CH4 Year Research Articles

Atmospheric methane uptake by tropical montane forest soils and the contribution of organic layers

Microbial oxidation in aerobic soils is the primary biotic sink for atmospheric methane (CH4), a powerful greenhouse gas. Although tropical forest soils are estimated to globally account for about 28% of annual soil CH4 consumption (6.2 Tg CH4 year−1), limited data are available on CH4 exchange from tropical montane forests. We present the results of an extensive study on CH4 exchange from tropical montane forest soils along an elevation gradient (1,000, 2,000, 3,000 m) at different topographic positions (lower slope, mid-slope, ridge position) in southern Ecuador. All soils were net atmospheric CH4 sinks, with decreasing annual uptake rates from 5.9 kg CH4–C ha−1 year−1 at 1,000 m to 0.6 kg CH4–C ha−1 year−1 at 3,000 m. Topography had no effect on soil atmospheric CH4 uptake. We detected some unexpected factors controlling net methane fluxes: positive correlations between CH4 uptake rates, mineral nitrogen content of the mineral soil and with CO2 emissions indicated that the largest CH4 uptake corresponded with favorable conditions for microbial activity. Furthermore, we found indications that CH4 uptake was N limited instead of inhibited by NH4 +. Finally, we showed that in contrast to temperate regions, substantial high affinity methane oxidation occurred in the thick organic layers which can influence the CH4 budget of these tropical montane forest soils. Inclusion of elevation as a co-variable will improve regional estimates of methane exchange in these tropical montane forests.

Assessment of methane and nitrous oxide flux from mangroves along Eastern coast of India

AbstractMangroves are considered to be a minor source of greenhouse gases (CH4 and N2O) in pristine environmental condition. However, estimates of efflux suggest that anthropogenic activities have led to a pronounced increase in greenhouse gas emission. Along the east coast of India, mangroves vary substantially in area, physiography and freshwater input, which ultimately modify the biogeochemical processes operating within this ecosystem. An attempt has here been made to elucidate the existing variation and role of climatic variability on the emission of greenhouse gases from mangroves. The flux estimates of CH4 and N2O have been quantified from Bhitarkanika mangrove accounting for spatial and temporal (seasonal) variation. The annual rates were estimated to be 0.096 × 10 9 g CH4 year−1 and 5.8 × 103g N2O year−1 for the whole mangrove area of the east coast of India. Upscaling these estimates yield an annual emission of 1.95 × 10 12 g CH4 year−1 and 1.1 × 10 11 g N2O year−1 from worldwide mangrove areas. The influence of elevated nutrient inputs through anthropogenic influence enhances the emission of greenhouse gas. The present article shows the need to develop an inventory on greenhouse gas flux from mangrove ecosystem.

Emission of methane from chalk streams has potential implications for agricultural practices

Summary1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function.2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 μm) dominated by the sand‐sized fraction (63–1000 μm).3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH4 transport through Ranunculus stems and an increase in water column CH4 concentration, while the sediment CH4 concentration increased 100‐fold between February and April. A marked seasonal enrichment in the δ15N of N2 dissolved in the pore water correlated with CH4 flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin.4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems.5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 × 10−6 Tg CH4 year−1), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.

Fluxes of N2O and CH4 from soils of savannas and seasonally‐dry ecosystems

AbstractAim Savannas and seasonally‐dry ecosystems cover a significant part of the world's land surface. If undisturbed, these ecosystems might be expected to show a net uptake of methane (CH4) and a limited emission of nitrous oxide (N2O). Land management has the potential to change dramatically the characteristics and gas exchange of ecosystems. The present work investigates the contribution of warm climate seasonally‐dry ecosystems to the atmospheric concentration of nitrous oxide and methane, and analyses the impact of land‐use change on N2O and CH4 fluxes from the ecosystems in question.Location Flux data reviewed here were collected from the literature; they come from savannas and seasonally‐dry ecosystems in warm climatic regions, including South America, India, Australasia and Mediterranean areas.Methods Data on gas fluxes were collected from the literature. Two factors were considered as determinants of the variation in gas fluxes: land management and season. Land management was grouped into: (1) control, (2) ‘burned only’ and (3) managed ecosystems. The season was categorized as dry or wet. In order to avoid the possibility that the influence of soil properties on gas fluxes might confound any differences caused by land management, sites were grouped in homogeneous clusters on the basis of soil properties, using multivariate analyses. Inter‐ and intra‐cluster analysis of gas fluxes were performed, taking into account the effects of season, land management and main vegetation types.Results Soils were often acid and nutrient‐poor, with low water retention. N2O emissions were generally very low (median flux 0.32 mg N2O m−2 day−1), and no significant differences were observed between woodland savannas and managed savannas. The highest fluxes (up to 12.9 mg N2O m−2 day−1) were those on relatively fertile soils with high air‐filled porosity and water retention. The effect of season on N2O production was evident only when sites were separated in homogeneous groups on the basis of soil properties. CH4 fluxes varied over a wide range (−22.9 to 3.15 mg CH4 m−2 day−1, where the negative sign denotes removal of gas from the atmosphere), with an annual average daily flux of −0.48 ± 0.96 (SD) mg CH4 m−2 day−1 in undisturbed (control) sites. Land‐use change dramatically reduced this CH4 sink. Managed sites were weak sinks of CH4 in the dry season and became sources of CH4 in the wet season. This was particularly evident for pastures. Burning alone did not reduce soil net CH4 oxidation, but decreased N2O production.Main conclusions Despite the low potential for N2O production, both in natural and managed conditions, tropical seasonally‐dry ecosystems represent a significant source of N2O (4.4 Tg N2O year−1) on a global scale, as a consequence of the large area they occupy. The same environments represent a potential CH4 sink of 5.17 Tg CH4 year−1. However, assuming that c. 30% of the tropical land is converted to different uses, the sink would be reduced to 3.2 Tg CH4 year−1. The limited information on fluxes from Mediterranean ecosystems does not allow a meaningful scaling up.

A comment on the quantitative significance of aerobic methane release by plants.

A recent study by Keppler et al. (2006; Nature 439, 187-191) demonstrated CH4 emission from living and dead plant tissues under aerobic conditions. This work included some calculations to extrapolate the findings from the laboratory to the global scale and led various commentators to question the value of planting trees as a greenhouse mitigation option. The experimental work of Keppler et al. (2006) appears to be largely sound, although some concerns remain about the quantification of emission rates. However, whilst accepting their basic findings, we are critical of the method used for extrapolating results to a global scale. Using the same basic information, we present alternative calculations to estimate global aerobic plant CH4 emissions as 10-60 Mt CH4 year-1. This estimate is much smaller than the 62-236 Mt CH4 year-1 reported in the original study and can be more readily reconciled within the uncertainties in the established sources and sinks in the global CH4 budget. We also assessed their findings in terms of their possible relevance for planting trees as a greenhouse mitigation option. We conclude that consideration of aerobic CH4 emissions from plants would reduce the benefit of planting trees by between 0 and 4.4%. Hence, any offset from CH4 emission is small in comparison to the significant benefit from carbon sequestration. However, much critical information is still lacking about aerobic CH4 emission from plants. For example, we do not yet know the underlying mechanism for aerobic CH4 emission, how CH4 emissions change with light, temperature and the physiological state of leaves, whether emissions change over time under constant conditions, whether they are related to photosynthesis and how they relate to the chemical composition of biomass. Therefore, the present calculations must be seen as a preliminary attempt to assess the global significance from a basis of limited information and are likely to be revised as further information becomes available.

Development of region‐specific emission factors and estimation of methane emission from rice fields in the East, Southeast and South Asian countries

AbstractRice cultivation areas in East, Southeast and South Asia account for 89% of the world total, and field measurements of methane (CH4) emission from rice cultivation have been widely performed in this area. In this paper, we assembled most of the measurements and developed region‐specific CH4 emission factors. Efforts were made in order to regionalize rice fields by climate and soil properties, and to incorporate the effect of organic input and water regime on emission. Data on rice cultivation areas of 1995 were collected at subdivision level (province, state, prefecture, etc.). Total emission from these areas was estimated at 25.1 Tg CH4 year−1, of which 7.67 Tg was emitted from China and 5.88 Tg from India. Irrigated and rainfed rice fields contributed 70.4 and 27.5% to the total emission, respectively. Deepwater rice fields had a very small share. A high‐resolution and quality emission distribution map was constructed as the emission was directly estimated at province level and below that, a 30‐second land‐use dataset was used in order to translate the emission to grid format. As the rice cultivation area in the study region accounts for 89% of the world total, extrapolating the estimate to the global scale indicates a global emission of 28.2 Tg CH4 year−1.The estimate was compared with country reports made by local scientists. For some countries – such as Indonesia, Myanmar, Thailand, Vietnam, Japan, South Korea, Pakistan and the Philippines – the results of this estimate agree reasonably well with their country reports (CV < 15%). For some other countries – such as China, India and Bangladesh – there is relatively large disagreement between our estimate and their country reports. The reasons for the discrepancies were discussed.

Human impacts on methane emission from mangrove ecosystems in India

This study deals with the emission of methane in relation to changing environmental conditions and human impact, in three mangrove ecosystems of south India. Time-varying fluxes of methane adopting the close chamber technique were used to estimate CH4 emission from an unpolluted site (Pichavaram mangroves) and two polluted sites viz. (1) Ennore Creek mangroves (affected by fertilizer effluents and crude oil discharges) and (2) Adyar estuary mangroves (affected by the discharges of organic and industrial wastes), covering monthly and seasonal variations. The results indicate annual average CH4 emissions of 7.4, 5.02 and 15.4 mg m−2 h−1 from the sediment–water interface of the Pichavaram, Ennore Creek and Adyar estuary respectively. Emission characteristics obtained at Pichavaram mangroves represent a natural variability with changing physico-chemical factors, whereas the emission characteristics at Ennore Creek and Adyar estuary mangroves show anthropogenic influence. Several environmental factors such as oxygen availability, organic matter, soil physical and chemical properties, in addition to human-mediated interventions have been identified as influencing emission rates in the mangrove ecosystems. Preliminary CH4 emission estimates for the mangrove ecosystems along the Indian sub- continent and the tropical and subtropical coastline of the world by linear extrapolation based on surface area range from 0.05 to 0.37 and 2.8 to 19.25 Tg CH4 year−1 respectively. Our results also highlight the impact of human activities on future emission of methane from the mangrove ecosystems.

Effects of nitrogen input and grazing on methane fluxes of extensively and intensively managed grasslands in the Netherlands

Generally, grasslands are considered as sinks for atmospheric CH4, and N input as a factor which reduces CH4 uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH4 emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300–500 kg N ha–1 year–1. Net CH4 emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3–9 years, on net CH4 emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH4 production. The highest atmospheric CH4 uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH4 uptake of 1.1 kg CH4 ha–1 year–1. This is assumed to be the upper limit of CH4 uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH4, with an estimated CH4 uptake of 0.5 Gg CH4 year–1. At the current rates of total N input, the overall effect of N fertilisation on net CH4 emissions from grasslands is thought to be small or negligible.