Spatially explicit estimates of N2O emissions from croplands suggest climate mitigation opportunities from improved fertilizer management

With increasing nitrogen (N) application to croplands required to support growing food demand, mitigating N₂O emissions from agricultural soils is a global challenge. National greenhouse gas emissions accounting typically estimates N₂O emissions at the country scale by aggregating all crops, under the assumption that N₂O emissions are linearly related to N application. However, field studies and meta‐analyses indicate a nonlinear relationship, in which N₂O emissions are relatively greater at higher N application rates. Here, we apply a super‐linear emissions response model to crop‐specific, spatially explicit synthetic N fertilizer and manure N inputs to provide subnational accounting of global N₂O emissions from croplands. We estimate 0.66 Tg of N₂O‐N direct global emissions circa 2000, with 50% of emissions concentrated in 13% of harvested area. Compared to estimates from the IPCC Tier 1 linear model, our updated N₂O emissions range from 20% to 40% lower throughout sub‐Saharan Africa and Eastern Europe, to >120% greater in some Western European countries. At low N application rates, the weak nonlinear response of N₂O emissions suggests that relatively large increases in N fertilizer application would generate relatively small increases in N₂O emissions. As aggregated fertilizer data generate underestimation bias in nonlinear models, high‐resolution N application data are critical to support accurate N₂O emissions estimates.     

Methane and nitrous oxide emissions under no‐till farming in China: a meta‐analysis

No‐till (NT) practices are among promising options toward adaptation and mitigation of climate change. However, the mitigation effectiveness of NT depends not only on its carbon sequestration potential but also on soil‐derived CH₄ and N₂O emissions. A meta‐analysis was conducted, using a dataset involving 136 comparisons from 39 studies in China, to identify site‐specific factors which influence CH₄ emission, CH₄ uptake, and N₂O emission under NT. Comparative treatments involved NT without residue retention (NT0), NT with residue retention (NTR), compared to plow tillage (PT) with residue removed (PT0). Overall, NT0 significantly decreased CH₄ emission by ~30% (P < 0.05) compared to PT0 with an average emission 218.8 kg ha⁻¹ for rice paddies. However, the increase in N₂O emission could partly offset the benefits of the decrease in CH₄ emission under NT compared to PT0. NTR significantly enhanced N₂O emission by 82.1%, 25.5%, and 20.8% (P < 0.05) compared to PT0 for rice paddies, acid soils, and the first 5 years of the experiments, respectively. The results from categorical meta‐analysis indicated that the higher N₂O emission could be mitigated by adopting NT within alkaline soils, for long‐term duration, and with less N fertilization input when compared to PT0. In addition, the natural log (lnR) of response ratio of CH₄ and N₂O emissions under NT correlated positively (enhancing emission) with climate factors (temperature and precipitation) and negatively (reducing emission) with experimental duration, suggesting that avoiding excess soil wetness and using NT for a long term could enhance the benefits of NT. Therefore, a thorough understanding of the conditions favoring greenhouse gas(es) reductions is essential to achieving climate change mitigation and advancing food security in China.     

垃圾填埋场N2O排放通量及测定方法研究进展

氧化亚氮(N₂O)是第三大温室气体和最主要的臭氧层破坏气体.填埋是目前城市生活垃圾处理处置的主要方式,而垃圾填埋场是N₂O的排放源之一.实验室研究和现场测定均表明,生活垃圾填埋场可以有高的N₂O释放通量,但不同填埋场测定数据差异很大.目前,对生活垃圾填埋场N₂O排放量的原位准确测定以及排放机理和重要性的认识仍有很多不足.本文概述了生活垃圾填埋场N₂O排放研究现状,从垃圾堆体和覆土层两部分探讨了传统厌氧卫生填埋场的N₂O产生和排放机理,并就此对新型脱氮型生物反应器填埋场做了相应探讨.最后,就静态箱法、涡度相关法等N₂O通量测定方法在填埋场的适用性进行了讨论,并展望了填埋场N₂O排放的研究方向.     

Impacts of natural factors and farming practices on greenhouse gas emissions in the North China Plain: A meta‐analysis

Requirements for mitigation of the continued increase in greenhouse gas (GHG ) emissions are much needed for the North China Plain (NCP ). We conducted a meta‐analysis of 76 published studies of 24 sites in the NCP to examine the effects of natural conditions and farming practices on GHG emissions in that region. We found that N₂O was the main component of the area‐scaled total GHG balance, and the CH ₄ contribution was <5%. Precipitation, temperature, soil pH , and texture had no significant impacts on annual GHG emissions, because of limited variation of these factors in the NCP . The N₂O emissions increased exponentially with mineral fertilizer N application rate, with y  =  0.2389e^0.0058x for wheat season and y  =  0.365e^0.0071x for maize season. Emission factors were estimated at 0.37% for wheat and 0.90% for maize at conventional fertilizer N application rates. The agronomic optimal N rates (241 and 185 kg N ha^?1 for wheat and maize, respectively) exhibited great potential for reducing N₂O emissions, by 0.39 (29%) and 1.71 (56%) kg N₂O‐N ha^?1 season^?1 for the wheat and maize seasons, respectively. Mixed application of organic manure with reduced mineral fertilizer N could reduce annual N₂O emissions by 16% relative to mineral N application alone while maintaining a high crop yield. Compared with conventional tillage, no‐tillage significantly reduced N₂O emissions by ~30% in the wheat season, whereas it increased those emissions by ~10% in the maize season. This may have resulted from the lower soil temperature in winter and increased soil moisture in summer under no‐tillage practice. Straw incorporation significantly increased annual N₂O emissions, by 26% relative to straw removal. Our analysis indicates that these farming practices could be further tested to mitigate GHG emission and maintain high crop yields in the NCP .     

Mitigation of ammonia,nitrous oxide and methane emissions from manure management chains: a meta‐analysis and integrated assessment

Livestock manure contributes considerably to global emissions of ammonia (NH₃) and greenhouse gases (GHG), especially methane (CH₄) and nitrous oxide (N₂O). Various measures have been developed to mitigate these emissions, but most of these focus on one specific gas and/or emission source. Here, we present a meta‐analysis and integrated assessment of the effects of mitigation measures on NH₃, CH₄ and (direct and indirect) N₂O emissions from the whole manure management chain. We analysed the effects of mitigation technologies on NH₃, CH₄ and N₂O emissions from individual sources statistically using results of 126 published studies. Whole‐chain effects on NH₃ and GHG emissions were assessed through scenario analysis. Significant NH₃ reduction efficiencies were observed for (i) housing via lowering the dietary crude protein (CP) content (24–65%, compared to the reference situation), for (ii) external slurry storages via acidification (83%) and covers of straw (78%) or artificial films (98%), for (iii) solid manure storages via compaction and covering (61%, compared to composting), and for (iv) manure application through band spreading (55%, compared to surface application), incorporation (70%) and injection (80%). Acidification decreased CH₄ emissions from stored slurry by 87%. Significant increases in N₂O emissions were found for straw‐covered slurry storages (by two orders of magnitude) and manure injection (by 26–199%). These side‐effects of straw covers and slurry injection on N₂O emission were relatively small when considering the total GHG emissions from the manure chain. Lowering the CP content of feed and acidifying slurry are strategies that consistently reduce NH₃ and GHG emissions in the whole chain. Other strategies may reduce emissions of a specific gas or emissions source, by which there is a risk of unwanted trade‐offs in the manure management chain. Proper farm‐scale combinations of mitigation measures are important to minimize impacts of livestock production on global emissions of NH₃ and GHG.     

Greenhouse gas emissions from dairy manure management: a review of field‐based studies

Livestock manure management accounts for almost 10% of greenhouse gas emissions from agriculture globally, and contributes an equal proportion to the US methane emission inventory. Current emissions inventories use emissions factors determined from small‐scale laboratory experiments that have not been compared to field‐scale measurements. We compiled published data on field‐scale measurements of greenhouse gas emissions from working and research dairies and compared these to rates predicted by the IPCC Tier 2 modeling approach. Anaerobic lagoons were the largest source of methane (368 ± 193 kg CH₄ hd^?1 yr^?1), more than three times that from enteric fermentation (~120 kg CH₄ hd^?1 yr^?1). Corrals and solid manure piles were large sources of nitrous oxide (1.5 ± 0.8 and 1.1 ± 0.7 kg N₂O hd^?1 yr^?1, respectively). Nitrous oxide emissions from anaerobic lagoons (0.9 ± 0.5 kg N₂O hd^?1 yr^?1) and barns (10 ± 6 kg N₂O hd^?1 yr^?1) were unexpectedly large. Modeled methane emissions underestimated field measurement means for most manure management practices. Modeled nitrous oxide emissions underestimated field measurement means for anaerobic lagoons and manure piles, but overestimated emissions from slurry storage. Revised emissions factors nearly doubled slurry CH₄ emissions for Europe and increased N₂O emissions from solid piles and lagoons in the United States by an order of magnitude. Our results suggest that current greenhouse gas emission factors generally underestimate emissions from dairy manure and highlight liquid manure systems as promising target areas for greenhouse gas mitigation.     

Application of Bacillus sp. TAT105 to reduce ammonia emissions during pilot-scale composting of swine manure

Thermophilic ammonium-tolerant bacterium Bacillus sp. TAT105 grows and reduces ammonia (NH₃) emissions by assimilating ammonium nitrogen during composting of swine feces. To evaluate the efficacy of a biological additive containing TAT105 at reducing NH₃ emissions, composting tests of swine manure on a pilot scale (1.8 m³) were conducted. In the TAT105-added treatment, NH₃ emissions and nitrogen loss were lower than those in the control treatment without TAT105. No significant difference was detected in losses in the weight and volatile solids between the treatments. Concentration of thermophilic ammonium-tolerant bacteria in the compost increased in both treatments at the initial stage of composting. In the TAT105-added treatment, bacterial concentration reached ~10⁹ colony-forming units per gram of dry matter, several-fold higher than that in the control and stayed at the same level until the end. These results suggest that TAT105 grows during composting and reduces NH₃ emissions in TAT105-added treatment.     

Product specific emissions from municipal solid waste landfills

For the inventory analysis of environmental impacts associated with products in Life Cycle Assessment (LCA) there is a great need for estimates of emissions from waste products disposed at municipal solid waste landfills (product specific emissions). Since product specific emissions can not be calculated or measured directly at the landfills, they must be estimated by modeling of landfill processes. This paper presents a landfill model based on a large number of assumptions and approximations concerning landfill properties, waste product properties and characteristics of various kinds of environmental protection systems (e.g. landfill gas combustion units and leachate treatment units). The model is useful for estimation of emissions from waste products disposed in landfills and it has been made operational in the computer tool LCA-LAND presented in a following paper. In the model, waste products are subdivided into five groups of components: general organic matter (e.g. paper), specific organic compounds (e.g. organic solvents), inert components (e.g. PVC), metals (e.g. cadmium), and inorganic non-metals (e.g. chlorine,) which are considered individually. The assumptions and approximations used in the model are to the extent possible scientifically based, but where scientific information has been missing, qualified estimates have been made to fulfill the aim of a complete tool for estimation of emissions. Due to several rough simplifications and missing links in our present understanding of landfills, the uncertainty associated with the model is relatively high.     

Product specific emissions from municipal solid waste landfills

This paper presents and verifies the computer tool LCA-LAND for estimation of emissions from specific waste products disposed in municipal solid waste landfills in European countries for use in the inventory analysis of LCA. Examples of input data (e.g. distribution of the waste product in different countries, composition of the product and physical/chemical/biological properties of waste product components) and output data (e.g. estimated emissions to atmosphere and water) are given for a fictive waste product made of representative types of components (toluene, cellulose, polyvinylchloride (PVC), copper and chloride). Since waste products from different processes in the product system may be disposed at different landfills where they are mixed with waste originating outside the product system, the estimated emissions from specific waste products cannot be compared with measured emissions from true landfills. Hence, the computer tool is verified in terms of mass balances and sensitivity analyses. The mass balances agree exactly and the sensitivity analyses show that different types of waste product components behave differently in different types of landfills. Emission of e.g. toluene is significantly reduced in the presence of landfill top-cover, landfill gas combustion units and leachate treatment units. Generally, the sensitivity analysis shows good agreement between the relative proportions of various types of emissions (based on properties of the waste and properties of landfills) and good agreement with emission levels that would be expected based on a general understanding of landfill processes.     

Soil greenhouse gas emissions from inorganic fertilizers and recycled oil palm waste products from Indonesian oil palm plantations

A continuous rise in the global demand for palm oil has resulted in the large‐scale expansion of oil palm plantations and generated environmental controversy. Efforts to increase the sustainability of oil palm cultivation include the recycling of oil mill and pruning residues in the field, but this may increase soil methane (CH₄) emissions. This study reports the results of yearlong field‐based measurements of soil nitrous oxide (N₂O) and CH₄ emissions from commercial plantations in North Sumatra, Indonesia. One experiment investigated the effects of soil‐water saturation on N₂O and CH₄ emissions from inorganic fertilizers and organic amendments by simulating 25 mm rainfall per day for 21 days. Three additional experiments focused on emissions from (a) inorganic fertilizer (urea), (b) combination of enriched mulch with urea and (c) organic amendments (empty fruit bunches, enriched mulch and pruned oil palm fronds) applied in different doses and spatial layouts (placed in inter‐row zones, piles, patches or bands) for a full year. The higher dose of urea led to a significantly higher N₂O emissions with the emission factors ranging from 2.4% to 2.7% in the long‐term experiment, which is considerably higher than the IPCC standard of 1%. Organic amendments were a significant source of both N₂O and CH₄ emissions, but N₂O emissions from organic amendments were 66%–86% lower than those from inorganic fertilizers. Organic amendments applied in piles emitted 63% and 71% more N₂O and CH₄, respectively, than when spread out. With twice the dose of organic amendments, cumulative emissions were up to three times greater. The (simulated) rainwater experiment showed that the increase in precipitation led to a significant increase in N₂O emissions significantly, suggesting that the time of fertilization is a critical management option for reducing emissions. The results from this study could therefore help guide residue and nutrient management practices to reduce emissions while ensuring better nutrient recycling for sustainable oil palm production systems.     

Methane and nitrous oxide emissions from rice paddy fields as affected by nitrogen fertilisers and water management

Methane and N₂O emissions affected by nitrogen fertilisers were measured simultaneously in rice paddy fields under intermittent irrigation in 1994. Ammonium sulphate and urea were applied at rates of 0 (control), 100 and 300 kg N ha^-1. The results showed that CH₄ emission, on the average, decreased by 42 and 60% in the ammonium sulphate treatments and 7 and 14% in the urea treatments at rates of 100 and 300 kg N ha^-1, respectively, compared to the control. N₂O emission increased significantly with the increase in the nitrogen application rate. N₂O emission was higher from ammonium sulphate treatments than from the urea treatments at the same application rate. A trade-off effect between CH₄ and N₂O emission was clearly observed. The N₂O flux was very small when the rice paddy plots were flooded, but peaked at the beginning of the disappearance of floodwater. In contrast, the CH₄ flux peaked during flooding and was significantly depressed by mid-season aeration (MSA). The results suggest that it is important to evaluate the integrative effects of water management and fertiliser application for mitigating greenhouse gas emissions in order to attenuate the greenhouse effect contributed by rice paddy fields.     

Activity and composition of ammonia oxidizing bacterial communities and emission dynamics of NH3 and N2O in a compost reactor treating organic household waste

Aims:  To monitor emissions of NH₃ and N₂O during composting and link these to ammonia oxidation rates and the community structure of ammonia oxidizing bacteria (AOB).
Methods and Results:  A laboratory-scale compost reactor treating organic household waste was run for 2 months. NH₃ emissions peaked when pH started to increase. Small amounts of N₂O and CH₄ were also produced. In total, 16% and less than 1% of the initial N was lost as NH₃-N and N₂O-N respectively. The potential ammonia oxidation rate, determined by a chlorate inhibition assay, increased fourfold during the first 9 days and then remained high. Initially, both Nitrosospira and Nitrosomonas populations were detected using DGGE analysis of AOB specific 16S rRNA fragments. Only Nitrosomonas europaea was detected under thermophilic conditions, but Nitrosospira populations re-established during the cooling phase.
Conclusions:  Thermophilic conditions favoured high potential ammonia oxidation rates, suggesting that ammonia oxidation contributed to reduced NH₃ emissions. Small but significant amounts of N₂O were emitted during the thermophilic phase. The significance of different AOBs detected in the compost for ammonia oxidation is not clear.
Significance and Impact of Study:  This study shows that ammonia oxidation occurs at high temperature composting and therefore most likely reduces NH₃ emissions.     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Meta‐analysis on the potential for increasing nitrogen losses from intensifying tropical agriculture

Fertilized temperate croplands export large amounts of reactive nitrogen (N), which degrades water and air quality and contributes to climate change. Fertilizer use is poised to increase in the tropics, where widespread food insecurity persists and increased agricultural productivity will be needed, but much less is known about the potential consequences of increased tropical N fertilizer application. We conducted a meta‐analysis of tropical field studies of nitrate leaching, nitrous oxide emissions, nitric oxide emissions, and ammonia volatilization totaling more than 1,000 observations. We found that the relationship between N inputs and losses differed little between temperate and tropical croplands, although total nitric oxide losses were higher in the tropics. Among the potential drivers we studied, the N input rate controlled all N losses, but soil texture and water inputs also controlled hydrological N losses. Irrigated systems had significantly higher losses of ammonia, and pasture agroecosystems had higher nitric oxide losses. Tripling of fertilizer N inputs to tropical croplands from 50 to 150 kg N ha^?1 year^?1 would have substantial environmental implications and would lead to increases in nitrate leaching (+30%), nitrous oxide emissions (+30%), nitric oxide (+66%) emissions, and ammonia volatilization (+74%), bringing tropical agricultural nitrate, nitrous oxide, and ammonia losses in line with temperate losses and raising nitric oxide losses above them.     

An agronomic assessment of greenhouse gas emissions from major cereal crops

Agricultural greenhouse gas (GHG) emissions contribute approximately 12% to total global anthropogenic GHG emissions. Cereals (rice, wheat, and maize) are the largest source of human calories, and it is estimated that world cereal production must increase by 1.3% annually to 2025 to meet growing demand. Sustainable intensification of cereal production systems will require maintaining high yields while reducing environmental costs. We conducted a meta‐analysis (57 published studies consisting of 62 study sites and 328 observations) to test the hypothesis that the global warming potential (GWP) of CH₄ and N₂O emissions from rice, wheat, and maize, when expressed per ton of grain (yield‐scaled GWP), is similar, and that the lowest value for each cereal is achieved at near optimal yields. Results show that the GWP of CH₄ and N₂O emissions from rice (3757 kg CO₂ eq ha^?1 season^?1) was higher than wheat (662 kg CO₂ eq ha^?1 season^?1) and maize (1399 kg CO₂ eq ha^?1 season^?1). The yield‐scaled GWP of rice was about four times higher (657 kg CO₂ eq Mg^?1) than wheat (166 kg CO₂ eq Mg^?1) and maize (185 kg CO₂ eq Mg^?1). Across cereals, the lowest yield‐scaled GWP values were achieved at 92% of maximal yield and were about twice as high for rice (279 kg CO₂ eq Mg^?1) than wheat (102 kg CO₂ eq Mg^?1) or maize (140 kg CO₂ eq Mg^?1), suggesting greater mitigation opportunities for rice systems. In rice, wheat and maize, 0.68%, 1.21%, and 1.06% of N applied was emitted as N₂O, respectively. In rice systems, there was no correlation between CH₄ emissions and N rate. In addition, when evaluating issues related to food security and environmental sustainability, other factors including cultural significance, the provisioning of ecosystem services, and human health and well‐being must also be considered.     

Nitrous oxide and methane emissions from cryptogamic covers

Cryptogamic covers, which comprise some of the oldest forms of terrestrial life on Earth (Lenton & Huntingford, 2003 ), have recently been found to fix large amounts of nitrogen and carbon dioxide from the atmosphere (Elbert et al., 2012 ). Here we show that they are also greenhouse gas sources with large nitrous oxide (N₂O) and small methane (CH₄) emissions. Whilst N₂O emission rates varied with temperature, humidity, and N deposition, an almost constant ratio with respect to respiratory CO₂ emissions was observed for numerous lichens and bryophytes. We employed this ratio together with respiration data to calculate global and regional N₂O emissions. If our laboratory measurements are typical for lichens and bryophytes living on ground and plant surfaces and scaled on a global basis, we estimate a N₂O source strength of 0.32–0.59 Tg year^?1 for the global N₂O emissions from cryptogamic covers. Thus, our emission estimate might account for 4–9% of the global N₂O budget from natural terrestrial sources. In a wide range of arid and forested regions, cryptogamic covers appear to be the dominant source of N₂O. We suggest that greenhouse gas emissions associated with this source might increase in the course of global change due to higher temperatures and enhanced nitrogen deposition.     

Stimulation of N2O emission by manure application to agricultural soils may largely offset carbon benefits: a global meta‐analysis

Animal manure application as organic fertilizer does not only sustain agricultural productivity and increase soil organic carbon (SOC) stocks, but also affects soil nitrogen cycling and nitrous oxide (N₂O) emissions. However, given that the sign and magnitude of manure effects on soil N₂O emissions is uncertain, the net climatic impact of manure application in arable land is unknown. Here, we performed a global meta‐analysis using field experimental data published in peer‐reviewed journals prior to December 2015. In this meta‐analysis, we quantified the responses of N₂O emissions to manure application relative to synthetic N fertilizer application from individual studies and analyzed manure characteristics, experimental duration, climate, and soil properties as explanatory factors. Manure application significantly increased N₂O emissions by an average 32.7% (95% confidence interval: 5.1–58.2%) compared to application of synthetic N fertilizer alone. The significant stimulation of N₂O emissions occurred following cattle and poultry manure applications, subsurface manure application, and raw manure application. Furthermore, the significant stimulatory effects on N₂O emissions were also observed for warm temperate climate, acid soils (pH < 6.5), and soil texture classes of sandy loam and clay loam. Average direct N₂O emission factors (EFs) of 1.87% and 0.24% were estimated for upland soils and rice paddy soils receiving manure application, respectively. Although manure application increased SOC stocks, our study suggested that the benefit of increasing SOC stocks as GHG sinks could be largely offset by stimulation of soil N₂O emissions and aggravated by CH₄ emissions if, particularly for rice paddy soils, the stimulation of CH₄ emissions by manure application was taken into account.     

Methane emissions from soils: synthesis and analysis of a large UK data set

Nearly 5000 chamber measurements of CH₄ flux were collated from 21 sites across the United Kingdom, covering a range of soil and vegetation types, to derive a parsimonious model that explains as much of the variability as possible, with the least input requirements. Mean fluxes ranged from ?0.3 to 27.4 nmol CH₄ m^?2 s^?1, with small emissions or low rates of net uptake in mineral soils (site means of ?0.3 to 0.7 nmol m^?2 s^?1) and much larger emissions from organic soils (site means of ?0.3 to 27.4 nmol m^?2 s^?1). Less than half of the observed variability in instantaneous fluxes could be explained by independent variables measured. The reasons for this include measurement error, stochastic processes and, probably most importantly, poor correspondence between the independent variables measured and the actual variables influencing the processes underlying methane production, transport and oxidation. When temporal variation was accounted for, and the fluxes averaged at larger spatial scales, simple models explained up to ca. 75% of the variance in CH₄ fluxes. Soil carbon, peat depth, soil moisture and pH together provided the best sub‐set of explanatory variables. However, where plant species composition data were available, this provided the highest explanatory power. Linear and nonlinear models generally fitted the data equally well, with the exception that soil moisture required a power transformation. To estimate the impact of changes in peatland water table on CH₄ emissions in the United Kingdom, an emission factor of +0.4 g CH₄ m^?2 yr^?1 per cm increase in water table height was derived from the data.     

Circulating trimethylamine N‐oxide and the risk of cardiovascular diseases: a systematic review and meta‐analysis of 11 prospective cohort studies

Circulating trimethylamine N‐oxide (TMAO), a canonical metabolite from gut flora, has been related to the risk of cardiovascular disorders. However, the association between circulating TMAO and the risk of cardiovascular events has not been quantitatively evaluated. We performed a systematic review and meta‐analysis of all available cohort studies regarding the association between baseline circulating TMAO and subsequent cardiovascular events. Embase and PubMed databases were searched for relevant cohort studies. The overall hazard ratios for the developing of cardiovascular events (CVEs) and mortality were extracted. Heterogeneity among the included studies was evaluated with Cochran's Q Test and I² statistics. A random‐effect model or a fixed‐effect model was applied depending on the heterogeneity. Subgroup analysis and meta‐regression were used to evaluate the source of heterogeneity. Among the 11 eligible studies, three reported both CVE and mortality outcome, one reported only CVEs and the other seven provided mortality data only. Higher circulating TMAO was associated with a 23% higher risk of CVEs (HR = 1.23, 95% CI: 1.07–1.42, I² = 31.4%) and a 55% higher risk of all‐cause mortality (HR = 1.55, 95% CI: 1.19–2.02, I² = 80.8%). Notably, the latter association may be blunted by potential publication bias, although sensitivity analysis by omitting one study at a time did not significantly change the results. Further subgroup analysis and meta‐regression did not support that the location of the study, follow‐up duration, publication year, population characteristics or the samples of TMAO affect the results significantly. Higher circulating TMAO may independently predict the risk of subsequent cardiovascular events and mortality.     

植物释放甲烷研究进展

植物是否在有氧条件下自身产生甲烷、其产生机制和释放速率等问题目前还存在很大争议,如果确证植物在有氧条件下产生较大量的甲烷,就必须重新认识和计算全球甲烷的源汇及其收支平衡。已有研究表明,植物排放的甲烷有一部分是由土壤或木本植物的根和树干内部产甲烷微生物产生,再通过植物传输进入大气中的;植物本身产生甲烷的机制可能主要是在活性氧自由基的作用下,将植物细胞壁成分果胶、木质素等中的甲氧基转化为甲烷,这一过程受到高温、强光和UV辐射等环境胁迫的刺激。根据植物排放速率或大气甲烷浓度与碳同位素组成的实测值,对区域和全球植物源甲烷排放率做出的估算还存在相当大的不确定性,需要对更多植物和更多地点开展实测研究,深入了解植物产甲烷的机制和过程,并结合大气传输模型才能进一步提高估算准确性。