Methane emissions from a landfill measured by eddy correlation using a fast response diode laser sensor

We describe a fast response methane sensor based on the absorption of radiation generated with a near-infrared InGaAsP diode laser. The sensor uses an open path absorption region 0.5 m long; multiple pass optics provide an optical path of 50 m. High frequency wavelength modulation methods give stable signals with detection sensitivity (S/N=1, 1 Hz bandwidth) for methane of 65 ppb at atmospheric pressure and room temperature. Improvements in the optical stability are expected to lower the current detection limit. We used the new sensor to measure, by eddy correlation, the CH₄ flux from a clay-capped sanitary landfill. Simultaneously we measured the flux of CO₂ and H₂O. From seven half-hourly periods of data collected after a rainstorm on November 23, 1991, the average flux of CH₄ was 17 mmol m^–2 hr^–1 (6400 mg CH₄ m^–2 d^–1) with a coefficient of variation of 25%. This measurement may underrepresent the flux by 15% due to roll-off of the sensor response at high frequency. The landfill was also a source of CO₂ with an average flux of 8.1 mmol m^–2 hr^–1 (8550 mg CO₂ m^–2 d^–1) and a coefficient of variation of 26%. A spectral analysis of the data collected from the CH₄, CO₂, and H₂O sensors showed a strong similarity in the turbulent transfer mechanisms.     

The Surface Energy Budget and Its Impact on the Freeze-thaw Processes of Active Layer in Permafrost Regions of the Qinghai-Tibetan Plateau

The surface energy budget is closely related to freeze-thaw processes and is also a key issue for land surface process research in permafrost regions.In this study,in situ data collected from 2005 to 2015 at the Tanggula site were used to analyze surface energy regimes,the interaction between surface energy budget and freeze-thaw processes.The results confirmed that surface energy flux in the permafrost region of the Qinghai-Tibetan Plateau exhibited obvious seasonal variations.Annual average net radiation(R_n)for 2010 was 86.5 W m^-2,with the largest being in July and smallest in November.Surface soil heat flux(G₀)was positive during warm seasons but negative in cold seasons with annual average value of 2.7 W m^-2.Variations in R_n and G₀ were closely related to freeze-thaw processes.Sensible heat flux(H)was the main energy budget component during cold seasons,whereas latent heat flux(LE)dominated surface energy distribution in warm seasons.Freeze-thaw processes,snow cover,precipitation,and surface conditions were important influence factors for surface energy flux.Albedo was strongly dependent on soil moisture content and ground surface state,increasing significantly when land surface was covered with deep snow,and exhibited negative correlation with surface soil moisture content.Energy variation was significantly related to active layer thaw depth.Soil heat balance coefficient K was>1 during the investigation time period,indicating the permafrost in the Tanggula area tended to degrade.     

Carbon dioxide seasonality in dynamically ventilated caves: the role of advective fluxes

The seasonality in cave CO₂ levels was studied based on (1) a new data set from the dynamically ventilated Comblain-au-Pont Cave (Dinant Karst Basin, Belgium), (2) archive data from Moravian Karst caves, and (3) published data from caves worldwide. A simplified dynamic model was proposed for testing the effect of all conceivable CO₂ fluxes on cave CO₂ levels. Considering generally accepted fluxes, i.e., the direct diffusive flux from soils/epikarst, the indirect flux derived from dripwater degassing, and the input/output fluxes linked to cave ventilation, gives the cave CO₂ level maxima of 1.9 × 10⁻² mol m⁻³ (i.e., ∼ 440 ppmv), which only slightly exceed external values. This indicates that an additional input CO₂ flux is necessary for reaching usual cave CO₂ level maxima. The modeling indicates that the additional flux could be a convective advective CO₂ flux from soil/epikarst driven by airflow (cave ventilation) and enhanced soil/epikarstic CO₂ concentrations. Such flux reaching up to 170 mol s⁻¹ is capable of providing the cave CO₂ level maxima up to 3 × 10⁻² mol m⁻³ (70,000 ppmv). This value corresponds to the maxima known from caves worldwide. Based on cave geometry, three types of dynamic caves were distinguished: (1) the caves with the advective CO₂ flux from soil/epikarst at downward airflow ventilation mode, (2) the caves with the advective soil/epikarstic flux at upward airflow ventilation mode, and (3) the caves without any soil/epikarstic advective flux. In addition to CO₂ seasonality, the model explains both the short-term and seasonal variations in δ¹³C in cave air CO₂.

     

Continuous measurement of methane flux over a larch forest using a relaxed eddy accumulation method

We measured the methane flux of a forest canopy throughout a year using a relaxed eddy accumulation (REA) method. This sampling system was carefully validated against heat and CO₂ fluxes measured by the eddy covariance method. Although the sampling system was robust, there were large uncertainties in the measured methane fluxes because of the limited precision of the methane gas analyzer. Based on the spectral characteristics of signals from the methane analyzer and the diurnal variations in the standard deviation of the vertical wind velocity, we found the daytime and nighttime precision of half-hourly methane flux measurements to be approximately 1.2 and 0.7?μg?CH₄?m^?2?s^?1, respectively. Additional uncertainties caused by the dilution effect were estimated to affect the accuracy by as much as 0.21?μg?CH₄?m^?2?s^?1 on a half-hourly basis. Diurnal and seasonal variations were observed in the measured fluxes. The biological emission from plant leaves was not observed in our studies, and thus could be negligible at the canopy-scale exchange. The annual methane sink was 835?±?175?mg?CH₄?m^?2?year^?1 (8.35?kg?CH₄?ha^?1?year^?1), which was comparable to the flux range of 379–2,478?mg?CH₄?m^?2?year^?1 previously measured in other Japanese forest soils. This study indicated that the REA method could be a promising technique to measure canopy scale methane fluxes over forests, but further improvement of precision of the analyzer will be required.     

Field measurements of NO and NO2 emissions from fertilized and unfertilized soils

Field measurements of NO and NO₂ emissions from soils have been performed in Finthen near Mainz (F.R.G.) and in Utrera near Seville (Spain). The applied method employed a flow box coupled with a chemiluminescent NO _x detector allowing the determination of minimum flux rates of 2 g N m^-2 h^-1 for NO and 3 g m^-2 h^-1 for NO₂.The NO and NO₂ flux rates were found to be strongly dependent on soil surface temperatures and showed strong daily variations with maximum values during the early afternoon and minimum values during the early morning. Between the daily variation patterns of NO and NO₂, there was a time lag of about 2 h which seem to be due to the different physico-chemical properties of NO and NO₂. The apparent activation energy of NO emission calculated from the Arrhenius equation ranged between 44 and 103 kJ per mole. The NO and NO₂ emission rates were positively correlated with soil moisture in the upper soil layer.The measurements carried out in August in Finthen clearly indicate the establishment of NO and NO₂ equilibrium mixing ratios which appeared to be on the order of 20 ppbv for NO and 10 ppbv for NO₂. The soil acted as a net sink for ambient air NO and NO₂ mixing ratios higher than the equilibrium values and a net source for NO and NO₂ mixing ratios lower than the equilibrium values. This behaviour as well as the observation of equilibrium mixing ratios clearly indicate that NO and NO₂ are formed and destroyed concurrently in the soil.Average flux rates measured on bare unfertilized soils were about 10 g N m^-2 h^-1 for NO₂ and 8 g N m^-2 h^-1 for NO. The NO and NO₂ flux rates were significantly reduced on plant covered soil plots. In some cases, the flux rates of both gases became negative indicating that the vegetation may act as a sink for atmospheric NO and NO₂.Application of mineral fertilizers increased the NO and NO₂ emission rates. Highest emission rates were observed for urea followed by NH₄Cl, NH₄NO₃ and NaNO₃. The fertilizer loss rates ranged from 0.1% for NaNO₃ to 5.4% for urea. Vegetation cover substantially reduced the fertilizer loss rate.The total NO _x emission from soil is estimated to be 11 Tg N yr^-1. This figure is an upper limit and includes the emission of 7 Tg N yr^-1 from natural unfertilized soils, 2 Tg N yr^-1 from fertilized soils as well as 2 Tg N yr^-1 from animal excreta. Despite its speculative character, this estimation indicates that NO _x emission by soil is important for tropospheric chemistry especially in remote areas where the NO _x production by other sources is comparatively small.     

A national landfill methane budget for Sweden based on field measurements, and an evaluation of IPCC models

Seven Swedish landfills were investigated from 2001 to 2003. On each landfill, a measure of the total methane production was calculated from data on: (1) methane emissions (leakage); (2) methane oxidation and (3) from gas recovery.
Methane emissions were determined via a tracer gas (N₂O) release-based remote sensing method. N₂O and CH₄ were measured with an Fourier Transform infrared detector at a distance of more than 1 km downwind from the landfills. Methane oxidation in the landfill covers was measured with the stable carbon isotope method. The efficiency in gas recovery systems proved to be highly variable, but on an average, 51% of the produced landfill gas was captured.
A first-order decay model, based on four fractions (waste from households and parks, sludges and industrial waste), showed that the use of a degradable organic carbon fraction (DOCf) value of 0.54, in accordance with the default value for DOCf of 0.50 in the latest IPCC model, gave an emission estimate similar to the official national reports.     

Carbon dioxide exchange in a temperate grassland ecosystem

Carbon dioxide exchange was measured, using the eddy correlation technique, over a tallgrass prairie in northeastern Kansas, U.S.A., during a six-month period in 1987. The diurnal patterns of daytime and nocturnal CO₂ fluxes are presented on eight selected days. These days were distributed throughout most of the growing season and covered a wide range of meteorological and soil water conditions. The midday CO₂ flux reached a maximum of 1.3 mg m^-2 (ground area) s^-1 during early July and was near zero during the dry period in late July. The dependence of the daytime carbon dioxide exchange on pertinent controlling variables, particularly photosynthetically active radiation, vapor pressure deficit and soil water content is discussed. The nocturnal CO₂ flux (soil plus plant respiration) averaged -0.4 mg m^-2 (ground area) s^-1 during early July and was about -0.2 mg m^-2 s^-1 during the dry period.Published as Paper No. 9061, Journal Series, Agricultural Research Division, University of Nebraska-Lincoln, U.S.A.Research Associate and Professor, respectively.     

Impact of intensive fish farming on methane emission in a tropical hydropower reservoir

Fisheries and aquaculture are important sources of food for hundreds of millions of people around the world. World fish production is projected to increase by 15% in the next 10 years, reaching around 200 million tonnes per year. The main driver of this increase will be based on fish farming management in developing countries. In Brazil, fish farming is increasing due to the climate conditions and large supply of water resources, with the production system based on Nile tilapia (Oreochromis niloticus) farming in reservoirs. Inland waters like reservoirs are a natural source of methane (CH₄) to the atmosphere. However, knowledge of the impact from intensive fish production in net cages on CH₄ fluxes is not well known. This paper presents in situ measurements of CH₄ fluxes and dissolved CH₄ (DM) in the Furnas Hydroelectric Reservoir in order to evaluate the impact of fish farming on methane emissions. Measurements were taken in a control area without fish production and three areas with fish farming. The overall mean of diffusive methane flux (DMF) (5.9?±?4.5 mg CH₄ m^?2 day^?1) was significantly lower when compared to the overall mean of bubble methane flux (BMF) (552.9?±?1003.9 mg CH₄ m^?2 day^?1). The DMF and DM were significantly higher in the two areas with fish farming, whereas the BMF was not significantly different. The DMF and DM were correlated to depth and chlorophyll-a. However, the low production of BMF did not allow the comparison with the limnological parameters measured. This case study shows that CH₄ emissions are influenced more by reservoir characteristics than fish production. Further investigation is necessary to assess the impact of fish farming on the greenhouse gas emissions.     

填埋场甲烷排放因素分析及甲烷减排研究进展

垃圾填埋场甲烷排放是全球人为温室气体排放的重要来源,对于整个大气中温室气体增加引起的气候效应的影响不容忽视,是世界各国现代化进程中迫切需要解决的一个严重的社会公害问题.文章从填埋场甲烷产生的相关因素、垃圾处理现状和填埋场甲烷减排技术等方面对国内外研究现状做了总结.甲烷的产生受填埋场中的垃圾特性、含水率、温度、pH值、填埋时间、渗滤液含量和其他因素影响.当前的填埋场减排技术包括原位减排、资源化利用和末端控制等,填埋场可以从多方面共同作用实现减排目标.     

The Synergism between Methanogens and Methanotrophs and the Nature of their Contributions to the Seasonal Variation of Methane Fluxes in a Wetland: The Case of Dajiuhu Subalpine Peatland

Wetland ecosystems are the most important natural methane (CH₄) sources, whose fluxes periodically fluctuate. Methanogens (methane producers) and methanotrophs (methane consumers) are considered key factors affecting CH₄ fluxes in wetlands. However, the symbiotic relationship between methanogens and methanotrophs remains unclear. To help close this research gap, we collected and analyzed samples from four soil depths in the Dajiuhu subalpine peatland in January, April, July, and October 2019 and acquired seasonal methane flux data from an eddy covariance (EC) system, and investigated relationships. A phylogenetic molecular ecological networks (pMENs) analysis was used to identify keystone species and the seasonal variations of the co-occurrence patterns of methanogenic and methanotrophic communities. The results indicate that the seasonal variations of the interactions between methanogenic and methanotrophic communities contributed to CH₄ emissions in wetlands. The keystone species discerned by the network analysis also showed their importance in mediating CH₄ fluxes. Methane (CH₄) emissions in wetlands were lowest in spring; during this period, the most complex interactions between microbes were observed, with intense competition among methanogens while methanotrophs demonstrated better cooperation. Reverse patterns manifested themselves in summer when the highest CH₄ flux was observed. Methanoregula formicica was negatively correlated with CH₄ fluxes and occupied the largest ecological niches in the spring network. In contrast, both Methanocella arvoryzae and Methylocystaceae demonstrated positive correlations with CH₄ fluxes and were better adapted to the microbial community in the summer. In addition, soil temperature and nitrogen were regarded as significant environmental factors to CH₄ fluxes. This study was successful in explaining the seasonal patterns and microbial driving mechanisms of CH₄ emissions in wetlands.     

Production of N2O,CH4, and CO2 from soils in the tropical savanna during the dry season

Emissions of N₂O, CH₄, and CO₂ from soils at two sites in the tropical savanna of central Venezuela were determined during the dry season in February 1987. Measured arithmetic mean fluxes of N₂O, CH₄, and CO₂ from undisturbed soil plots to the atmosphere were 2.5×10⁹, 4.3×10¹⁰, and 3.0×10¹³ molecules cm^-2 s^-1, respectively. These fluxes were not significantly affected by burning the grass layer. Emissions of N₂O increased fourfold after simulated rainfall, suggesting that production of N₂O in savanna soils during the rainy season may be an important source for atmospheric N₂O. The CH₄ flux measurements indicate that these savanna soils were not a sink, but a small source, for atmospheric methane. Fluxes of CO₂ from savanna soils increased ninefold two hours after simulated rainfall, and remained three times higher than normal after 16 hours. More research is needed to clarify the significance of savannas in the global cycles of N₂O, CH₄, CO₂, and other trace gases, especially during the rainy season.     

Model-based estimates of water loss from “patches” of the understory mosaic of the Hartheim Scots pine plantation

Summary During the Hartheim Experiment (HartX) 1992 conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory and the forest floor by several methods. At the vegetation patch level, direct estimates were made with small weighing lysimeters, and water loss was scaled-up to the stand level based on vegetation patchtype distribution. At the leaf level, transpiration flux was determined with a CO₂/H₂O porometer for the dominant understory plant species,Brachypodium pinnatum, Carex alba, andCarex flacca. Measured leaf transpiration was scaled-up to patch level with a canopy light interception and leaf gas exchange model, and then to stand level as in the case of lysimeter data, but with further consideration of patchtype leaf area index (LAI). On two days, total understory latent heat flux was estimated by eddy correlation methods below the tree canopy.The understory vegetation was subdivided into five major patch-types which covered 62% of the ground area and resulted in a cumulative LAI of approx. 1.54 when averaged over total stand ground area and compared to the average tree canopy LAI of 2.8. The remaining 38% of ground area was unvegetated bare soil and/or covered by moss (mainly byScleropodium purum) or litter. The evapotranspiration from the understory and unvegetated areas equaled approx. 20% of total forest stand transpiration during the HartX period. The understory vegetation transpired about 0.4 mm d^–1 (13%) estimated over the period of May 13 to 21, whereas evaporation from moss and soil patches amounted 0.23 mm d^–1 (7.0%). On dry, sunny days, total water vapor flux below the tree canopy exceeded 0.66 mm d^–1. Using the transpiration rates derived from the GAS-FLUX model together with estimates of evaporation from moss and soil areas and a modified application of the Penman-Monteith equation, the average daily maximum conductance of the understory and the forest floor was 1.7 mm s^–1 as compared to 5.5 mm s^–1 for the tree canopy.With 6 Figures     

Application of stable isotope analysis for improved understanding of the methane budget: comparison of TROICA measurements with TM3 model simulations

Presented is a detailed comparison of CH₄ and δ¹³C–CH₄ measurements with simulations of the global transport model TM3. Experimental data were obtained during campaigns along the Trans-Siberian railroad in the framework of the TROICA project. Two summer (1999 and 2001) and one spring (2003) expeditions are evaluated. Model simulations include sensitivity tests to further investigate the isotopic composition of natural gas and emissions from Siberian wetlands. Comparison of the average mixing ratio of methane and its isotopic composition (δ¹³C) has been performed for different geographic zones, including the European part of Russia, Western Siberia and Central Siberia. Simulations are in reasonable agreement with the measurements for the European part of Russia and confirm a high contribution of natural gas to the observed methane levels. An increase of emission from bogs shifts the simulated methane isotopic composition closer to the observations. The relative importance of the Western Siberia emissions in current inventories is underestimated in comparison with other wetland regions in the former USSR. Simulated average mixing ratios are in a good agreement with the observations in Central Siberia, while ¹³C(CH₄) values tend to be higher than measured in all considered scenarios. These results point to a bias in the modeled source mixture over Russia, which could be repaired by shifting emissions from isotopically heavy methane sources (e.g. coal, oil or biomass burning) to light sources (e.g. wetlands, ruminants, waste treatment). Alternatively, the average isotopic signature of Siberian wetlands may be lighter than expected.     

CH4 emission from various rice fields in P.R. China

Summary The CH₄ emission rates from Chinese rice fields have been measured in five typical areas representing all of the five major rice culture regions in People's Republic of China (P.R. China). Four types of diurnal variations (afternoon peak, night peak, afternoon-night double peaks and random pattern) of CH₄ emission rates have been found. The first pattern was normally found in clear weather, the second and the third types were only found occasionally in particular place, while the fourth were found in cloudy or rainy weather. Due to the irregular pattern of the methane production observed in the morning-afternoon comparison experiment, the transport pathway influenced by certain factors, may be the major factor governing the diurnal variation of CH₄ emission. Seasonal variation patterns of CH₄ emission differ slightly with different field locations, where climate system, cropping system and other factors are different. Two and three emission peaks were generally found during single and early rice vegetation periods, with the peak magnitude and time of appearance differing to small degree in individual sites. A decreasing trend of seasonal variation was always observed in late rice season. A combination of seasonal change of transport efficiency and that of CH₄ production rate in the paddy soil explains well the CH₄ emission. The role of rice plant in transporting CH₄ varied over a large range in different rice growing stages. The reasons for internnual changes of CH₄ flux are not yet clear.Great spatial variation of the CH₄ emission has been found, which can be attributed to the differences in soil type and soil properties, local climate condition, rice species, fertilizer and water treatment. Experiments showed that while the application of some mineral fertilizers will reduce the CH₄ emission and CH₄ production in the soil, the application of organic manure will enhance CH₄ emission and CH₄ production in the soil. Any measures which can get off easily decomposed carbon from organic manure may reduce C supply for CH₄ production, and hence reduce CH₄ emission. Fermented sludges from biogas generators and farmyard-stored manure seem to be promising. In some parts of China, separate application of the organic and mineral manure instead of mixed application could be another option. Frequent Scientific drainage and ridge cultivation, which are often used water management techniques in Chinese rice agriculture, have been proved in the experiments to be a very efficient mitigation measures to reduce CH₄ emission from rice fields.By summarizing the present available data, China's rice fields contribute about 13.3 Tg yr^–1 (11.4–15.2) CH₄ to the atmosphere. The total methane emission from global rice fields can be estimated 33–60 Tg yr^–1, much less than the estimates made before.If we extrapolate the measured data in China with a consideration of measured data in other Asian country, the total global emission of CH₄ from rice fields are estimated to be about 35–60 Tg yr^–1 With 2 Figures     

Measurement of methane flux over an evergreen coniferous forest canopy using a relaxed eddy accumulation system with tuneable diode laser spectroscopy detection

Very few studies have conducted long-term observations of methane (CH₄) flux over forest canopies. In this study, we continuously measured CH₄ fluxes over an evergreen coniferous (Japanese cypress) forest canopy throughout 1?year, using a micrometeorological relaxed eddy accumulation (REA) system with tuneable diode laser spectroscopy (TDLS) detection. The Japanese cypress forest, which is a common forest type in warm-temperate Asian monsoon regions with a wet summer, switched seasonally between a sink and source of CH₄ probably because of competition by methanogens and methanotrophs, which are both influenced by soil conditions (e.g., soil temperature and soil moisture). At hourly to daily timescales, the CH₄ fluxes were sensitive to rainfall, probably because CH₄ emission increased and/or absorption decreased during and after rainfall. The observed canopy-scale fluxes showed complex behaviours beyond those expected from previous plot-scale measurements and the CH₄ fluxes changed from sink to source and vice versa.     

Net carbon dioxide exchange in canopies of burned and unburned tallgrass prairie

Summary Net carbon dioxide exchange (NCE) rates were measured in a tallgrass prairie, a grassland with high productivity, to determine photosynthetic rates of the canopy. Canopy measurements were made in large, plexiglass chambers (1.21 m long; 0.91 m wide; 1.40 m tall) placed on burned and unburned areas of the prairie. The NCE rates of the canopy were compared with those of individual leaves ofAndropogon gerardii Vitman (big bluestem). In addition, CO₂ flux from the soil was quantified and compared with net photosynthetic flux. The canopy NCE rates were generally lower than those made on individual leaves. In mid-summer (11 July 1987), the maximum canopy NCE rates were 55% and 64% of those measured on individual leaves in burned and unburned treatments, respectively. Canopy NCE rates were lower than individual-leaf NCE rates for two reasons. First, the individualleaf measurements were made on young, unshaded, healthy leaves, while the canopy measurements were made on all types of leaves including senescing, shaded, and damaged leaves. Second, soil CO₂ flux into the chambers lowered NCE values. The CO₂ flux from the soil ranged from 7.2% to 28.4% of the total NCE. One needs to add soil CO₂ flux rates to the measured canopy NCE rates to obtain canopy NCE rates closer to individual-leaf NCE rates. Soil CO₂ flux decreased when conditions became dry, reaching a low of 0.06 mg CO₂m^–2s^–1, but increased after rain to 0.16 mg CO₂m^–2s^–1. Also, after rain, when plants were well watered, they were not light saturated at 1 900 µEm^–2s^–1. The NCE rates on the burned treatment were either higher or similar to those on the unburned treatment. For example, on 11 July 1987, NCE rates were higher on the burned treatment (0.66 mg CO₂m^–2s^–1) compared to the unburned treatment (0.47 mg CO₂m^–2s^–1). During the rest of July and August, the rates of the two treatments were not significantly different. But in September and October, the NCE rates were again higher on the burned treatment compared to the unburned treatment. The results indicated that canopy NCE rates may be more indicative of the productivity of the prairie than individual-leaf measurements made only on young, highly productive leaves.Contribution No. 89-82-J from the Kansas Agricultural Experiment Station. This research was supported, in part, by Grant No. DE-FG02-84ER60253.A000.With 4 Figures     

The Parameterisation of Scalar Transfer over Rough Ice

We test a surface renewal model that is widely used over snow and ice surfaces to calculate the scalar roughness length (z _s ), one of the key parameters in the bulk aerodynamic method. For the first time, the model is tested against observations that cover a wide range of aerodynamic roughness lengths (z ₀). During the experiments, performed in the ablation areas of the Greenland ice sheet and the Vatnajökull ice cap in Iceland, the surface varied from smooth snow to very rough hummocky ice. Over relatively smooth snow and ice with z ₀ below a threshold value of approximately 10^?3 m, the model performs well and in accord with earlier studies. However, with growing hummock size, z ₀ increases well above the threshold and the bulk aerodynamic flux becomes significantly smaller than the eddy-correlation flux (e.g. for z ₀ = 0.01 m, the bulk aerodynamic flux is about 50% smaller). Apparently, the model severely underpredicts z _s over hummocky ice. We argue that the surface renewal model does not account for the deep inhomogeneous roughness sublayer (RSL) that is generated by the hummocks. As a consequence, the homogeneous substrate ice grain cover becomes more efficiently ‘ventilated’. Calculations with an alternative model that includes the RSL and was adapted for use over hummocky ice, qualitatively confirms our observations. We suggest that, whenever exceedance of the threshold occurs (z ₀  >  10^?3 m, i.e., an ice surface covered with at least 0.3-m high hummocks), the following relation should be used to calculate scalar roughness lengths, ln (z _s /z ₀)  =  1.5  ? 0.2 ln (Re _*)  ? 0.11(ln (Re _*))².     

CO2 efflux from agricultural soils in Eastern Germany – comparison of a closed chamber system with eddy covariance measurements

Summary In order to quantify the effects of temperature and soil water content on soil respiration, during June and July 2002 CO₂ soil efflux was measured with a closed chamber (non-steady state, flow through) system in the field. The amount of CO₂ emission was highly dependent on the land-use in the observation area, which consisted of meadow soil and brownfield. The CO₂ emission from the brownfield ranged from 0.9 to 5.5µmol CO₂ m^–2s^–1, and that for meadow soil from 1.1 to 12.6µmol CO₂ m^–2s^–1. Soil respiration, as a function of soil temperature (T_soil), relative soil water content (RSWC), soil pH, and the soil carbon/nitrogen ratio (C/N), was analysed by a modified closed non-linear regression model. Between 63% and 81% of the variation of soil CO₂ emission could be explained with changes of T_soil, RSWC, pH, and C/N for the individual chambers on the brownfield.Subsequent analysis involved a comparison of the soil chamber results with eddy covariance (EC) measurements of one week, and included a footprint analysis to account for the influence of the different land use types on the measurements. For this, EC data (143 measurements after quality check) were restricted to those originating from the brownfield area with more than 90% of the flux. For a second comparison, the net ecosystem exchange (NEE) was calculated for different parts of the meadow using the SVAT model PROXEL. Together with the respiration from the brownfield, a weighted average of model NEE was produced using the flux contribution determined by the footprint model. Acceptable agreement (r²=0.69) was found between the modelled data and individual EC measurements, except during situations where the performance of the footprint model was disturbed by internal boundary layer effects.     

锡林浩特草原CO2通量特征及其影响因素分析

利用锡林浩特国家气候观象台开路涡度相关系统、辐射土壤观测系统,测得的长期连续通量观测数据,对锡林浩特草原2009—2011年期间的CO₂通量观测特征进行了分析。结果表明:CO₂通量存在明显的年际、季节和日变化特征。3 a中NEE年际变率达到200 g·m^-2,季节变率最大达到460 g·m^-2,日变化幅度生长季最大达到0.25 mg·m^-2·s^-1。通过不同时间尺度碳通量与温度、水分、辐射等环境因子的分析,认为CO₂通量日变化主要受温度和光合有效辐射影响,而季节变化和年变化主要受降水和土壤含水量的影响。降水强度及时间分布是制约牧草CO₂吸收的关键因素,大于15%的土壤含水量有利于促进牧草生长。     

Eddy correlation measurements of methane flux in a northern peatland ecosystem

A pilot study to measure methane flux using eddy correlation sensors was conducted in a peatland ecosystem in north central Minnesota. A prototype tunable diode laser spectrometer system was employed to measure the fluctuations in methane concentration.The logarithmic cospectrum of methane concentration and vertical wind velocity fluctuations under moderately unstable conditions had a peak nearf = 0.10 (wheref is the nondimensional frequency) and was quite similar to the cospectra of water vapor and sensible heat. Daytime methane flux during the first two weeks of August ranged from 120 to 270 mg m^-2 day^-1. The temporal variation in methane fluxes was consistent with changes in peat temperature and water table elevation. Our results compared well with the range of values obtained in previous studies in Minnesota peatlands.These field observations demonstrate the utility of the micrometeorological eddy correlation technique for measuring surface fluxes of methane. The current state-of-the-art in tunable diode laser spectroscopy makes this approach practical for use in key ecosystems.Published as Paper No. 9556, Journal Series, Nebraska Agricultural Research Division.