Salinity determines the blue carbon sequestration capacity of Phragmites australis in coastal ecosystems

Salinity is a well-known environmental factor that profoundly influences vegetation growth and ecological functions of coastal salt marshes. This study conducted an in-situ control experiment to assess the effects of salinity on the morphological and physiological traits of coastal Phragmites australis, as well as its carbon sequestration capacity (including CO₂ uptake, CH₄ emissions, and vegetation and soil organic carbon densities). Field investigations and microbial abundance analyses were integrated to provide a comprehensive assessment. The results showed that the growth characteristics and photosynthetic activity of P. australis increased initially but declined as salinity rose, peaking at a moderate level (5‰). Despite concurrent peaks in CO₂ uptake and CH₄ emissions at 5‰ salinity, the net negative daytime CO₂-eq flux indicated that this salinity level provided the strongest net cooling effect, driven by stronger CO₂ uptake relative to CH₄-induced warming. Under higher salinity levels (> 10‰), P. australis exhibited an adaptive strategy of reduced carbon allocation to roots, leading to a significant decrease in soil organic carbon density. Through the identification of salinity thresholds that optimize growth and carbon sequestration of P. australis, this study delivers a mechanistic understanding for advancing adaptive management and restoration efforts of coastal salt marsh ecosystems to enhance their blue carbon sequestration, particularly in the context of sea-level rise.