Explore the vast range of titles available.

Temperate forest soils are considered significant methane (CH4) sinks, but other methane sources and sinks within these forests, such as trees, litter, deadwood, and the production of volatile organic compounds are not well understood. Improved understanding of all CH4 fluxes in temperate forests could help mitigate CH4 emissions from other sources and improve the accuracy of global greenhouse gas budgets. This review highlights the characteristics of temperate forests that influence CH4 flux and assesses the current understanding of the CH4 cycle in temperate forests, with a focus on those managed for specific purposes. Methane fluxes from trees, litter, deadwood, and soil, as well as the interaction of canopy-released volatile organic compounds on atmospheric methane chemistry are quantified, the processes involved and factors (biological, climatic, management) affecting the magnitude and variance of these fluxes are discussed. Temperate forests are unique in that they are extremely variable due to strong seasonality and significant human intervention. These features control CH4 flux and need to be considered in CH4 budgets. The literature confirmed that temperate planted forest soils are a significant CH4 sink, but tree stems are a small CH4 source. CH4 fluxes from foliage and deadwood vary, and litter fluxes are negligible. The production of volatile organic compounds could increase CH4’s lifetime in the atmosphere, but current in-forest measurements are insufficient to determine the magnitude of any effect. For all sources and sinks more research is required into the mechanisms and microbial community driving CH4 fluxes. The variability in CH4 fluxes within each component of the forest, is also not well understood and has led to overestimation of CH4 fluxes when scaling up measurements to a forest or global scale. A roadmap for sampling and scaling is required to ensure that all CH4 sinks and sources within temperate forests are accurately accounted for and able to be included in CH4 budgets and models to ensure accurate estimates of the contribution of temperate planted forests to the global CH4 cycle.

Atmospheric methane is a potent greenhouse gas that plays a major role in controlling the Earth’s climate. The causes of the renewed increase of methane concentration since 2007 are uncertain given the multiple sources and complex biogeochemistry. Here, we present a metadata analysis of methane fluxes from all major natural, impacted and human-made aquatic ecosystems. Our revised bottom-up global aquatic methane emissions combine diffusive, ebullitive and/or plant-mediated fluxes from 15 aquatic ecosystems. We emphasize the high variability of methane fluxes within and between aquatic ecosystems and a positively skewed distribution of empirical data, making global estimates sensitive to statistical assumptions and sampling design. We find aquatic ecosystems contribute (median) 41% or (mean) 53% of total global methane emissions from anthropogenic and natural sources. We show that methane emissions increase from natural to impacted aquatic ecosystems and from coastal to freshwater ecosystems. We argue that aquatic emissions will probably increase due to urbanization, eutrophication and positive climate feedbacks and suggest changes in land-use management as potential mitigation strategies to reduce aquatic methane emissions.

This work presents the development of a low-cost and high-performance multi-sensory gas detection instrument named the AIMNet Sensor, with the integration of a machine learning-based data processing method. The compact and low-power instrument (8.5 × 11.5 cm, 1.4 W) houses the core sensing hardware module, Senseair K96, that integrates both a non-dispersive infrared (NDIR)-based gas sensing unit and a BME280 environmental sensing unit. To address the outdoor operation challenges caused by environmental fluctuation due to the varying temperature, humidity, and pressure, from the software aspect, multiple machine learning-based regression models were trained in this work on 13,125 calibration data points collected under controlled laboratory conditions. Among ten tested algorithms, the Multilayer Perceptron (MLP) and Elastic Net models achieved the highest accuracy, with R-squared coefficient R2>0.8 on both indoor and outdoor scenarios, and with inter-sensor root mean square error (RMSE) within 1.5 ppm across four identical instruments. Moreover, field mobile validation was performed near a wastewater management facility using this solution, confirming a strong correlation with LI-COR reference measurements and a reliable detection of CH4 leaks with concentrations up to 18 ppm at the test site. Overall, this machine learning-integrated NDIR sensing solution (i.e., AIMNet) offers a practical and scalable solution towards a more robust distributed CH4 monitoring network for real-world field-deployable applications.

Separation of impurities (CO2 and N2) from CH4 is an important issue for natural gas alternatives (such as coalbed gas, biogas, and landfill gas) upgrading. It is notably challenging to synthesize high N-doped porous carbon with an appropriate porous structure. In this work, high N content (14.48 wt %) porous carbon with micropore size of 0.52 and 1.2 nm and specific surface area of 862 m2 g−1 has been synthesized from potassium hydroxide (KOH) activated waste wool upon the urea modification. Pure component adsorption isotherms of CO2, CH4, and N2 are systematically measured on this enhanced N-doped porous carbon at 0 and 25 °C, up to 1 bar, to evaluate the gases adsorption capability, and correlated with the Langmuir model. These data are used to estimate the separation selectivities for binary mixtures of CO2/CH4 and CH4/N2 at different mixing ratios according to the ideal adsorbed solution theory (IAST) model. At an ambient condition of 25 °C and 1 bar, the predicted selectivities for equimolar CO2/CH4 and CH4/N2 are 3.19 and 7.62, respectively, and the adsorption capacities for CO2, CH4, and N2 are 2.91, 1.01, and 0.13 mmol g−1, respectively. This report introduces a simple pathway to obtain enhanced N-doped porous carbon with large adsorption capacities for gas separation of CO2/CH4 and CH4/N2.

Forest ecosystem methane (CH₄) research has focused on soils, but trees are also important sources and sinks in forest CH₄ budgets. Living and dead trees transport and emit CH₄ produced in soils; living trees and dead wood emit CH₄ produced inside trees by microorganisms; and trees produce CH₄ through an abiotic photochemical process. Here, we review the state of the science on the production, consumption, transport, and emission of CH₄ by living and dead trees, and the spatial and temporal dynamics of these processes across hydrologic gradients inclusive of wetland and upland ecosystems. Emerging research demonstrates that tree CH₄ emissions can significantly increase the source strength of wetland forests, and modestly decrease the sink strength of upland forests. Scaling from stem or leaf measurements to trees or forests is limited by knowledge of the mechanisms by which trees transport soil-produced CH₄, microbial processes produce and oxidize CH₄ inside trees, a lack of mechanistic models, the diffuse nature of forest CH₄ fluxes, complex overlap between sources and sinks, and extreme variation across individuals. Understanding the complex processes that regulate CH₄ source–sink dynamics in trees and forests requires cross-disciplinary research and new conceptual models that transcend the traditional binary classification of wetland vs upland forest.

Tree stems from wetland, floodplain and upland forests can produce and emit methane (CH₄). Tree CH₄ stem emissions have high spatial and temporal variability, but there is no consensus on the biophysical mechanisms that drive stem CH₄ production and emissions. Here, we summarize up to 30 opportunities and challenges for stem CH₄ emissions research, which, when addressed, will improve estimates of the magnitudes, patterns and drivers of CH₄ emissions and trace their potential origin.We identified the need: (1) for both long-term, high-frequency measurements of stem CH₄ emissions to understand the fine-scale processes, alongside rapid large-scale measurements designed to understand the variability across individuals, species and ecosystems; (2) to identify microorganisms and biogeochemical pathways associated with CH₄ production; and (3) to develop a mechanistic model including passive and active transport of CH₄ from the soil–tree–atmosphere continuum. Addressing these challenges will help to constrain the magnitudes and patterns of CH₄ emissions, and allow for the integration of pathways and mechanisms of CH₄ production and emissions into process-based models. These advances will facilitate the upscaling of stem CH₄ emissions to the ecosystem level and quantify the role of stem CH₄ emissions for the local to global CH₄ budget.

Cellulosic biofuels are intended to improve future energy and climate security. Nitrogen (N) fertilizer is commonly recommended to stimulate yields but can increase losses of the greenhouse gas nitrous oxide (N2O) and other forms of reactive N, including nitrate. We measured soil N2O emissions and nitrate leaching along a switchgrass (Panicum virgatum) high resolution N-fertilizer gradient for three years post-establishment. Results revealed an exponential increase in annual N2O emissions that each year became stronger (R2 > 0.9, P < 0.001) and deviated further from the fixed percentage assumed for IPCC Tier 1 emission factors. Concomitantly, switchgrass yields became less responsive each year to N fertilizer. Nitrate leaching (and calculated indirect N2O emissions) also increased exponentially in response to N inputs, but neither methane (CH4) uptake nor soil organic carbon changed detectably. Overall, N fertilizer inputs at rates greater than crop need curtailed the climate benefit of ethanol production almost two-fold, from a maximum mitigation capacity of −5.71 0.22 Mg CO2e ha−1 yr−1 in switchgrass fertilized at 56 kg N ha−1 to only −2.97 0.18 Mg CO2e ha−1 yr−1 in switchgrass fertilized at 196 kg N ha−1. Minimizing N fertilizer use will be an important strategy for fully realizing the climate benefits of cellulosic biofuel production.

The potent greenhouse gas methane (CH₄) is readily emitted from tropical reservoirs, often via ebullition (bubbles). This highly stochastic emission pathway varies in space and time, however, hampering efforts to accurately assess total CH₄ emissions from water bodies. We systematically studied both the spatial and temporal scales of ebullition variability in a river inflow bay of a tropical Brazilian reservoir. We conducted multiple highly resolved spatial surveys of CH₄ ebullition using a hydroacoustic approach supplemented with bubble traps over a 12-month and a 2-week timescale to evaluate which scale of variation was more important. To quantify the spatial and temporal variability of CH₄ ebullition, we used the quartile coefficients of dispersion at each point in space and time and compared their frequency distributions across the various temporal and spatial scales. We found that CH₄ ebullition varied more temporally than spatially and that the intra-annual variability was stronger than daily variability within 2 weeks. We also found that CH₄ ebullition was positively related to water temperature increase and pressure decrease, but no consistent relationship with water column depth or sediment characteristics was found, further highlighting that temporal drivers of emissions were stronger than spatial drivers. Annual estimates of CH₄ ebullition from our study area may vary by 75–174% if ebullition is not resolved in time and space, but at a minimum we recommend conducting spatially resolved measurements at least once during each major hydrologic season in tropical regions (i.e., in dry and rainy season when water levels are falling and rising, respectively).

The root-associated methanotrophs contribute to N 2 fixation and CH 4 oxidation in paddy fields under N-limited conditions. However, the impact of nitrogen inputs on N₂ fixation and CH₄ oxidation by methanotrophs is largely unknown, especially in saline-alkali paddy fields with higher nitrogen application. This study explored the impact of nitrogen fertilization on N₂ fixation and CH₄ oxidation by root-associated active diazotrophic and methanotrophic communities in a saline-alkali paddy field using 15 N–N 2 and 13 C–CH 4 isotope feeding experiments along with RNA-based sequencing. The 15 N and 13 C isotope feeding experiments showed that the CH₄ oxidation-dependent nitrogen fixation rate of methanotrophs ( 15 N and 13 C) in the roots of two rice cultivars was significantly higher than the CH₄ oxidation-independent nitrogen fixation rate of heterotrophic diazotrophs (only 15 N) under nitrogen fertilization (SN) in a saline-alkali environment ( P  < 0.05). For Kongyu131 rice, the CH₄ oxidation-dependent nitrogen fixation rate ranged from 1.17 to 4.15 μmol/h/g, while the CH₄ oxidation-independent nitrogen fixation rate was determined to be 1.10 to 3.17 μmol/h/g. In J3 rice, these rates were 7.30 to 9.22 μmol/h/g and 5.76 to 4.85 μmol/h/g, respectively ( P  < 0.05). Moreover, both N 2 fixation and CH 4 oxidation rates of methanotrophs in the roots of salt-alkali tolerant J3 cultivar (9.22 μmol/h/g for N 2 fixation; 0.09 μmol/h/g for CH 4 oxidation) were significantly higher than those in the roots of the common rice cultivar Kongyu131 (4.15 μmol/h/g for N₂ fixation; 0.03 μmol/h/g for CH₄ oxidation) under nitrogen fertilization ( P  < 0.01). Thus, methanotrophs associated with J3 rice roots demonstrated improved N 2 fixation and CH 4 oxidation activities under saline-alkali stress in the presence of nitrogen fertilizer. Even heterotrophic diazotrophs in J3 rice roots showed enhanced N 2 fixation with (SN) or without (LN) nitrogen inputs. The RNA-based amplicon sequencing showed that nitrogen fertilizer had a greater influence on diazotrophic and methanotrophic communities than the differences between rice cultivars. Further, active Methylomonas (type I methanotrophs) dominated the root-associated diazotrophic (9.8–20.9%) and methanotrophic (46.8–80.3%) communities. Within these, Methylomonas methanica (13.3 vs. 3.8%) and Methylomonas paludis (8.8 vs. 27.4%) were determined as the common genera in the diazotrophic and methanotrophic communities, respectively, with both proportions undergoing significant shifts under SN conditions. Whereas the LN condition led to high CH 4 oxidation activity and a relatively high abundance of Methylocystis (26.0%) in the roots of Kongyu131 rice, which sharply decreased under the SN condition (0.3%). The findings revealed that CH 4 oxidation-dependent N 2 fixation and CH 4 oxidation activities of root-associated type I methanotrophs were significantly affected under nitrogen fertilization, with a more pronounced effect in the salt-alkali tolerant J3 rice cultivar compared to Kongyu131. This study highlights the potential of aerobic diazotrophic methanotrophs in enhancing symbiotic diversity and environmental adaptability while contributing to CH 4 emission reduction and bioavailable nitrogen accumulation in saline-alkali paddy fields.