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.