Biomarker analysis revealed tidal organic carbon input enhanced soil respiration and weakened carbon sequestration function of estuarine wetland: Field validation of the Jiuduansha Wetland in the Yangtze River estuary

In estuarine wetlands, tidal water is an important source of organic carbon input besides plants. However, it remains unclear whether tidal organic carbon input can affect soil organic carbon turnover processes in estuarine wetlands, especially soil respiration (SR) and carbon output. N-alkanes and lignin can be used as biomarker compounds to reflect the input and retention of organic carbon from tidal and plant sources. In the experiment, the contents of n-alkanes and lignin in tidal water, soil and plant samples were measured from upstream to downstream in Jiuduansha Wetland in the Yangtze River Estuary to compare the input intensity and retention capacity of tidal organic carbon with plant organic carbon in soils, and try to verify whether exogenous tidal organic carbon input enhanced SR and weakened carbon sequestration function. N-alkanes analysis revealed tidal organic carbon input gradually reduced from upstream to downstream of Jiuduansha wetland, and its degradability also reduced, which is an important reason for higher SR in upstream wetland. Lignin analysis showed the plant organic carbon input decreased with decreasing elevation, however the retention ratio of plant organic carbon was the highest in low tidal flats which has a high tidal organic carbon input. These results indicated tidal organic carbon was more easily degraded to CO2 than plant organic carbon. N-alkanes analysis also indicated only 11.94%–13.76% of the tidal organic carbon originated from CO2 (phytoplankton organic carbon (OC)), therefore the tidal organic carbon input did not increase the wetland CO2 input significantly, but significantly increase SR and CO2 emission of the wetland, this will weaken carbon sequestration function of estuarine wetland. Therefore, future studies should explore strategies to balance carbon sequestration and tidal organic matter interception and purification functions of estuarine wetlands.