Factors influencing methane emission from peat soils: Comparison of tropical and temperate wetlands

Methane (CH₄) emissions from peat soils in tropical and temperate wetlands were compared. Annual CH₄ emission rates in Ozegahara, the largest wetland on Honshu main island, Japan, were higher than in drained forest wetland areas examined in Indonesia. Methane emissions from the lowland paddy fields examined in Indonesia were higher than those of peaty paddy fields in Japan. There was generally a positive correlation (r² = 0.09; P < 0.1) between CH₄ emissions and CH₄ production activities in wetland soils. In Ozegahara, there was a positive relationship (r² = 0.80; P < 0.01) between CH₄ production activities and soil pH, but there was no such relationship in Indonesia. The range of soil pH in Ozegahara was 5–7, while pH values in the Indonesian sites were lower than 5. There was a positive response of CH₄ emission with respect to groundwater level in all of these areas. In Indonesia, land-use change from swamp and drained forest to cassava or coconut field lowered groundwater levels and decreased CH₄ emission, while change to lowland paddy raised the groundwater level and increased CH₄ emission. Addition of acetate generally inhibited CH₄ production during the early period (until 2 weeks) of incubation, then enhanced it afterward in both Ozegahara and Indonesian wetland soils. Addition of hydrogen mostly enhanced CH₄ production. From the results of this study, CH₄ fluxes from peat soil to the atmosphere were positively correlated with CH₄ production activities, and CH₄ production activity in peat soil was regulated by soil pH, while land-use change from wetland to upland crop lowered groundwater level and thus reduced CH₄ production and enhanced CH₄ oxidation.     

Greenhouse gas emissions from tropical peatlands of Kalimantan,Indonesia

Greenhouse gas emissions were measured from tropical peatlands of Kalimantan, Indonesia. The effect of hydrological zone and land-use on the emission of N₂O, CH₄ and CO₂ were examined. Temporal and annual N₂O, CH₄ and CO₂ were then measured. The results showed that the emissions of these gases were strongly affected by land-use and hydrological zone. The emissions exhibited seasonal changes. Annual emission of N₂O was the highest (nearly 1.4 g N m^–2y^–1) from site A-1 (secondary forest), while there was no signi.cant difference in annual N₂O emission from site A-2 (paddy field) and site A-3 (rice-soybean rotation field). Multiplying the areas of forest and non-forest in Kalimantan with the emission of N₂O from corresponding land-uses, the annual N₂O emissions from peat forest and peat non-forest of Kalimantan were estimated as 0.046 and 0.004 Tg N y^–1, respectively. The emissions of CH₄ from paddy field and non-paddy field were estimated similarly as 0.14 and 0.21 Tg C y^–1, respectively. Total annual CO₂ emission was estimated to be 182 Tg C y^–1. Peatlands of Kalimantan, Indonesia, contributed less than 0.3 of the total global N₂O, CO₂ or CH₄ emission, indicating that the gaseous losses of soil N and C from the study area to the atmosphere were small.     

Spatial patterns of greenhouse gas emission in a tropical rainforest in Indonesia

Spatial patterns of CO₂, CH₄, and N₂O flux were analyzed in the soil of a primary forest in Sumatra, Indonesia. The fluxes were measured at 3-m intervals on a sampling grid of 8 rows by 10 columns, with fluxes found to be below the minimum detection level at 12 points for CH₄ and 29 points for N₂O. All three gas fluxes distributed log-normally. The means and standard deviations of CO₂ and CH₄ fluxes calculated by the maximum likelihood method were 3.68 ± 1.32 g C m^–2 d^–1 and 0.79 ± 0.60 mg C m^–2 d^–1, respectively. The mean and standard deviation of N₂O fluxes using a maximum likelihood estimator for the censored data set was 2.99 ± 3.26 g N m^–2 h^–1. The spatial dependency of CH₄ fluxes was not detected in 3-m intervals, while weak spatial dependency was observed in CO₂ and N₂O fluxes. The coefficients of variation of CH₄ and N₂O were higher than that of CO₂. Some hot spots where high levels of CH₄ and N₂O were generated in the studied field may increase the variability of these gases. The resulting patterns of variability suggest that sampling distances of >10 m and > 20 m are required to obtain statistically independent samples for CO₂ and N₂O flux in the studied field, respectively. But because of weak or no spatial dependency of each flux, a sampling distance of more than 10 m intervals is enough to prevent a significant problem of autocorrelation for each flux measurement.     

Effects of rice cultivars on methane fluxes in a paddy soil

CH₄ emission and its relevant processes involved (i.e. CH₄ production, rhizospheric CH₄ oxidation and plant-mediated CH₄ transport) were studied simultaneously to comprehensively understand how rice cultivars (Yanxuan, 72031, and 9516) at growth stages (early and late tillering, panicle initiation, ripening, and harvest stage) affect CH₄ emission in a paddy soil. Over the entire rice-growing season, Yanxuan had the highest CH₄ emission flux with 5.98 g CH₄ m^–2 h^–1 followed by 72031 (4.48 g CH₄ m^–2 h^–1) and 9516 (3.41 g CH₄ m^–2 h^–1). The highest CH₄ production rate of paddy soils planted to Yanxuan was observed with 18.0 g CH₄ kg_{{ (d.w.soil)}} h^–1 followed by the soil planted to 9516 (17.5 g CH₄ kg_{{ (d.w.soil)}} h^–1). For each cultivar, both rhizospheric CH₄ oxidation ability and plant-mediated CH₄ transport efficiency varied widely with a range of 9.81–76.8% and 15.5–80.5% over the duration of crop growth, respectively. Multiple regression analyses showed that CH₄ emission flux was positively related with CH₄ production rate and rice plant-mediated CH₄ transport efficiency, but negatively with rhizospheric CH₄ oxidation (R ²=0.425 for Yanxuan, P<0.01; R ²=0.426 for 72031, P<0.01; R ²=0.564 for 9516, P<0.01). The contribution of rice plants to CH₄ production seems to be more important than to rhizospheric CH₄ oxidation and plant-mediated transport in impact of rice plants on CH₄ emission.     

Fluxes of CO2 and CH4 in soil profiles of a mountainous watershed of Nepal as influenced by land use, temperature, moisture and substrate addition

Effects of land use, moisture, temperature and substrate on production of CO₂ and consumption of CH₄ were measured in a series of laboratory incubation experiments on bulk soil samples from 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths under four dominant land uses [forest, grazing land, irrigated rice on level terraces (Khet), and upland maize–millet on sloping terraces (Bari)] of Mardi watershed (area=144 km²), Nepal. In addition, baseline physical and chemical properties of these soils were measured. The production of CO₂-C day^–1 per unit soil organic carbon (SOC) content in topsoil was lowest in grazing land, indicating a possibility of higher C sequestration with this land use than with other land uses. There was a decreasing trend of CO₂ emission with soil depth in all land uses, as was also the case with the SOC content. The CO₂ emission was increased by 90, 58, 27 and 23% for Bari, Khet, grazing and forest soil, respectively, with increase in moisture level from 40 to 60% (w/w). The CO₂ release from forest soil went up from 15 to 50 g CO₂ g^–1 dry soil with increase in temperature from 5 to 15 °C and it further increased to 67 g CO₂ g^–1 at 20 °C with estimated Q ₁₀ values of 3.4 and 1.8, respectively. Significantly higher amounts of CO₂ were emitted from all the land use types upon addition of glucose to the soil, indicating high potential of microbial activity. Consumption of CH₄ was more rapid in the soil from 10 to 20 cm depth for all the land use types. There was a 89% increase in CH₄ consumption in forest soil with increase in moisture level from 40 to 60%, while it was decreased by 38, 73, and 40% for Khet, Bari and grazing soil, respectively. Addition of (NH₄)₂SO₄ inhibited CH₄ oxidation in soils of all land uses, indicating a negative effect of N fertiliser input on CH₄ uptake in soil.     

Estimation of Regional Methane Emission from Rice Fields Using Simple Atmospheric Diffusion Models

Two atmospheric diffusion models, the box model ad the ATDL (Atmospheric Turbulent and Diffusion Laboratory) model, were used to calculate regional methane (CH₄) emissions of rice fields in the Beijing area. Compared with conventional closed chamber measurements, the box model overestimated CH₄ emission because of meteorological conditions--the ground inverse layer was not favorable for the application of the model during the rice-growing season. The ATDL model, on the other hand, handled this unfavorable meteorological condition and gave reasonable CH₄ emission estimates (about 6.1–8.5 mg m^–2 h^–1) close to conventional measurements (about 0.3–14.3 mg m^–2 h^–1) in June, a period generally characterized by significant CH₄ emission from rice fields. In September, CH₄ emission as measured with closed chambers was negligible (about 0–0.3 mg m^–2 h^–1), but the ATDL model still calculated it to be about 2.8–5.3 mg m^–2 h^–1, albeit at a low level and considerably below the June emission level. This discrepancy cannot be explained at present and needs further stuy. Most likely causes are measurement artifacts and/or the presence of minor local CH₄ sources (ditches, field depressions) in the study area. The application of atmospheric diffusion models for regional CH₄ emission estimation depends greatly on meteorological conditions. Moreover, the models tend to give much more reliable results during periods of rather high CH₄ emission. This coincides with the time that such regional CH₄ emission estimates are most valuable. The atmospheric diffusion models complement the closed chamber method by providing integrated CH₄ emission estimates from 1–100-km² rice areas. Detailed information about agricultural management of rice fields and other potential CH₄ sources within the study region are necessary to better understand the integrated regional emission estimates.     

Quantitative dependence of methane emission on soil properties

To identify the key soil parameters influencing CH₄ emission from rice paddies, an outdoor pot experiment with a total of 18 paddy soils was conducted in Nanjing Agricultural University during the 2000 rice growing season. The seasonal average rate of CH₄ emission for all 18 soils was 6.42±2.70 mg m^–2 h^–1, with a range of 1.96 to 11.06 mg m^–2 h^–1. Correlation analysis indicated that the seasonal average of CH₄ emission was positively dependent on soil sand content and negatively on total N as well as NH₄ ⁺-N determined before rice transplanting. Copper content of soils had a significant negative impact on CH₄ emission. No clear relationship existed between CH₄ emission and soil carbon content. In addition, soil type cannot explain the variability in CH₄ emission. Soil parameters influencing CH₄ emission were different as rice growth and development proceeded. A further investigation suggested that the seasonal average rate of CH₄ emission could be quantitatively determined by a linear combination of soil NH₄ ⁺-N, available copper, the ratio of available to total sulphur, and the ratio of available to total iron. Moreover, the average rates of CH₄ emission in the vegetative, reproductive and ripening stages could be also respectively described by a linear combination of different soil variables.     

Refinement in methodologies for methane budget estimation from rice paddies

To reduce the involved uncertainties in the methane budget estimation from rice paddy fields, the methodologies of methane budget estimation have been revised mainly on the basis of measurements undertaken in the Methane Asia Campaign (MAC-98). Studies from other continuous measurements of methane emission from rice paddy fields over last few years in other Asian countries were also used. The Asian Development Bank (ADB) sponsored Methane Asia Campaign (MAC-98) in which India, China, Indonesia, Philippines, Vietnam and Thailand participated during 1998–99.The resulting CH₄ measurements have shown that apart from water management, soil organic carbon also plays a significant role in determination of methane emission factors from rice paddy fields. The available data from participating countries reveal that paddy soils can be broadly classified into low soil organic carbon (<0.7%C) and high soil organic carbon (>0.7% C) classes which show average methane emission factors of 12 (5–29) and 36 (22–57) g m^–2 respectively for continuously flooded (CF) fields without organic amendments compared to the IPCC–96 emission factor of 20 g m^–2. Similarly for irrigated paddy fields with intermittently flooded multiple aeration (IF-MA) without organic amendments, the MAC-98 gives average emission factors of 2 (0.06–3) and 6 (0.6–24) g m^–2, respectively, for low and high organic carbon soils compared to IPCC–96 emission factor of 4 (0–10) g m^–2. Incorporation of soil organic carbon along with classification based on water management and organic amendments in the estimation of CH₄ emissions from rice paddy fields yields more characteristic emission factors for low and high organic carbon soils and is, therefore, capable of reducing uncertainties.     

Fluxes of NO and N2O from temperate forest soils: impact of forest type, N deposition and of liming on the NO and N2O emissions

Annual cycles of NO, NO₂ and N₂O emission rates from soil were determined with high temporal resolution at a spruce (control and limed plot) and beech forest site (Höglwald) in Southern Germany (Bavaria) by use of fully automated measuring systems. The fully automated measuring system used for the determination of NO and NO₂ flux rates is described in detail. In addition, NO, NO₂ and N₂O emission rates from soils of different pine forest ecosystems of Northeastern Germany (Brandenburg) were determined during 2 measuring campaigns in 1995. Mean monthly NO and N₂O emission rates (July 1994–June 1995) of the untreated spruce plot at the Höglwald site were in the range of 20–130 µg NO-N m^-2 h^-1 and 3.5–16.4 µg N₂O-N m^-2 h^-1, respectively. Generally, NO emission exceeded N₂O emission. Liming of a spruce plot resulted in a reduction of NO emission rates (monthly means: 15–140 µg NO-N m^-2 h^-1) by 25-30% as compared to the control spruce plot. On the other hand, liming of a spruce plot significantly enhanced over the entire observation period N₂O emission rates (monthly means: 6.2–22.1 µg N₂O-N m^-2 h^-1). Contrary to the spruce stand, mean monthly N₂O emission rates from soil of the beech plot (range: 7.9–102 µg N₂O-N m^-2 h^-1) were generally significantly higher than NO emission rates (range: 6.1–47.0 µg NO-N m^-2 h^-1). Results obtained from measuring campaigns in three different pine forest ecosystems revealed mean N₂O emission rates between 6.0 and 53.0 µg N₂O-N m^-2 h^-1 and mean NO emission rates between 2.6 and 31.1 µg NO-N m^-2 h^-1. The NO and N₂O flux rates reported here for the different measuring sites are high compared to other reported fluxes from temperate forests. Ratios of NO/N₂O emission rates were >> 1 for the spruce control and limed plot of the Höglwald site and << 1 for the beech plot. The pine forest ecosystems showed ratios of NO/N₂O emission rates of 0.9 ± 0.4. These results indicate a strong differentiating impact of tree species on the ratio of NO to N₂O emitted from soil.     

Nitrous oxide emissions from paddy soil in three rice-based cropping systems in China

Rice-flooding fallow, rice-wheat, and double rice-wheat systems were adopted in pot experiment in an annual rotation to investigate the effects of cropping system on N₂O emission from rice-based cropping systems. The annual N₂O emission from the rice-wheat and the double rice-wheat cropping systems were 4.3 kg N ha^–1 and 3.9 kg N ha^–1, respectively, higher than that from rice-flooding fallow cropping system, 1.4 kg N ha^–1. The average N₂O flux was 115 and 118 g N m^–2 h^–1 for rice season in rice-wheat system and early rice season in double rice-wheat system, respectively, 68.6 and 35.3 g N m^–2 h^–1 for the late rice season in double rice-wheat system and rice season in rice-flooding fallow, respectively, and only 3.1–5.3 g N m^–2 h^–1 for winter wheat or flooding fallow season. Temporal variations of N₂O emission during rice growing seasons differed and high N₂O emission occurred when soil conditions changed from upland crop to flooded rice.     

Scientific basis for establishing country greenhouse gas estimates for rice-based agriculture: An Indian case study

A comprehensive scientific assessment of CH₄ budget estimation for Indian rice paddies, based on a decade of measurements in India, is presented. Indian paddy cultivation areas contain soils that have low to medium levels of soil organic carbon. The average seasonally integrated CH₄ flux (E _sif) values calculated from these measurements were 15.3 ± 2.6 g m^–2 for continuously flooded (CF), 6.9 ± 4.3 g m^–2 for intermittently flooded (IF) single aeration (SA) and 2.2 ± 1.5 g m^–2 for IF multiple aeration (MA) rice ecosystems. For CF and IF (MA) rice ecosystems having high soil organic carbon, without organic amendments, the CH₄ flux (E _sif) may be increased by 1.7 times relative to low soil organic carbon, whereas it may enhance by 5.3 for CF if amended organically. Organic amendment and high soil organic carbon paddy areas do not alter the methane budget estimates for India (3.6±1.4 TgY^–1) much, due to their small paddy harvested area. Methane estimated using average emission factors (E _sif) for all paddy water regimes, which include harvested areas having soils with high organic carbon and organic amendments, may give a budget of 5 TgY^–1 for India.     

Preliminary studies on N2O emission fluxes from upland soils and paddy soils in China

In this paper, we presented the preliminary results of N₂O fluxes from Chinese upland and rice paddy fields. The mean N₂O flux from upland fields of North China is 30.6 μg N₂O-N m^-2 h^-1; the average N₂O flux from Chinese rice paddy field is 39.5 μg N₂O-N m^-2 h^-1. The effects of cropping system, water management and application of N fertilizer and organic manure on N₂O emission from rice paddy field have also been presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Overwinter greenhouse gas fluxes in two contrasting agricultural habitats

Mid-day field fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) were measured during late winter/early spring in an arable field and an adjacent fallow in southern Germany. On the arable field, 2 dm high ridges, drawn as seed-beds for potato, were exposed to mild, partly diurnal freezing–thawing. Substantially elevated N₂O emission rates (6–750 µg N₂O-N m^–2 h^–1) were observed throughout the investigation period which coincided with freezing–thawing events in the surface soil (0–5 cm). Soil temperatures in the densely vegetated fallow were more isothermal due to an insulating snow/ice cover, resulting in much lower N₂O emission rates (0–57 µg N₂O-N m^–2 h^–1). CH₄ uptake rates were low in both habitats during soil frost (+2 to –7.5 µg CH₄-C m^–2 h^–1) but increased markedly in the fallow after spring thaw. Our data suggest that N₂O emission peaks may occur recurrently throughout the winter when soils are subjected to diurnal surface thawing. We concluded that microclimatic conditions strongly control N₂O winter loss, thus overriding ecosystem-level differences in off-season nutrient cycling. To further characterize winter-time nutrient cycling and habitat functioning in our sites, we determined NO₃ ^– and NH₄ ⁺ contents, fumigation-extractable carbon (C_mic) and nitrogen (N_mic) and enumerated protozoa and nematoda throughout the investigation period. C_mic and microbial C:N ratios in the fallow were higher in winter than during the rest of the year as indicated by a 2-year study, reflecting favorable conditions for microbial C assimilation at low temperatures in the absence of freeze–thaw perturbation. In the arable soil, C_mic contents were significantly reduced during soil freezing but recovered quickly upon warming of the soil. Dynamics of C_mic in the arable soil were paralleled by protozoan biomass and transient shifts in functional composition of the nematode community, indicating that microfaunal predation played an important role in nutrient cycling after freeze–thaw perturbation. Only minor microfaunal dynamics were observed in the climatically more stable fallow, essentially confirming the absence of perturbation at this site. Our findings provide strong evidence that overwinter N₂O formation is regulated by both the physical freeze–thaw susceptibility of the soil and the ecological functioning of the habitat.     

Assessment of CH4 and N2O fluxes in a Danish beech (Fagus sylvatica) forest and an adjacent N-fertilised barley (Hordeum vulgare) field: effects of sewage sludge amendments

Fluxes of CH₄ and N₂O were measured regularly in an agricultural field treated with 280 g m⁻² of sewage sludge. In a nearby beech forest N₂O and CH₄ fluxes were measured in a well-drained (dry) area and in a wet area adjacent to a drainage canal. We observed brief increases of both CH₄ and N₂O emissions immediately following soil applications of digested sewage sludge. Cumulated values for CH₄ emissions over the course of 328 days after sludge applications indicated a small net source in sludge treated plots (7.6 mg C m⁻²) whereas sludge-free soil constituted a small sink (-0.9 mg C m⁻²). The CH₄ emission amounted 0.01% of the sludge-C. Extrapolated to current rates of sludge applications in Danish agriculture this amounts to 0.1% of the total agricultural derived CH₄. Sludge applications did not affect cumulated fluxes of N₂O showing 312 mg N₂O–N m⁻² and 304 mg N m⁻² with and without sludge, respectively. Four months after the sludge applications a significant effect on CO₂ and NO emissions was still obvious in the field, the latter perhaps due to elevated nitrification. Nitrous oxide emission in the beech forest was about six times smaller (45 mg N m⁻²) than in the field and independent of drainage status. Methane oxidation was observed all-year round in the forest cumulating to -225 mg C m⁻² and -84 mg C m⁻² in dry and wet areas. In a model experiment with incubated soil cores, nitrogen amendment (NH₄Cl) and perturbation significantly reduced CH₄ oxidation in the forest soil, presumably as a result of increased nitrification activity. Sludge also induced net CH₄ production in the otherwise strong CH₄ oxidising forest soil. This emphasises the potential for CH₄ emissions from sewage sludge applications onto land. The study shows, however, that emissions of N₂O and CH₄ induced by sewage sludge in the field is of minor importance and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH₄ and N₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of soil water contents in the non-rice growth season on CH4 emission during the following rice-growing period

A pot experiment was carried out to investigate the effect of soil water content in the non-rice growth season (winter season) on CH₄ emission during the following rice-growing period. The results showed that CH₄ fluxes increased significantly with the increase of soil water content in the winter season, except air-dry water condition. The mean CH₄ fluxes of treatments with soil water contents in the winter of 3.89–5.37% (air-dry), 25–35%, 50–60%, 75–85% and 107% (flooded) of field water capacity (FWC) were 13.04, 4.04, 8.61, 13.26 and 20.47 mg m^–2 h^–1, respectively. Antecedent soil water contents also markedly affected temporal variation patterns of CH₄ fluxes and soil redox potential (E_h) during the rice-growing period. The higher soil water contents in the winter season were, the quicker soil E_h decreased, and the earlier CH₄ emission occurred after rice transplanting, except air-dry water condition. Though the seasonal mean CH₄ flux was significantly correlated with the seasonal mean soil E_h, the seasonal variation of CH₄ fluxes was not always significantly correlated with soil E_h. For the treatment flooded in the fallow season, there was no significant correlation between CH₄ flux and soil E_h, but there was significant correlation between CH₄ flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil E_h, not soil temperature that significantly affected the temporal variation of CH₄ emissions. Soil water contents in the fallow season significantly influenced concentrations of soil labile organic carbon (including undecomposed plant debris), active Fe and Mn immediately before rice transplanting. The mean CH₄ fluxes during rice-growing period were significantly correlated with soil labile organic carbon contents (positively) and contents of soil active Fe and Mn (negatively).     

The variation of greenhouse gas emissions from soils of various land-use/cover types in Jambi province,Indonesia

We measured fluxes of three greenhouse gases (N₂O, CO₂O and CH₄) from soils of six different land-use types at 27 temporary field sites in Jambi Province, Sumatra, Indonesia. Study sites included natural and logged-over forests; rubber plantation; oil palm plantation; cinnamon plantation; and grassland field. The ranges of N₂O, CO₂ and CH₄ fluxes were 0.13–55.8 gN m-2h-1; 1.38–5.16 g C m-2d-1; –1.27–1.18 mg C m-2d-1, respectively. The averages of N₂O, CO₂ and CH₄ fluxes at 27 sites were 9.4 gN m-2h-1,3.65 g C m-2d-1, –0.45 mg C m-2d-1, respectively. The values of CO₂ and CH₄ fluxes were comparable with those in the reports regarding other humid tropical forests, while the N₂O flux was relatively lower than those of previous reports. The N₂O fluxes in each soil type were correlated with the nitrification rates of soils of 0–5 cm depth. In Andisols, the ratio of the N₂O emission rate to the nitrification rate was possibly smaller than that of the other soil types. There was no clear relationship between N₂O flux and the soil water condition, such as water-filled pore space. Seventeen percent of CH₄ fluxes were positive; according to these positive fluxes, we did not find a good correlation between CH₄ uptake rate and soil properties. Although we performed a chronosequence analysis to produce some hypotheses about the effect of land-use change by a limited amount of sampling at one point in time, further tests are required for the future.     

A kinetic study of methane and carbon dioxide interconversion over 0.5%Pt/SrTiO3 catalysts

The kinetics of interconversion of methane with carbon dioxide was studied over a 0.5%Pt/SrTiO₃ solid catalyst in the temperature range 813–893 K and partial pressure range 0.083<P_CH4,P_CO2<0.667. The fitting of the experimental data for the rate of methane conversion, R_CH4, using the empirical equation R_CH4=k₁(P_CH4)^m(P_CO2)ⁿ showed that both reaction orders n and m are steady and obtain values equal to m ≈ 1 and n ≈ 0. The results are explained using Langmuir–Hinshelwood kinetics with the reactants adsorbed on distinct and discreet active sites of the solids, namely the methane is weakly adsorbed on the metallic phase and the carbon dioxide is strongly adsorbed on the oxidic phase of the catalyst. The apparent activation energy for the reforming of methane was estimated to be 123 kJ mol⁻¹.The rate of conversion of the carbon dioxide, R_CO2, was also fitted using a similar empirical equation R_CO2=k₂(P_CH4)^m(P_CO2)ⁿ. The results indicate that there is a positive but variable dependence on both reaction orders which increases in the temperature range 813–893 K from m ≈ 0.0 to m ≈ 0.30 and from n ≈ 0.3 to n ≈ 0.6. This variation is attributed to the variable participation of the rate of the reverse water gas shift reaction, R_rwgs, to the overall rate R_CO2 of CO₂ conversion. The dependence of R_rwgs on the partial pressure of CO₂ appears similar to that of R_CH4 on the same reactant but shows strong inhibition by the reaction products. The results are discussed using Langmuir–Hinshelwood kinetics with the reactants and products adsorbed competitively on similar active sites of the catalyst.     

The Electro-Reduction of Carbon Dioxide in a Continuous Reactor

This paper reports an investigation into the electro-reduction of CO₂ in a laboratory bench-scale continuous reactor with co-current flow of reactant gas and catholyte liquid through a flow-by 3D cathode of 30^# mesh tinned-copper. Factorial and parametric experiments were carried out in this apparatus with the variables: current (1–8 A), gas phase CO₂ concentration (16–100 vol%) and operating time (10–180 min), using a cathode feed of [CO₂ + N₂] gas and 0.45 m KHCO₃(aq) with an anolyte feed of 1 m KOH(aq), in operation near ambient conditions (ca. 115 kPa(abs), 300 K). The primary and secondary reactions here were respectively the reduction of CO₂ to formate (HCOO⁻) and of water to hydrogen, while up to ca. 5% of the current went to production of CO, CH₄ and C₂H₄. The current efficiency for formate depended on the current density and CO₂ pressure, coupled with the hydrogen over-potential plus mass transfer capacity of the cathode, and decreased with operating time, as tin was lost from the cathode surface. For superficial current densities ranging from 0.22 to 1.78 kA m⁻², the measured values of the performance indicators are: current efficiency for HCOO⁻ = 86–13%, reactor voltage = 3–6 Volt, specific energy for HCOO⁻ = 300–1300 kWh kmol⁻¹, space-time yield of HCOO⁻ = 2 × 10⁻⁴–6 × 10⁻⁴ kmol m⁻³ s⁻¹, conversion of CO₂ = 20–80% and yield of organic products from CO₂ = 6–17%.     

Fluxes of methane and nitrous oxide in water-saving rice production in north China

Lowland rice production is currently facing serious water shortages in numerous Asian countries. In the North China Plain water limitations are severe. Water-saving rice production techniques are therefore increasingly searched for. Here we present the first study of trace gas emissions from a water-saving rice production system where soils are mulched and are kept close to field capacity in order to compare their contribution to global warming with traditional paddy rice. In a two-year field experiment close to Beijing, CH₄ and N₂O fluxes were monitored in two forms of the Ground Cover Rice Production System (GCRPS) and in traditional paddy fields using closed chambers. With paddy rice the observed CH₄ emissions were very low, about 0.3 g CH₄ m⁻² a⁻¹ in 2001 and about 1 g CH₄ m⁻² a⁻¹ in 2002. In GCRPS, the CH₄ emissions were negligible. N₂O fluxes in GCRPS were similar, 0.5 to 0.6 g N₂O m⁻² a⁻¹ in 2001 and 2002, and emission peaks mainly followed fertilizer applications. In paddy rice, N₂O fluxes were unexpectedly low throughout the year 2001 (0.03 g N₂O m⁻² a⁻¹), and in 2002 larger emissions occurred during the drainage period. So with 0.4 g N₂O m⁻² a⁻¹ the cumulative flux was similar to emissions in GCRPS. Total CO₂ equivalent fluxes calculated according to IPCC methodology were tenfold higher in GCRPS compared to paddy in 2001. In 2002, fluxes from both systems were similar with 175 and 141 g CO₂ equivalents m⁻² a⁻¹ from GCRPS and paddy. Burkhard Sattelmacher deceased.     

The Effects of Cultural Practices on Methane Emission from Rice Fields

A field experiment was conducted in a clayey soil to determine the effects of cultural practices on methane (CH₄) emissions from rice fields. The factors evaluated were a) direct seeding on dry vs wet soil, b) age of transplanted seedlings (8 d old and 30 d old), and c) fall vs spring plowing. Methane emissions were measured weekly throughout the rice-growing season using a closed static chamber technique. Transplanted 8-d-old seedlings showed the highest emission of 42.4 g CH₄ m^–2 season^–1, followed by transplanted 30-d-seedlings (40.3 g CH₄ m^–2 season^–1), and direct seeding on wet soil (37.1 g CH₄ m^–2 season^–1). Direct seeding on dry soil registered the least emission of 26.9 g CH₄ m^–2 season^–1. Thus transplanting 30-d-old seedlings, direct seeding on wet soil, and direct seeding on dry soil reduced CH₄ emission by 5%, 13%, and 37%, respectively, when compared with transplanting 8-d-old seedlings. Methane emission under spring plowing was 42.0 g CH₄ m^–2 season^–1 and that under fall plowing was 31.3 g CH₄ m^–2 seasons^–1. The 26% lower emission in the field plowed in spring was caused by degradation of organic matter over the winter.