Natural and regenerated saltmarshes exhibit different bulk soil and aggregate-associated organic and inorganic carbon contents but similar total carbon contents

Saltmarshes are considered to be one of the planet's most efficient carbon sinks. The continued loss of saltmarshes and induced ecological consequences promoted their restoration worldwide. Previous efforts aimed to evaluate the success of restoration in terms of organic carbon accumulation, but inorganic carbon and carbon contents within soil aggregates, which are essential for making a comprehensive assessment of the carbon sink function, were rarely studied. To fill this gap, a range of metrics including bulk and aggregate-associated soil organic and inorganic carbon contents together with the soil's physical, chemical and microbiological parameters were measured to compare natural and a 15-year restoration effort in saltmarsh habitats within the Yellow River Delta region in eastern China. The results showed that regenerated saltmarsh exhibited significantly higher soil organic carbon (SOC) contents but significantly lower soil inorganic carbon contents, resulting in no notable change in total carbon contents between the regenerated and natural saltmarshes. SOC contents within the silt and clay fractions and their contribution to the bulk SOC contents were significantly lower in the regenerated saltmarsh than those in the natural ones (P < 0.05). In regenerated saltmarsh, significantly lower soil microbial biomass and distinct microbial community composition with reduced Gram-negative to Gram-positive bacteria ratios were observed compared to natural saltmarsh. These findings indicate the stability of SOC fraction and soil microbe-mediated carbon biogeochemical processes differed between naturally occurring and artificially regenerated saltmarshes. As interest in blue carbon programs gains global attention, further research on the generation and transformation processes of different carbon fractions during restoration are needed, which can be conducive to elucidating more details in coastal carbon cycling processes.