This study aimed to reveal the species-specific patterns of soil cation exchange capacity (CEC) in typical mangrove forests within the specific environment of coastal wetlands, elucidate the multi-scale coupled driving mechanisms involving soil physical properties, chemical properties, and ecological stoichiometric ratios, and verify the ecological suitability of CEC as a core indicator of soil nutrient retention capacity in mangrove ecosystems, thereby providing a scientific basis for precisely enhancing the efficiency of soil fertility regulation in mangrove ecological restoration.

Taking the three typical communities of Aegiceras corniculatum, Sonneratia apetala, and Kandelia obovata as study objects in Maowei Sea, four plots of 20 m × 20 m were set up in each forest, soil samples were collected within each plot using a zigzag pattern to a depth of 0-60 cm, 12 physical and chemical indicators including soil CEC, pH value, organic matter, total nitrogen, and total phosphorus were determined. Correlation analysis, principal component analysis (PCA), variance partitioning analysis (VPA), and partial least squares structural equation modeling (PLS-SEM) were used to explore the key influencing factors of CEC and its coupling mechanism with nutrient retention capacity.

1) Except for nitrate nitrogen, the other 11 soil physical and chemical indicators showed significant differences among tree species (P<0.05); CEC and most nutrient indicators in the 0-20 cm topsoil were significantly higher than those in the subsoil (P<0.05), exhibited an obvious surface accumulation effect; 2) CEC was jointly influenced by soil physical properties, chemical characteristics and ecological stoichiometric ratios. Among them, soil physical properties had the largest total effect (regulating chemical properties through indirect effects), chemical properties had the strongest direct effect, and the two synergistically drove CEC changes, while ecological stoichiometric ratios had the weakest impact; 3) The integrated fertility index (IFI) showed the same order as CEC (Aegiceras corniculatum > Sonneratia apetala > Kandelia obovata), soil CEC and fertility levels were significantly affected by tree species differences, meanwhile, the reliability of CEC as a core indicator of nutrient retention capacity was verified.

The comprehensive soil fertility level of mangroves in the study area was generally medium to low. The soil was rich in organic matter but deficient in nitrogen and phosphorus. The key to maintaining appropriate CEC lies in protecting the natural accumulation of organic matter. It is suggested to adopt artificial regulation measures to accelerate vegetation restoration, promote vegetation succession, improve nitrogen and phosphorus levels, and enhance soil nutrient supply and retention capacity.

Ecological stoichiometry is an efficient tool to explore the balance and cycling of coupled elements, this study aims to provide a theoretical basis for the scientific management of alien plants by investigating the contents of key elements and their ecological stoichiometric characteristics in the surface sediments of Sonneratia apetala plantations of different ages.

The study focused on three artificially established S. apetala forests of different ages (10, 15, and 20 years) in the Qinzhou bay coastal wetland. Three plots of 20 m × 20 m were set up in each forest age group. Surface sediment samples were collected within each plot using a zigzag pattern to a depth of 0-20 cm. Samples were analyzed to determine the contents of total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), total sulfur(TS), available sulfur (AS), total potassium(TK), available potassium (AK) and so on. The ecological stoichiometric characteristics of C, N, P and other elements in surface sediments and their influencing factors were studied.

1) The age of S. apetala forests had a significant influence on the physicochemical properties that included TOC, TN, TP, TS and AS, as well as the ecological stoichiometry that contained C/N, C/P, C/S, C/K, N/S, P/S, P/K, and S/K in sediments; 2) The average C/N values of sediments in the study area (20.41) were slightly higher than those of Chinese wetland soils (18.22). Conversely, the mean C/P values (64.91) and N/P values (3.19) were much lower than those of Chinese wetland soils(245.22 and 13.6, respectively). The average C∶N∶P ratio of 20.3 fell within the range of 7.08 to 41.44 that was observed in Chinese wetland soils. (3) Correlation analysis and redundancy analysis revealed that TOC, TN, and TP were the key factors influencing the variation in ecological stoichiometry of surface sediments.

The sediments in the study area were characterized by rich C and P but less N, with a higher mineralization capacity of sedimentary phosphorus. The growth of S. apetala requires a large amount of phosphorus while being limited by nitrogen. During the management process of S. apetala plantations, the diffusion rate of S. apetala can be slowed down by scientifically regulating the sediment organic carbon content and reducing the influx of nitrogen and phosphorus into the estuary, thus reducing ecological risks.