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This study investigates the ecological stoichiometric characteristics and driving factors of carbon (C), nitrogen (N), and phosphorus (P) in deep sediments within the critical zone of South Dongting Lake Wetland. Correlation analysis, partial least squares structural equation modeling (PLS-SEM), and gradient boosted decision tree (GBDT) algorithm were employed for this investigation. The results showed that the mean values of the total carbon (TC), soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP) contents in sediments are 9.0, 7.3, 0.9, and 0.5 g kg− 1, respectively. Sediment C, N, and P contents tended to decrease with increasing burial depth (H). The mean ratios of C/N, C/P, and N/P in sediments were 10.5, 46.5, and 5.1, respectively, notably lower than the averages in wetland soils across China. Lower C/N and C/P ratios indicate that the decomposition rate of organic matter is relatively fast and organic P is prone to mineralization in sediments. Additionally, the lower N/P ratio implies N limitation within the sediments. The TC, SOC, TN, and TP exhibited significant negative correlations with both H and redox potential (Eh), while showing positive associations with water content (W). Moreover, these factors influence ecological stoichiometric ratios (ESR) by directly affecting C, N, and P contents in sediments. The GBDT modelling revealed that TN primarily influenced C/N ratios, while TP predominantly controlled C/P and N/P ratios. The contents of C, N, and P, as well as their ESR in deep sediments of wetland are mainly controlled by H, Eh, and W.

As an essential component of proteins and genetic material for all organisms, nitrogen (N) is one of the major limiting factors that control the dynamics, biodiversity and functioning of lacustrine wetlands, in which intensified N biogeochemical activities take place. Reactive N loaded into wetland ecosystems has been doubled due to various human activities, including industrial, agricultural activities and urbanization. The main driving mechanisms of N transport and transformation in lacustrine wetlands are categorized to pushing forces and pulling forces in this study. Geomorphology, wetland age, N concentrations, and temperature are the main pushing forces (passive forces); whereas water table variation, oxygen concentration, other elements availability, oxidation-reduction potential (Eh) and pH, and microorganisms are the predominant pulling forces (active forces). The direction and kinetic energy of reactions are determined by pulling forces and then are stimulated by pushing forces. These two types of forces are analyzed and discussed separately. Based on the analysis of driving mechanisms, possible solutions to wetland N pollutions are proposed at individual, regional and global scales, respectively. Additional research needs are addressed to obtain a thorough understanding of N transport and transformations in wetlands and to reduce detrimental impacts of excessive N on such fragile ecosystems.

The enclosure engineering of tidal flats behind Changxing Island is taken as the research background. In order to explore the undrained cyclic response of natural marine clay on tidal level periodic changing, the stress-controlled low frequency cyclic tri-axial tests and static creep tests are conducted on natural K0-consolidated Shanghai clay. In the series I tests, samples are cyclically sheared by low-frequency cyclic loading until stability or failure, the accumulative behavior and reversible strain are studied. The effects of confining pressure and stress ratio are also investigated. Significantly, loading frequency is a key factor in controlling the cyclic behavior. In the series II tests, special attention is given to the low frequency cyclic vibration effect on the undrained cyclic tests by comparison with the static creep tests. The critical stress ratio of low frequency cyclic behavior is less than static creep strength.