An essential strategy to enhance carbon accumulation within coastal wetlands: Introducing Spartina alterniflora-biochar facilitating microbial carbon sequestration in subtidal zone

Coastal wetlands serve as vital global carbon sinks but face degradation from threats like Spartina alterniflora invasions and habitat loss, diminishing their carbon sequestration capacity. While biochar enhances terrestrial carbon storage, its efficacy in coastal wetlands under tidal-induced aerobic/anaerobic conditions remains unclear. Therefore, we produced biochar from S. alterniflora and conducted indoor simulations, replicating the different environmental conditions in coastal wetland tidal zones: bare (B), alternating bare and flooding (BF), and flooding (F). Our goal was to determine the best application zones and understand how S. alterniflora biochar (SBC) works at different doses. Results revealed that 3 % SBC in subtidal zones (F) maximized carbon accumulation (0.80 ± 0.16 mg/g), followed by intertidal zones (BF: 0.26 ± 0.08 mg/g), while supratidal zones (B) exhibited carbon loss (−0.81 mg/g). Anaerobic conditions in subtidal zones suppressed microbial metabolism, e.g., Actinomycetota and Bacteroidetes, which are commonly recognized as rapidly proliferating eutrophic bacteria, resulting in the decreased carbon consumption and the accumulation of recalcitrant organic carbon and microbial necromass carbon. Conversely, employing SBC in supratidal zones within aerobic environments stimulated microbial activity, accelerating native soil organic matter degradation. Fluorescence spectroscopy confirmed higher humification and aromaticity of dissolved organic matter under SBC amendment, enhancing carbon stability. The study concludes that subtidal zones are optimal for SBC application, as flooding conditions mitigate carbon loss and stabilize microbial-derived organic carbon. These findings provide a strategic framework for utilizing invasive S. alterniflora as biochar to enhance wetland carbon sinks, addressing both ecological restoration and climate mitigation goals.