A re-examination of closed flux chamber methods for the measurement of trace gas emissions from soils to the atmosphere

The most widely used method for measuring the emission of a trace gas such as N₂O from soil to the atmosphere involves the accumulation of the gas under closed chambers followed by sampling and analysis (by gas chromatography or infrared methods). These chambers can affect the gas exchange, and so improved designs have been proposed. We have tested their performance. One design includes a vent tube to allow ambient pressure fluctuations to occur also inside the chamber. We tested it against a sealed version on two different grassland sites during N₂O peak emissions in spring 1997. On a welldrained soil with a fairly large air permeability vented chambers yielded fluxes as much as five times those of sealed chambers, depending on wind speed. By contrast, on a heavier and wetter soil with smaller air permeability vented chambers averaged only 88% of the fluxes observed with sealed chambers. The effects of venting cannot be explained solely on the basis of mean pressure differences inside and outside the chamber. It seems more likely that wind blowing over the vent depressurizes the chamber (Venturi effect), resulting in significant gas flow from the more permeable soil into the interior of the chamber. The opposite trend for the less permeable soil suggests that diffusion losses through the vent tube are greater than the increase in concentration due to soil gas flow. Venting can create larger errors than the ones it is supposed to overcome.     

影响静态箱检测开放式气体排放源N2O排放通量的关键因子

为研究影响静态箱检测开放式气体排放源氧化亚氮（N2O）排放通量的关键因子,以提高静态箱检测气体排放通量的准确性,该文在实验室条件下,探究了箱体配置（有无通气孔、有无风扇）和检测条件（不同密闭时间：30、40、50和60 min;不同排放源表面风速：0、0.5、1.0、1.5和2.0 m/s）对300 mm（直径）×300 mm（高度）（D300 mm×H300 mm）的静态箱检测N2O排放通量准确性的影响规律。结果表明,不同配置的静态箱测量结果偏差率随时间的变化趋势均相同,其中有通气孔和风扇的箱体在不同风速下的检测稳定性较好,检测准确性最高。当排放源表面风速为0～2 m/s时,风扇对静态箱检测准确性无显著性影响,排放源表面的风主要通过通气孔影响静态箱的检测准确性。静态箱检测的气体排放通量与实际排放通量的偏差率随排放源表面风速和箱体密闭时间的增加而显著降低。该试验推荐在排放源表面风速小于2 m/s的无粪便堆积的奶牛运动场以及排放源介质相似的开放式气体排放系统中使用有通气孔和风扇的静态箱对N2O排放通量进行检测,密闭50 min。     

Small lateral air pressure gradients generated by a large chamber system have a strong effect on CO2 transport in soil

Gas transport in soils is usually assumed to be purely diffusive, although several studies have shown that non-diffusive processes can significantly enhance soil gas transport. These processes include barometric air pressure changes, wind-induced pressure pumping and static air pressure fields generated by wind interacting with obstacles. The associated pressure gradients in the soil can cause advective gas fluxes that are much larger than diffusive fluxes. However, the contributions of the respective transport processes are difficult to separate. We developed a large chamber system to simulate pressure fields and investigate their influence on soil gas transport. The chamber consists of four subspaces in which pressure is regulated by fans that blow air in or out of the chamber. With this setup, we conducted experiments with oscillating and static pressure fields. CO₂ concentrations were measured along two soil profiles beneath the chamber. We found a significant relationship between static lateral pressure gradients and the change in the CO₂ profiles (R² = 0.53; p-value <2e-16). Even small pressure gradients between −1 and 1 Pa relative to ambient pressure resulted in an increase or decrease in CO₂ concentrations of 8% on average in the upper soil, indicating advective flow of air in the pore space. Positive pressure gradients resulted in decreasing, negative pressure gradients in increasing CO₂ concentrations. The concentration changes were probably caused by an advective flow field in the soil beneath the chamber generated by the pressure gradients. No effect of oscillating pressure fields was observed in this study. The results indicate that static lateral pressure gradients have a substantial impact on soil gas transport and therefore are an important driver of gas exchange between soil and atmosphere. Lateral pressure gradients in a comparable range can be induced under windy conditions when wind interacts with terrain features. They can also be caused by chambers used for flux measurements at high wind speed or by fans used for head-space mixing within the chambers, which yields biased flux estimates.     

Measuring forest floor CO2 fluxes in a Douglas-fir forest

CO₂ exchange was measured on the forest floor of a coastal temperate Douglas-fir forest located near Campbell River, British Columbia, Canada. Continuous measurements were obtained at six locations using an automated chamber system between April and December, 2000. Fluxes were measured every half hour by circulating chamber headspace air through a sampling manifold assembly and a closed-path infrared gas analyzer. Maximum CO₂ fluxes measured varied by a factor of almost 3 between the chamber locations, while the highest daily average fluxes observed at two chamber locations occasionally reached values near 15 μmol C m⁻² s⁻¹. Generally, fluxes ranged between 2 and 10 μmol C m⁻² s⁻¹ during the measurement period. CO₂ flux from the forest floor was strongly related to soil temperature with the highest correlation found with 5 cm depth temperature. A simple temperature dependent exponential model fit to the nighttime fluxes revealed Q₁₀ values in the normal range of 2–3 during the warmer parts of the year, but values of 4–5 during cooler periods. Moss photosynthesis was negligible in four of the six chambers, while at the other locations, it reduced daytime half-hourly net CO₂ flux by about 25%. Soil moisture had very little effect on forest floor CO₂ flux. Hysteresis in the annual relationship between chamber fluxes and soil temperatures was observed. Net exchange from the six chambers was estimated to be 1920±530 g C m⁻² per year, the higher estimates exceeding measurement of ecosystem respiration using year-round eddy correlation above the canopy at this site. This discrepancy is attributed to the inadequate number of chambers to obtain a reliable estimate of the spatial average soil CO₂ flux at the site and uncertainty in the eddy covariance respiration measurements.     

贵阳市及其郊区土壤大气界面间汞交换通量的初步研究

利用动力学通量箱与高时间分辨率测汞仪联用技术对贵阳市市区及其郊区4个点的土壤大气界面间的汞交换通量进行了测定,用小型多功能气象仪记录了采样期间的气象参数。结果显示:4个采样点的大气汞含量都明显高于全球背景值(1.5~2.0 ng m-3),显示出贵阳市市区及郊区都遭受了不同程度的大气汞污染。土壤大气界面间的汞交换通量与光照强度具有显著的相关关系,与土壤温度也有很好的相关性。     

田间土壤氨挥发的原位测定——风洞法

详细介绍了农田土壤氨挥发风洞法测定系统的原理、构造和特点,并通过回收率试验和田间试验进行了验证。所选用的德国风洞主要包括采样箱、采样系统和控制系统3部分,系统内部的气压、温度、湿度、风速等微气象条件接近自然环境条件,测量结果有较好的代表性。回收实验结果表明,回收率为90%,说明风洞的密闭性和浓度分布的均匀程度较好,适合于土壤氨挥发的多处理、多重复的田间原位测定,尤其适用于多因子对比实验。风洞法测定不受天气的影响,对实验区面积要求不高,重复性及可靠性较好,不仅可以测定农田土壤的氨挥发,还可以测定有机肥贮存和施用以及各种肥料形态的氨挥发状况。     

Mercury Flux Measurements over Air and Water in Kejimkujik National Park,Nova Scotia

Mercury flux measurements were conducted at two lakes and three soil sites in Kejimkujik National Park, located in the eastern Canadian province of Nova Scotia. One of the lakes had high levels of both mercury and Dissolved Organic Carbon (DOC). Two of the soil sites were located under the forestcanopy, while the other was in a small clearing surrounded by forest. Flux measurements were performed using the dynamic chamber method. Mercury concentrations in the air were measured with a TEKRAN mercury analyzer. Mercury fluxes over the two lakes were most strongly correlated with solar radiation, although the flux was also significantly correlated with water temperature, air temperature, and negatively correlated with relative humidity. The flux from the high DOC lake (Big Dam West) was especially high when the conditions were both sunny and windy (wind speed greater than 1.3 m s^-1) and the average flux measured was 5.4 ng m^-2 h^-1. The mercury flux from this lake was wellparameterized in terms of a simple expression involving solar radiation and a nonlinear dependence on wind speed. The flux measured from the low DOC lake (North Cranberry) tended to be lower than from the high DOC lake. The averageflux measured was 1.1 ng m^-2 h^-1, but was again strongly correlated with solar radiation. The flux was low during windy conditions in the absence of sunlight, suggesting that wind enhances mercury evasion from lakes only in the presence of solar radiation. Mercury fluxes measured over the soil sites tended to be smaller than those over water (a range of –1.4–4.3 ng m^-2 h^-1). At one of the forest sites, mercury flux was very strongly correlated with 0.5 cm soil temperature, and this dependence was well described by an Arrhenius-typeexpression with an activation energy of 14.6 kcal^-1 mole, quite close to the heat of vaporization of mercury (14.5 kcal^-1 mol^-1 at 20 °C). At the clearing, where there was direct exposure to the sun, the mercury flux was most strongly correlated with solar radiation.     

Wind erosion in agricultural soils: an example of limited supply of particles available for erosion

Soil erosion by wind is a complex process since many interacting factors are involved. In addition, wind erosion can show a considerable spatial and temporal variability associated with changes in soil surface conditions. During a wind erosion experiment conducted in August 1995 within an agricultural field of Central Aragón (NE Spain) [López, M.V., Sabre, M., Gracia, R., Arrúe, J.L., Gomes, L., 1998. Tillage effects on soil surface conditions and dust emission by wind erosion in semiarid Aragón (NE Spain). Soil Tillage Res. (in press)], a decay in dust emission (vertical dust flux) with an increase in wind speed was observed at the end of the experimental period. A further analysis of the evolution of the vertical flux with time in response to changes in soil erodibility is shown in the present study. The analysis is based on the comparison of the measured flux with the potential flux predicted for identical wind conditions assuming that the supply of erodible material at the soil surface was unlimited. The potential flux was estimated by using the dust emission model developed by Marticorena and Bergametti [Maticorena, B., Bergametti, G., 1995. Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme. J. Geophys. Res. 100, pp. 16415–16430]. The model is based on the parameterization of the threshold wind shear velocity as a function of the aggregate size distribution and the roughness length of soil surface. The results indicate that the observed reduction in soil erodibility with time was probably due to variations in the aggregate size distribution and, more precisely, to a limited supply of erodible particles at the soil surface. This study underlines the need to consider the temporal variability of the surface conditions in wind erosion research and derived models.     

高浓度CO2下长白松幼苗土壤和根系呼吸

The objectives of this study were to investigate the effect of higher CO2 concentrations （500 and 700 μmol mol^-1） in atmosphere on total soil respiration and the contribution of root respiration to total soil respiration during seedling growth of Pinus sylvestris vat. sylvestriformis. During the four growing seasons （May-October） from 1999 to 2003, the seedlings were exposed to elevated concentrations of CO2 in open-top chambers. The total soil respiration and contribution of root respiration were measured using an LI-6400-09 soil CO2 flux chamber on June 15 and October 8, 2003. To separate root respiration from total soil respiration, three PVC cylinders were inserted approximately 30 cm deep into the soil in each chamber. There were marked diurnal changes in air and soil temperatures on June 15. Both the total soil respiration and the soil respiration without roots showed a strong diurnal pattern, increasing from before sunrise to about 14：00 in the afternoon and then decreasing before the next sunrise. No increase in the mean total soil respiration and mean soil respiration with roots severed was observed under the elevated CO2 treatments on June 15, 2003, as compared to the open field and control chamber with ambient CO2. However, on October 8, 2003, the total soil respiration and soil respiration with roots severed in the open field were lower than those in the control and elevated CO2 chambers. The mean contribution of root respiration measured on June 15, 2003, ranged from 8.3% to 30.5% and on October 8, 2003, from 20.6% to 48.6%.     

Vents and seals in non-steady-state chambers used for measuring gas exchange between soil and the atmosphere

Despite decades of research to define optimal chamber design and deployment protocol for measuring gas exchange between the Earth's surface and the atmosphere, controversy still surrounds the procedures for applying this method. Using a numerical simulation model we demonstrated that (i) all non‐steady‐state chambers should include a properly sized and properly located vent tube; (ii) even seemingly trivial leakiness of the seals between elements of a multiple‐component chamber results in significant risk of measurement error; (iii) a leaking seal is a poor substitute for a properly designed vent tube, because the shorter path length through the seal supports much greater diffusive gas loss per unit of conductance to mass flow; (iv) the depth to which chamber walls must be inserted to minimize gas loss by lateral diffusion is smaller than is customary in fine‐textured, wet or compact soil, but much larger than is customary in highly porous soils, and (v) repetitive sampling at the same location is not a major source of error when using non‐steady‐state chambers. Finally, we discuss problems associated with computing the flux of a gas from the non‐linear increase in its concentration in the headspace of a non‐steady‐state chamber.     

Quantification of uncertainty in trace gas fluxes measured by the static chamber method

Fluxes of methane (CH₄) and nitrous oxide (N₂O) are commonly measured with closed static chambers. Here, we analyse several of the uncertainties inherent in these measurements, including the accuracy of calibration gases, repeatability of the concentration measurements, choice of model used to calculate the flux and lack of fit to the model, as well as inaccuracies in measurements of sampling time, temperature, pressure and chamber volume. In an analysis of almost 1000 flux measurements from six sites in the UK, the choice of model for calculating the flux and model lack‐of‐fit were the largest sources of uncertainty. The analysis provides confidence intervals based on the measurement errors, which are typically 20%. Our best estimate, using the best‐fitting model, but substituting the linear model in the case of concave fits, gave a mean flux that is 25% greater than that calculated with the linear model. The best‐fit non‐linear model provided a better (convex) fit to the data than linear regression in 36% of the measurements. We demonstrate a method to balance the number of gas samples per chamber (n_samples) and the number of chambers, so as to minimize the total uncertainty in the estimate of the mean flux for a site with a fixed number of gas samples. The standard error generally decreased over the available range in n_samples, suggesting that more samples per chamber (at the expense of proportionally fewer chambers) would improve estimates of the mean flux at these sites.     

塔里木盆地多种沙源类型沙通量异变特征研究

以塔里木盆地周边地区多种沙尘源地为研究区,通过塔中、肖塘、若羌、策勒四个风沙观测场获取的沙尘暴过程中贴地层风向、风速数据以及风蚀传感器实测的撞击颗粒数等数据计算出各地起沙临界摩擦速度,利用Gillette经典的垂直沙尘通量及Lettau水平沙尘通量公式对若羌、策勒、塔中、肖塘沙尘通量进行了估算,并通过各层实测的梯度风速作为背景值界定出塔里木盆地多种沙源类型沙通量变化比对特征,结果表明:垂直沙尘通量F量级为10-8kg/(m2.s),水平沙尘通量Q量级为10-4 kg/(m.s);风速相同情况下,F若羌最大,策勒次之,塔中、肖塘较小;Q若羌值最大,策勒次之,塔中第三,肖塘最小。垂直沙通量F与风速都表现出较好的线性相关,风速递增,垂直沙通量递增;风速递减,垂直沙通量递减。在风速递增幅度相同条件下,若羌Q值递增幅度最大,策勒次之,塔中、肖塘基本相同。     

Wind-splash erosion of bare peat on UK upland moorlands

Peat is a common land surface material in many countries of the world and is particularly important in upland regions of the UK. Peat landscapes represent an important land use for hill farming, water management, and shooting, and are a globally scarce resource. Wind is a fundamental characteristic of upland environments in the UK and has long been recognised as a significant factor in peat erosion. This paper presents the first results of a project that aims to determine the significance of wind action in the erosion of upland peat. Wind erosion monitoring is being undertaken at Moor House in the North Pennines on a 3-ha area of relatively flat, sparsely vegetated peat. Measurements using arrays of passive horizontal mass flux gauges (fixed orientation vertical slot gauges), together with a vertical array of mass flux samplers (directional), provide estimates of sediment flux. A micrometeorological station records local wind speed (four heights), wind direction, rainfall, soil moisture, and temperature conditions. For 1999 and 2000, the annual horizontal net erosion flux is 0.46 and 0.48 t ha⁻¹, respectively. Results of detailed monitoring over a 10-month period demonstrate that the peat sediment flux collected in windward- and leeward-oriented sediment traps on 10 separate occasions is between 3 and 12 times greater in the windward-facing traps. The concentration of peat with height decays rapidly and the majority of the peat is transported close to the ground surface. Above 0.3 m, very little peat is found. Significant horizontal fluxes of peat occur in both wet and dry periods. This is evaluated using the local micrometeorological data to try and predict sediment yields. Correlations among time-averaged friction velocity measurements, surface conditions, and sediment flux are complex. Event-based measurements, as opposed to cumulative sediment yields, are required to resolve this. These results quantify for the first time the significance of wind action in the erosion of peat in a UK upland environment.     

An explanation of linear increases in gas concentration under closed chambers used to measure gas exchange between soil and the atmosphere

Earlier models describing the accumulation of gases under closed chambers are based on the assumption of a constant concentration source that does not change during the time of chamber deployment. A new model is proposed which is based on the assumption of a constant production source, and takes into account possible changes in gas concentrations at the source during chamber deployment. Using N₂O as an example, simulations have been carried out for different source strength and depth, diffusivities and air porosities. The main finding was a chamber‐induced increase in gas concentrations in the upper part of the soil profile, including the depth where the N₂O source is located. The increase started immediately after chamber closure. Nevertheless, fluxes calculated from increasing concentrations within the chamber's headspace were always less than those expected under undisturbed conditions, i.e. in the absence of a chamber. This was due to a proportion of the gas produced being stored within the soil profile while the chamber was in place. The discrepancy caused by this effect increased with increasing air‐filled porosity and decreasing height of the chamber, and a procedure for correcting chamber flux measurements accordingly is proposed. The increase in soil gas concentrations after chamber closure was confirmed in a laboratory experiment.     

Variations in the Diurnal Flux of Greenhouse Gases from Soil and Optimizing the Sampling Protocol for Closed Static Chambers

There is no standardized method for the sampling of greenhouse gas fluxes from soil. Two methods are primarily used: closed dynamic chamber (CDC) and closed static chamber (CSC) systems. The most complex and costly are the CDC systems, which can sample gases in situ. However, the low-cost CSC systems are being increasingly used in which the gas samples are collected manually and analyzed off-site at a later date. Given their growing popularity, it is important to optimize the sampling procedure of the CSC systems to ensure that the measurements are both repeatable and representative. Samples from a commercial potato crop were collected in the morning and afternoon at 0, 15, 30, 60, 90, and 120 min after the chambers were closed, and the concentrations of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) were determined using gas chromatography. The concentrations of CO₂ and N₂O inside chambers increased linearly over time, whereas the concentration of CH₄ remained constant. The fluxes of CO₂ and N₂O from soil were greater in the afternoon than the morning, whereas the flux of CH₄ was greater in the morning. For longer-term single-point soil flux monitoring using CSCs with a volume of 6.3 L, it is recommended that samples are collected in the morning at 0, 30, and 60 min after chambers are closed. This approach will ensure that the concentration of the gases are representative and will allow for a high level of repeatability and certainty in the results.     

Calibrating soil respiration measures with a dynamic flux apparatus using artificial soil media of varying porosity

Measurement of soil respiration to quantify ecosystem carbon cycling requires absolute, not relative, estimates of soil CO₂ efflux. We describe a novel, automated efflux apparatus that can be used to test the accuracy of chamber‐based soil respiration measurements by generating known CO₂ fluxes. Artificial soil is supported above an air‐filled footspace wherein the CO₂ concentration is manipulated by mass flow controllers. The footspace is not pressurized so that the diffusion gradient between it and the air at the soil surface drives CO₂ efflux. Chamber designs or measurement techniques can be affected by soil air volume, hence properties of the soil medium are critical. We characterized and utilized three artificial soils with diffusion coefficients ranging from 2.7 × 10^?7 to 11.9 × 10^?7 m² s^?1 and porosities of 0.26 to 0.46. Soil CO₂ efflux rates were measured using a commercial dynamic closed‐chamber system (Li‐Cor 6400 photosynthesis system equipped with a 6400‐09 soil CO₂ flux chamber). On the least porous soil, small underestimates (< 5%) of CO₂ effluxes were observed, which increased as soil diffusivity and soil porosity increased, leading to underestimates as high as 25%. Differential measurement bias across media types illustrates the need for testing systems on several types of soil media.     

Iso-FD: A novel method for measuring the isotopic signature of surface flux

Stable carbon isotopes have become a critical and often used tool in understanding ecological and physical processes affecting gas production and emissions in soil. While the insights gained using chamber based flux methods have been significant, it is known now that many of these chamber methods have an inherent bias that complicates the interpretation of their measurements. Here we present a new chamber method that uses diffusive membranes to control CO₂ flow into and out of the chamber, and can measure the isotopic composition of soil flux without inducing a bias. We present numerical modeling, followed by laboratory calibration and field measurements using this new method coupled to a Cavity Ring Down Spectrometer (CRDS). Simulations, as well was lab and field results showed that the method is both robust over a range of environmental conditions and can be unbiased, unlike other chamber approaches. Finally, we discuss possibilities for future improvements and variations on the measurement approaches we used.     

黄土高原水蚀风蚀交错带不同立地条件下土壤呼吸特征

水蚀风蚀交错带是黄土高原土壤侵蚀最严重地区,该地区立地条件复杂,土壤质地有较大的空间变异性,对生态系统碳循环过程产生重要影响。该文选取土壤质地为立地条件的主要参考因子,利用便携式CO2分析仪对黄土高原水蚀风蚀交错带不同立地条件下2种典型灌木(柠条和沙柳)土壤呼吸特征进行了连续2a的野外对比研究,分析不同立地条件下灌木林地土壤呼吸之间的差异,阐明土壤呼吸的动态变化及其对土壤温度的敏感性。结果表明:在不同立地条件下,柠条林地黄绵土土壤呼吸高于风沙土,相反,沙柳林地风沙土土壤呼吸高于黄绵土。每种灌木林地在不同立地条件下土壤呼吸呈现明显的季节性变化,土壤呼吸速率的高峰值出现在7、8月份。不同立地条件下土壤呼吸与土壤温度呈现显著指数函数关系,每种灌木林地黄绵土土壤呼吸的温度敏感性系数Q10高于风沙土,从而对土壤温度的响应更为敏感。     

Photodegradation effects on CO2 emissions from litter and SOM and photo-facilitation of microbial decomposition in a California grassland

Decomposition of soil organic matter (SOM) and plant litter has been shown to be affected by high solar radiation; this could partly explain why biogeochemical models underestimate decomposition in arid and semi-arid ecosystems. We set out to test the effect of using traditional PVC chambers for measuring soil gas fluxes versus quartz chambers that allowed passage of light during field measurements in a dry-land field in Davis, CA. Results showed that fluxes from quartz-top chambers were on average 29% higher than from opaque chambers. We also studied the effect of solar light exposure on decomposition of native grass litter and SOM in a field experiment where plots were shaded or left exposed for 157 days during summer; litter did not seem to be affected by exposure to light. However, we concluded that SOM decomposition was affected by light exposure since shaded soil had similar respiration to sunlight-exposed soil indicating that microbial respiration occurred under the shade while photo-degradation likely occurred under the sun. Additionally, ¹⁵N-labeled grass was placed in litter bags in the field with either clear filters to allow light or aluminum covers to block light; 3-month exposure caused a change in lignin degradability as indicated by the change in the Ad/Al ratio. Incubation of that litter showed 9.3% more CO₂ produced from litter in clear and aluminum bags than unexposed litter. This showed that photo-facilitation occurred although to a small degree and was a result of light exposure and/or heat degradation. We attributed the similar respiration from clear- and aluminum-exposed litter to heat degradation of the aluminum-exposed litter. In conclusion, our results show that in hot dry ecosystems conventional PVC chambers underestimate measured CO₂ flux rates; sunlight exposure changes litter chemistry and appears to affect the degradation of soil organic matter, but the magnitude of degradation depends on an interaction of factors such as soil temperature and moisture.     

Estimating nocturnal ecosystem respiration from the vertical turbulent flux and change in storage of CO2

Micrometeorological measurements of nighttime ecosystem respiration can be systematically biased when stable atmospheric conditions lead to drainage flows associated with decoupling of air flow above and within plant canopies. The associated horizontal and vertical advective fluxes cannot be measured using instrumentation on the single towers typically used at micrometeorological sites. A common approach to minimize bias is to use a threshold in friction velocity, u_*, to exclude periods when advection is assumed to be important, but this is problematic in situations when in-canopy flows are decoupled from the flow above. Using data from 25 flux stations in a wide variety of forest ecosystems globally, we examine the generality of a novel approach to estimating nocturnal respiration developed by van Gorsel et al. (van Gorsel, E., Leuning, R., Cleugh, H.A., Keith, H., Suni, T., 2007. Nocturnal carbon efflux: reconciliation of eddy covariance and chamber measurements using an alternative to the u_*-threshold filtering technique. Tellus 59B, 397–403, Tellus, 59B, 307-403). The approach is based on the assumption that advection is small relative to the vertical turbulent flux (F_C) and change in storage (F_S) of CO₂ in the few hours after sundown. The sum of F_C and F_S reach a maximum during this period which is used to derive a temperature response function for ecosystem respiration. Measured hourly soil temperatures are then used with this function to estimate respiration R_Rmax. The new approach yielded excellent agreement with (1) independent measurements using respiration chambers, (2) with estimates using ecosystem light-response curves of F_c + F_s extrapolated to zero light, R_LRC, and (3) with a detailed process-based forest ecosystem model, R_cast. At most sites respiration rates estimated using the u_*-filter, R_ust, were smaller than R_Rmax and R_LRC. Agreement of our approach with independent measurements indicates that R_Rmax provides an excellent estimate of nighttime ecosystem respiration.