Spatio-temporal patterns and environmental controls of soil organic carbon stocks in global tidal wetlands since 2009

Tidal wetlands are important blue carbon ecosystems, yet global estimates of soil organic carbon (SOC) stocks and dynamics across mangroves, salt marshes, and tidal flats remain unclear. Using 42,154 records and 63 environmental covariates, we generate global tidal wetland SOC maps for two periods (2009-2014 and 2015-2020) at three depths. Here we show that SOC stocks decrease from ${8.80}_{2.61}^{20.94}$ Pg (mean, 5^th-95^th percentile prediction interval) to ${8.27}_{2.27}^{20.61}$ Pg, representing a 6.11% decline, with greater losses in deeper layers. Low-latitude regions, particularly South Asia and Africa, are loss hotspots, and lower-middle and upper-middle income countries experience pronounced losses. Mangroves suffer the largest losses primarily due to SOC density decline, whereas salt marshes gain in both area and SOC density. Climate, elevation, and total suspended matter drive mangrove SOC changes; oceanographic factors influence salt marshes and dominate tidal flats. This work reveals the magnitude of blue carbon spatio-temporal change, underscoring the need for targeted conservation.