Labile and recalcitrant sediment organic carbon pools in the Pearl River Estuary, southern China.

Little is known about the labile and recalcitrant sediment organic carbon (SOC) in estuarine ecosystem, and the effects of human activities on SOC sequestration also remain poorly understood. In this study, sediment cores in the Pearl River Estuary (PRE) and adjacent coastal areas in the South China Sea were collected to analyse variations in the sources of SOC and its labile and recalcitrant pools. Concentrations of SOC, microbial biomass carbon (MBC), and recalcitrant organic carbon (ROC) ranged from 4.37 to 10.4 g/kg, 0.522 to 1.53 g/kg, and 1.59 to 5.42 g/kg, respectively, with their corresponding mean values as 7.20 ± 1.43 g/kg, 0.896 ± 0.228 g/kg, and 3.71 ± 1.01 g/kg, respectively. ROC was the chief fraction of SOC, and SOC, MBC and ROC has a similar source. Higher SOC and MBC were observed in the upper sediments, which might be attributable to the enhancement of seawater nutrient and particulate organic carbon concentrations in recent decades. Higher concentrations of SOC, ROC, and SOC stock were found in the inner estuary relative to the outer estuary due to a higher terrigenous organic carbon contribution, while the ratio of water-soluble organic carbon, salt-extractable carbon, and MBC to SOC exhibited a contrasting trend caused by a higher autochthonous contribution. Sand excavation reduced SOC, ROC, ROC/SOC, and SOC stock. The estimated SOC stock of the top 75 cm of sediment in the PRE was 34.0 Mg C ha, and the reduction of SOC stock in the PRE due to sea reclamation from 1973 to 2015 was 1,171,159.6 Mg C. Therefore, measures should be taken to control sea reclamation and sand excavation activities in the PRE to enhance carbon sequestration capacity.