Distribution and deposition characteristics of carbon and nitrogen in sediments in a semi-closed bay area, southeast China

• The effect of particle size about TOC and TN contents was not discovered.
• δ13C indicated organic matter source was dominated by marine sources.
• The carbon and nitrogen accumulations were affected by Spartina alterniflora.
• The carbon sequestration rate was consistent with the study on the global marshes.

In this study, we analyzed carbon and nitrogen parameters (e.g., total organic carbon (TOC), total nitrogen (TN), stable isotopes of organic matter (δ13C, δ15N), and carbon–nitrogen ratio (C/N)), grain-size parameters and deposition rate, as well as their variations in the surface layer and on the profile of the sediment cores in various ecological zones of Luoyuan Bay. The results showed that the sedimentary type of Luoyuan Bay was clay silt. The TOC, TN, δ13C, and δ15N were in the range of 0.450–0.955%, 0.054–0.101%, −23.75 to −19.47‰, and 3.57–6.72‰, respectively; the C/N was in the range of 8.80–13.78. The grain-size parameters of the Spartina alterniflora marsh and transition zone were similar, whereas a similarity in the carbon and nitrogen parameters between the transition zone and mudflat was observed. The correlation of TOC and TN was different between the fresh organic matter and the obsolete organic matter. The particle size was not the main factor that controlled the TOC and TN contents in the sediments; the δ13C indicated the organic matter was dominated by marine sources. The average deposition rates in the Spartina alterniflora marsh, transition zone, and mudflat were 2.47, 2.79, 1.16 g cm−2 y−1, respectively. In the Spartina alterniflora marsh, the TOC and TN content increased by 96% and 104%, respectively, from 1955 to the present. Compared with the mudflat, the TOC and TN content in the layer between the surface and the 40-cm depth of the Spartina alterniflora marsh were 26% and 13% higher, respectively. The introduction of Spartina alterniflora and the metabolism of their roots had a significant effect on the carbon and nitrogen deposition in the layer at 0–40 cm depth. The carbon sequestration rate of the salt marsh wetland in Luoyuan Bay was comparable to the carbon sequestration of global marshes. The deposition rates of TOC and TN in the Spartina alterniflora marsh and transition zone were greater than twice that in the mudflat.