Deep peat warming increases surface methane and carbon dioxide emissions in a black spruce‐dominated ombrotrophic bog

Boreal peatlands contain approximately 500 Pg carbon (C) in the soil, emit globally significant quantities of methane (CH₄), and are highly sensitive to climate change. Warming associated with global climate change is likely to increase the rate of the temperature‐sensitive processes that decompose stored organic carbon and release carbon dioxide (CO₂) and CH₄. Variation in the temperature sensitivity of CO₂ and CH₄ production and increased peat aerobicity due to enhanced growing‐season evapotranspiration may alter the nature of peatland trace gas emission. As CH₄ is a powerful greenhouse gas with 34 times the warming potential of CO₂, it is critical to understand how factors associated with global change will influence surface CO₂ and CH₄ fluxes. Here, we leverage the Spruce and Peatland Responses Under Changing Environments (SPRUCE) climate change manipulation experiment to understand the impact of a 0–9°C gradient in deep belowground warming (“Deep Peat Heat”, DPH) on peat surface CO₂ and CH₄ fluxes. We find that DPH treatments increased both CO₂ and CH₄ emission. Methane production was more sensitive to warming than CO₂ production, decreasing the C‐CO₂:C‐CH₄ of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ¹³C of CH₄ suggesting an increasing contribution of acetoclastic methanogenesis to total CH₄ production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH₄ is <2%, CH₄ represents >50% of seasonal C emissions in the highest‐warming treatments when adjusted for CO₂ equivalents on a 100‐year timescale. These results suggest that warming in boreal regions may increase CH₄ emissions from peatlands and result in a positive feedback to ongoing warming.     

基于IPCC Tier 1层级的中国反刍家畜胃肠道甲烷排放格局变化分析

依据政府间气候变化委员会(IPCC)2006年公布的反刍家畜胃肠道CH4排放系数(奶牛61 kg CH4头-1a-1,肉役牛47 kg CH4头-1a-1,绵羊和山羊均为5 kg CH4头-1a-1)和IPCC Tier 1计算方法,结合《中国统计年鉴》和《中国畜牧业统计年鉴》,估算了中国以及各个省份1990—2010年CH4的排放总量,绘制出中国反刍家畜胃肠道CH4排放格局及历史变化图。研究结果表明:11990—2010年期间,中国反刍家畜胃肠道CH4排放量(5.90—7.65 Tg)总体呈现先上升(1995年最高)后下降的趋势,其中肉役牛(主要是黄牛和水牛)胃肠道CH4排放量(4.33 Tg)及其所占比例(65%)最大。奶牛胃肠道CH4排放量及其比例呈现逐年增加的趋势,2006年(0.83 Tg,12.7%)超过山羊和绵羊胃肠道CH4排放量及其比例,成为中国反刍家畜胃肠道CH4排放第二大源。但是,奶牛单位产奶量所排放的CH4量逐年降低,表明中国奶牛饲养技术与生产性能不断提高。2中国反刍家畜胃肠道CH4排放格局呈现区域集中特点,主要集中在中西部和北部的各个省份,其中四川、内蒙古、新疆、河南、西藏、山东、河北、黑龙江、云南和甘肃等10个省份的排放量占中国排放总量的50%以上。总之,1990—2010年间,中国反刍家畜胃肠道CH4排放总量为(6.77±0.46)Tg(肉役牛为主要排放源),随时间推移呈现先上升后下降的趋势;中国反刍家畜胃肠道CH4排放的格局呈区域集中分布,中西部和北部的各个省份占60%以上。     

Methane emission from global livestock sector during 1890–2014: Magnitude,trends and spatiotemporal patterns

Human demand for livestock products has increased rapidly during the past few decades largely due to dietary transition and population growth, with significant impact on climate and the environment. The contribution of ruminant livestock to greenhouse gas (GHG) emissions has been investigated extensively at various scales from regional to global, but the long‐term trend, regional variation and drivers of methane (CH₄) emission remain unclear. In this study, we use Intergovernmental Panel on Climate Change (IPCC) Tier II guidelines to quantify the evolution of CH₄ emissions from ruminant livestock during 1890–2014. We estimate that total CH₄ emissions in 2014 was 97.1 million tonnes (MT) CH₄ or 2.72 Gigatonnes (Gt) CO₂‐eq (1 MT = 10¹² g, 1 Gt = 10¹⁵ g) from ruminant livestock, which accounted for 47%–54% of all non‐CO₂ GHG emissions from the agricultural sector. Our estimate shows that CH₄ emissions from the ruminant livestock had increased by 332% (73.6 MT CH₄ or 2.06 Gt CO₂‐eq) since the 1890s. Our results further indicate that livestock sector in drylands had 36% higher emission intensity (CH₄ emissions/km²) compared to that in nondrylands in 2014, due to the combined effect of higher rate of increase in livestock population and low feed quality. We also find that the contribution of developing regions (Africa, Asia and Latin America) to the total CH₄ emissions had increased from 51.7% in the 1890s to 72.5% in the 2010s. These changes were driven by increases in livestock numbers (LU units) by up to 121% in developing regions, but decreases in livestock numbers and emission intensity (emission/km²) by up to 47% and 32%, respectively, in developed regions. Our results indicate that future increases in livestock production would likely contribute to higher CH₄ emissions, unless effective strategies to mitigate GHG emissions in livestock system are implemented.     

Punching above their weight: Large release of greenhouse gases from small agricultural dams

Freshwater ecosystems play a major role in global carbon cycling through the breakdown of organic material and release of greenhouse gases (GHGs). Carbon dioxide (CO₂) and methane (CH₄) emissions from lakes, wetlands, reservoirs and small natural ponds have been well studied, however, the GHG emissions of highly abundant, small‐scale (<0.01 km²) agricultural dams (small stream and run‐off impoundments) are still unknown. Here, we measured the diffusive CO₂ and CH₄ flux of 77 small agricultural dams within south‐east Australia. The GHG emissions from these waterbodies, which are currently unaccounted for in GHG inventories, amounted to 11.12 ± 2.59 g CO₂‐equivalent m²/day, a value 3.43 times higher than temperate reservoir emissions. Upscaling these results to the entire state of Victoria, Australia, resulted in a farm dam CO₂‐equivalent/day emission rate of 4,853 tons, 3.1 times higher than state‐wide reservoir emissions in spite of farm dams covering only 0.94 times the comparative area. We also show that CO₂ and CH₄ emission rates were both significantly positively correlated with dissolved nitrate concentrations, and significantly higher in livestock rearing farm dams when compared to cropping farm dams. The results from this study demonstrate that small agricultural farm dams can be a major source of greenhouse gas emissions, thereby justifying their inclusion in global carbon budgets.     

Modeling methane emissions from irrigated rice cultivation in China from 1960 to 2050

Rice paddy is a major source of anthropogenic terrestrial methane (CH₄). China has the second‐largest area of rice cultivation in the world, accounting for ca. 19% of the world's rice‐producing area. Recognizing the significance of China's rice cultivation in the global CH₄ budget, we estimated the CH₄ emissions resulting from irrigated rice cultivation in China from 1960 to 2050 using a CH4MOD model. The model estimates suggest that the annual CH₄ emissions decreased from 5.62 Tg yr^?1 in 1960 to 4.13 Tg yr^?1 in 1970, and this decrease was attributed to changes in water management from continuous flooding to mid‐season drainage irrigation. Since the early 1970s, the amount of CH₄ emissions gradually increased to 6.85 Tg yr^?1 by 2009 because of significant improvements in crop production that led to high‐crop residue retention. Higher levels of CH₄ emissions occurred in southern China, where double rice cropping systems are most common. For the A1B and B1 scenarios of the IPCC Special Report on Emissions Scenarios (SRES), the amount of CH₄ emissions from 2010 to 2050 is predicted to increase at an average rate of 1.2 kg ha^?1 yr^?1 in response to global warming. Compared to 2009, the CH₄ flux is predicted to increase by ca. 14% by the late 2040s, and the increase in these emissions in northeastern China is estimated to become more significant than in the other rice‐growing regions of the country. Under the assumptions that the rice‐producing land area will remain the same, decrease by 25% or increase by 38% by the late 2040s, the CH₄ emissions are projected to be 7.8, 5.6 or 11.7 Tg yr^?1, respectively.     

Prediction of enteric methane production,yield, and intensity in dairy cattle using an intercontinental database

Enteric methane (CH₄) production from cattle contributes to global greenhouse gas emissions. Measurement of enteric CH₄ is complex, expensive, and impractical at large scales; therefore, models are commonly used to predict CH₄ production. However, building robust prediction models requires extensive data from animals under different management systems worldwide. The objectives of this study were to (1) collate a global database of enteric CH₄ production from individual lactating dairy cattle; (2) determine the availability of key variables for predicting enteric CH₄ production (g/day per cow), yield [g/kg dry matter intake (DMI)], and intensity (g/kg energy corrected milk) and their respective relationships; (3) develop intercontinental and regional models and cross‐validate their performance; and (4) assess the trade‐off between availability of on‐farm inputs and CH₄ prediction accuracy. The intercontinental database covered Europe (EU), the United States (US), and Australia (AU). A sequential approach was taken by incrementally adding key variables to develop models with increasing complexity. Methane emissions were predicted by fitting linear mixed models. Within model categories, an intercontinental model with the most available independent variables performed best with root mean square prediction error (RMSPE) as a percentage of mean observed value of 16.6%, 14.7%, and 19.8% for intercontinental, EU, and United States regions, respectively. Less complex models requiring only DMI had predictive ability comparable to complex models. Enteric CH₄ production, yield, and intensity prediction models developed on an intercontinental basis had similar performance across regions, however, intercepts and slopes were different with implications for prediction. Revised CH₄ emission conversion factors for specific regions are required to improve CH₄ production estimates in national inventories. In conclusion, information on DMI is required for good prediction, and other factors such as dietary neutral detergent fiber (NDF) concentration, improve the prediction. For enteric CH₄ yield and intensity prediction, information on milk yield and composition is required for better estimation.     

The role of rice cultivation in changes in atmospheric methane concentration and the Global Methane Pledge

Resumption of the increase in atmospheric methane (CH₄) concentrations since 2007 is of global concern and may partly have resulted from emissions from rice cultivation. Estimates of CH₄ emissions from rice fields and abatement potential are essential to assess the contribution of improved rice management in achieving the targets of the Global Methane Pledge agreed upon by over 100 countries at COP26. However, the contribution of CH₄ emissions from rice fields to the resumed CH₄ growth and the global abatement potential remains unclear. In this study, we estimated the global CH₄ emissions from rice fields to be 27 ± 6 Tg CH₄ year⁻¹ in the recent decade (2008–2017) based on the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. The trend of CH₄ emissions from rice cultivation showed an increase followed by no significant change and then, a stabilization over 1990–2020. Consequently, the contribution of CH₄ emissions from rice fields to the renewed increase in atmospheric CH₄ concentrations since 2007 was minor. We summarized the existing low-cost measures and showed that improved water and straw management could reduce one-third of global CH₄ emissions from rice fields. Straw returned as biochar could reduce CH₄ emissions by 12 Tg CH₄ year⁻¹, equivalent to 10% of the total reduction of all anthropogenic emissions. We conclude that other sectors than rice cultivation must have contributed to the renewed increase in atmospheric CH₄ concentrations, and that optimizing multiple mitigation measures in rice fields could contribute significantly to the abatement goal outlined in the Global Methane Pledge.     

中国农业系统近40年温室气体排放核算

基于排放因子法构建了包含种植业和牲畜养殖业的农业系统温室气体排放核算体系,系统核算了1980-2020年我国全国尺度上的农业系统温室气体排放总量和变化趋势,并在区县级尺度下对1980、2000、2011年的中国农业系统的温室气体排放量进行核算,对比不同阶段农业系统温室气体排放变化的时空异质性规律。研究发现：1980-2020年我国农业系统温室气体排放量呈波动增长趋势,增长了近46%。CH₄是农业系统排放贡献最大的温室气体,占总排放量的47.33%。我国农业系统温室气体排放与不同地区农业生产方式有关,CH₄排放量高的地区主要位于我国主要水稻产区以及旱地作物产区。CO₂排放量高的地区主要位于东北、西北等地区以及华东地区。N₂O排放量较高地区主要位于西北的主要畜牧养殖地区,以及我国农业经济发展水平高的中南部地区。研究有助于揭示我国农业温室气体排放的动态特征,现状规律,以及空间差异性特征,从农业减排角度为实现双碳目标提供科学参考。     

Higher yields and lower methane emissions with new rice cultivars

Breeding high‐yielding rice cultivars through increasing biomass is a key strategy to meet rising global food demands. Yet, increasing rice growth can stimulate methane (CH₄) emissions, exacerbating global climate change, as rice cultivation is a major source of this powerful greenhouse gas. Here, we show in a series of experiments that high‐yielding rice cultivars actually reduce CH₄ emissions from typical paddy soils. Averaged across 33 rice cultivars, a biomass increase of 10% resulted in a 10.3% decrease in CH₄ emissions in a soil with a high carbon (C) content. Compared to a low‐yielding cultivar, a high‐yielding cultivar significantly increased root porosity and the abundance of methane‐consuming microorganisms, suggesting that the larger and more porous root systems of high‐yielding cultivars facilitated CH₄ oxidation by promoting O₂ transport to soils. Our results were further supported by a meta‐analysis, showing that high‐yielding rice cultivars strongly decrease CH₄ emissions from paddy soils with high organic C contents. Based on our results, increasing rice biomass by 10% could reduce annual CH₄ emissions from Chinese rice agriculture by 7.1%. Our findings suggest that modern rice breeding strategies for high‐yielding cultivars can substantially mitigate paddy CH₄ emission in China and other rice growing regions.     

基于过程模型的青藏高原湿地甲烷排放格局评估

甲烷(CH_4)是大气中最丰富的碳氢化合物,是仅次于二氧化碳(CO_2)的温室气体。湿地是甲烷的重要来源,在全球碳循环中发挥着重要作用,其排放的甲烷占所有天然甲烷排放源的70%,占全球甲烷排放总量的24.8%。青藏高原平均海拔4000 m以上,占有中国约三分之一的湿地。近几十年来,由于全球气候变暖和降水增加,该地区甲烷排放率和湿地面积都发生着巨大变化,因此,青藏高原湿地CH_4排放的长期变化在很大程度上仍存在较大的不确定性。利用TRIPLEX-GHG模型模拟了青藏高原湿地1978—2008年CH_4排放的动态特征,研究结果表明:(1)1978—2008年青藏高原湿地CH_4排放速率呈逐渐增加趋势。(2)青藏高原大多数湿地区域CH_4排放速率为0—6.13 g CH_4 m~(-2 )a~(-1);东北部分湿地区域CH_4排放速率为6.14—20.19 g CH_4 m~(-2 )a~(-1);较高的CH_4排放速率分布于青藏高原南部湿地区域,为56.14—74.97 g CH_4 m~(-2 )a~(-1)。(3)青藏高原湿地CH_4排放量在1978、1990、2000年和2008年分别为0.21、0.23、0.27和0.32 Tg CH_4 a~(-1)。在1978—1990年,尽管CH_4排放速率增加,但湿地面积减少,因此这一时期青藏高原湿地CH_4排放量并未发生明显变化。随后由于降水增加和冰川融化,使得湿地面积逐渐增加,青藏高原湿地CH_4排放量也呈现增加趋势。     

Regionally differentiated estimates of cropland N2O emissions reduce uncertainty in global calculations

Nitrogen (N) additions to cropland soils are the largest source of anthropogenic nitrous oxide (N₂O) emissions and are an important contributor to global greenhouse gas radiative forcing. Progress in understanding controls on N₂O fluxes from soils is demonstrated in increasingly sophisticated emissions estimates with improved spatial and source resolution. These methods build upon ongoing field, laboratory, and modeling advances that are restricted to just a handful of countries. Thus, burgeoning new knowledge is of limited utility for improving N₂O emissions estimates for the rest of the world where prospects for near‐term advances are constrained by the limited breadth of observations and availability of model driver data. Here, we use Bayesian inversion to leverage information from recent national‐level N₂O emission inventories and reduce uncertainty by up to 65% for estimates of regional and global direct cropland N₂O emissions. Our estimates for the proportion of N inputs lost as N₂O vary by a factor of two between regions and depart from existing default emission factors, yet regional emissions estimates based on these factors are consistent with global, regional, and local observations. Improved regional emission factors will enhance national greenhouse gas inventories in information‐poor countries and guide efforts to reduce agricultural N₂O emissions.     

Methane emissions from wetlands: biogeochemical,microbial, and modeling perspectives from local to global scales

Understanding the dynamics of methane (CH₄) emissions is of paramount importance because CH₄ has 25 times the global warming potential of carbon dioxide (CO₂) and is currently the second most important anthropogenic greenhouse gas. Wetlands are the single largest natural CH₄ source with median emissions from published studies of 164 Tg yr^?1, which is about a third of total global emissions. We provide a perspective on important new frontiers in obtaining a better understanding of CH₄ dynamics in natural systems, with a focus on wetlands. One of the most exciting recent developments in this field is the attempt to integrate the different methodologies and spatial scales of biogeochemistry, molecular microbiology, and modeling, and thus this is a major focus of this review. Our specific objectives are to provide an up‐to‐date synthesis of estimates of global CH₄ emissions from wetlands and other freshwater aquatic ecosystems, briefly summarize major biogeophysical controls over CH₄ emissions from wetlands, suggest new frontiers in CH₄ biogeochemistry, examine relationships between methanogen community structure and CH₄ dynamics in situ, and to review the current generation of CH₄ models. We highlight throughout some of the most pressing issues concerning global change and feedbacks on CH₄ emissions from natural ecosystems. Major uncertainties in estimating current and future CH₄ emissions from natural ecosystems include the following: (i) A number of important controls over CH₄ production, consumption, and transport have not been, or are inadequately, incorporated into existing CH₄ biogeochemistry models. (ii) Significant errors in regional and global emission estimates are derived from large spatial‐scale extrapolations from highly heterogeneous and often poorly mapped wetland complexes. (iii) The limited number of observations of CH₄ fluxes and their associated environmental variables loosely constrains the parameterization of process‐based biogeochemistry models.     

Methyl bromide emissions to the atmosphere from temperate woodland ecosystems

The environmental importance of methyl bromide (CH₃Br) arises from its contribution to stratospheric ozone loss processes and, as a consequence, its emissions from anthropogenic sources are subject to the Montreal Protocol. A better understanding of the natural budget of CH₃Br is required for assessing the benefit of anthropogenic emission reductions and for understanding any potential effects of environmental change on global CH₃Br concentrations. Measurements of CH₃Br flux in temperate woodland ecosystems, in particular, are very sparse, yet these cover a large fraction of terrestrial land surface. Results presented here from 18 months of field measurements of CH₃Br fluxes in four static flux chambers in a woodland in Scotland and from enclosures of rotting wood and deciduous and coniferous leaf litter suggest net emissions from temperate woodlands. Net CH₃Br fluxes in the woodland varied between the chambers, fluctuating between net uptake and net emissions (?73 to 279 ng m^?2 h^?1 across 161 individual measurements), and with no strong seasonality, but with time‐averaged net emission overall [27±57 (1 SD)] ng m^?2 h^?1]. This work demonstrates that scale‐up needs to be based on sufficient individual measurements to provide a reasonably constrained estimate of the long‐term mean. Mean (±1 SD) net CH₃Br emissions from deciduous and coniferous leaf litter were 43 (±33) ng kg^?1 (dry weight) h^?1 and 80 (±37) ng kg^?1 (dry weight) h^?1, respectively, and ～1–2 ng kg^?1 (fresh weight) h^?1 from rotting woody litter. Despite the intrinsic variability, data obtained here consistently point to the conclusion that the temperate forest soil/litter ecosystem is a net source of CH₃Br to the atmosphere.     

Observational constraints reduce model spread but not uncertainty in global wetland methane emission estimates

The recent rise in atmospheric methane (CH₄) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH₄ source, estimates of global wetland CH₄ emissions vary widely among approaches taken by bottom-up (BU) process-based biogeochemical models and top-down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi-model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH₄ emission estimates and model performance. We find that using better-performing models identified by observational constraints reduces the spread of wetland CH₄ emission estimates by 62% and 39% for BU- and TD-based approaches, respectively. However, global BU and TD CH₄ emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH₄ year⁻¹) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter-site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH₄ models to move beyond static benchmarking and focus on evaluating site-specific and ecosystem-specific variabilities inferred from observations.     

Emission of Methane and Nitrous Oxide by Australian Mangrove Ecosystems

Abstract: The fluxes of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) were measured in mangrove wetlands in Queensland, Australia, using the closed chamber technique. Large differences in the fluxes of both gases from different study sites were observed, which presumably depended on differences in substrate availability. CH₄ emission rates were in the range of 20 to 350 μg m^‐2 h^‐1, whereas N₂O fluxes were lower, amounting to ‐ 2 to 14 μg m^‐2 h^‐1. In general, the field sites with high substrate availability showed higher emissions than sites with poor nutrient supply. This assumption is supported by the observation of dramatically increased N₂O emissions (150 ‐ 400 μg m^‐2 h^‐1) if study sites were artificially fertilised with additional N. As expected, N fertilisation did not alter CH₄ fluxes during the period of investigation. In the present study, it was confirmed that the mangrove vegetation may play a role as a transport path for CH₄ and N₂O by facilitating diffusion out of the soil. Prop roots from Rhizophora stylosa emitted CH₄ and N₂O at rates of 2.6 and 3.3 μg m^‐2 root surface h^‐1, respectively, whereas the soil of this stand acted as a sink for CH₄. As a consequence, the ecosystem as a whole could constitute a CH₄ source despite CH₄ uptake by the soil. In contrast to prop roots, the presence of pneumatophores in Avicennia marina led to a significant increase in CH₄ emissions from mangrove soils, but did not enhance N₂O emissions. These findings indicate that mangrove ecosystems may be considered a significant source of N₂O and that anthropogenic nutrient input into these ecosystems will lead to enhanced source strengths. For an up‐scaling of greenhouse gas emissions from mangrove forests to a global scale, more information is needed, particularly on the significance of vegetation.     

Measurement and prediction of enteric methane emission

The greenhouse gas (GHG) emissions from the agricultural sector account for about 25.5% of total global anthropogenic emission. While CO₂ receives the most attention as a factor relative to global warming, CH₄, N₂O and chlorofluorocarbons (CFCs) also cause significant radiative forcing. With the relative global warming potential of 25 compared with CO₂, CH₄ is one of the most important GHGs. This article reviews the prediction models, estimation methodology and strategies for reducing enteric CH₄ emissions. Emission of CH₄ in ruminants differs among developed and developing countries, depending on factors like animal species, breed, pH of rumen fluid, ratio of acetate:propionate, methanogen population, composition of diet and amount of concentrate fed. Among the ruminant animals, cattle contribute the most towards the greenhouse effect through methane emission followed by sheep, goats and buffalos, respectively. The estimated CH₄ emission rate per cattle, buffaloe, sheep and goat in developed countries are 150.7, 137, 21.9 and 13.7 (g/animal/day) respectively. However, the estimated rates in developing countries are significantly lower at 95.9 and 13.7 (g/animal/day) per cattle and sheep, respectively. There exists a strong interest in developing new and improving the existing CH₄ prediction models to identify mitigation strategies for reducing the overall CH₄ emissions. A synthesis of the available literature suggests that the mechanistic models are superior to empirical models in accurately predicting the CH₄ emission from dairy farms. The latest development in prediction model is the integrated farm system model which is a process-based whole-farm simulation technique. Several techniques are used to quantify enteric CH₄ emissions starting from whole animal chambers to sulfur hexafluoride (SF6) tracer techniques. The latest technology developed to estimate CH₄ more accurately is the micrometeorological mass difference technique. Because the conditions under which animals are managed vary greatly by country, CH₄ emissions reduction strategies must be tailored to country-specific circumstances. Strategies that are cost effective, improve productivity, and have limited potential negative effects on livestock production hold a greater chance of being adopted by producers. It is also important to evaluate CH₄ mitigation strategies in terms of the total GHG budget and to consider the economics of various strategies. Although reductions in GHG emissions from livestock industries are seen as high priorities, strategies for reducing emissions should not reduce the economic viability of enterprises.     

Modeled production,oxidation, and transport processes of wetland methane emissions in temperate,boreal, and Arctic regions

Wetlands are the largest natural source of methane (CH₄) to the atmosphere. The eddy covariance method provides robust measurements of net ecosystem exchange of CH₄, but interpreting its spatiotemporal variations is challenging due to the co-occurrence of CH₄ production, oxidation, and transport dynamics. Here, we estimate these three processes using a data-model fusion approach across 25 wetlands in temperate, boreal, and Arctic regions. Our data-constrained model—iPEACE—reasonably reproduced CH₄ emissions at 19 of the 25 sites with normalized root mean square error of 0.59, correlation coefficient of 0.82, and normalized standard deviation of 0.87. Among the three processes, CH₄ production appeared to be the most important process, followed by oxidation in explaining inter-site variations in CH₄ emissions. Based on a sensitivity analysis, CH₄ emissions were generally more sensitive to decreased water table than to increased gross primary productivity or soil temperature. For periods with leaf area index (LAI) of ≥20% of its annual peak, plant-mediated transport appeared to be the major pathway for CH₄ transport. Contributions from ebullition and diffusion were relatively high during low LAI (<20%) periods. The lag time between CH₄ production and CH₄ emissions tended to be short in fen sites (3 ± 2 days) and long in bog sites (13 ± 10 days). Based on a principal component analysis, we found that parameters for CH₄ production, plant-mediated transport, and diffusion through water explained 77% of the variance in the parameters across the 19 sites, highlighting the importance of these parameters for predicting wetland CH₄ emissions across biomes. These processes and associated parameters for CH₄ emissions among and within the wetlands provide useful insights for interpreting observed net CH₄ fluxes, estimating sensitivities to biophysical variables, and modeling global CH₄ fluxes.     

Regional-scale measurements of CH4 exchange from a tall tower over a mixed temperate/boreal lowland and wetland forest

The biosphere–atmosphere exchange of methane (CH₄) was estimated for a temperate/boreal lowland and wetland forest ecosystem in northern Wisconsin for 1997–1999 using the modified Bowen ratio (MBR) method. Gradients of CH₄ and CO₂ and CO₂ flux were measured on the 447‐m WLEF‐TV tower as part of the Chequamegon Ecosystem–Atmosphere Study (ChEAS). No systematic diurnal variability was observed in regional CH₄ fluxes measured using the MBR method. In all 3 years, regional CH₄ emissions reached maximum values during June–August (24±14.4 mg m^?2 day^?1), coinciding with periods of maximum soil temperatures. In 1997 and 1998, the onset in CH₄ emission was coincident with increases in ground temperatures following the melting of the snow cover. The onset of emission in 1999 lagged 100 days behind the 1997 and 1998 onsets, and was likely related to postdrought recovery of the regional water table to typical levels. The net regional emissions were 3.0, 3.1, and 2.1 g CH₄ m^?2 for 1997, 1998, and 1999, respectively. Annual emissions for wetland regions within the source area (28% of the land area) were 13.2, 13.8, and 10.3 g CH₄ m^?2 assuming moderate rates of oxidation of CH₄ in upland regions in 1997, 1998, and 1999, respectively. Scaling these measurements to the Chequamegon Ecosystem (CNNF) and comparing with average wetland emissions between 40°N and 50°N suggests that wetlands in the CNNF emit approximately 40% less than average wetlands at this latitude. Differences in mean monthly air temperatures did not affect the magnitude of CH₄ emissions; however, reduced precipitation and water table levels suppressed CH₄ emission during 1999, suggesting that long‐term climatic changes that reduce the water table will likely transform this landscape to a reduced source or possibly a sink for atmospheric CH₄.     

Methane emissions from contrasting urban freshwaters: Rates,drivers, and a whole‐city footprint

Global urbanization trends impose major alterations on surface waters. This includes impacts on ecosystem functioning that can involve feedbacks on climate through changes in rates of greenhouse gas emissions. The combination of high nutrient supply and shallow depth typical of urban freshwaters is particularly conducive to high rates of methane (CH₄) production and emission, suggesting a potentially important role in the global CH₄ cycle. However, there is a lack of comprehensive flux data from diverse urban water bodies, of information on the underlying drivers, and of estimates for whole cities. Based on measurements over four seasons in a total of 32 water bodies in the city of Berlin, Germany, we calculate the total CH₄ emission from various types of surface waters of a large city in temperate climate at 2.6 ± 1.7 Gg CH₄/year. The average total emission was 219 ± 490 mg CH₄ m^?2 day^?1. Water chemical variables were surprisingly poor predictors of total CH₄ emissions, and proxies of productivity and oxygen conditions had low explanatory power as well, suggesting a complex combination of factors governing CH₄ fluxes from urban surface waters. However, small water bodies (area <1 ha) typically located in urban green spaces were identified as emission hotspots. These results help constrain assessments of CH₄ emissions from freshwaters in the world's growing cities, facilitating extrapolation of urban emissions to large areas, including at the global scale.     

Magnitude and biophysical regulators of methane emission and consumption in the Australian agricultural, forest, and submerged landscapes: a review

Increases in the concentrations of atmospheric greenhouse gases, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) due to human activities are associated with global climate change. CO₂ concentration in the atmosphere has increased by 33% (to 380 ppm) since 1750 ad, whilst CH₄ concentration has increased by 75% (to 1,750 ppb), and as the global warming potential (GWP) of CH₄ is 25 fold greater than CO₂ it represents about 20% of the global warming effect. The purpose of this review is to: (a) address recent findings regarding biophysical factors governing production and consumption of CH₄, (b) identify the current level of knowledge regarding the main sources and sinks of CH₄ in Australia, and (c) identify CH₄ mitigation options and their potential application in Australian ecosystems. Almost one-third of CH₄ emissions are from natural sources such as wetlands and lake sediments, which is poorly documented in Australia. For Australia, the major anthropogenic sources of CH₄ emissions include energy production from fossil fuels (~24%), enteric fermentation in the guts of ruminant animals (~59%), landfills, animal wastes and domestic sewage (~15%), and biomass burning (~5%), with minor contributions from manure management (1.7%), land use, land-use change and forestry (1.6%), and rice cultivation (0.2%). A significant sink exists for CH₄ (~6%) in aerobic soils, including agricultural and forestry soils, and potentially large areas of arid soils, however, due to limited information available in Australia, it is not accounted for in the Australian National Greenhouse Gas Inventory. CH₄ emission rates from submerged soils vary greatly, but mean values ≤10 mg m^?2 h^?1 are common. Landfill sites may emit CH₄ at one to three orders of magnitude greater than submerged soils. CH₄ consumption rates in non-flooded, aerobic agricultural, pastoral and forest soils also vary greatly, but mean values are restricted to ≤100 μg m^?2 h^?1, and generally greatest in forest soils and least in agricultural soils, and decrease from temperate to tropical regions. Mitigation options for soil CH₄ production primarily relate to enhancing soil oxygen diffusion through water management, land use change, minimised compaction and soil fertility management. Improved management of animal manure could include biogas capture for energy production or arable composting as opposed to open stockpiling or pond storage. Balanced fertiliser use may increase soil CH₄ uptake, reduce soil N₂O emissions whilst improving nutrient and water use efficiency, with a positive net greenhouse gas (CO₂-e) effect. Similarly, the conversion of agricultural land to pasture, and pastoral land to forestry should increase soil CH₄ sink. Conservation of native forests and afforestation of degraded agricultural land would effectively mitigate CH₄ emissions by maintaining and enhancing CH₄ consumption in these soils, but also by reducing N₂O emissions and increasing C sequestration. The overall impact of climate change on methanogenesis and methanotrophy is poorly understood in Australia, with a lack of data highlighting the need for long-term research and process understanding in this area. For policy addressing land-based greenhouse gas mitigation, all three major greenhouse gases (CO₂, CH₄ and N₂O) should be monitored simultaneously, combined with improved understanding at process-level.