores were analyzed for total, inorganic and organic carbon and nitrogen contents, as well as for molecular SOM composition by analytical pyrolysis (<2 mm fraction). Bacteria and archaea communities were assessed by 16S rRNA gene metabarcoding. The results showed the mineralization of polysaccharides and guaiacyl and syringyl lignin and a selective preservation of p-hydroxybenzoic acid moieties and methylene chain compounds. Soil depth and plant cover showed significant effects on microbial richness and diversity. Spatial variations in SOM inputs and microbial community composition between cores highlighted the importance of intrameadow variability. Cover loss resulted in a release of soil organic carbon (1.8 fold decrease) and the boost of archaeal communities in the rhizosphere, whereas no effect was found in deeper layers. Our study demonstrates that seagrass cover loss enhances SOM mineralization and highlights the importance of soil stabilization against erosion to avoid the release of millenary SOM stocks.