Unvegetated Tidal Flats: A Critical Yet Vulnerable Coastal Blue Carbon Sink

Unvegetated tidal flats, despite accounting for approximately 24.9% of global blue carbon ecosystem area, remain underrepresented in blue carbon budgets. Here, we synthesize data from 184 sites across 23 countries to quantify their carbon stocks, carbon accumulation rate (CAR) and greenhouse gas (GHG, CO₂ and CH₄) emissions. Our results reveal that tidal flats sequester 8.6 (4.5-9.9) Tg C yr^-1 and store 1.2 (0.7-2.0) Pg C in the top meter of sediments, increasing global blue carbon estimates by 6%-17%. However, they simultaneously emit 0.073 Tg CH₄ yr^-1 (3.3 Tg CO₂eq yr^-1), 27.9% of the CH₄ emissions from salt marshes, offsetting 14.1% (1.8%-20.4%) of their carbon sequestration benefits. Mudflats exhibit the highest carbon stocks and CO₂ fluxes among different sediment types, primarily due to their abundant binding sites of fine-grained particles and stronger microbial activities. Both CAR and CH₄ fluxes decline poleward, with tropical zone as hotspots. Our findings highlight the urgency of integrating tidal flats into climate mitigation strategies, particularly given their vulnerability to land-use changes and sea-level rise. Conservation efforts must balance carbon sequestration gains against CH₄ trade-offs to optimize their climate regulation potential.