Evaluating the vulnerability of salt marsh soil organic matter to microbial decomposition

Coastal salt marshes are increasingly impacted by global change and anthropogenic disturbances that can destabilize soils, leading to a loss of buried organic carbon and ecosystem subsidence. The vulnerability of soil organic matter (SOM) to microbial decomposition following disturbances is an important unknown that is often determined through short-term experiments. However, shallow ponds situated on the salt marsh platform offer a natural experiment to evaluate the composition of SOM that persists following decades of microbial decomposition. Here, we compared soil properties, accretion rates, and organic matter composition in soil cores collected from the salt marsh platform and nearby ponds at three sites within the Plum Island Ecosystems-LTER site (MA, USA). Total organic carbon densities decreased with depth into the salt marsh platform and surface horizons of pond soils had lower values than the surrounding marsh platform at similar elevations, indicating the loss of organic matter content during sediment decomposition. Downcore trends of lipid biomarkers produced by microalgae illustrated a loss of reactive SOM from surface to depth within the marsh and pond soils. Abundances of bacterial biomarkers remained constant within the pond core, consistent with bacterial decomposition occurring throughout the pond soil column. Correspondingly, pond surface soils exhibited higher ratios of vascular plant hydrocarbons relative to algal-derived hydrocarbons, demonstrating an accumulation of structurally complex SOM in relation to the surrounding marsh platform. Finally, pond surface soils had greater evidence of degradation based on decomposition indices obtained from Fourier transform infrared spectroscopy illustrating the preferential loss of aliphatic functional groups relative to aromatic functional groups as a result of sediment respiration. These techniques allowed us to comprehensively evaluate the composition and quality of salt marsh SOM that remained buried following sustained microbial decomposition, providing further insight into the future stability of this blue carbon ecosystem.