Species-specific effects of vascular plants on methane emissions from estuarine saltmarshes in Hangzhou Bay, China

Coastal saltmarshes are key blue carbon (C) ecosystems for climate change mitigation, yet their climate benefits can be partially offset by methane (CH 4 ) emissions. How species differences among vascular plants regulate CH 4 emissions in estuarine saltmarshes under comparable environmental settings remains insufficiently understood. Here, we measured annual CH 4 fluxes from saltmarshes dominated by Spartina alterniflora, Phragmites australis , and Scirpus mariqueter , and from an adjacent bare tidal flat in Hangzhou Bay, China. Our results indicated that CH 4 emissions differed markedly in both magnitude and seasonal patterns among sites. The exotic S. alterniflora marsh exhibited the highest mean CH 4 flux (8.25 ± 2.06 mg CH 4 m −2 h −1 ) and the longest emission period, resulting in annual emissions of 76.04 ± 4.86 g CH 4 m −2 , which were 2.65 and 59.9 times those from native P. australis (28.69 ± 1.74 g CH 4 m −2 ) and S. mariqueter (1.27 ± 0.06 g CH 4 m −2 ) marshes, respectively, and two orders of magnitude higher than those from bare tidal flat (0.83 ± 0.03 g CH 4 m −2 ). Stepwise regression identified air temperature and dissolved organic C (DOC) as key drivers of seasonal CH 4 dynamics. Across sites, annual CH 4 emissions were positively correlated with plant biomass, DOC, and soil organic C (SOC), but not with salinity, likely due to its narrow range among sites (6.18–9.26 ppt). A synthesis of 98 global in situ observations further revealed that the aboveground biomass (AGB)-to-salinity ratio ( R 2 = 0.58, P < 0.001) and SOC-to-salinity ratio ( R 2 = 0.34, p < 0.001) were stronger predictors of CH 4 emission than salinity alone ( R 2 = 0.21, P < 0.001). These findings highlight the key role of vascular plants in regulating saltmarsh CH 4 emissions through C supply and gas transport, and suggest that incorporating plant traits and soil C status into blue C assessments.