Coupling of soil methane emissions at different depths under typical coastal wetland vegetation types

Tidal and seasonal effects on sediment methane emissions from three different mangrove species

The anaerobic environment of mangrove sediments due to periodic tides is conducive to methane (CH4) production, but processes and mechanisms of CH4 emission from mangrove sediments are not yet well understood. We used in situ field monitoring and laboratory experiments to investigate the effects of tides and seasons on CH4 emissions from the sediments of Sonneratia apetala (SA), Kandelia obovata (KO), and Avicennia marina (AM), respectively. Methane emissions from the sediments of all mangrove species were significantly higher in summer than in winter, with overall CH4 fluxes being 2.14 times higher during the after-ebb tide compared to the pre-flood tide. Among the mangrove species, AM (16.77 ± 13.73 mg m-2 h-1) exhibited the highest emissions, followed by SA (1.45 ± 0.90 mg m-2 h-1) and KO (0.14 ± 0.16 mg m-2 h-1). CH4 emissions in three mangrove species were mainly driven directly by abiotic factors, including sediment organic carbon (SOC) that could provide substrate for methanogens to generate CH4, and dissolved CH4 concentration in porewater likely served as a carbon source or turnover state for CH4 to eventually enter the atmosphere. Also, sediment CH4 emissions were suppressed by the α-diversity of methanogenic communities. In addition, pH, CH4 flux, SOC, and redox potential significantly shaped structure of the methanogenic communities, potentially regulating sediment CH4 emissions. This study result highlights that abiotic factors can greatly influence CH4 emissions from mangrove sediments, as well as emphasizes the important role of the sediment-porewater-atmosphere pathway on CH4 emissions.

Study on methane flux variation characteristics and regulation mechanisms in urban wetlands under different restoration years and moisture gradients

Urban wetlands play a crucial role in the regional carbon balance and regulation of greenhouse gas (GHG) emissions; however, the effects of restoration year and moisture gradient on methane (CH4) fluxes during wetland restoration and their driving mechanisms are unclear. This study investigated CH4 fluxes, environmental factors, and methanogenic communities in urban wetlands under different restoration years (2 years, 9 years, and natural wetland without restoration) and moisture gradients (perennial waterlogging, seasonal waterlogging, and no surface waterlogging) to reveal the spatial and temporal variation characteristics of CH4 fluxes in urban wetlands and their regulatory mechanisms. The results indicated that wetlands with short-term restoration (2 years) exhibited significantly higher CH4 fluxes under conditions of sufficient moisture (such as perennial waterlogging), whereas wetlands with long-term restoration (9 years) exhibited lower CH4 fluxes. Furthermore, the average CH4 fluxes under seasonal waterlogging (1.80 mg m-2·h-1) and no surface waterlogging gradients (0.04 mg m-2·h-1) were significantly lower than that under perennial waterlogging gradient (87.36 mg m-2·h-1). Variations in CH4 flux were significantly regulated by key environmental factors (soil water content, soil organic matter, and soil temperature) and specific methanogenic communities (Methanospirillum and Methanoregula). Moreover, methanogenic communities can be altered by environmental factors that vary with restoration years and moisture gradients, leading to differences in wetland CH4 fluxes. The results provide a new perspective on the spatial and temporal variations in CH4 emissions from urban wetlands and identify key regulatory factors affecting CH4 fluxes, providing a reference for evaluating the restoration effects of urban wetlands and improving governance.

The significant role of vegetation activity in regulating wetland methane emission in China

Accurate quantifying of methane (CH4) emissions is a critical aspect of current research on regional carbon budgets. However, due to limitations in observational data, research methodologies, and an incomplete understanding of process mechanisms, significant uncertainties persist in the assessment of wetland CH4 fluxes in China. In this study, we developed a machine learning model by integrating measured CH4 fluxes with related environmental data to produce a high-resolution (1 km) dataset of CH4 fluxes from China's wetlands for the period 2000-2020. Our results estimate that the wetland CH4 flux in China is approximately 1.54 ± 0.03 mg CH4 m-2 h-1, with total annual emissions of 10.85 ± 0.26 Tg CH4 yr-1. Yangtze River Basin (6.01 Tg CH4 yr-1), Northeastern China (1.65 Tg CH4 yr-1), and the Qinghai-Tibetan Plateau (1.34 Tg CH4 yr⁻1) were identified as the primary contributing regions. Notably, total CH4 emissions from China's wetlands exhibited a significant declining trend from 2000 to 2020, primarily driven by a substantial decrease in emissions from the Yangtze River Basin and Southern China, where paddy field wetlands are predominant. In contrast, an increasing trend was observed in Northeastern China and the Tibetan Plateau, characterized by natural wetlands. Further analysis revealed that the spatial and temporal dynamics of CH4 emissions from China's wetlands are closely linked to vegetation activity. This study highlights the spatial and temporal patterns of wetland CH4 fluxes in China and investigates their potential driving mechanisms, offering valuable data support and a theoretical foundation for national CH4 emission reduction strategies and wetland management programs.

Multiple microbial guilds mediate soil methane cycling along a wetland salinity gradient

Estuarine wetlands harbor considerable carbon stocks, but rising sea levels could affect their ability to sequester soil carbon as well as their potential to emit methane (CH4). While sulfate loading from seawater intrusion may reduce CH4 production due to the higher energy yield of microbial sulfate reduction, existing studies suggest other factors are likely at play. Our study of 11 wetland complexes spanning a natural salinity and productivity gradient across the San Francisco Bay and Delta found that while CH4 fluxes generally declined with salinity, they were highest in oligohaline wetlands (ca. 3-ppt salinity). Methanogens and methanogenesis genes were weakly correlated with CH4 fluxes but alone did not explain the highest rates observed. Taxonomic and functional gene data suggested that other microbial guilds that influence carbon and nitrogen cycling need to be accounted for to better predict CH4 fluxes at landscape scales. Higher methane production occurring near the freshwater boundary with slight salinization (and sulfate incursion) might result from increased sulfate-reducing fermenter and syntrophic populations, which can produce substrates used by methanogens. Moreover, higher salinities can solubilize ionically bound ammonium abundant in the lower salinity wetland soils examined here, which could inhibit methanotrophs and potentially contribute to greater CH4 fluxes observed in oligohaline sediments.IMPORTANCELow-level salinity intrusion could increase CH4 flux in tidal freshwater wetlands, while higher levels of salinization might instead decrease CH4 fluxes. High CH4 emissions in oligohaline sites are concerning because seawater intrusion will cause tidal freshwater wetlands to become oligohaline. Methanogenesis genes alone did not account for landscape patterns of CH4 fluxes, suggesting mechanisms altering methanogenesis, methanotrophy, nitrogen cycling, and ammonium release, and increasing decomposition and syntrophic bacterial populations could contribute to increases in net CH4 flux at oligohaline salinities. Improved understanding of these influences on net CH4 emissions could improve restoration efforts and accounting of carbon sequestration in estuarine wetlands. More pristine reference sites may have older and more abundant organic matter with higher carbon:nitrogen compared to wetlands impacted by agricultural activity and may present different interactions between salinity and CH4. This distinction might be critical for modeling efforts to scale up biogeochemical process interactions in estuarine wetlands.

Metabolic diversity shapes vegetation-enhanced methane oxidation in landfill covers: Multi-omics study of rhizosphere microorganisms

Vegetation root exudates have the ability to shape soil microbial community structures, thereby enhancing CH4 bio-oxidation capacity in landfill cover systems. In this study, the CH4 oxidation capacity of indigenous vegetation rhizosphere microorganisms within operational landfill covers in Chongqing, China, was investigated for the first time, with the objective of identifying suitable plant candidates for CH4 mitigation initiatives within landfill cover systems. Furthermore, a multi-omics methodology was employed to explore microbial community structures and metabolic variances within the rhizospheric environment of diverse vegetation types. The primary aim was to elucidate the fundamental factors contributing to divergent CH4 oxidation capacities observed in rhizosphere soils. The findings demonstrated that herbaceous vegetation predominated in landfill covers. Notably, Rumex acetosa exhibited the highest CH4 oxidation capacity in the rhizosphere soil, approximately 20 times greater than that in non-rhizosphere soil. Root exudates played a crucial role in inducing the colonization of CH4-oxidizing functional microorganisms in the rhizosphere, subsequently prompting the development of specific metabolic pathways. This process, in turn, enhanced the functional activity of the microorganisms while concurrently bolstering their tolerance to microbial pollutants. Consequently, the addition of substances like Limonexic acid strengthened the CH4 bio-oxidation process, thereby underscoring the suitability of Rumex acetosa and similar vegetation species as preferred choices for landfill cover vegetation restoration.