Annual CH4 Emissions Research Articles

A Lake Biogeochemistry Model for Global Methane Emissions: Model Development, Site‐Level Validation, and Global Applicability

AbstractLakes are important sentinels of climate change and may contribute over 30% of natural methane (CH4) emissions; however, no earth system model (ESM) has represented lake CH4 dynamics. To fill this gap, we refined a process‐based lake biogeochemical model to simulate global lake CH4 emissions, including representation of lake bathymetry, oxic methane production (OMP), the effect of water level on ebullition, new non‐linear CH4 oxidation kinetics, and the coupling of sediment carbon pools with in‐lake primary production and terrigenous carbon loadings. We compiled a lake CH4 data set for model validation. The model shows promising performance in capturing the seasonal and inter‐annual variabilities of CH4 emissions at 10 representative lakes for different lake types and the variations in mean annual CH4 emissions among 106 lakes across the globe. The model reproduces the variations of the observed surface CH4 diffusion and ebullition along the gradients of lake latitude, depth, and surface area. The results suggest that OMP could play an important role in surface CH4 diffusion, and its relative importance is higher in less productive and/or deeper lakes. The model performance is improved for capturing CH4 outgassing events in non‐floodplain lakes and the seasonal variability of CH4 ebullition in floodplain lakes by representing the effect of water level on ebullition. The model can be integrated into ESMs to constrain global lake CH4 emissions and climate‐CH4 feedback.

Tree stem-atmosphere greenhouse gas fluxes in a boreal riparian forest

Tree stems exchange greenhouse gases with the atmosphere but the magnitude, variability and drivers of these fluxes remain poorly understood. Here, we report stem fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in a boreal riparian forest, and investigate their spatiotemporal variability and ecosystem level importance. For two years, we measured CO2 and CH4 fluxes on a monthly basis in 14 spruces (Picea abies) and 14 birches (Betula pendula) growing near a headwater stream affected by historic ditching. We also measured N2O fluxes on three occasions. All tree stems were net emitters of CO2 and CH4, while N2O fluxes were around zero. CO2 fluxes correlated strongly with air temperature and peaked in summer. CH4 fluxes correlated modestly with air temperature and solar radiation and peaked in late winter and summer. Trees with larger stem diameter emitted more CO2 and less CH4 and trees closer to the stream emitted more CO2 and CH4. The CO2 and CH4 fluxes did not differ between spruce and birch, but correlations of CO2 fluxes with stem diameter and distance to stream differed between the tree species. The absence of vertical trends in CO2 and CH4 fluxes along the stems and their low correlation with groundwater levels and soil CO2 and CH4 partial pressures suggest tree internal production as the primary source of stem emissions. At the ecosystem level, the stem CO2, CH4 and N2O emissions represented 52 ± 16 % of the forest floor CO2 emissions and 3 ± 1 % and 11 ± 40 % of the forest floor CH4 and N2O uptake, respectively, during the snow-free period (median ± SE). The six month snow-cover period contributed 11 ± 45 % and 40 ± 29 % to annual stem CO2 and CH4 emissions, respectively. Overall, the stem gas fluxes were more typical for upland rather than wetland ecosystems likely due to historic ditching and subsequent groundwater level decrease.

Global Wetland Methane Emissions From 2001 to 2020: Magnitude, Dynamics and Controls

AbstractThe large uncertainties in estimating CH4 emissions from wetland ecosystems, the leading natural source to the atmosphere, substantially hinder the quantification of the global CH4 budget. This study used the IBIS‐CH4 (Integrated BIosphere Simulator‐Methane) model, a process‐based model integrating microbial mechanisms associated with CH4 production and oxidation processes, to simulate global wetland CH4 emissions from 2001 to 2020. Initially, we employed the IBIS‐CH4 model to evaluate its performance across 26 diverse wetland sites worldwide. The results showed that the magnitude and seasonality of observed CH4 fluxes over various wetland sites were well reproduced. We then used this model to estimate the annual global wetland CH4 emissions from 2001 to 2020, averaging 152.67 Tg CH4 yr−1, with a range of 135.72–167.57 Tg CH4 yr−1. The estimated global wetland CH4 emissions are generally in agreement with the current bottom‐up estimates (117–256 Tg CH4 yr−1) and closely overlap with independent top‐down estimates (139–183 Tg CH4 yr−1). During 2001–2020, the estimated global wetland CH4 emissions initially showed an increasing trend, followed by a decline. The peak of CH4 emissions reached in 2010, coinciding with the peak of wetland area. The majority of global wetland CH4 emissions were concentrated in tropical regions, which exhibited a clear seasonality and had a peak in July. The impact of meteorological factors on wetland CH4 emissions was greater than that of leaf area index, indicating the importance of soil hydrothermal conditions on wetland CH4 emissions.

Temporal Variations in Methane Emissions from a Restored Mangrove Ecosystem in Southern China

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide has been increasingly investigated in recent years. While studies have shown that mangroves are weak sources of methane (CH4) emissions, measurements of CH4 fluxes from these ecosystems remain scarce. In this study, we examined the temporal variation and biophysical drivers of ecosystem-scale CH4 fluxes in China’s northernmost mangrove ecosystem based on eddy covariance measurements obtained over a 3-year period. In this mangrove, the annual CH4 emissions ranged from 6.15 to 9.07 g C m−2 year−1. The daily CH4 flux reached a peak of over 0.07 g C m−2 day−1 during the summer, while the winter CH4 flux was negligible. Latent heat, soil temperature, photosynthetically active radiation, and tide water level were the primary factors controlling CH4 emissions. This study not only elucidates the mechanisms influencing CH4 emissions from mangroves, strengthening the understanding of these processes but also provides a valuable benchmark dataset to validate the model-derived carbon budget estimates for these ecosystems.

Identifying a sustainable rice-based cropping system via on-farm evaluation of grain yield, carbon sequestration capacity and carbon footprints in Central China

ContextRice-based cropping system is a major anthropogenic source of direct greenhouse gases (GHG) emissions, agricultural inputs also produces numerous indirect GHG emissions and environmental problems. Identification of rice cropping systems with lower GHG emissions and higher grain yields is of great significance to ensure food security while minimizing agricultural carbon footprint (CF). ObjectiveThis study aimed to identify the optimal rice-based cropping system with lower greenhouse effects and comparable annual yield in central China. MethodsUsed life cycle assessment, a two-year field experiment was performed to evaluate the direct and indirect GHG emissions, CF, and carbon sequestration capacity in the six rice-based cropping systems widely used in central China, which consisted of fallow-early rice-late rice, rapeseed-early rice-late rice, fallow-ratoon rice, rapeseed-ratoon rice (RRaR), fallow-middle rice, and rapeseed-middle rice. ResultsThe results showed that compared with the three systems with fallow season, the three systems with rapeseed had lower annual CH4 emissions. The two systems with ratoon rice had lower annual indirect and direct GHG emissions than the two systems with double rice. The three systems with rapeseed had higher annual biomass, grain yields, and fixed carbon in biomass and significantly lower CF than the three systems with fallow season, and RRaR had the highest value of fixed carbon in biomass. The systems with ratoon rice had lower average CF than those systems with double rice. CH4 emissions from paddy fields were the primary source of total annual GHG and the main component of CF. Compared with the three systems with fallow season, the three systems with rapeseed had significantly higher annual carbon sequestration capacity based on net ecosystem production (NEP). Ratoon rice systems had higher average NEP than double- and middle-rice systems. RRaR had the lowest CF per unit yield and highest NEP among all the six systems. ConclusionsThe results indicated that appropriate rice-based cropping systems may alleviate the greenhouse effect via reducing GHG emissions and enhancing paddy NEP, RRaR could achieve relatively higher annual grain yield and carbon sequestration capacity as well as lower CF. SignificanceThis study highlight conversion of double rice and middle rice systems to RRaR may help improve grain yield and mitigate greenhouse effect. The findings may provide a theoretical basis for selection of sustainable development of rice based cropping systems in central China.

To Harvest or not to Harvest: Management Intensity did not Affect Greenhouse Gas Balances of Phalaris Arundinacea Paludiculture

The cultivation of flooding-tolerant grasses on wet or rewetted peatlands is a priority in climate change mitigation, balancing the trade-off between atmospheric decarbonisation and biomass production. However, effects of management intensities on greenhouse gas (GHG) emissions and the global warming potential (GWP) are widely unknown. This study assessed whether intensities of two and five annual harvest occurrences at fertilisation rates of 200 kg nitrogen ha− 1 yr− 1 affects GHG exchange dynamics compared to a ‘nature scenario’ with neither harvest nor fertilisation. Fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), using opaque and transparent chambers, were measured on a wet fen peatland with a mean water table depth of -10 cm below soil surface. Overall, no treatment effect was found on biomass yields and GHG emissions. Annual cumulative CH4 emissions were low, ranging between 0.3 and 0.5 t CO2-C eq ha− 1 yr− 1. Contrary to this, emissions of N2O were high, ranging between 1.1 and 1.5 t CO2-C eq ha− 1 yr− 1. For magnitudes of CH4 and N2O, soil moisture conditions and electrical peat properties were critical proxies. Atmospheric uptake of CO2 by net ecosystem exchange was higher for the treatments with management. However, this benefit was offset by the export of carbon in biomass compared to the treatment without management. In conclusion, the results highlighted a near-equal GWP in the range of 10.5–11.5 t CO2-C eq t ha− 1 yr− 1 for all treatments irrespectively of management. In a climate context, a restoration scenario but also intensive paludiculture practices were equal land-use options.

Changing climatic controls on the greenhouse gas balance of thermokarst bogs during succession after permafrost thaw.

Permafrost thaw in northern peatlands causes collapse of permafrost peat plateaus and thermokarst bog development, with potential impacts on atmospheric greenhouse gas exchange. Here, we measured methane and carbon dioxide fluxes over 3 years (including winters) using static chambers along two permafrost thaw transects in northwestern Canada, spanning young (~30 years since thaw), intermediate and mature thermokarst bogs (~200 years since thaw). Young bogs were wetter, warmer and had more hydrophilic vegetation than mature bogs. Methane emissions increased with wetness and soil temperature (40 cm depth) and modelled annual estimates were greatest in the young bog during the warmest year and lowest in the mature bog during the coolest year (21 and 7 g C-CH4 m-2 year-1, respectively). The dominant control on net ecosystem exchange (NEE) in the mature bog (between +20 and -54 g C-CO2 m-2 year-1) was soil temperature (5 cm), causing net CO2 loss due to higher ecosystem respiration (ER) in warmer years. In contrast, wetness controlled NEE in the young and intermediate bogs (between +55 and -95 g C-CO2 m-2 year-1), where years with periodic inundation at the beginning of the growing season caused greater reduction in gross primary productivity than in ER leading to CO2 loss. Winter fluxes (November-April) represented 16% of annual ER and 38% of annual CH4 emissions. Our study found NEE of thermokarst bogs to be close to neutral and rules out large CO2 losses under current conditions. However, high CH4 emissions after thaw caused a positive net radiative forcing effect. While wet conditions favouring high CH4 emissions only persist for the initial young bog period, we showed that continued climate warming with increased ER, and thus, CO2 losses from the mature bog can cause net positive radiative forcing which would last for centuries after permafrost thaw.

Three years of CO2, CH4 and N2O fluxes from different sheepfolds in a semiarid steppe region, China

To better assess greenhouse gas (GHG) emissions from livestock folds in semi-arid steppe zones and reduce uncertainties in regional and national GHG emission inventories, we measured the fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) from sheepfolds under contrasting management regimes (i.e., summer sheepfolds under continuous and rotational grazing strategies and the winter sheepfold) for 3 consecutive years. Our results showed that these GHG fluxes had high intra-annual and interannual variations, emphasizing the importance of multi-year measurement for achieving temporally representative annual budgets. Sheep presence and temperature appeared to be the key factors driving CH4, CO2 and N2O fluxes from sheepfolds, e.g., higher GHG emissions usually occurred in seasons with sheep presence. However, the sheepfold type exerted a distinct influence on the temperature sensitivity of GHG fluxes, i.e., the Q10 values for GHG fluxes were generally higher in summer sheepfolds than in winter sheepfold. The annual CH4, CO2 and N2O emissions for the 3 sheepfolds were estimated to be 1.5–16.5 kg C ha−1 yr−1 (or 1.9–2.6 g C yr−1sheep−1), 8.6–16.0 t C ha−1 yr−1 (or 5.1–6.6 kg C yr−1sheep−1) and 28.3–41.9 kg N ha−1 yr−1 (or 19.0–26.8 g N yr−1sheep−1), respectively. Averaging across the 3 years, the annual net GHG emissions (CH4 + CO2 + N2O) for all sheepfolds ranged from 47 to 71 t CO2-eq ha−1 yr−1 (or 27–36 kg CO2-eq yr−1 sheep−1), of which CO2 and N2O emissions contributed the most; moreover, the annual net GHG emissions had no significant differences between sheepfold types or grazing strategies. Given that local steppe soils have a lower magnitude of soil respiration (CO2) and N2O emissions and are also net sink for atmospheric CH4, the sheepfold sites in this region are undoubtedly one of the significant hotspots for GHG emissions and could be key areas to focus mitigation action.

Quantification of Methane Emissions from Cold Heavy Oil Production with Sand Extraction in Alberta and Saskatchewan, Canada.

Cold heavy oil production with sand (CHOPS) is an extraction process for heavy oil in Canada, with the potential to lead to higher CH4 venting than conventional oil sites, that have not been adequately characterized. In order to quantify CH4 emissions from CHOPS activities, a focused aerial measurement campaign was conducted in the Canadian provinces of Alberta and Saskatchewan in June 2018. Total CH4 emissions from each of 10 clusters of CHOPS wells (containing 22-167 well sites per cluster) were derived using a mass balance computation algorithm that uses in situ wind data measurement on board aircraft. Results show that there is no statistically significant difference in CH4 emissions from CHOPS wells between the two provinces. Cluster-aggregated emission factors (EF) were determined using correspondingly aggregated production volumes. The average CH4 EF was 70.4 ± 36.9 kg/m3 produced oil for the Alberta wells and 55.1 ± 13.7 kg/m3 produced oil for the Saskatchewan wells. Using these EF and heavy oil production volumes reported to provincial regulators, the annual CH4 emissions from CHOPS were estimated to be 121% larger than CHOPS emissions extracted from Canada's National Inventory Report (NIR) for Saskatchewan. The EF were found to be positively correlated with the percentage of nonpiped production volumes in each cluster, indicating higher emissions for nonpiped wells while suggesting an avenue for methane emission reductions. A comparison with recent measurements indicates relatively limited effectiveness of regulations for Saskatchewan compared to those in Alberta. The results of this study indicate the substantial contribution of CHOPS operations to the underreporting observed in the NIR and provide measurement-based EF that can be used to develop improved emissions inventories for this sector and mitigate CH4 emissions from CHOPS operations.

Effects of Ratoon Rice Cropping Patterns on Greenhouse Gas Emissions and Yield in Double-Season Rice Regions.

The ratoon rice cropping pattern is an alternative to the double-season rice cropping pattern in central China due to its comparable annual yield and relatively lower cost and labor requirements. However, the impact of the ratoon rice cropping pattern on greenhouse gas (GHG) emissions and yields in the double-season rice region requires further investigation. Here, we compared two cropping patterns, fallow-double season rice (DR) and fallow-ratoon rice (RR), by using two early-season rice varieties (ZJZ17, LY287) and two late-season rice varieties (WY103, TY390) for DR, and two ratoon rice varieties (YLY911, LY6326) for RR. The six varieties constituted four treatments, including DR1 (ZJZ17 + WY103), DR2 (LY287 + TY390), RR1 (YLY911), and RR2 (LY6326). The experimental results showed that conversion from DR to RR cropping pattern significantly altered the GHG emissions, global warming potential (GWP), and GWP per unit yield (yield-scaled GWP). Compared with DR, the RR cropping pattern significantly increased cumulative methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) emissions by 65.73%, 30.56%, and 47.13%, respectively, in the first cropping season. Conversely, in the second cropping season, the RR cropping pattern effectively reduced cumulative CH4, N2O, and CO2 emissions by 79.86%, 27.18%, and 30.31%, respectively. RR led to significantly lower annual cumulative CH4 emissions, but no significant difference in cumulative annual N2O and CO2 emissions compared with DR. In total, the RR cropping pattern reduced the annual GWP by 7.38% and the annual yield-scaled GWP by 2.48% when compared to the DR cropping pattern. Rice variety also showed certain effects on the yields and GHG emissions in different RR cropping patterns. Compared with RR1, RR2 significantly increased annual yield while decreasing annual GWP and annual yield-scaled GWP. In conclusion, the LY6326 RR cropping pattern may be a highly promising strategy to simultaneously reduce GWP and maintain high grain yield in double-season rice regions in central China.

Life Cycle Greenhouse Gas Emissions in Maize No-Till Agroecosystems in Southern Brazil Based on a Long-Term Experiment

Life Cycle Greenhouse Gas Emissions in Maize No-Till Agroecosystems in Southern Brazil Based on a Long-Term Experiment

Effects of Controlled-release Blended Fertilizer on Crop Yield and Greenhouse Gas Emissions in Wheat-maize Rotation System

The increasing use of nitrogen fertilizers exerts extreme pressure on the environment (e.g., greenhouse gas emissions, GHGs) for winter wheat-summer maize rotation systems in the North China Plain. The application of controlled-release fertilizers is considered as an effective measure to improve crop yield and nitrogen fertilizer utilization efficiency. To explore the impact of one-time fertilization of controlled-release blended fertilizer on crop yield and GHGs of a wheat-maize rotation system, field experiments were carried out in Dezhou Modern Agricultural Science and Technology Park from 2020 to 2022. Five treatments were established for both winter wheat and summer maize, including no nitrogen control (CK), farmers' conventional nitrogen application (FFP), optimized nitrogen application (OPT), CRU1 (the blending ratio of coated urea and traditional urea on winter wheat and summer maize was 5:5 and 3:7, respectively), and CRU2 (the blending ratio of coated urea and traditional urea on winter wheat and summer maize was 7:3 and 5:5, respectively). The differences in yield, nitrogen fertilizer utilization efficiency, fertilization economic benefits, and GHGs among different treatments were compared and analyzed. The results showed that nitrogen application significantly increased the single season and annual crop yields of the wheat-maize rotation system (P < 0.05). Compared with those of FFP, the CRU1 and CRU2 treatments increased the yields of summer maize by 0.4% to 5.6%, winter wheat by -5.4% to 4.1%, and annual yields by -1.1% to 3.9% (P > 0.05). N recovery efficiency (NRE), N agronomic efficiency (NAE), and N partial factor productivity (NPFP) were increased by -8.6%-43.4%, 2.05-6.24 kg·kg-1, and 4.24-10.13 kg·kg-1, respectively. Annual net income increased by 0.2% to 6.3%. Nitrogen application significantly increased the annual emissions of soil N2O and CO2 in the rotation system (P < 0.05) but had no effect on the annual emissions of CH4 (except for in the FFP treatment in the first year). The annual total N2O emissions under the CRU1 and CRU2 treatments were significantly reduced by 23.4% to 30.2% compared to those under the FFP treatment (P < 0.05). Additionally, nitrogen application significantly increased the annual global warming potential (GWP) of the rotation system (P < 0.05), but the intensity of greenhouse gas emissions was reduced due to the increase in crop yields. Compared with that under FFP, the annual GWP under the CRU1 and CRU2 treatments decreased by 9.6% to 11.5% (P < 0.05), and the annual GHGs decreased by 11.2% to 13.8% (P > 0.05). In summary, the one-time application of controlled-release blended fertilizer had a positive role in improving crop yield and economic benefits, reducing nitrogen fertilizer input and labor costs, and GHGs, which is an effective nitrogen fertilizer management measure to promote cleaner production of food crops in the North China Plain.

Methane emissions from residential natural gas meter set assemblies

Residential natural gas meter set assemblies (MSAs) emit methane (CH4), but reported emissions factors vary. To test existing emissions factors, we quantified CH4 emissions from 37 residential MSAs in Calgary, Alberta, Canada. A notable difference with previous studies is the targeted measurement of regulator vents in this study, which were measured with a static chamber, while fugitives were measured with a modified hi-flow sampler. Emissions were dominated by pressure regulator vents (emissions factor = 1.18 g CH4/h/MSA), but 7 fugitives were found (emissions factor = 0.018 g CH4/h/MSA). Six regulator vents were emitting at notably higher rates (≥ 1.79 g CH4/h/MSA). The total empirical emissions factor was 1.20 g CH4/h/MSA (95 % CI, 1.03 to 1.37 g/h/MSA). This is ∼7 times higher than the emissions factor for residential MSAs used in the U.S. EPA's Greenhouse Gas Inventory, which may not include emissions from regulator vents. Upscaling to annual CH4 emissions in Calgary indicates 3234.6 t CH4/yr (95 % CI, 2776.4 t to 3692.9 t CH4/yr) could be emitted from MSAs. This is equivalent to 4.1 % (95 % CI, 3.5 % to 4.7 %) of total city-level CH4 emissions as estimated with satellite data. Results suggest residential MSA emissions may be under-estimated and further study isolating root causes of regulator vent emissions is required to guide mitigation and improve emissions modeling.

Study of spatiotemporal variation and annual emission of CH4 in Shaoxing Yangtze River Delta, China, Using a portable CH4 detector on the UAV

Methane (CH4) is the second greenhouse gas and has a profound impact on global climate change due to its high global warming potential and concentration. By 2022, the CH4 concentration was approximately 1.9 ppm, which was 264% of the pre-industrial level. The spatiotemporal distribution of CH4 was investigated by a portable CH4 detector on an unmanned aerial vehicle and electric bicycles in Shaoxing, a city situated in the Yangtze River Delta, China. The vertical distribution revealed CH4 concentration generally decreased slowly with height. However, the inversion condition and low atmospheric boundary layer height (ABLH) leaded to the enhancement of CH4 with height. The highest CH4 concentration (2.2 ± 0.1 ppm, n = 1428) was observed in winter and the lowest (2.0 ± 0.2 ppm, n = 1530) in spring. Regarding the daily variation, CH4 concentration peaked at 5:00 local time (LT) and reached its lowest level at 14:00 LT, which was attributed to the daily variation of ABLH, lowest in the early morning and highest in the noon. In urban areas, CH4 concentrations showed higher levels near restaurants, natural gas stations and sewerage well, with a maximum value of 13.1 ppm, which was caused by CH4 emission and natural gas leakage from these places. The annual CH4 emission in Shaoxing were estimated to be approximately 69 ton/(km2·year) by the mass balance approach. Compared with other cities in the world, the CH4 emission is in higher level which imply some control measures should be conducted to reduce CH4 emission in Shaoxing.

Methane Emissions From the Qinghai‐Tibet Plateau Ponds and Lakes: Roles of Ice Thaw and Vegetation Zone

AbstractComprehensive seasonal observation is essential for accurately quantifying methane (CH4) emissions from ponds and lakes in permafrost regions. Although CH4 emissions during ice thaw are important and highly variable in high‐latitude freshwater ponds and lakes (north of ∼50°N), their contribution is seldom included in estimates of aquatic‐atmospheric CH4 exchange across different alpine ecosystems. Here, we characterized annual CH4 emissions, including emissions during ice thaw, from ponds and lakes across four alpine vegetation zones in the Qinghai‐Tibet Plateau (QTP) permafrost region. We observed significant spatial variability in annual CH4 emission rates (8.44−421.05 mmol m−2 yr−1), CH4 emission rates during ice thaw (0.26−144.39 mmol m−2 yr−1), and the contribution of CH4 emissions during ice thaw to annual emissions (3−33%) across different vegetation zones. Dissolved oxygen concentration under ice, along with substrate availability and water salinity, played critical roles in influencing CH4 flux during ice thaw. We estimated annual CH4 emissions from ponds and lakes in the QTP permafrost region as 0.04 (0.03−0.05) Tg CH4 yr−1 (median (first quartile−third quartile)), with approximately 20% occurring during ice thaw. Notably, the average areal CH4 emission rate from ponds and lakes in the QTP permafrost region amounts to only 8% of that from high‐latitude waterbodies, primarily due to the dominance of large saline lakes with lower CH4 emission rates in the alpine permafrost region. Our findings emphasize the significance of incorporating comprehensive seasonal observation of CH4 emissions across different vegetation zones in better predicting CH4 emissions from alpine ponds and lakes.

Greenhouse Gas Emissions, Carbon Footprint, and Grain Yields of Rice-Based Cropping Systems in Eastern China

A multiple cropping system is beneficial for utilizing natural resources, while increasing the grain production and economic outputs. However, its impact on greenhouse gas emissions is unclear. The objective of this study was to evaluate the influence of rice-based cropping systems on methane (CH4) and nitrous oxide (N2O) emissions, the carbon footprint (CF), grain yields, and net economic returns in eastern China. Four treatments were applied: rice–fallow (as a control), rice–milk vetch, rice–wheat, and rice–rapeseed. Methane and N2O emissions were measured every 7 days via static chamber and gas chromatography methods from the 2019 rice season to the 2021 non-rice season. The CF was calculated based on the life cycle assessment. The results showed that multiple cropping systems significantly increased the annual grain yield by 1.2–6.4 t ha−1 and the annual CH4 and N2O emissions by 38–101 kg CH4-C ha−1 and 0.58–1.06 kg N2O-N ha−1, respectively. The average annual net returns for rice–wheat and rice–rapeseed were 131–150% greater than those for rice–milk vetch and rice–fallow. The annual CFs increased in the following order: rice–wheat (19.2 t CO2-eq ha−1) > rice–rapeseed (16.6 t CO2-eq ha−1) > rice–milk vetch (13.9 t CO2-eq ha−1) > rice–fallow (11.5 t CO2-eq ha−1). The CH4 emissions contributed to the largest share of the CF (60.4–68.8%), followed by agricultural inputs (27.2–33.7%) and N2O emissions (2.9–5.9%). Moreover, nitrogen fertilizer accounted for 65.6–72.4% of the indirect greenhouse gas emissions from agricultural inputs. No significant difference in the CF per unit grain yield was observed between the four rice-based cropping systems. The CF per net return of rice–wheat and rice–rapeseed significantly decreased by 37–50% relative to that of rice–fallow and rice–milk vetch. These findings suggest the potential to optimize rice-based cropping systems for environmental sustainability and grain security.

Boreal–Arctic wetland methane emissions modulated by warming and vegetation activity

Wetland methane (CH4) emissions over the Boreal–Arctic region are vulnerable to climate change and linked to climate feedbacks, yet understanding of their long-term dynamics remains uncertain. Here, we upscaled and analysed two decades (2002–2021) of Boreal–Arctic wetland CH4 emissions, representing an unprecedented compilation of eddy covariance and chamber observations. We found a robust increasing trend of CH4 emissions (+8.9%) with strong inter-annual variability. The majority of emission increases occurred in early summer (June and July) and were mainly driven by warming (52.3%) and ecosystem productivity (40.7%). Moreover, a 2 °C temperature anomaly in 2016 led to the highest recorded annual CH4 emissions (22.3 Tg CH4 yr−1) over this region, driven primarily by high emissions over Western Siberian lowlands. However, current-generation models from the Global Carbon Project failed to capture the emission magnitude and trend, and may bias the estimates in future wetland CH4 emission driven by amplified Boreal–Arctic warming and greening.

An appropriate amount of straw replaced chemical fertilizers returning reduced net greenhouse gas emissions and improved net ecological economic benefits

With the increasing application of crop straw, the greenhouse gas emissions caused due to crop straw have received increasing attention. However, the current studies have not explored the greenhouse gas emissions from on-farmland consumption of returned straw. Based on a long-term positioning experiment, three treatments were set up in double-cropping rice fields: chemical fertilizer (CF), straw replacing 1/3 nitrogen fertilizer (MS), and straw replacing 2/3 nitrogen fertilizer (HS). We measured the biomass, soil properties, and greenhouse gas emissions of double-cropping rice and then comprehensively evaluated the global warming potential, carbon footprint, and net ecological and economic benefits. The straw treatments (MS and HS) increased the cumulative annual emissions of CH4 (98.44% and 261.23%), CO2 (30.85% and 122.29%), and N2O (7.37% and 52.50%), the cumulative annual global warming potential (74.15% and 206.12%), average GHG intensity (43.26% and 138.07%), and the annual cumulative net ecosystem carbon budget (52.96% and 100.97%) in the early and late rice growing seasons, respectively. We observed that the real-time greenhouse gas emissions were significantly correlated to the soil microbial functional genes and total carbon, NH4+-N, and NO3−-N. The results of the random forest model showed that total carbon (13.87%) and nirK abundance (9.80%) were the highest predictors of global warming potential at the booting and maturity stages, respectively during rice growing season. Combining the resource inputs for agricultural production and the greenhouse gas emission potential of the returned straw on its own, MS showed the lowest net greenhouse gas emission, and MS and HS showed significantly increased annual cumulative net ecological economic benefits (38.08% and 34.30%). Overall, MS showed the lowest net greenhouse gas emission and the highest net ecological economic benefits, which is a straw-returning measure with low environmental impact and high economic returns.

Impact of Reservoir Operation Policies on Spatiotemporal Dynamics of Sediment Methane Production and Release in a Large Reservoir

AbstractReservoir operation policies affect sediment methane (CH4) production and pathways through complex influence on hydrodynamic and biogeochemical processes in reservoir water and sediment. However, the underlying influencing mechanisms, especially the impact of operations on sediment CH4 production and pathways, remain poorly understood. This study combines a physical‐biogeochemical model with a reservoir operation model to evaluate operation impacts on spatiotemporal sediment CH4 production and release dynamics and to analyze affiliated processes and driving factors. Three typical operation policies (i.e., standard, hedging, eco‐friendly) are selected to create the operation scenarios, and a coupled physical‐biogeochemical model is adopted to simulate and compare sediment CH4 production, consumption, diffusion, and ebullition under these scenarios. The methods are applied in the second largest reservoir in China (i.e., Danjiangkou Reservoir) as a case study. Results show that annual total sediment CH4 production among the scenarios ranges from 285.9 to 298.3 Gg C year−1, while operation scenario outcomes significantly differ. Specifically, a significant CH4 percentage (34%–39%) is oxidized in surface sediment while the remaining escapes sediment through diffusion and ebullition. Although diffusion accounts for >60% of the annual sediment CH4 release, sediment ebullition ultimately dominates atmospheric CH4 emissions. Annual total atmospheric CH4 emissions range from 42.8 to 71.1 Gg C year−1 and account for 14%–25% of the annual total sediment production among the three scenarios. The differences demonstrate that reservoir operations significantly impact CH4 pathways and atmospheric emissions. This study provides implications for reservoir managers to coordinate socioeconomic and ecological protection while mitigating CH4 emission targets.

Methane emission mapping and quantification in two medium-sized cities in Germany: Heidelberg and Schwetzingen

Estimating the contribution of cities to regional and global methane (CH4) budgets is challenging due to the complex infrastructures of cities. Mobile measurement devices are well suited to detect CH4 sources via real-time ambient air measurements. Surveys involving mobile CH4 measurements were conducted from May 2020 to July 2021 at the street level in two medium-sized cities in Germany. With coverage levels of 30% and 65% of the road networks in Heidelberg and Schwetzingen, respectively, Leak Indications (LIs) for CH4 were observed with mole fractions of 100 to 9500 ppb above background. A minor portion of leaks (2 out of 70) was attributed to the sewer system, but most leaks originated from the gas distribution network with 0.48 LIs per km obtained in Heidelberg and 0.08 LIs per km determined in Schwetzingen. A method to assign an emission rate to all LIs developed by Weller et al. (2019) was assessed and adapted via controlled CH4 release experiments in Heidelberg. The method was modified for cities with smaller street widths and smaller distances from the leak to the measurement device. The annual total street-level CH4 emissions calculated for Heidelberg and Schwetzingen were 42 ± 17 and 1.5 ± 0.5 t CH4 yr−1 (1sigma), respectively, corresponding to 0.26 ± 0.11 and 0.03 ± 0.01 kg CH4 yr−1 per capita, respectively.