Source Identification and Estimation of Organic Carbon in the Intertidal Wetlands of the Eastern Coast of China

Intertidal wetlands are important carbon reservoirs that play a significant role in climate change mitigation. However, the lack of large‐scale quantification and source identification of sediment organic carbon (SOC) in different discharge estuaries hampers the assessment of the carbon storage potential in these systems. In this study, based on the elemental ratios and stable carbon isotopes of the core sediment from the intertidal wetlands along the east coast of China, we quantified the contribution of organic carbon (OC) derived from terrestrial/estuarine particulate organic matter (POM), marine phytoplankton, and local plants, such as mangrove and salt marsh plants in the study area. We explored the hydrological and plant drivers controlling the variation in the contribution of OC sources among different coastal environmental settings. We found that SOC in high discharge estuaries (river runoff more than 50 billion m3/a) originated predominantly from terrestrial/estuarine POM (45 ± 6%), whereas the primary source for low discharge estuaries was marine phytoplankton OC (51 ± 14%). Moreover, our estimates revealed a sharp increase in the contribution of OC from mangroves to deep sediments compared with surface sediments, owing to the infiltration of mangrove roots at greater depths and the slow degradation of roots contributing to the substantial refractory OC buried in the deep sediments. These findings indicate that carbon storage in the intertidal wetlands varies among contrasting coastal environmental conditions, which provides implications for intertidal wetlands as a critical carbon sink in the global carbon budget. Plain Language Summary Better constraints on the sources of sediment organic carbon (SOC) in intertidal wetlands are very important for calculating the global carbon budget. Many researchers have studied the distribution, source, burial of SOC, but still, there is an obvious lack of quantification of the contributions of multiple SOC sources in different discharge estuaries across a large geographical area. Terrestrial/estuarine particulate organic matter (POM), marine phytoplankton, and local plants contribute organic carbon (OC) to sediments, varying in different places. In this study, by comparing the total organic carbon and total nitrogen content, δ13C values, and C/N ratios of the sediment cores, we quantified the relative contributions of three OC sources to sediments from 12 intertidal wetlands along the east coast of China. We