Species-specific difference of CO₂ emissions in mangroves: coupling sediment physicochemistry and microbial communities

Mangrove ecosystems function simultaneously as carbon sinks and carbon sources. While their contribution to biomass accumulation and long-term carbon sequestration have been extensively studied, the mechanisms driving carbon emissions, particularly those mediated by tree species and microbial communities, remain poorly understood. In this study, we investigated <i>Kandelia obovate</i> (KO), <i>Sonneratia apetala</i> (SA), and an adjacent mudflat in the Hanjiang River Estuary, southern China, to evaluate seasonal changes in sediment physicochemistry, microbial community structure, and CO₂ fluxes, and to evaluate the influence of vegetation on carbon emissions. This research shows that mangrove colonization significantly altered sediment conditions, with <i>K. obovata</i> exhibiting higher salinity, water content, and total carbon concentration than <i>S. apetala</i>. Sediment CO₂ fluxes were consistently greater in mangrove habitats than in mudflats and displayed clear seasonal variation. In summer, sediment CO₂ fluxes in <i>S. apetala</i> and <i>K. obovata</i> were 4.3- and 2.5-fold higher than in winter, respectively. Concurrently, root respiration intensified in <i>S. apetala</i> during summer, whereas <i>K. obovata</i> root respiration remained stable across seasons. Microbial communities were dominated by Proteobacteria and Chloroflexi across sites, however, their network structures differed. <i>S. apetala</i> supported tighter microbial interactions, while <i>K. obovata</i> exhibited higher modularity and functional specialization. Additionally, partial least squares structural equation modeling revealed that sediment physicochemical properties strongly constrained microbial diversity and regulated CO₂ flux both directly and indirectly. These findings highlight the importance of sediment and root respiration in mangrove carbon cycling and demonstrate how species identity modulates CO₂ fluxes by shaping the interactions between sediment conditions and microbial communities.