Accounting for spectroscopic effects in laser‐based open‐path eddy covariance flux measurements

A significant portion of the production and consumption of trace gases (e.g. CO₂, CH₄, N₂O, NH₃, etc.) by world ecosystems occurs in areas without sufficient infrastructure or easily available grid power to run traditional closed‐path flux stations. Open‐path analyzer design allows such measurements with power consumption 10–150 times below present closed‐path technologies, helping to considerably expand the global coverage and improve the estimates of gas emissions and budgets, informing the remote sensing and modeling communities and policy decisions, all the way to IPCC reports. Broad‐band nondispersive infrared devices have been used for open‐path CO₂ and H₂O measurements since the late 1970s, but since recently, a growing number of new narrow‐band laser‐based instruments are being rapidly developed. The new design comes with its own challenges, specifically: (a) mirror contamination, and (b) uncontrolled air temperature, pressure and humidity, affecting both the gas density and the laser spectroscopy of the measurements. While the contamination can be addressed via automated cleaning, and density effects can be addressed via the Webb‐Pearman‐Leuning approach, the spectroscopic effects of the in situ temperature, pressure and humidity fluctuations on laser‐measured densities remain a standing methodological question. Here we propose a concept accounting for such effects in the same manner as Webb et al. proposed to account for respective density effects. Derivations are provided for a general case of flux of any gas, examined using a specific example of CH₄ fluxes from a commercially available analyzer, and then tested using “zero‐flux” experiment. The proposed approach helps reduce errors in open‐path, enclosed, and temperature‐ or pressure‐uncontrolled closed‐path laser‐based flux measurements due to the spectroscopic effects from few percents to multiple folds, leading to methodological advancement and geographical expansion of the use of such systems providing reliable and consistent results for process‐level studies, remote sensing and Earth modeling applications, and GHG policy decision‐making.     

Addressing the influence of instrument surface heat exchange on the measurements of CO2 flux from open-path gas analyzers

There is a growing concern in the flux community that using the eddy covariance method with open‐path CO₂ analyzers often leads to measurements of an apparent ecosystem CO₂ uptake during off‐season periods, especially in cold climates. Such uptake has not been observed when measurements were made with closed‐path analyzers, chambers, or profile methods, suggesting it is an artifact due in some way to the use of open‐path analyzers. In this study, a series of laboratory tests and field experiments were conducted to determine the magnitude of the instrument surface heat exchange in the open path and its relationship with the measured CO₂ flux. Results showed that (1) the surface of an open‐path instrument became substantially warmer than ambient due to electronics and radiation load during daytime, while at night, radiative cooling moderated temperature increases in the path; (2) high‐frequency temperature measurements inside the path were correlated with vertical wind speed producing sensible heat flux inside the instrument path exceeding the ambient heat flux by up to 14%; (3) enclosing the open‐path instrument eliminated the sensible heat flux in the path, and caused measured CO₂ flux to match a closed‐path reference; (4) using sensible heat flux measured directly inside the open path in the WPL term instead of the ambient sensible heat flux also led to a match in CO₂ flux between open‐path instrument and closed‐path reference; and (5) correcting previously collected open‐path CO₂ flux data was possible by estimating the instrument heating effect with a semi‐empirical model using standard weather variables. Results showed that all proposed techniques led to a significant reduction in apparent CO₂ uptake during off‐season periods and to a reduction of the underestimation of CO₂ release in other periods. Close agreement between the open‐path measurements and closed‐path references was achieved in all cases.     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Evaluation of photoacoustic infrared spectroscopy for simultaneous measurement of N2O and CO2 gas concentrations and fluxes at the soil surface

Simultaneous measurement of N₂O and CO₂ flux at the soil surface with photoacoustic infrared spectroscopy (PAS) is gaining popularity due to portability, low maintenance, and ease‐of‐operation. However, the ability of PAS to measure N₂O with accuracy and precision similar to gas chromatography (GC) is uncertain due to overlap in N₂O, CO₂, and H₂O absorbance spectra combined with the large range in analyte concentrations. We tested the ability of six PAS units to simultaneously measure N₂O and CO₂ gas concentrations and fluxes with accuracy and precision similar to two GC units. We also evaluated H₂O vapor and CO₂ interferences with N₂O measurement. The accuracy and precision of standard gas concentration measurements with PAS and GC were similar. High water vapor (~26 600 ppm) and CO₂ concentrations (~4500 ppm) did not interfere with N₂O measurement across the concentration range typically observed in static flux chambers at the soil surface (~0.5–3.0 ppm N₂O). On average, N₂O fluxes measured with the six PAS were 4.7% higher than one GC and 9.9% lower than the second GC.     

Assessing the performance of the photo‐acoustic infrared gas monitor for measuring CO2, N2O,and CH4 fluxes in two major cereal rotations

Rapid, precise, and globally comparable methods for monitoring greenhouse gas (GHG) fluxes are required for accurate GHG inventories from different cropping systems and management practices. Manual gas sampling followed by gas chromatography (GC) is widely used for measuring GHG fluxes in agricultural fields, but is laborious and time‐consuming. The photo‐acoustic infrared gas monitoring system (PAS) with on‐line gas sampling is an attractive option, although it has not been evaluated for measuring GHG fluxes in cereals in general and rice in particular. We compared N₂O, CO_2, and CH₄ fluxes measured by GC and PAS from agricultural fields under the rice–wheat and maize–wheat systems during the wheat (winter), and maize/rice (monsoon) seasons in Haryana, India. All the PAS readings were corrected for baseline drifts over time and PAS‐CH₄ (PCH₄) readings in flooded rice were corrected for water vapor interferences. The PCH₄ readings in ambient air increased by 2.3 ppm for every 1000 mg cm^?3 increase in water vapor. The daily CO₂, N₂O, and CH₄ fluxes measured by GC and PAS from the same chamber were not different in 93–98% of all the measurements made but the PAS exhibited greater precision for estimates of CO₂ and N₂O fluxes in wheat and maize, and lower precision for CH₄ flux in rice, than GC. The seasonal GC‐ and PAS‐N₂O (PN₂O) fluxes in wheat and maize were not different but the PAS‐CO₂ (PCO₂) flux in wheat was 14–39% higher than that of GC. In flooded rice, the seasonal PCH₄ and PN₂O fluxes across N levels were higher than those of GC‐CH₄ and GC‐N₂O fluxes by about 2‐ and 4fold, respectively. The PAS (i) proved to be a suitable alternative to GC for N₂O and CO₂ flux measurements in wheat, and (ii) showed potential for obtaining accurate measurements of CH₄ fluxes in flooded rice after making correction for changes in humidity.     

温带针阔混交林土壤碳氮气体通量的主控因子与耦合关系

中高纬度森林地区由于气候条件变化剧烈,土壤温室气体排放量的估算存在很大的不确定性,并且不同碳氮气体通量的主控因子与耦合关系尚不明确。以长白山温带针阔混交林为研究对象,采用静态箱-气相色谱法连续4a(2005—2009年)测定土壤二氧化碳(CO2)、甲烷(CH4)和氧化亚氮(N2O)净交换通量以及温度、水分等相关环境因子。研究结果表明:温带针阔混交林土壤整体上表现为CO2和N2O的排放源和CH4的吸收汇。土壤CH4、CO2和N2O通量的年均值分别为-1.3 kg CH4hm-2a-1、15102.2 kg CO2hm-2a-1和6.13 kg N2O hm-2a-1。土壤CO2通量呈现明显的季节性规律,主要受土壤温度的影响,水分次之;土壤CH4通量的季节变化不明显,与土壤水分显著正相关;土壤N2O通量季节变化与土壤CO2通量相似,与土壤水分、温度显著正相关。土壤CO2通量和CH4通量不存在任何类型的耦合关系,与N2O通量也不存在耦合关系;土壤CH4和N2O通量之间表现为消长型耦合关系。这项研究显示温带针阔混交林土壤碳氮气体通量主要受环境因子驱动,不同气体通量产生与消耗之间存在复杂的耦合关系,下一步研究需要深入探讨环境变化对其耦合关系的影响以及内在的生物驱动机制。     

Net grassland carbon flux over a subambient to superambient CO2 gradient

Increasing atmospheric CO₂ concentrations may have a profound effect on the structure and function of plant communities. A previously grazed, central Texas grassland was exposed to a 200‐µmol mol^?1 to 550 µmol mol^?1 CO₂ gradient from March to mid‐December in 1998 and 1999 using two, 60‐m long, polyethylene‐ covered chambers built directly onto the site. One chamber was operated at subambient CO₂ concentrations (200–360 µmol mol^?1 daytime) and the other was regulated at superambient concentrations (360–550 µmol mol^?1). Continuous CO₂ gradients were maintained in each chamber by photosynthesis during the day and respiration at night. Net ecosystem CO₂ flux and end‐of‐year biomass were measured in each of 10, 5‐m long sections in each chamber. Net CO₂ fluxes were maximal in late May (c. day 150) in 1998 and in late August in 1999 (c. day 240). In both years, fluxes were near zero and similar in both chambers at the beginning and end of the growing season. Average daily CO₂ flux in 1998 was 13 g CO₂ m^?2 day^?1 in the subambient chamber and 20 g CO₂ m^?2 day^?1 in the superambient chamber; comparable averages were 15 and 26 g CO₂ m^?2 day^?1 in 1999. Flux was positively and linearly correlated with end‐of‐year above‐ground biomass but flux was not linearly correlated with CO₂ concentration; a finding likely to be explained by inherent differences in vegetation. Because C₃ plants were the dominant functional group, we adjusted average daily flux in each section by dividing the flux by the average percentage C₃ cover. Adjusted fluxes were better correlated with CO₂ concentration, although scatter remained. Our results indicate that after accounting for vegetation differences, CO₂ flux increased linearly with CO₂ concentration. This trend was more evident at subambient than superambient CO₂ concentrations.     

Canopy radiation‐ and water‐use efficiencies as affected by elevated [CO2]

This study used an environmentally controlled plant growth facility, EcoCELLs, to measure canopy gas exchanges directly and to examine the effects of elevated [CO₂] on canopy radiation‐ and water‐use efficiencies. Sunflowers (Helianthus annus var. Mammoth) were grown at ambient (399 μmol mol^?1) and elevated [CO₂] (746 μmol mol^?1) for 53 days in EcoCELLs. Whole canopy carbon‐ and water‐fluxes were measured continuously during the period of the experiment. The results indicated that elevated [CO₂] enhanced daily total canopy carbon‐ and water‐fluxes by 53% and 11%, respectively, on a ground‐area basis, resulting in a 54% increase in radiation‐use efficiency (RUE) based on intercepted photosynthetic active radiation and a 26% increase in water‐use efficiency (WUE) by the end of the experiment. Canopy carbon‐ and water‐fluxes at both CO₂ treatments varied with canopy development. They were small at 22 days after planting (DAP) and gradually increased to the maxima at 46 DAP. When canopy carbon‐ and water‐fluxes were expressed on a leaf‐area basis, no effect of CO₂ was found for canopy water‐flux while elevated [CO₂] still enhanced canopy carbon‐flux by 29%, on average. Night‐time canopy carbon‐flux was 32% higher at elevated than at ambient [CO₂]. In addition, RUE and WUE displayed strong diurnal variations, high at noon and low in the morning or afternoon for WUE but opposite for RUE. This study provided direct evidence that plant canopy may consume more, instead of less, water but utilize both water and radiation more efficiently at elevated than at ambient [CO₂], at least during the exponential growth period as illustrated in this experiment.     

Boundary layer budgets for regional estimates of scalar fluxes

The paper considers the theory and application of budget techniques for regional scalar flux estimation using the daytime convective boundary layer (CBL) and the nocturnal boundary layer (NBL). CBL techniques treat the well mixed layer of air between heights of, say, 100 m and 1000 m as an integrator of surface fluxes along the path of a column of air moving over the landscape. They calculate the average surface flux from the scalar concentration in and above the mixed layer, and the CBL height. The flux estimates are averaged over regions of 10–10⁴ km² extending 10 to 100 km upwind. An integral form of the CBL budget is used to estimate daily regional rates of CO₂ uptake and evaporation from three data sets. There was plausible agreement between the estimates and locally measured fluxes. CBL budgets have great potential for estimating regional scalar fluxes, but there is an urgent need for validation through direct measurements of fluxes and budget parameters. NBL budgets are useful when low-level, radiative inversions inhibit vertical mixing. Surface scalar fluxes can then be estimated from the rate of concentration change below the inversion. An example application for estimating the nocturnal CO₂ flux is given. While simple in concept, NBL budgets are more difficult to apply in practice because of the unpredictability of the depth of the layer and sometimes, its absence altogether. On the other hand, the depth of the atmospheric mixing chamber is better defined, few assumptions are required and the concentration changes usually will be larger and hence more easily detectable than in CBL budgetting.     

温带混交林碳水通量模拟及其对冠层分层方式的响应——耦合的气孔导度-光合作用-能量平衡模型

利用Leuning建立的耦合的光合作用、气孔导度和能量平衡方程,以将冠层分成多层的方式,包括Gaussian五点积分法、将冠层平均分为多层的方法,逐层计算温带混交林的碳水通量,最后累加至冠层尺度,以模拟CO2和H2O通量。该模型以常规气象观测数据作为驱动变量,计算出冠层与大气之间的碳水交换,与涡动相关系统的通量观测数据进行比较,分析了不同的冠层分层方式对多层模型模拟结果的影响。从3个温带混交林通量站涡动相关系统的能量平衡闭合度来看,中国长白山站CBS、韩国GDK和日本MMF站点的能量平衡比率(EBR)分别为0.76、0.66和1.07,居于国际同类观测范围(0.34—1.2)的中上水平,因此,涡动相关系统的观测数据较为可靠。从碳水通量的日变化来看,用Gaussian五点积分法将冠层分为五层的模型能较好的模拟碳水通量的"单峰形"日变化趋势。夜间Fc为负值且变化趋势较为平缓,表明生态系统进行呼吸作用释放CO2,从日出开始Fc逐渐变为正值,表明生态系统进行光合作用吸收CO2,Fc在中午时分达到最大值,下午Fc逐渐减小,日落之后又回复到夜间的负值。H2O通量的日变化曲线与CO2通量相似,且模拟值与涡动相关实测值具有较好的一致性。在多层模型中,对冠层采用不同的分层方法,对碳水通量模拟结果有一定的影响。以Gaussian五点积分法将冠层分为五层的方法作为对照,分别将冠层平均分为2、5、10、20层的方法得到的碳水通量与其进行比较。从平均值来看,分层越多,H2O通量模拟值越低,而CO2通量模拟值越高。不同的分层方法产生的差异,主要来自于不同层的辐射吸收、温湿度、风速等环境要素的垂直廓线差异,且叶片光合作用对光的响应是非线性的。     

The relative merits of open- and closed-path analysers for measurement of eddy fluxes

Theoretical and practical aspects of measuring eddy fluxes of trace gases using open-and closed-path analysers are presented. Trace gas fluxes measured with an open-path analyser require the concurrent measurement of sensible and latent heat fluxes to correct for density fluctuations in trace gas concentration caused by these fluxes. A closed-path analyser eliminates the corrections due to sensible heat flux, but not for water vapour, provided temperature fluctuations are completely removed without significantly reducing fluctuations in the trace gas mixing ratio. Theory for the design of heat exchangers and for the attenuation of concentration fluctuations during air flow through tubes is used to provide design criteria for closed-path systems. Spectral transfer functions are used to estimate flux losses caused by flow through the sampling tube and gas analyser. Other factors considered include cross-sensitivity of infrared CO₂ analysers to water vapour, and deterioration of system performance caused by contaminants on the walls of sampling tubes. Of two open-path, infrared CO₂ analysers tested, one showed a strong interaction between CO₂ and water vapour, while the other showed little sensitivity to the presence of water vapour, other than caused by dilution. A commercial closed-path CO₂ analyser also showed little cross-sensitivity to water vapour. Compared to results for a clean sampling tube, the spectral bandwidth for water vapour fluctuations decreased significantly after several weeks of sampling. No such deterioration in bandwidth was observed for CO₂. These findings are attributed to differential adsorption/desorption of water vapour by dust or salt on the tubing walls. Rain and dust must be removed from open-path analysers to obtain satisfactory measurements. Careful system design and maintenance is required for both open- and closed-path systems to ensure satisfactory long-term measurement of trace gas fluxes. With these precautions, both approaches will provide satisfactory flux measurements.     

Evaluating atmospheric CO2 inversions at multiple scales over a highly inventoried agricultural landscape

An intensive regional research campaign was conducted by the North American Carbon Program (NACP) in 2007 to study the carbon cycle of the highly productive agricultural regions of the Midwestern United States. Forty‐five different associated projects were conducted across five US agencies over the course of nearly a decade involving hundreds of researchers. One of the primary objectives of the intensive campaign was to investigate the ability of atmospheric inversion techniques to use highly calibrated CO₂ mixing ratio data to estimate CO₂ flux over the major croplands of the United States by comparing the results to an inventory of CO₂ fluxes. Statistics from densely monitored crop production, consisting primarily of corn and soybeans, provided the backbone of a well studied bottom‐up inventory flux estimate that was used to evaluate the atmospheric inversion results. Estimates were compared to the inventory from three different inversion systems, representing spatial scales varying from high resolution mesoscale (PSU), to continental (CSU) and global (CarbonTracker), coupled to different transport models and optimization techniques. The inversion‐based mean CO₂‐C sink estimates were generally slightly larger, 8–20% for PSU, 10–20% for CSU, and 21% for CarbonTracker, but statistically indistinguishable, from the inventory estimate of 135 TgC. While the comparisons show that the MCI region‐wide C sink is robust across inversion system and spatial scale, only the continental and mesoscale inversions were able to reproduce the spatial patterns within the region. In general, the results demonstrate that inversions can recover CO₂ fluxes at sub‐regional scales with a relatively high density of CO₂ observations and adequate information on atmospheric transport in the region.     

CO2 emissions from an undrained tropical peatland: Interacting influences of temperature,shading and water table depth

Emission of CO₂ from tropical peatlands is an important component of the global carbon budget. Over days to months, these fluxes are largely controlled by water table depth. However, the diurnal cycle is less well understood, in part, because most measurements have been collected daily at midday. We used an automated chamber system to make hourly measurements of peat surface CO₂ emissions from chambers root‐cut to 30 cm. We then used these data to disentangle the relationship between temperature, water table and heterotrophic respiration (R_het). We made two central observations. First, we found strong diurnal cycles in CO₂ flux and near‐surface peat temperature (<10 cm depth), both peaking at midday. The magnitude of diurnal oscillations was strongly influenced by shading and water table depth, highlighting the limitations of relying on daytime measurements and/or a single correction factor to remove daytime bias in flux measurements. Second, we found mean daily R_het had a strong linear relationship to the depth of the water table, and under flooded conditions, R_het was small and constant. We used this relationship between R_het and water table depth to estimate carbon export from both R_het and dissolved organic carbon over the course of a year based on water table records. R_het dominates annual carbon export, demonstrating the potential for peatland drainage to increase regional CO₂ emissions. Finally, we discuss an apparent incompatibility between hourly and daily average observations of CO₂ flux, water table and temperature: water table and daily average flux data suggest that CO₂ is produced across the entire unsaturated peat profile, whereas temperature and hourly flux data appear to suggest that CO₂ fluxes are controlled by very near surface peat. We explore how temperature‐, moisture‐ and gas transport‐related mechanisms could cause mean CO₂ emissions to increase linearly with water table depth and also have a large diurnal cycle.     

Methane emissions from sheep pasture,measured with an open‐path eddy covariance system

Methane (CH₄) is an important greenhouse gas, contributing 0.4–0.5 W m^?2 to global warming. Methane emissions originate from several sources, including wetlands, rice paddies, termites and ruminating animals. Previous measurements of methane flux from farm animals have been carried out on animals in unnatural conditions, in laboratory chambers or fitted with cumbersome masks. This study introduces eddy covariance measurements of CH₄, using the newly developed LI‐COR LI‐7700 open‐path methane analyser, to measure field‐scale fluxes from sheep grazing freely on pasture. Under summer conditions, fluxes of methane in the morning averaged 30 nmol m^?2 s^?1, whereas those in the afternoon were above 100 nmol m^?2 s^?1, and were roughly two orders of magnitude larger than the small methane emissions from the soil. Methane emissions showed no clear relationship with air temperature or photosynthetically active radiation, but some diurnal pattern was apparent, probably linked to sheep grazing behaviour and metabolism. Over the measurement period (days 60–277, year 2010), cumulative methane fluxes were 0.34 mol CH₄ m^?2, equating to 134.3 g CO₂ equivalents m^?2. By comparison, a carbon dioxide (CO₂) sink of 819 g CO₂ equivalents m^?2 was measured over the same period, but it is likely that much of this would be released back to the atmosphere during the winter or as off‐site losses (through microbial and animal respiration). By dividing methane fluxes by the number of sheep in the field each day, we calculated CH₄ emissions per head of livestock as 7.4 kg CH₄ sheep^?1 yr^?1, close to the published IPCC emission factor of 8 kg CH₄ sheep^?1 yr^?1.     

Tundra ecosystems observed to be CO2 sources due to differential amplification of the carbon cycle

Are tundra ecosystems currently a carbon source or sink? What is the future trajectory of tundra carbon fluxes in response to climate change? These questions are of global importance because of the vast quantities of organic carbon stored in permafrost soils. In this meta‐analysis, we compile 40 years of CO₂ flux observations from 54 studies spanning 32 sites across northern high latitudes. Using time‐series analysis, we investigated if seasonal or annual CO₂ fluxes have changed over time, and whether spatial differences in mean annual temperature could help explain temporal changes in CO₂ flux. Growing season net CO₂ uptake has definitely increased since the 1990s; the data also suggest (albeit less definitively) an increase in winter CO₂ emissions, especially in the last decade. In spite of the uncertainty in the winter trend, we estimate that tundra sites were annual CO₂ sources from the mid‐1980s until the 2000s, and data from the last 7 years show that tundra continue to emit CO₂ annually. CO₂ emissions exceed CO₂ uptake across the range of temperatures that occur in the tundra biome. Taken together, these data suggest that despite increases in growing season uptake, tundra ecosystems are currently CO₂ sources on an annual basis.     

Legacy effects override soil properties for CO2 and N2O but not CH4 emissions following digestate application to soil

The application of organic materials to soil can recycle nutrients and increase organic matter in agricultural lands. Digestate can be used as a nutrient source for crop production but it has also been shown to stimulate greenhouse gas (GHG) emissions from amended soils. While edaphic factors, such as soil texture and pH, have been shown to be strong determinants of soil GHG fluxes, the impact of the legacy of previous management practices is less well understood. Here we aim to investigate the impact of such legacy effects and to contrast them against soil properties to identify the key determinants of soil GHG fluxes following digestate application. Soil from an already established field experiment was used to set up a pot experiment, to evaluate N₂O, CH₄ and CO₂ fluxes from cattle‐slurry‐digestate amended soils. The soil had been treated with farmyard manure, green manure or synthetic N‐fertilizer, 18 months before the pot experiment was set up. Following homogenization and a preincubation stage, digestate was added at a concentration of 250 kg total N/ha eq. Soil GHG fluxes were then sampled over a 64 day period. The digestate stimulated emissions of the three GHGs compared to controls. The legacy of previous soil management was found to be a key determinant of CO₂ and N₂O flux while edaphic variables did not have a significant effect across the range of variables included in this experiment. Conversely, edaphic variables, in particular texture, were the main determinant of CH₄ flux from soil following digestate application. Results demonstrate that edaphic factors and current soil management regime alone are not effective predictors of soil GHG flux response following digestate application. Knowledge of the site management in terms of organic amendments is required to make robust predictions of the likely soil GHG flux response following digestate application to soil.     

Carbon limitation of soil respiration under winter snowpacks: potential feedbacks between growing season and winter carbon fluxes

A reduction in the length of the snow‐covered season in response to a warming of high‐latitude and high‐elevation ecosystems may increase soil carbon availability both through increased litter fall following longer growing seasons and by allowing early winter soil frosts that lyse plant and microbial cells. To evaluate how an increase in labile carbon during winter may affect ecosystem carbon balance we investigated the relationship between carbon availability and winter CO₂ fluxes at several locations in the Colorado Rockies. Landscape‐scale surveys of winter CO₂ fluxes from sites with different soil carbon content indicated that winter CO₂ fluxes were positively related to carbon availability and experimental additions of glucose to soil confirmed that CO₂ fluxes from snow‐covered soil at temperatures between 0 and ?3°C were carbon limited. Glucose added to snow‐covered soil increased CO₂ fluxes by 52–160% relative to control sites within 24 h and remained 62–70% higher after 30 days. Concurrently a shift in the δ¹³C values of emitted CO₂ toward the glucose value indicated preferential utilization of the added carbon confirming the presence of active heterotrophic respiration in soils at temperatures below 0°C. The sensitivity of these winter fluxes to substrate availability, coupled with predicted changes in winter snow cover, suggests that feedbacks between growing season carbon uptake and winter heterotrophic activity may have unforeseen consequences for carbon and nutrient cycling in northern forests. For example, published winter CO₂ fluxes indicate that on average 50% of growing season carbon uptake currently is respired during the winter; changes in winter CO₂ flux in response to climate change have the potential to reduce substantially the net carbon sink in these ecosystems.     

Aircraft‐derived regional scale CO2 fluxes from vegetated drained thaw‐lake basins and interstitial tundra on the Arctic Coastal Plain of Alaska

The landscape surface of the Barrow Peninsula of Alaska is a mosaic of small ponds, thaw lakes, different aged vegetated drained thaw‐lake basins (VDTLBs), and interstitial tundra which have been dynamically formed by both short‐ and long‐term processes. We used a combination of tower‐ and aircraft‐based eddy covariance measurements to characterize the spatial and temporal patterns of CO₂, latent, and sensible heat fluxes along with MODIS NDVI, and were able to scale the aircraft‐based CO₂ fluxes to the 1802 km² Barrow Peninsula region. During typical 2006 summer conditions, the midday hourly CO₂ flux over the region was ?2.04 × 10⁵ kg CO₂ h^?1. The CO₂ fluxes among the interstitial tundra, Ancient, and Old VDTLBs, as well as between the Medium and Young VDTLBs were not significantly different. Combined, the interstitial tundra and Old and Ancient VDTLBs represent～67% of the Barrow Peninsula surface area, accounting for ～59% of the regional flux signal. Although the Medium and Young VDTLBs represent ～11% of the surface area, they account for a large portion, ～35%, of the total regional flux. The remaining ～22% of the surface area are lakes and contributed the remaining ～6% of the total regional flux. Previous studies treated vegetated areas of the region as a single surface type with measurements from a few study sites; doing so could underestimate the regional flux by ～22%. Here, we demonstrate that aircraft‐based systems have the ability to cover large spatial scales while measuring the turbulent fluxes across a number of surfaces and combined with ground‐ and satellite‐based measurements provide a valuable tool for both scaling and validation of regional‐scale fluxes.     

Effects of rainfall events on soil CO2 flux in a cool temperate deciduous broad-leaved forest

The effects of rainfall events on soil CO₂ fluxes were examined in a cool temperate Quercus/Betula forest in Japan. The soil CO₂ fluxes were measured using an open-flow gas exchange system with an infrared gas analyzer in the snow-free season from August 1999 to November 2000. Soil CO₂ flux showed no significant diurnal trend on days without rain. In contrast, rainfall events caused a significant increase in soil CO₂ flux. To determine the effect of rainfall events and to evaluate more precisely the daily and annual soil carbon flux, we constructed a multiple polynomial regression model that included two variables, soil temperature and soil water content, using the soil CO₂ flux data recorded on sunny days. Daily soil carbon fluxes on sunny days calculated by the model were almost the same as those determined by the field measurements. On the contrary, the fluxes measured on rainy days were significantly higher than those calculated daily from the soil carbon fluxes by the model. Annual soil carbon fluxes in 1999 and 2000 were estimated using models that both do and do not take rainfall effects into consideration. The result indicates that post-rainfall increases in soil CO₂ flux represent approximately 16–21% of the annual soil carbon flux in this cool temperate deciduous forest.     

Process-level controls on CO<Subscript>2</Subscript> fluxes from a seasonally snow-covered subalpine meadow soil,Niwot Ridge,Colorado

Fluxes of CO₂ during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO₂ fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO₂ fluxes of 0.71 μmol m⁻² s⁻¹ in 2006 and 0.86 μmol m⁻² s⁻¹ in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m⁻² emitted over the winter, ~30% of the annual soil CO₂ efflux at this site. In general, the CO₂ flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO₂ flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO₂ flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO₂ flux in each zone changes from periodic pulses of CO₂ during thaw events (zone I), to CO₂ fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO₂ fluxes increasing gradually as soil moisture increases (zone III), to CO₂ fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO₂ flux from seasonally snow-covered soils.