Methane emissions respond to soil temperature in convergent patterns but divergent sensitivities across wetlands along altitude

Recent advances on greenhouse gas emissions from wetlands: Mechanism, global warming potential, and environmental drivers

Greenhouse gas (GHG) emissions from wetlands have exacerbated global warming, attracting worldwide attention. However, the research process and development trends in this field remain unknown. Herein, 1865 papers related to wetlands GHG emissions published from January 2000 to December 2023 were selected, and CiteSpace and VOSviewer were used for bibliometric analysis to visually analyze the publications distribution, research authors, organizations and countries, core journal and keywords, and discussed the research progress, trends and hotspots in the fields. Over the past 24 years, the research has gone through three phases: the "embryonic" stage (2000-2006), the accumulation stage (2007-2014), and the acceleration stage (2015-2023). China has played a pivotal role in this domain, publishing the most papers and working closely with the United States, United Kingdom, Canada, Germany, and Australia. In addition, this study synthesized 311 field observations from 123 publications to analyze the variability in GHG emissions and their driving factors in four different types of natural wetlands. The results suggested that the average carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes in different wetlands were significantly different. River wetlands exhibited the highest GHG fluxes, while marsh wetlands demonstrated greater global warming potential (GWP). The average CO2, CH4 and N2O fluxes were 60.41 mg m-2·h-1, 2.52 mg m-2·h-1 and 0.05 mg m-2·h-1, respectively. The GWP of Chinese natural wetlands was estimated as 648.72 Tg·CO2-eq·yr-1, and CH4 contributed the largest warming effect, accounting for 57.43%. Correlation analysis showed that geographical location, climate factors, and soil conditions collectively regulated GHG emissions from wetlands. The findings provide a new perspective on sustainable wetland management and reducing GHG emissions.

Methane emissions and microbial communities under differing flooding conditions and seasons in littoral wetlands of urban lake

Wetlands are the largest natural sources of methane (CH4) emissions worldwide. Littoral wetlands of urban lakes represent an ecotone between aquatic and terrestrial ecosystems and are strongly influenced by water levels, environmental conditions, and anthropogenic activities. Despite these littoral zones being potential "hotspots" of CH4 emissions, the status of CH4 emissions therein and the role of physicochemical properties and microbial communities regulating these emissions remain unclear. This study compared the CH4 fluxes, physicochemical properties, and CH4-cycling microbial communities (methanogens and methanotrophs) of three zones (a non-flooded supralittoral zone, a semi-flooded eulittoral zone, and a flooded infralittoral zone) in the littoral wetlands of Lake Pipa, Jiangsu Province, China, for two seasons (summer and winter). The eulittoral zone was a CH4 source (median: 11.49 and 0.02 mg m-2 h-1 in summer and winter, respectively), whereas the supralittoral zone acted as a CH4 sink (median: -0.78 and -0.09 mg m-2 h-1 in summer and winter, respectively). The infralittoral zone shifted from CH4 sink to source between the summer (median: -10.65 mg m-2 h-1) and winter (median: 0.11 mg m-2 h-1). The analysis of the functional genes of methanogenesis (mcrA) and methanotrophy (pmoA) and path analysis showed that CH4 fluxes were strongly regulated by biotic factors (abundance of the mcrA gene and alpha diversity of CH4-cycling microbial communities) and abiotic factors (ammonia nitrogen, moisture, and soil organic carbon). In particular, biotic factors had a major influence on the variation in the CH4 flux, whereas abiotic factors had a minor influence. Our findings provide novel insights into the spatial and seasonal variations in CH4-cycling microbial communities and identify the key factors influencing CH4 fluxes in littoral wetlands. These results are important for managing nutrient inputs and regulating the hydrological regimes of urban lakes.

Plant-mediated CH4 exchange in wetlands: A review of mechanisms and measurement methods with implications for modelling

Plant-mediated CH4 transport (PMT) is the dominant pathway through which soil-produced CH4 can escape into the atmosphere and thus plays an important role in controlling ecosystem CH4 emission. PMT is affected by abiotic and biotic factors simultaneously, and the effects of biotic factors, such as the dominant plant species and their traits, can override the effects of abiotic factors. Increasing evidence shows that plant-mediated CH4 fluxes include not only PMT, but also within-plant CH4 production and oxidation due to the detection of methanogens and methanotrophs attached to the shoots. Despite the inter-species and seasonal differences, and the probable contribution of within-plant microbes to total plant-mediated CH4 exchange (PME), current process-based ecosystem models only estimate PMT based on the bulk biomass or leaf area index of aerenchymatous plants. We highlight five knowledge gaps to which more research efforts should be devoted. First, large between-species variation, even within the same family, complicates general estimation of PMT, and calls for further work on the key dominant species in different types of wetlands. Second, the interface (rhizosphere-root, root-shoot, or leaf-atmosphere) and plant traits controlling PMT remain poorly documented, but would be required for generalizations from species to relevant functional groups. Third, the main environmental controls of PMT across species remain uncertain. Fourth, the role of within-plant CH4 production and oxidation is poorly quantified. Fifth, the simplistic description of PMT in current process models results in uncertainty and potentially high errors in predictions of the ecosystem CH4 flux. Our review suggest that flux measurements should be conducted over multiple growing seasons and be paired with trait assessment and microbial analysis, and that trait-based models should be developed. Only then we are capable to accurately estimate plant-mediated CH4 emissions, and eventually ecosystem total CH4 emissions at both regional and global scales.

Recent increases in annual, seasonal, and extreme methane fluxes driven by changes in climate and vegetation in boreal and temperate wetland ecosystems

Climate warming is expected to increase global methane (CH4 ) emissions from wetland ecosystems. Although in situ eddy covariance (EC) measurements at ecosystem scales can potentially detect CH4 flux changes, most EC systems have only a few years of data collected, so temporal trends in CH4 remain uncertain. Here, we use established drivers to hindcast changes in CH4 fluxes (FCH4 ) since the early 1980s. We trained a machine learning (ML) model on CH4 flux measurements from 22 [methane-producing sites] in wetland, upland, and lake sites of the FLUXNET-CH4 database with at least two full years of measurements across temperate and boreal biomes. The gradient boosting decision tree ML model then hindcasted daily FCH4 over 1981-2018 using meteorological reanalysis data. We found that, mainly driven by rising temperature, half of the sites (n = 11) showed significant increases in annual, seasonal, and extreme FCH4 , with increases in FCH4 of ca. 10% or higher found in the fall from 1981-1989 to 2010-2018. The annual trends were driven by increases during summer and fall, particularly at high-CH4 -emitting fen sites dominated by aerenchymatous plants. We also found that the distribution of days of extremely high FCH4 (defined according to the 95th percentile of the daily FCH4 values over a reference period) have become more frequent during the last four decades and currently account for 10-40% of the total seasonal fluxes. The share of extreme FCH4 days in the total seasonal fluxes was greatest in winter for boreal/taiga sites and in spring for temperate sites, which highlights the increasing importance of the non-growing seasons in annual budgets. Our results shed light on the effects of climate warming on wetlands, which appears to be extending the CH4 emission seasons and boosting extreme emissions.