Carbon and nitrogen characteristics and sediment organic matter sources tracing across vegetation zones of liaohe estuarine saltmarsh, Northeast China

Improving our understanding of carbon and nitrogen cycling in coastal wetlands and their response mechanism to environmental changes caused by anthropogenic activity requires analyzing the distribution and source changes of sediment organic matter (SOM). In this study, three sediment core samples were collected from different zones of reed marsh, seepweed marsh and bare beach in the Liaohe estuarine saltmarsh. As key factors related to SOM, sediment grain size, total organic carbon (TOC), total nitrogen (TN), and stable isotopes (δ¹³C and δ¹⁵N) were measured, their vertical changes in stages of depth-dependent transitions were analyzed. In addition, SOM sources were investigated based on the TOC/TN ratio and δ¹³C/δ¹⁵N fingerprint. The results show that the distribution of SOM is controlled by vegetation conditions, altitude and material input. Based on an ocean–vegetation–river triple-end-member model quantifying, the sources of SOM showed the characteristics of a mixture of both terrestrial and marine inputs, with the marine input being more significant. The SOM contributions in the reed marsh and bare beach derived from the ocean with ranging from 48.6 % to 65.9 % and 45.3 % to 73.9 %, respectively. The contributions from vegetation and river sources are comparable, but their changes with depth are significant, reflecting the regional stage evolutionary trend of environment change in the estuarine area. The depleted surface δ¹⁵N_total signature (0.78 ‰) in the seepweed marsh, combined with stratigraphic discontinuities and SOM sources patterns suggests anthropogenic impact.