Variation In Methane Emissions Research Articles

Short communication: Comparison of the GreenFeed system with the sulfur hexafluoride tracer technique for measuring enteric methane emissions from dairy cows

The objective of this study was to compare 2 commonly used techniques for measuring methane emissions from ruminant animals: the GreenFeed (GF) system and the sulfur hexafluoride (SF6) technique. The study was part of a larger experiment in which a methane inhibitor, 3-nitrooxypropanol, fed at 4 application rates (0, 40, 60, and 80 mg/kg of feed dry matter) decreased enteric methane emission by an average of 30% (measured by both GF and SF6) in a 12-wk experiment with 48 lactating Holstein cows fed a total mixed ration. The larger experiment used a randomized block design and was conducted in 2 phases (February to May, phase 1, and June to August, phase 2), with 2 sets of 24 cows in each phase. Using both GF and SF6 techniques, methane emission data were collected simultaneously during experimental wk 2, 6, and 12 (phase 1) and 2, 9, and 12 (phase 2), which corresponded to a total of 6 sampling periods. During each sampling period, 8 spot samples of gas emissions (staggered over a 3-d period) were collected from each cow using GF, as well as 3×24-h collections using the SF6 technique. Methane emission data were averaged per cow for the statistical analysis. The mean methane emission was 373 (standard deviation=96.3) and 405 (standard deviation=156) g/cow per day for GF and SF6, respectively. Coefficients of variation for the 2 methods were 25.8 and 38.6%, respectively; correlation and concordance between the 2 methods were 0.40 and 0.34, respectively. The difference in methane emission between the 2 methods (SF6 – GF) within treatment was from 46 to 144 and 24 to 27 g/d for phases 1 and 2, respectively. In the conditions of this experiment, the SF6 technique produced larger variability in methane emissions than the GF method. The overall difference between the 2 methods was on average about 8%, but was not consistent over time, likely influenced by barn ventilation and background methane and SF6 concentrations.

Interannual variations in methane emission from an irrigated rice paddy caused by rainfalls during the aeration period

Mid-season drainage (MSD) decreases methane emission from an irrigated rice paddy. For this reason, the 2006 IPCC Guidelines and several methane estimation models consider MSD as a static input factor. However, if rainfall occurs during the MSD period, the reduction effect on methane emission may become small. In this paper, we report the methane emission from an intermittently irrigated rice paddy with MSD for three growing seasons. Methane flux was measured using eddy covariance technique with an open-path methane analyzer. Our objective is to ascertain the seasonal and interannual variations in methane emission caused by rainfalls during the MSD period. The growing season (from June to Oct.) methane emission during the three year study period ranged from 198 to 450kg CH4 ha−1, showing significant interannual variability with changes in rainfall during the MSD period. In order to implement the effect of rainfall during the MSD on methane emission (not considered in the IPCC method), we developed a simple method for a scaling factor. The estimated and the observed CH4 emissions agreed within 40%, and additional consideration of soil pH further improved the agreement within 10%. These results suggest that the rainfall effect should be considered when estimating national or global methane emission from irrigated rice paddies.

The spatial variability of methane emission from subtaiga and forest–steppe grass–moss fens of Western Siberia

Methane emission from the grass–moss fens of the Western Siberia subtaiga was studied using a static chamber method. It was established that CH4 flux median ± half of the interquartile range in the studied wetland ecosystems constituted 4.9 ± 2.9 mg of CH4/(m2 h). It was shown that such a high spatial variability of emission is caused mainly by the difference in the water table level. It was found that, in these observations, a higher water table level correlates with lower emission values. The causes of this phenomenon are discussed, and recommendations for conducting field studies for estimating the regional flux are given.

Natural variation in methane emission of sheep fed on a lucerne pellet diet is unrelated to rumen ciliate community type.

Only limited information is available on the roles of different rumen ciliate community types, first described by Eadie in 1962, in enteric methane (CH4) formation by their ruminant hosts. If the different types were differentially associated with CH4 formation, then ciliate community typing could be used to identify naturally high and low CH4-emitting animals. Here we measured the CH4 yields [g CH4 (kg feed dry matter intake, DMI)(-1)] of 118 sheep fed a standard pelleted lucerne diet at two different times, at least 2 weeks apart. There were significant differences (P < 2.2 × 10(-16), Wilcoxon rank sum test) in the CH4 yields (± sd) from sheep selected as high [16.7 ± 1.5 g CH4 (kg DMI)(-1)] and low emitters [13.3 ± 1.5 g CH4 (kg DMI)(-1)]. A rumen sample was collected after each of the two measurements, and ciliate composition was analysed using barcoded 454 Titanium pyrosequencing of 18S rRNA genes. The genera found, in order of mean relative abundance, were Epidinium, Entodinium, Dasytricha, Eudiplodinium, Polyplastron, Isotricha and Anoplodinium-Diplodinium, none of which was significantly correlated with the CH4 emissions ranking associated with the rumen sample. Ciliate communities naturally assembled into four types (A, AB, B and O), characterized by the presence and absence of key genera. There was no difference in CH4 yield between sheep that harboured different ciliate community types, suggesting that these did not underlie the natural variation in CH4 yields. Further research is needed to unravel the nature of interactions between ciliate protozoa and other rumen micro-organisms, which may ultimately lead to contrasting CH4 emission phenotypes.

Productivity and depth regulate lake contributions to atmospheric methane

AbstractDespite significant contributions of inland lakes to the global methane cycle, we lack a process‐based understanding of what regulates inter‐lake variation in methane emissions. Previous comparative work has identified a potential link between lake primary productivity and methane emissions; also, lab‐scale experiments suggest that the addition of algal substrate to anoxic sediments rapidly enhances rates of methanogenesis. This existing work indicates that primary productivity could enhance lake contributions to the global methane cycle. However, a more systematic investigation of the links between lake primary production, methanogenesis, and methane emission to the atmosphere is required to quantify the implications of increased cultural eutrophication for methane evasion from lakes. Using paired measurements of methanogenesis and methane emissions on 16 north temperate lakes, we documented a positive relationship between lake productivity and sediment methanogenesis rates. However, increased methanogenesis rates did not result in an increase in diffusive methane emissions. Rather, they generated greater methane storage during summer stratification and enhanced methane emission to the atmosphere via bubbling (ebullition), dependent on site depth. Ebullition most frequently occurred at sites less than 6 m deep and where methanogenesis rates were high. The relationships between lake productivity, methanogenesis, and depth‐dependent ebullition suggests it is likely that shallow, productive lakes contribute significantly more methane to the atmosphere than deep, clear lakes and will continue to do so in light of the growing prevalence of lake eutrophication.

Modeling methane emissions from arctic lakes: Model development and site‐level study

AbstractTo date, methane emissions from lakes in the pan‐arctic region are poorly quantified. In order to investigate the response of methane emissions from this region to global warming, a process‐based climate‐sensitive lake biogeochemical model was developed. The processes of methane production, oxidation, and transport were modeled within a one‐dimensional sediment and water column. The sizes of 14C‐enriched and 14C‐depleted carbon pools were explicitly parameterized. The model was validated using observational data from five lakes located in Siberia and Alaska, representing a large variety of environmental conditions in the arctic. The model simulations agreed well with the measured water temperature and dissolved CH4 concentration (mean error less than 1°C and 0.2 μM, respectively). The modeled CH4 fluxes were consistent with observations in these lakes. We found that bubbling‐rate‐controlling nitrogen (N2) stripping was the most important factor in determining CH4 fraction in bubbles. Lake depth and ice cover thickness in shallow waters were also controlling factors. This study demonstrated that the thawing of Pleistocene‐aged organic‐rich yedoma can fuel sediment methanogenesis by supplying a large quantity of labile organic carbon. Observations and modeling results both confirmed that methane emission rate at thermokarst margins of yedoma lakes was much larger (up to 538 mg CH4 m−2 d−1) than that at nonthermokarst zones in the same lakes and a nonyedoma, nonthermokarst lake (less than 42 mg CH4 m−2 d−1). The seasonal variability of methane emissions can be explained primarily by energy input and organic carbon availability.

Within field spatial variation in methane emissions from lowland rice in Myanmar.

An assessment of within field spatial variations in grain yield and methane (CH4) emission was conducted in lowland rice fields of Myanmar. Two successive rice fields (1st field and 2nd field) were divided into fertilized and non-fertilized parts and CH4 measurements were conducted at the inlet, middle and outlet positions of each field. The results showed that CH4 emissions at non-fertilized parts were higher than those at fertilized part in both rice fields. The average CH4 emissions ranged from 8.7 to 26.6 mg m-2 h-1 in all positions in both rice fields. The spatial variation in CH4 emission among the positions was high in both rice fields with the highest emissions in the outlet of the 1st field and the inlet of the 2nd field. The CH4 emissions at these two positions showed 2 - 2.5 times higher than those at other positions in both rice fields. Stepwise regression analysis indicates that soil total carbon content is the primary factor for CH4 emission. The average CH4 emissions during rice growing season were 13.5 mg m-2 h-1 for the 1st field and 15.7 mg m-2 h-1 for the 2nd field. Spearman rank order correlation analysis showed that CH4 emission was significantly and positively correlated with soil temperature, surface water depth and negatively correlated with soil redox potential. The result indicated that high within field spatial variation in CH4 emissions required different site specific management practices to mitigate CH4 emissions in lowland paddy rice soil.

Measurements of methane emissions from natural gas gathering facilities and processing plants: measurement results.

Facility-level methane emissions were measured at 114 gathering facilities and 16 processing plants in the United States natural gas system. At gathering facilities, the measured methane emission rates ranged from 0.7 to 700 kg per hour (kg/h) (0.6 to 600 standard cubic feet per minute (scfm)). Normalized emissions (as a % of total methane throughput) were less than 1% for 85 gathering facilities and 19 had normalized emissions less than 0.1%. The range of methane emissions rates for processing plants was 3 to 600 kg/h (3 to 524 scfm), corresponding to normalized methane emissions rates <1% in all cases. The distributions of methane emissions, particularly for gathering facilities, are skewed. For example, 30% of gathering facilities contribute 80% of the total emissions. Normalized emissions rates are negatively correlated with facility throughput. The variation in methane emissions also appears driven by differences between inlet and outlet pressure, as well as venting and leaking equipment. Substantial venting from liquids storage tanks was observed at 20% of gathering facilities. Emissions rates at these facilities were, on average, around four times the rates observed at similar facilities without substantial venting.

A comparative study of daytime-based methane emission from two wetlands of Nepal Himalaya

Natural wetlands constitute one of the major sources of methane emission to the atmosphere. Data on methane emission from wetlands on southern slopes of the Himalaya (SSH) have not been reported so far. Such data are very valuable for filling the gap and generating the whole emission patterns at regional or even global scale. We selected two wetlands at different altitudinal locations in Nepal, i.e. Beeshazar Lake (286 m a.s.l.) and Dhaap Lake (2089 m a.s.l.), to monitor the daytime methane emissions in monsoon season and dry season separately. Daytime methane emission varied between monsoon and dry seasons and also across different plant communities. The daytime methane emission variations were stronger in dry season than in monsoon season. The source/sink strengths of the two selected plant communities in each wetland were significantly different, presenting the strong spatial variation of methane emission within wetland. The methane emissions recorded in monsoon season were significantly higher (7.74 ± 6.49 mg CH4m−2 h−1 and 1.00 ± 1.23 mg CH4m−2 h−1 in low and high altitude wetlands, respectively) than those in dry season (1.84 ± 4.57 mg CH4m−2 h−1 and 0.27 ± 0.71 mg CH4m−2 h−1 in low and high altitude wetlands, respectively). Methane emissions from the low altitude wetland were significantly higher than those from the high altitude wetland in both of the seasons. Plant community height, standing water depth and soil temperature correlated to the methane emission from wetlands in this region.

Spatial and temporal variability in methane emissions from tree stems of Fraxinus mandshurica in a cool-temperate floodplain forest

We performed field measurements on the spatial and temporal variability in CH4 emissions from stem surfaces of mature Fraxinus mandshurica Rupr. trees in a floodplain forest of northern Japan. Stem CH4 fluxes were measured by a static closed-chamber method at ca. 15 cm above ground on ten selected trees to test among-individual variability, and the diurnal and seasonal changes in three representative trees. Daytime stem CH4 emission rates varied between 81 and 1,305 µg CH4 m−2 h−1 among individual trees, and showed a spatial gradient apparently corresponding to the difference in water table depth at the experimental site. Stem CH4 fluxes were quite stable throughout a 24 h period for foliated trees in August and were similar for defoliated trees in November. Large differences were observed in the magnitude of seasonal changes in stem CH4 flux among individual trees; one sampled tree showed no clear seasonal changes in stem CH4 flux, while another tree exhibited drastic seasonal changes ranging larger than one order of magnitude. Results demonstrated the high variability in stem CH4 emissions in space and time, and suggested the importance of soil temperature, water table depth and porewater CH4 concentration as possible environmental factors controlling stem CH4 emissions from temperate forested wetlands.

Gross photosynthesis explains the ‘artificial bias’ of methane fluxes by static chamber (opaque versus transparent) at the hummocks in a boreal peatland

The closed chamber technique has been widely employed to detect methane emissions, despite little being known about whether the absence or presence of light will impact the flux estimation. Here, we employed a laser greenhouse gas analyzer with an opaque—transparent chamber pair to measure the methane emission rate in a boreal peatland complex. Microtopography (i.e., hummocks and hollows) in natural and drained peatlands, and plant communities (i.e., grasses and shrubs) in a pasture converted from natural peatlands, were considered to cover the local heterogeneity. Our results indicated that opaque chambers (0.58–0.78 g CH4 m−2 during the growing season) measured a significantly higher (∼2–3 times) methane emission at the hummocks than transparent chambers (∼0.24 g CH4 m−2); however, a similar phenomenon was not found at the hollows or at other measurement plots. Gross photosynthesis explained 44%–47% of the temporal variation of the ‘artificial bias’ (the difference in methane flux obtained by the opaque versus transparent chambers) at the hummocks. Additionally, both water table depth and surface soil moisture significantly explained spatial variations of methane emissions. Our study suggests that microtopography has a significant influence on the artificial bias in methane emission estimation and the artificial properties of a chamber (transparency/opacity) method can be vitally important in some cases (i.e., hummocks), and negligible in others (i.e., hollows). The observed connection between the photosynthesis process and the ‘artificial bias’ of closed chambers (opaque versus transparent) can be used to improve methane flux modeling. Separate parameterization schemes are needed for methane transportation under the presence or absence of light.

Spatial Variability of Annual Estimates of Methane Emissions in a Phragmites Australis (Cav.) Trin. ex Steud. Dominated Restored Coastal Brackish Fen

Methane is a major greenhouse gas with a global warming potential 25 times higher than carbon dioxide over a 100 year time horizon. Determining annual emission estimates, usually specified for different vegetation types under particular land use, requires the use of chamber measurements. These emission estimates may be strongly biased towards the lower or upper end by the spatial arrangement of measurement spots in the ecosystem. Here, we analyze the spatial variability of annual methane emission estimates based on non-steady state closed chamber measurements in pure and mixed stands of Phragmites australis (Cav.) Trin ex Steud. in a restored coastal brackish fen. Annual methane emission estimates per measurement location vary largely between 46 and 1,329 kg ha−1 a−1 CH4 but they do not differ significantly between pure and mixed stands of P. australis. Mantel tests show a significant correlation of distances between spots and the variation in methane emission estimates (p < 0.05). Spatial correlation of water levels and annual methane emission estimates is not significant. Empirical variograms suggest that variance in annual methane emission estimates is not increasing with increasing spatial distance. However, spots that share larger distances may differ considerably in their annual methane emission estimates. At the same time, spots that share smaller distances–though these exceed the distances common to many closed chamber measurement setups–may differ less in their annual methane emission estimates. Thus, a huge part of the natural variation in annual methane emissions in a particular ecosystem or vegetation type may be missed when using the typical clustered arrangement of measurement locations in closed chamber studies. Therefore, we suggest that measurement locations should cover a wide spatial extent to improve the reliability of annual emission estimates per vegetation type and ecosystem, and that the number of measurement locations appropriate should be determined by pre-studies whenever possible.

Spatial variability of methane emissions from Swiss alpine fens

Wetland ecosystems are a major natural source of the important greenhouse gas methane (CH4). Among these ecosystems, fens have been shown to release high quantities of CH4. Data on CH4 emissions from alpine fens are scarce and mainly limited to the United States and China. Therefore, static chambers were used to quantify CH4 emissions from 14 fens located in the Swiss Alps. The aims of this study were to determine the spatial variability of the emissions and to identify potential key factors which influence CH4 turnover. The fens were located at altitudes between 1,800 and 2,600 m a.s.l., the pore water varied from acidic to slightly acidic (pH 4.5–6.4) and the vegetation was dominated by plants of the genus Carex. In addition, the underlying bedrock was either siliceous or calcareous. Methane emissions ranged from 74 ± 43 to 711 ± 212 mg CH4 m−2 day−1. The type of bedrock, the plant biomass above the water table and the CH4 pore water concentrations at depths from 0 to 20 cm were the main factors influencing CH4 emissions. Detailed measurements in three selected fens suggested that more than 98 % of the total CH4 emissions are due to plant-mediated transport.

Influence of rumen protozoa on methane emission in ruminants: a meta-analysis approach

A meta-analysis was conducted to evaluate the effects of protozoa concentration on methane emission from ruminants. A database was built from 59 publications reporting data from 76 in vivo experiments. The experiments included in the database recorded methane production and rumen protozoa concentration measured on the same groups of animals. Quantitative data such as diet chemical composition, rumen fermentation and microbial parameters, and qualitative information such as methane mitigation strategies were also collected. In the database, 31% of the experiments reported a concomitant reduction of both protozoa concentration and methane emission (g/kg dry matter intake). Nearly all of these experiments tested lipids as methane mitigation strategies. By contrast, 21% of the experiments reported a variation in methane emission without changes in protozoa numbers, indicating that methanogenesis is also regulated by other mechanisms not involving protozoa. Experiments that used chemical compounds as an antimethanogenic treatment belonged to this group. The relationship between methane emission and protozoa concentration was studied with a variance−covariance model, with experiment as a fixed effect. The experiments included in the analysis had a within-experiment variation of protozoa concentration higher than 5.3 log10 cells/ml corresponding to the average s.e.m. of the database for this variable. To detect potential interfering factors for the relationship, the influence of several qualitative and quantitative secondary factors was tested. This meta-analysis showed a significant linear relationship between methane emission and protozoa concentration: methane (g/kg dry matter intake)=−30.7+8.14×protozoa (log10 cells/ml) with 28 experiments (91 treatments), residual mean square error=1.94 and adjusted R2=0.90. The proportion of butyrate in the rumen positively influenced the least square means of this relationship.

Decreasing methane emissions from ruminants grazing forages: a fit with productive and financial realities?

Ruminants contribute to human food supply and also anthropogenic greenhouse gas (GHG) emissions. An understanding of production systems and information on animal populations has enabled global inventories of ruminant GHG emissions (methane and nitrous oxide), and dietary strategies are being developed to reduce GHG emissions from ruminants. Mitigation strategies need to consider the management/feeding systems used to ensure that these strategies will be readily accepted and adopted by farmers. Housed systems allow diets to be formulated in ways that may reduce GHG production, but the challenge is much greater for systems where animals graze outdoors for long periods. A methane mitigation option in the form of fresh forage would be desirable in livestock production systems with high reliance on grazing. A brief summary of New Zealand research, carried out on fresh grasses, legumes, herbs and crops, suggest that we have an incomplete understanding of the feed characteristics that define a ‘high’ or a ‘low’ methane feed. The variation in methane emissions measured between feeds, individual animals and experiment is large, even in controlled conditions, and the dynamic nature of sward-animal interactions will only exacerbate this variation, creating challenges beyond the identification of mitigants. Furthermore, implementation of knowledge gained from controlled studies requires validation under grazing systems to identify any trade-offs between methane reduction and animal productivity or emission of other pollutants. Therefore, investment and research should be targeted at mitigation options that can and will be adopted on-farm, and the characteristics of temperate grasslands farming suggest that these options may differ from those for intensive (high input/output) or extensive (low input/output) systems.

Spatial variations in methane emissions from natural wetlands in China

Natural wetlands are thought to be one of the largest natural sources of atmospheric methane concentrations. Although numerous studies referred to the rate of methane fluxes in different geophysical regions, only a few had estimates of the overall geographical methane emissions in China. This study estimated the spatial variations of annual methane emissions with the pixel size of 1 km × 1 km from natural wetlands, excluding water surface, in China. The natural wetland areas were extracted from the database of the 2000 land covers, and geophysical divisions were used to represent different climate conditions. Methane emission in every geophysical region was calculated based on methane release factors obtained from an extensive overview of published literature and the data of elevation and vegetation proportion. The estimated annual methane emissions ranged from 0 to 5,702.8 kg per pixel within the area of 1 km2, and the spatial variation in methane emissions was strongly correlated with proportion of wetlands in the area. The total methane emission from natural wetland in China ranged from 3.48 to 7.16 Tg (terrogram, unit of weight) per year, with the mean value of 4.94 Tg per year, based on the area 133,000 km2 of natural wetlands. Specifically, the wetland in Northeast China had the highest contribution in China (39 %). Inner Mongolia and Qinghai-Tibet highland represented for about 25 and 21 %, respectively. The other 15 % of the measured methane was released in Northwest, North, Central, and South China.

Stable atmospheric methane in the 2000s: key-role of emissions from natural wetlands

Abstract. Two atmospheric inversions (one fine-resolved and one process-discriminating) and a process-based model for land surface exchanges are brought together to analyse the variations of methane emissions from 1990 to 2009. A focus is put on the role of natural wetlands and on the years 2000–2006, a period of stable atmospheric concentrations. From 1990 to 2000, the top-down and bottom-up visions agree on the time-phasing of global total and wetland emission anomalies. The process-discriminating inversion indicates that wetlands dominate the time-variability of methane emissions (90% of the total variability). The contribution of tropical wetlands to the anomalies is found to be large, especially during the post-Pinatubo years (global negative anomalies with minima between −41 and −19 Tg yr−1 in 1992) and during the alternate 1997–1998 El-Niño/1998–1999 La-Niña (maximal anomalies in tropical regions between +16 and +22 Tg yr−1 for the inversions and anomalies due to tropical wetlands between +12 and +17 Tg yr−1 for the process-based model). Between 2000 and 2006, during the stagnation of methane concentrations in the atmosphere, the top-down and bottom-up approaches agree on the fact that South America is the main region contributing to anomalies in natural wetland emissions, but they disagree on the sign and magnitude of the flux trend in the Amazon basin. A negative trend (−3.9 ± 1.3 Tg yr−1) is inferred by the process-discriminating inversion whereas a positive trend (+1.3 ± 0.3 Tg yr−1) is found by the process model. Although processed-based models have their own caveats and may not take into account all processes, the positive trend found by the B-U approach is considered more likely because it is a robust feature of the process-based model, consistent with analysed precipitations and the satellite-derived extent of inundated areas. On the contrary, the surface-data based inversions lack constraints for South America. This result suggests the need for a re-interpretation of the large increase found in anthropogenic methane inventories after 2000.

Temporal and spatial variability of methane emissions in a northern temperate marsh

Although methane (CH4) fluxes from northern wetlands in Asia have been described in previous research at different temporal and spatial scales, integrated studies at the ecosystem scale were scarce. In this study, CH4 fluxes were measured using eddy covariance (EC) technique and the chamber method in a cool temperate marsh in northeast China during the growing season (May–September) of 2011. CH4 emissions were highly variable, both temporally and spatially during the measurement period. According to the EC observation data, CH4 fluxes showed a significant diurnal cycle during the mid-growing season with nighttime average flux about 67% of the average daytime values. Daily cumulative CH4 fluxes varied from 54 to 250 mg CH4 m−2 d−1 with an average flux of 136.2 mg CH4 m−2 d−1. The observations of chamber method showed that CH4 emissions differed markedly among the three main plant communities. Average flux at the Carex lasiocarpa site was about 4 times and 13.5 times of that at the Glyceria spiculosa site and Deyeuxia angustifolia site, respectively. The spatial variability of CH4 flux was mainly controlled by the varying water table level as well as the spatial distribution of different vascular plants, while the seasonal dynamic of CH4 emission could be best explained by the change of surface soil temperature and air pressure. A comparison was made between EC measurements and the upscaled chamber based model. The results from the model overestimated CH4 emission by 28% compared to the EC data. Considering the large variability of methane emission, it is necessary to conduct continuous observations on CH4 emission from northern wetlands at different temporal and spatial scales to comprehend the variability and also to predict responses to climate change.

Toposequential variation in methane emissions from double-cropping paddy rice in Northwest Vietnam

Understanding the spatial and temporal variations in toposequential methane (CH4) emission is essential for assessing and mitigating CH4 emission from rice cascades in mountainous watersheds. To assess the toposequential variation in CH4 emission among different field positions, two cascades of double-cropping paddy rice fields were investigated in Yen Chau district, Northwest Vietnam. The cascades were divided into fertilized and non-fertilized parts and CH4 measurements at 10days intervals were conducted at top, middle and bottom fields of each part. The results showed that the rate and cumulative amount of CH4 emissions in non-fertilized part were higher than that of fertilized one in both spring and summer rice seasons due to the stimulation of CH4 oxidation by urea and sulfate containing fertilizers. The spatial variation in CH4 emissions among the field positions was high in both cropping seasons with the highest emissions in the bottom fields and the lowest emissions were found in the top fields (i.e. bottom field CH4 emissions 1.8–3.0 times higher than the top field). The differences among field positions were influenced by clay content, total nitrogen and total carbon content which showed toposequential differences. The average CH4 fluxes ranged from 1.0 to 5.1mgCH4m−2h−1 being largest at later growth stages for spring rice and during early growth stages for summer rice. Cumulative CH4 emissions for spring rice ranged from 3.1 to 13.7gCH4m−2 and that for summer rice from 4.3 to 23.5gCH4m−2. 61.7% was emitted during summer rice season and 38.1% from spring rice season. The higher values for summer crops were due to higher availability of fresh organic substrates under higher soil temperature during the early growing period. The average total CH4 emissions from double-cropping paddy rice fields were 14.8gCH4m−2 for cascade 1 and 27.3gCH4m−2 for cascade 2. The higher emission for cascade 2 might be due to the lower soil Eh and higher clay content especially in the lower lying fields. The results highlight that large toposequence differences in CH4 emissions require different site specific management practices for each toposequence position in order to mitigate CH4 emission in paddies in mountainous watersheds.

ASSESSMENT OF METHANE ESTIMATION FROM VOLATILE FATTY ACID STOICHIOMETRY IN THE RUMEN IN VITRO

Rumen microbes breakdown feed to produce volatile fatty acids (VFA), carbon dioxide, ammoniaand methane (CH4). Metabolic hydrogen in the form of reduced protons is used during CH4 formation aswell as during VFA synthesis. Therefore, VFA concentration in the rumen may stoichiometrically berelated to CH4 emission. The aim of this study was to evaluate methane emission between experimentaland model estimates. Two stoichiometrical models for predicting CH4 from VFA were assessed, i.e.Moss et al. (2000) and Hegarty and Nolan (2007) models. The data sets were obtained from a publishedliterature. Samples used were leaves from 27 tropical plant species. Prediction error was conducted bycomputing root mean square prediction error (RMSPE). Results showed that estimation model of Mosset al. (2000) had lower RMSPE value, i.e. 8.01%, than that of Hegarty and Nolan (2007) model, i.e.10.73%. Variation of methane emission, i.e. the low or high methane can be estimated by VFAcomposition with a sufficient accuracy. Adjusment by considering H2 recovery lowered the biassignificantly. It can be concluded that Moss model had better accuracy in predicting CH4 emission fromVFA composition than that of Hegarty and Nolan model.