Higher dynamic and turnover yet enhanced organic carbon burial in Spartina alterniflora sediments: A case study in a temperate coastal wetland

Coastal wetlands are natural blue carbon sinks that contribute significantly to global carbon storage. The effects of different vegetation types and their growth on the quantity and quality of sediment organic carbon (OC) are substantial, yet past studies often directly compared different sediments under varying hydrological conditions, which may overlook their potential entangling influence, introducing uncertainties in the assessment of potential carbon storage and dynamics in coastal wetlands. Therefore, in this study, an estuarine wetland was selected with generally similar hydrological conditions between different plant species (Spartina alterniflora, and Phragmites australis). The quantity and quality differences among S. alterniflora, P. australis, and mudflat sediments in total organic carbon, labile OC, and relatively stable OC (Fe-bound OC, OC-Fe) and their causes were explored and further amino sugar biomarkers were used to characterize the transformation of a portion of OC-Fe. The results revealed that S. alterniflora sediments accumulated greater amounts of total Fe and stored higher total OC, labile OC, and OC-Fe than P. australis, and mudflat sediments. While the proportions of labile and relatively stable OC in S. alterniflora sediments were comparable to those in P. australis, this indicated that S. alterniflora contributed equally or more significantly to both stable and labile OC pools. Notably, in the bottom layer of S. alterniflora sediment, a decrease in OC-Fe during the non-growing to growing period was observed, this is different from the conventional understanding of OC-Fe and suggests OC-Fe could be highly dynamic and transform into the microbial necromass as revealed by amino sugar biomarkers. Moreover, the OC contributed by S. alterniflora may decompose more easily, exhibit a higher turnover, and accumulate more microbial-derived OC in sediments. Overall, this study focused on the potential impact of plant types and growth on sediment OC pools, providing theoretical insights for blue carbon conservation and invasive plant management strategies in coastal wetlands.