Methane (CH₄) emissions from estuarine wetlands were proved to be influenced by tide movement and inundation conditions notably in many previous studies. Although there have been several researches focusing on the seasonal or annual CH₄ emissions, the short-term CH₄ emissions during the tide cycles were rarely studied up to now in this area. In order to investigate the CH₄ emission pattern during a tide cycle in Yangtze Estuary salt marshes, frequent fixed-point observations of methane flux were carried out using the in-situ static closed chamber technique. The results indicated that the daily average CH₄ fluxes varied from 0.68 mgCH₄·m⁻²·h⁻¹ to 4.22 mgCH₄·m⁻²·h⁻¹ with the average flux reaching 1.78 mgCH₄·m⁻²·h⁻¹ from small tide to spring tide in summer. CH₄ fluxes did not show consistent variation with both tide levels and inundation time but increased steadily during almost the whole research period. By Pearson correlation analysis, CH₄ fluxes were not correlated with both tide levels (R = −0.014, p = 0.979) and solar radiation (R = 0.024, p = 0.865), but significantly correlated with ambient temperature. It is temperature rather than the tide level mainly controlling CH₄ emissions during the tide cycles. Besides, CH₄ fluxes also showed no significant correlation with the underground pore-water CH₄ concentrations, indicating that plant-mediated transport played a more important role in CH₄ fluxes compared with its production and consumption.

The role of wetland plant (Scirpus mariqueter) on methane (CH₄) emissions from a subtropical tidal saltmarsh of Yangtze estuary was investigated over a year. Monthly CH₄ flux and pore-water CH₄ concentration were characterized using static closed chamber technique and pore-water extraction. Measured chamber CH₄ fluxes indicated that saltmarsh of the Yangtze estuary acted as a net source of atmospheric CH₄ with annual average flux of 24.0 mgCH₄·m⁻²·day⁻¹. The maximum chamber CH₄ flux was in August (91.2 mgCH₄·m⁻²·day⁻¹), whereas the minimum was observed in March (2.30 mgCH₄·m⁻²·day⁻¹). Calculated diffusion CH₄ fluxes were generally less than 6% of the chamber fluxes. Significant correlations were observed between the chamber CH₄ flux and rhizospheric pore-water CH₄ concentration (11–15 cm: p < 0.05, R = 0.732; 16–20 cm: p < 0.05, R = 0.777). In addition, chamber CH₄ fluxes from July to September constituted more than 80% of the total annual emission and were closely correlated with aboveground biomass yield of S. mariqueter. The results indicated that S. mariqueter transportation was the dominant CH₄ emission pathway and it provided an efficient route for the belowground CH₄ to escape into the atmosphere while avoiding oxidation, leading to CH₄ emissions.

Hangzhou Bay has long experienced excessive nitrogen loading coupled with limited hydrodynamic exchange, leading to some of the highest nitrogen concentrations in China’s coastal waters. As critical land-sea ecotones, intertidal wetlands play a crucial role in mitigating nitrogen pollution across the bay. However, rapid urbanization and extensive reclamation since 1990 have led to a loss of over 50% of the intertidal wetlands in southern Hangzhou Bay. In this study we measured sediment denitrification and anammox potentials across key habitats: salt marshes (vegetated by Spartina alterniflora, Phragmites australis, and Scirpus mariqueter), bare mudflats, and shellfish aquaculture zones. We used ¹⁵N isotope tracing techniques coupled with slurry incubation experiments. Analysis of sediment physicochemical properties was conducted to elucidate the driving mechanisms of nitrogen removal. By integrating wetland landscape evolution with regional nitrogen budgets, we evaluated the nitrogen sink function of these intertidal wetlands. Our results revealed a distinct spatial hierarchy in denitrification potential, decreasing in the order: S. alterniflora (13.02 ± 3.54 μmol·N·kg⁻¹·h⁻¹) > shellfish aquaculture zones (12.86 ± 7.50 μmol·N·kg⁻¹·h⁻¹) > P. australis (11.54 ± 1.80 μmol·N·kg⁻¹·h⁻¹) > S. mariqueter (7.33 ± 2.08 μmol·N·kg⁻¹·h⁻¹) > bare mudflats (5.99 ± 1.62 μmol·N·kg⁻¹·h⁻¹). S. alterniflora has higher primary productivity, biomass accumulation, and a more robust root system structure. It regulates the content and availability of sediment organic carbon, the supply of nitrate nitrogen, pH, and water content. These regulations subsequently enhance denitrification. In contrast, shellfish aquaculture zones enhance denitrification potential primarily through bioturbation, which increases water content and lowers pH conditions. An integrated assessment of denitrification potential and landscape patterns revealed that, despite ongoing habitat loss, the remaining intertidal wetlands in southern Hangzhou Bay still remove about 30.65% of exogenous inorganic nitrogen. This finding underscores their critical role as effective pollution buffers under high nitrogen loading.