Organic matter in the productive mangrove–estuary ecosystem plays an important role in global climate changes. In recent years, the eutrophication in such areas caused by anthropogenic inputs of sewage has been revealed, highlighting the need to understand its influence on organic matter. In this study, δ¹³C and δ¹⁵N were used to reveal the effects of sewage on organic matters in the Zhangjiang mangrove–estuary ecosystem. Our results indicate that sewage contributed the most of the total particulate organic matter (41%) in the Zhangjiang estuary, while mangrove plants contributed most of the soil organic matter (45%) in the neighboring Yunxiao mangrove. Phytoplankton was another important source of organic matter, accounting for about 21.8% and 49.8% of the particulate and soil organic matter, respectively. Sewage was also a major source of dissolved inorganic nitrogen, providing 28.9%, 12.2%, and 100% of the total NO₃⁻, NO₂⁻, and NH₄⁺ in the Zhangjiang estuary, respectively. This may be the major reason for the productive phytoplankton here, which contributed 21.8% and 49.8% of the total particulate and soil organic matter in the study area. Our results reveal the direct contribution and the potential effects of sewage on the contents and bioavailability of organic matter in mangrove–estuary ecosystems, providing new insights into understanding the response of coastal areas to the influence of human activities.

The decomposition of mangrove litter plays a crucial role in material circulation and energy flow within mangrove forests. Evaluating the decomposition-based variation in biogenic elements in litter is important for improving our understanding about their biogeochemical cycling in ecosystems. The main objective of this study was to examine the interaction effect during the decomposition process of mixed Kandelia obovata and Avicennia marina litter. Variations in C and N were also determined in the decomposing leaf litter mixtures. Our findings revealed that the decomposition rates were faster in summer than in winter, and increased with the proportion of A. marina litter. After 35 days of decomposition in summer, the remaining weights for different proportions of K. obovata (KO) and A. marina (AM) were 22.9% (KO:AM = 1:2), 27.2% (KO:AM = 1:1), and 31.2% (KO:AM = 2:1), respectively. Similarly, after 49 days of decomposition in winter, the remaining weights for the different KO:AM proportions were 27.7%, 35.4%, and 44.0%, respectively. Additionally, the decomposition of mixed K. obovata and A. marina litter had an influence on C content and N release dynamics. These results provide a scientific basis for understanding the decomposition of mixed mangrove litter and its implications for material circulation and energy flow within these ecosystems.

Estuary ecosystems serve as crucial connectors between terrestrial and marine environments, thus playing vital roles in maintaining the ecological balance of coastal marine ecosystems. In recent years, the eutrophication in estuaries caused by aquaculture sewage has been revealed, highlighting the necessity to understand its influence on the nutrient conditions and carbon storage of estuaries. In this study, δ¹⁵N and δ¹⁸O were used to indicate the contribution of aquaculture-derived sewage to dissolved inorganic nitrogen in Zhangjiang Estuary, and δ¹³C and C:N ratio were used to reveal its effects on the particulate organic matter. The major results are as follows: (1) Aquaculture water contributed 62~86% and 60~100% of the total nitrate and ammonium in Zhangjiang Estuary, respectively, and the drainage periods of the cultured species has a great influence on the content and composition of dissolved inorganic nitrogen. (2) Aquaculture water was also the major source of particulate organic matter (24~33% of the total content) here, most of which may be derived from crab ponds. (3) The imports of nutrients by aquaculture water may potentially regulate particulate organic matter in Zhangjiang Estuary by promoting the growth of phytoplankton and zooplankton. Our study revealed the coupling effects of aquaculture activities on the nitrogen and carbon storage in an estuarine ecosystem. It also indicates that isotopes may be efficient in the monitoring of a coastal environment, which may further aid the management of inshore cultivation.

Coastal wetlands are characterized by high production and thus play an important role in global climate change. In past decades, the invasion of Spartina alterniflora has caused many problems of coastal wetlands in southeastern China, and the restoration of such areas was mainly conducted by replacing Spartina alterniflora with mangrove plants. This may impact the carbon storage dynamics in such areas. In this study, stable isotopes (δ¹³C and δ¹⁵N) and molecular analysis were used to reveal the impact of artificial restoration on the carbon storage of Quanzhou Bay Estuary Wetland Natural Reserve. The major results are as follows: (1) the change in dominant plants results in a changing major source of soil organic matter, from external sources to mangrove plants; (2) the decrease in soil organic matter following the removal of Spartina alterniflora may be primarily caused by the loss of external organic matter, while the production of mangroves may offset such loss and enhance the content and stability of carbon storage over the long term; (3) microbial CO₂ assimilation may serve as an alternative source of bioavailable carbon and thus support the activity of benthic community. Our results revealed the long-term benefits of such restoration on the carbon storage function of wetlands invaded by Spartina alterniflora. Furthermore, the integrating of isotopic tracers and molecular technology may provide new insights in understanding the response of the carbon storage in coastal areas to human activity.

Mangrove ecosystems play a critical role in sequestering heavy metals pollutants, yet the dynamics of heavy metals accumulation during mixed litter decomposition remain poorly understood. This study investigated the seasonal and species-specific variations in heavy metals accumulation during the decomposition of Kandelia obovata (KO) and Avicennia marina (AM) leaf litter mixtures in a subtropical mangrove forest in the Jiulong River Estuary, Fujian, China. Using the litterbag technique, we monitored eight heavy metals (V, Cr, Ni, Cu, Zn, As, Se, Cd) across three mixing ratios (KO:AM = 1:2, 1:1, 2:1) in summer and winter. Results revealed that V concentrations were influenced by both season and litter ratio, with higher KO proportions enhancing V accumulation in summer but reducing it in winter. In contrast, Cr, Ni, Cu, As, Se, and Cd were primarily regulated by litter ratios: KO-dominated mixtures promoted Cr and Ni accumulation, while AM-dominated mixtures favored Cu, As, Se, and Cd. Zn exhibited the highest variability and was unaffected by season or ratio. Total organic carbon (TOC) and carbon/metal (C/M) ratios significantly correlated with reduced bioavailability of most heavy metals, whereas total nitrogen (TN) and C/N ratios showed no consistent relationship. The heavy metals accumulation index (MAI) indicated higher accumulation in summer than in winter, with the highest MAI observed in the KO:AM = 2:1 treatment group during summer (MAI = 1.36), whereas winter decomposition slowed accumulation rates. These findings highlight the dual regulatory roles of species composition and environmental factors in mangrove heavy metals cycling, offering critical insights for ecological risk assessment and contaminated soil remediation strategies in coastal ecosystems.

Mangroves are characterized by high productivity, thus playing crucial roles in combating global climate change. In recent decades, the invasion of Spartina alterniflora has led to significant degradation of mangrove vegetation. Currently, the main restoration measure for such damaged mangroves is to remove the invasive S. alterniflora. Furthermore, monitoring of S. alterniflora regeneration after restoration is also of great significance. In this study, an indicator of the presence of S. alterniflora in the soil was measured using a stable isotopic mixing model and further used to predict the potential regeneration of S. alterniflora in the natural Zhangjiang Estuary mangrove forest and the artificially planted Quanzhou Bay mangrove forest. The key findings are as follows: (1) The regeneration of S. alterniflora was observed in the Quanzhou Bay mangrove forest after observing an increased indication of its underground biomass (from 2.5% to 10.6%). This was not observed in the Zhangjiang Estuary mangrove forest, indicating its higher resistance against S. alterniflora regeneration. (2) The removal of S. alterniflora affected the diversity of the soil microbes, possibly by regulating the available organic matter, thus further altering the levels of S. alterniflora regeneration after restoration. (3) The higher functional redundancy and co-occurrence of soil microbes in the natural ZJE mangrove forest may be one major reason for its higher resistance to S. alterniflora invasion/regeneration. This study reveals potential effects of soil microbial communities on the stability of mangrove wetlands, which may provide new insights for future research on mangrove restoration programs.