Low organic carbon storage of sediment due to frequent river course changes in Yellow River Delta

Coastal alluvial plains in river deltas can store significant carbon by burying upland sediments and former topsoil. The Yellow River Delta, shaped by continuous upstream sediment deposition and shifting river courses, offers a unique case to study this. We collected five 2 m soil cores along a sediment sequence reflecting 80 years of deposition and river course changes. Our analysis of soil organic carbon (SOC) stocks and sources revealed that older river courses hold less SOC than newer ones. SOC was generally low, except near the surface. Upstream-derived organic carbon contributed more to SOC stocks than local vegetation, especially in younger sediments. All sediments showed high inorganic carbon and alkaline conditions, pointing to loess-derived subsoils from the uplands as the primary sediment source. These subsoils, low in organic carbon and weathering, indicate rapid erosion and a degraded terrestrial carbon cycle upstream. Although deltaic settings could protect deposited carbon, our findings highlight the limited increase in SOC due to the input of C-depleted materials. Therefore, soil conservation is critical not only in the delta but also in its erosion-prone upland sources.