H2O Fluxes Research Articles

Temporal dynamics of alfalfa water use efficiency under hyper arid conditions of Saudi Arabia

A field study was carried out to investigate the seasonal variations in alfalfa (Medicago sativa L.) water use efficiency (WUE) using Eddy Covariance (EC) measured CO2 and H2O fluxes, aiming at optimizing the use of irrigation water under hyper arid conditions. The EC system used for this study was installed on a center pivot-irrigated 50 ha alfalfa field. Results revealed that the net EC estimated CO2 uptake ranged from 65,00 kg ha−1 (in winter) to 21,500 kg ha−1 (in summer). While, H2O flux was 4,147 m3 ha−1 (in winter) and 20,157 m3 ha−1 (in summer). This resulted in an estimated alfalfa WUE of 1.57 and 1.07 kg m−3 for winter and summer seasons, respectively. However, the actual WUE of harvested alfalfa was calculated at 0.70 and 0.71 kg m−3 for winter and summer, respectively. Therefore, attaining an actual crop WUE of 33–55% lower than the EC measurement (i.e. more water losses were due to leaching and deep-percolation processes, as the EC system could only estimate evapotranspiration over agricultural fields) emphasizes the need of precision irrigation practices, which will enable farmers to apply irrigation water and agrochemicals more precisely and site-specifically to match soil and plant status and needs.

Changes in net ecosystem exchange of CO2, latent and sensible heat fluxes in a recently clear-cut spruce forest in western Russia: results from an experimental and modeling analysis

Ecosystem carbon dioxide, energy, and water fluxes were measured using eddy covariance in a fresh clear-cut surrounded by a mixed spruce-birch-aspen forest in the boreal zone of European Russia. Measurements were initiated in spring 2016 following timber harvest and continued for five months. The influence of surrounding forest on air flow and turbulent fluxes within the clear-cut were examined using a process-based two-dimensional (2D) hydrodynamic turbulent exchange model. The clear-cut was a source of CO2 to the atmosphere prior to onset of vegetation growth during early spring. During this period the mean daily latent (LE) and sensible (H) heat fluxes were very similar and the Bowen ratio (β = H/LE) averaged about 1.0. Daily net ecosystem exchange of CO2 (NEE) was around 0 gC m−2 d−1 following onset of vegetation growth from mid-spring through summer, while β declined to 0.6–0.7. There was strong diurnal variability in NEE, LE and H over the measurement period that was governed by solar radiation and temperature as well as the leaf area index (LAI) of regrown vegetation. Modeled vertical CO2 and H2O fluxes along a transect that crossed the clear-cut and coincided with the dominate wind direction showed that the clear-cut strongly influenced turbulent fluxes within the atmospheric surface layer. Furthermore, modeled atmospheric dynamics suggested that the clear-cut had a large influence on turbulent fluxes in the downwind forest, but little impact on the upwind side. An aggregated approach including field measurements and process-based models can be a useful approach to estimate energy, water and carbon dioxide fluxes in non-uniform forest landscapes.

Circadian rhythms have significant effects on leaf-to-canopy scale gas exchange under field conditions.

BackgroundMolecular clocks drive oscillations in leaf photosynthesis, stomatal conductance, and other cell and leaf-level processes over ~24 h under controlled laboratory conditions. The influence of such circadian regulation over whole-canopy fluxes remains uncertain; diurnal CO2 and H2O vapor flux dynamics in the field are currently interpreted as resulting almost exclusively from direct physiological responses to variations in light, temperature and other environmental factors. We tested whether circadian regulation would affect plant and canopy gas exchange at the Montpellier European Ecotron. Canopy and leaf-level fluxes were constantly monitored under field-like environmental conditions, and under constant environmental conditions (no variation in temperature, radiation, or other environmental cues).ResultsWe show direct experimental evidence at canopy scales of the circadian regulation of daytime gas exchange: 20–79 % of the daily variation range in CO2 and H2O fluxes occurred under circadian entrainment in canopies of an annual herb (bean) and of a perennial shrub (cotton). We also observed that considering circadian regulation improved performance by 8–17 % in commonly used stomatal conductance models.ConclusionsOur results show that circadian controls affect diurnal CO2 and H2O flux patterns in entire canopies in field-like conditions, and its consideration significantly improves model performance. Circadian controls act as a ‘memory’ of the past conditions experienced by the plant, which synchronizes metabolism across entire plant canopies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13742-016-0149-y) contains supplementary material, which is available to authorized users.

Measurements of methane emissions from a beef cattle feedlot using the eddy covariance technique

The eddy covariance (EC) technique has been extensively used in several sites around the world to measure energy fluxes and CO2 exchange at the ecosystem scale. Recent advances in optical sensors have allowed the use of the EC approach to measure other trace gases (e.g. CH4, NH3 and N2O), which has expanded the use of eddy covariance for other applications, including measuring gas emissions from livestock production systems. The main objectives of this study were to assess the performance of a closed-path EC system for measuring CH4, CO2, and H2O fluxes in a beef cattle feedlot and to investigate the spatial variability of eddy covariance fluxes measured above the surface of a feedlot using an analytical flux footprint analysis. A closed-path EC system was used to measure CH4, CO2, and H2O fluxes. To evaluate the performance of this closed-path system, an open-path EC system was also deployed on the flux tower to measure CO2 and H2O exchange. The performance assessment of the closed-path EC system showed that this system was suitable for EC measurements. The frequency attenuations, observed for the closed-path system CO2 and CH4 cospectra in this study, are in agreement with results from previous instrument comparison studies. For the water vapor closed-path cospectra, larger attenuations were likely caused by water vapor molecule interaction with the sampling tube walls. Values of R2 for the relationship between H2O and CO2 fluxes, measured by open-path and closed-path systems, were 0.94–0.98, respectively. The closed-path EC system overestimated the CO2 by approximately 5% and underestimated the latent heat fluxes by about 10% when compared with the open-path system measurements. Measured CH4 and CO2 fluxes during the study period from the feedlot averaged 2.63μmolm−2s−1 and 103.8μmolm−2s−1, respectively. Flux values were quite variable during the field experiment and the footprint analysis was useful to interpret flux temporal and spatial variation. This study shows indication that consideration of atmospheric stability condition, wind direction and animal movement are important to improve estimates of CH4 emissions per pen surface or per head of cattle.

The fluid budget of a continental plate boundary fault: Quantification from the Alpine Fault, New Zealand

Abstract Fluids play a key role in modifying the chemical and physical properties of fault zones, which may prime them for repeated rupture by the generation of high pore fluid pressures and precipitation of commonly weak, secondary minerals. Fluid flow paths, sources and fluxes, and the permeability evolution of fault zones throughout their seismic cycles remain poorly constrained, despite their importance to understanding fault zone behaviour. Here we use geochemical tracers of fluid–rock exchange to determine budgets for meteoric, metamorphic and mantle fluids on a major compressional tectonic plate boundary. The Alpine Fault marks the transpressional Pacific–Australian plate boundary through South Island, New Zealand and appears to fail in regular ( 329 ± 68 yrs ) large earthquakes ( M w ∼ 8 ) with the most recent event in 1717 AD. Significant convergent motion has formed the Southern Alps and elevated geothermal gradients in the hangingwall, which drive crustal fluid flow. Along the Alpine Fault the Alpine Schist of the Pacific Plate is thrust over radiogenic metasedimentary rocks on the Australian plate. The absence of highly radiogenic (87Sr/86Sr > 0.7200) strontium isotope ratios of hangingwall hot springs and hydrothermal minerals formed at a range of depths in the Alpine Fault damage zone indicates that the fluid flow is restricted to the hangingwall by a cross-fault fluid flow barrier throughout the seismogenic crust. Helium isotope ratios measured in hot springs near to the Alpine Fault (0.15–0.81 R A ) indicate the fault is a crustal-scale feature that acts as a conduit for fluids from the mantle. Rock-exchanged oxygen, but meteoric water-like hydrogen isotope signatures of hydrothermal veins indicate that partially rock-exchanged meteoric fluids dominate down to the top of the brittle to ductile transition zone at ∼6 km. Geochemical tracer transport modelling suggests only ∼0.02 to 0.05% of total rainfall west of the Main Divide penetrates to depth, yet this recharge flux is sufficient to overwhelm other fluid contributions. Calculated mantle fluid fluxes of CO2 and H2O (0.2 and 3 to 13 mol/m2/yr respectively) and metamorphic H2O fluxes (4 to 750 mol/m2/yr) are considerably lower than the focused meteoric water discharge flux up the Alpine Fault (4 × 103 to 7 × 104 mol/m2/yr), driven by the >3000 m hydrologic head of the Southern Alps. Meteoric waters are primarily responsible for modifying fault zone permeability during fluid–rock interactions and may facilitate the generation of high pore fluid pressures that could assist episodic earthquake rupture.

Optimization of a biochemical model with eddy covariance measurements in black spruce forests of Alaska for estimating CO2 fertilization effects

Abstract Understanding how high-latitude terrestrial productivity and evapotranspiration change in association with rising atmospheric CO2 concentration ([CO2]), also known as ‘CO2 fertilization’, is important for predicting future climate change. To quantify the magnitude of this fertilization effect, we have developed a big-leaf model that couples photosynthesis and stomatal conductance processes. This model was inverted by inputting eddy covariance CO2 and H2O fluxes from four black spruce forests in Alaska to infer spatially representative ecophysiological parameters using a global optimization technique. Inferred seasonal variations in a maximum carboxylation rate at 25 °C per unit leaf area and stomatal conductance suggest greater photosynthetic capacity per unit leaf area during the mid-growing season, compared to spring and autumn. The interannual variability of parameters suggest that warm summers stimulate photosynthetic capacity and dry summers force stomatal regulation. Based on the model with optimized parameters, small but clear increases in gross primary productivity (GPP) and decreases in latent heat flux (LE) were estimated to be associated with rising [CO2] from 2002 to 2014 (p

植物气孔导度的环境响应模拟及其尺度扩展

PDF HTML阅读 XML下载 导出引用 引用提醒 植物气孔导度的环境响应模拟及其尺度扩展 DOI: 10.5846/stxb201408211652 作者: 作者单位: 中国科学院 寒区旱区环境与工程研究所,中国科学院 寒区旱区环境与工程研究所,中国科学院 寒区旱区环境与工程研究所,中国科学院 寒区旱区环境与工程研究所,浙江瑞启检测技术有限公司;浙江瑞启检测技术有限公司 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金项目(41271037);国家自然科学基金青年科学基金项目(41401033);中国科学院内陆河流域生态水文重点实验室(90Y290F41) Environmental response simulation and the up-scaling of plant stomatal conductance Author: Affiliation: Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences,Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences,,, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:气孔导度是衡量植物和大气间水分、能量及CO2平衡和循环的重要指标,探讨气孔导度与环境因子的关系及其模拟,以及气孔导度在叶片、冠层及区域尺度间的尺度转换及累积效应,对更好地认识植被与大气间的水热运移过程,合理评价植被在陆面过程中的地位和作用都具有重要意义。从植物气孔导度与环境因子的关系、气孔导度模拟以及尺度扩展三个方面,对前人的研究成果进行了概括总结。从叶片和冠层两个尺度出发,归纳总结了前人对于不同植物气孔导度与环境因子关系的研究成果,发现由于不同植物的遗传特性、测定时的环境、时间尺度的不同,以及未考虑各个环境因子的相互作用对气孔导度的影响,由此得到的气孔导度与环境因子之间的关系也不尽一致。对各单一环境因子与气孔导度的关系,给出了生理学解释,从根本上说明了环境因子变化对气孔导度的影响,而研究环境因子对气孔导度的综合影响时,应对各环境因子进行系统控制与同步观测。模拟计算植物气孔导度的模型主要有Jarvis模型和BWB模型两类,这些模型的模拟能力随着研究对象、试验区域、环境条件的改变而存在一定的差异,在具体使用时应结合实际情况选择最优模型进行模拟。除上述常用模型外,还总结了其他学者分别从不同角度提出的新的模型,对现有气孔导度模型进行了全面的总结。从叶片-冠层、冠层-区域两个方面归纳总结了前人关于气孔导度尺度扩展的研究成果,发现叶片-冠层的尺度扩展研究较成熟而冠层-区域的尺度扩展在模拟精度的验证方面存在困难。针对以下几个方面提出了今后气孔导度的研究重点:(1)结合研究对象所在的区域及环境条件,选择最优模型进行模拟;(2)综合考虑环境因子之间的相互作用及其对气孔导度的累积影响;(3)BWB模型与光合模型的耦合;(4)提高大尺度范围内的气孔导度模拟精度。 Abstract:Stomatal conductance is a key indicator of the balance and cycles of heat, H2O, and CO2 fluxes at the vegetation-atmosphere interface. Thus, it is essential to study the heat and H2O transfer in the soil, vegetation, and atmosphere andevaluate the status and role of vegetation, by considering the relationships between stomatal conductance and environmental factors, and by modeling and extrapolating the cumulative effect of stomatal conductance at the leaf, canopy, and regional levels. This paper summarized three aspects of research in this area: (1) the relationships between stomatal conductance and environmental factors at the leaf and canopy levels, (2) the simulation of stomatal conductance, and (3) the up-scalingof stomatal conductance from the leaf to canopy level and from the canopy to regional level. The results showed that the relationships between stomatal conductance and environmental factors were not consistent, because hereditary characteristics, environmental conditions, and time-scales varied; in addition, the comprehensive effects of environmental factors were not considered. A hypothesis was made concerning the mechanism underlying the relationship between the environmental factors and stomatal conductance. Systematic control and simultaneous observation should receive more attention when studying these combined influences. At present, Jarvis and BWB are the most commonly used stomatal conductance models. The suitability of these two models differed among research goals, study sites, and environmental conditions, so model selection should be carried out on a case-by-case basis. This paper also summarized other models. The results of up-scaling stomatal conductance from the leaf to canopy level and from canopy to the regional level showed that while research on the former is well established, the latter is still difficult to validate. Finally, we recommend that future research should focus on the following: (1) choosing the best model to simulate stomatal conductance, based on the environmental conditions; (2) considering interactions between environmental factors, and take into consideration their combined influence on stomatal conductance; (3) studying the coupling between the BWB and photosynthetic models; and (4) improving simulation accuracy at large scales. 参考文献 相似文献 引证文献

Response of CO<sub>2</sub> and H<sub>2</sub>O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Niño events

Abstract. The possible impact of El Niño–Southern Oscillation (ENSO) events on the main components of CO2 and H2O fluxes between the tropical rainforest and the atmosphere is investigated. The fluxes were continuously measured in an old-growth mountainous tropical rainforest in Central Sulawesi in Indonesia using the eddy covariance method for the period from January 2004 to June 2008. During this period, two episodes of El Niño and one episode of La Niña were observed. All these ENSO episodes had moderate intensity and were of the central Pacific type. The temporal variability analysis of the main meteorological parameters and components of CO2 and H2O exchange showed a high sensitivity of evapotranspiration (ET) and gross primary production (GPP) of the tropical rainforest to meteorological variations caused by both El Niño and La Niña episodes. Incoming solar radiation is the main governing factor that is responsible for ET and GPP variability. Ecosystem respiration (RE) dynamics depend mainly on the air temperature changes and are almost insensitive to ENSO. Changes in precipitation due to moderate ENSO events did not have any notable effect on ET and GPP, mainly because of sufficient soil moisture conditions even in periods of an anomalous reduction in precipitation in the region.

Eddy-covariance data with low signal-to-noise ratio: time-lag determination, uncertainties and limit of detection

Abstract. All eddy-covariance flux measurements are associated with random uncertainties which are a combination of sampling error due to natural variability in turbulence and sensor noise. The former is the principal error for systems where the signal-to-noise ratio of the analyser is high, as is usually the case when measuring fluxes of heat, CO2 or H2O. Where signal is limited, which is often the case for measurements of other trace gases and aerosols, instrument uncertainties dominate. Here, we are applying a consistent approach based on auto- and cross-covariance functions to quantify the total random flux error and the random error due to instrument noise separately. As with previous approaches, the random error quantification assumes that the time lag between wind and concentration measurement is known. However, if combined with commonly used automated methods that identify the individual time lag by looking for the maximum in the cross-covariance function of the two entities, analyser noise additionally leads to a systematic bias in the fluxes. Combining data sets from several analysers and using simulations, we show that the method of time-lag determination becomes increasingly important as the magnitude of the instrument error approaches that of the sampling error. The flux bias can be particularly significant for disjunct data, whereas using a prescribed time lag eliminates these effects (provided the time lag does not fluctuate unduly over time). We also demonstrate that when sampling at higher elevations, where low frequency turbulence dominates and covariance peaks are broader, both the probability and magnitude of bias are magnified. We show that the statistical significance of noisy flux data can be increased (limit of detection can be decreased) by appropriate averaging of individual fluxes, but only if systematic biases are avoided by using a prescribed time lag. Finally, we make recommendations for the analysis and reporting of data with low signal-to-noise and their associated errors.

First determination of magma-derived gas emissions from Bromo volcano, eastern Java (Indonesia)

The composition and fluxes of volcanic gases released by persistent open-vent degassing at Bromo Volcano, east Java (Indonesia), were characterised in September 2014 from both in-situ Multi-GAS analysis and remote spectroscopic (dual UV camera) measurements of volcanic plume emissions. Our results demonstrate that Bromo volcanic gas is water-rich (H2O/SO2 ratios of 56–160) and has CO2/SO2 (4.1±0.7) and CO2/Stot (3.2±0.7) ratios within the compositional range of other high-temperature magma-derived gases in Indonesia. H2/H2O and H2S/SO2 ratios constrain a magmatic gas source with minimal temperature of ~700°C and oxygen fugacity of 10-17–10-18 bars. UV camera sensing on September 20 and 21, 2014 indicates a steady daily mean SO2 output of 166±38td−1, which is ten times higher than reported from few previous studies. Our results indicate that Bromo ranks amongst the strongest sources of quiescent volcanic SO2 emission measured to date in Indonesia, being comparable to Merapi volcano in central Java. By combining our results for the gas composition with the SO2 plume flux, we assess for the first time the fluxes of H2O (4725±2292td−1), CO2 (466±83td−1), H2S (25±12td−1) and H2 (1.1±0.8) from Bromo. Our study thus contributes a new piece of information to the still limited data base for volcanic gas emissions in Indonesia, and confirms that much remain to be done to fully assess the contribution of this very active arc region to global volcanic gas fluxes.

Fluxes of energy, H2O, and CO2 between the atmosphere and the monsoon tropical forest in Southern Vietnam.

The 2.5-year dynamics of heat, water and carbon dioxide fluxes in the tropical monsoon semi-evergreen forest in Southern Vietnam obtained by means of continuous eddy covariance observations using standard procedures of FLUXNET global network are analyzed. The features of wet seasons during the measurement period were close to long-term average ones, but dry seasons had a great heterogeneity. The maximal duration of the period with little precipitation was 4 months. The annual radiation balance in the south of Vietnam exceeded the balance at all stations of FLUXNET in tropical forests, except one. Annual evapotranspiration in monsoon forest of south of Vietnam is approximately equal to the evaporation of the rain forests of Central Amazonia. During the wet season evapotranspiration spent 80% of the radiation balance, and in the driest months this value decreased to 50%. In the dry season reduction of evapotranspiration and gross primary production was relatively small due to photosynthesizing trees of 2-4 canopy sub-layers. For the first time a large net sink of carbon dioxide from the atmosphere in the monsoon forest ecosystem was identified.

Coupling boreal forest CO2, H2O and energy flows by a vertically structured forest canopy – Soil model with separate bryophyte layer

A 1-dimensional multi-layer, multi-species soil-vegetation-atmosphere transfer model APES (Atmosphere-Plant Exchange Simulator) with a separate moss layer at the forest floor was developed and evaluated for a boreal Scots pine forest situated in Hyytiälä, Southern Finland. The APES is based on biophysical principles for up-scaling CO2, H2O, heat and momentum exchange from canopy element level to a stand scale. The functional descriptions of sub-models were parametrized by literature values, previous model approaches and leaf and moss gas exchange measurements, and stand structural characteristics derived from multi-scale measurements. The model was independently tested against eddy-covariance fluxes of CO2, H2O and sensible heat measured above and within the canopy, and against soil heat flux and temperature and moisture profiles. The model was shown to well reproduce fluxes and resulting scalar gradients at diurnal and seasonal timescales. Also predictions for moss moisture content and soil moisture and temperature dynamics were acceptable considering the heterogeneity in soil hydraulic and thermal properties and uncertainties in boundary conditions.The model framework allows for (1) coupling above-ground with the soil domains through the feedbacks between soil water and vegetation mediated by the moss layer, (2) several vascular plant species or cohorts in a multi-species canopy, and (3) explicit treatment of bryophyte layer energy and water balance and bottom layer – atmosphere exchange. These features make APES well-suited for exploring feedbacks between boreal forest structure, site conditions and vegetation processes controlling ecosystem-atmosphere exchange.

Land–atmosphere carbon and water flux relationships to vapor pressure deficit, soil moisture, and stream flow

Climatic change is exerting considerable influence on the hydrologic and biogeochemical cycles of snow- dominated montane forest ecosystems. Growing season drought stress is a common occurrence after snowmelt-derived soil water content (WC) and stream flow (Q) have declined, leading to an increase in atmospheric water demand (i.e., vapor pressure deficit, VPD). Here, we analyzed a 6-year record (2006–2011) of H2O and CO2 fluxes from the Tenderfoot Creek Experimental Forest, a montane forest in the northern Rocky Mountains to examine (1) how growing season evapotranspiration (ET), net ecosystem production (NEP), and water-use efficiency (WUE, NEP/ET) respond to changing WC and VPD, (2) how stream flow (Q), an integrated measure of catchment-level water availability, relates to NEP, and (3) how annual NEP is related to annual precipitation and the temperature-defined growing season length (GSL). Growing season NEP exhibited a linear relationship with WC and a log-linear relationship with Q, indicative of persistent water limitations when streamflow and soil moisture reach their annual minima late in the growing season. Nevertheless, years with long GSLs had relatively higher NEP, with a small net carbon sink maintained even at low levels of WC and Q, suggesting that trees are able to obtain water from deeper portions of the soil profile (>30cm) during droughts. However, the warmer, drier climate projected for this region could bring this system closer to a critical threshold of GSL, WC, and VPD, introducing vegetation water stress that could alter the current relationship between GSL and annual NEP.

Analysis of spatio-temporal patterns of CO2 and H2O fluxes in relation to crop growth under field conditions

This study aimed at observing and analyzing spatial and temporal patterns of growth and CO2 and H2O fluxes in winter wheat (Triticum aestivum L.) and winter barley (Hordeum vulgare L.) under heterogeneous soil conditions. Canopy level gas exchange, dry matter, leaf area development and plant available water were measured for this purpose at eight different observation points in three consecutive years (2011–2013) in a 0.72ha farmer's field with variable soil conditions. Results show that the dynamics of crop growth and carbon and water fluxes can have a high spatial and inter-annual variability depending on the underlying water supply patterns. Vegetation patterns arise from responses of growth processes and LAI development to drought stress. Flux patterns of CO2 could be linked to soil heterogeneity and leaf area development patterns. This has implications for the choice of scale and model complexity in applying crop growth models at the field level, suggesting a variable selection of scale and model setup depending on the variability of environmental drivers of plant growth and gas exchange.

The importance of micrometeorological variations for photosynthesis and transpiration in a boreal coniferous forest

Abstract. Plant canopies affect the canopy micrometeorology, and thereby alter canopy exchange processes. For the simulation of these exchange processes on a regional or global scale, large-scale vegetation models often assume homogeneous environmental conditions within the canopy. In this study, we address the importance of vertical variations in light, temperature, CO2 concentration and humidity within the canopy for fluxes of photosynthesis and transpiration of a boreal coniferous forest in central Sweden. A leaf-level photosynthesis-stomatal conductance model was used for aggregating these processes to canopy level while applying the within-canopy distributions of these driving variables. The simulation model showed good agreement with eddy covariance-derived gross primary production (GPP) estimates on daily and annual timescales, and showed a reasonable agreement between transpiration and observed H2O fluxes, where discrepancies are largely attributable to a lack of forest floor evaporation in the model. Simulations in which vertical heterogeneity was artificially suppressed revealed that the vertical distribution of light is the driver of vertical heterogeneity. Despite large differences between above-canopy and within-canopy humidity, and despite large gradients in CO2 concentration during early morning hours after nights with stable conditions, neither humidity nor CO2 played an important role for vertical heterogeneity of photosynthesis and transpiration.

Methane and nitrous oxide exchange over a managed hay meadow.

The methane (CH4) and nitrous oxide (N2O) exchange of a temperate mountain grassland near Neustift, Austria, was measured during 2010–2012 over a time period of 22 months using the eddy covariance method. Exchange rates of both compounds at the site were low, with 97% of all half-hourly CH4 and N2O fluxes ranging between ±200 and ±50 ng m−2 s−1, respectively. The meadow acted as a sink for both compounds during certain time periods, but was a clear source of CH4 and N2O on an annual timescale. Therefore, both gases contributed to an increase of the global warming potential (GWP), effectively reducing the sink strength in terms of CO2 equivalents of the investigated grassland site. In 2011, our best guess estimate showed a net greenhouse gas (GHG) sink of −32 g CO2 equ. m−2 yr−1 for the meadow, whereby 55% of the CO2 sink strength of −71 g CO2m−2 yr−1 was offset by CH4 (N2O) emissions of 7 (32) g CO2 equ. m−2 yr−1. When all data were pooled, the ancillary parameters explained 27 (42)% of observed CH4 (N2O) flux variability, and up to 62 (76)% on shorter timescales in-between management dates. In the case of N2O fluxes, we found the highest emissions at intermediate soil water contents and at soil temperatures close to 0 or above 14 °C.In comparison to CO2, H2O and energy fluxes, the interpretation of CH4 and N2O exchange was challenging due to footprint heterogeneity regarding their sources and sinks, uncertainties regarding post-processing and quality control. Our results emphasize that CH4 and N2O fluxes over supposedly well-aerated and moderately fertilized soils cannot be neglected when evaluating the GHG impact of temperate managed grasslands.

Soil CO2 efflux and production rates as influenced by evapotranspiration in a dry grassland

Our aim was to study the effect of potential biotic drivers, including evapotranspiration (ET) and gross primary production (GPP), on the soil CO2 production and efflux on the diel time scale. Eddy covariance, soil respiration and soil CO2 gradient systems were used to measure the CO2 and H2O fluxes in a dry, sandy grassland in Hungary. The contribution of CO2 production from three soil layers to plot-scale soil respiration was quantified. CO2 production and efflux residuals after subtracting the effects of the main abiotic and biotic drivers were analysed. Soil CO2 production showed a strong negative correlation with ET rates with a time lag of 0.5 h in the two upper layers, whereas less strong, but still significant time-lagged and positive correlations were found between GPP and soil CO2 production. Our results suggest a rapid negative response of soil CO2 production rates to transpiration changes, and a delayed positive response to GPP. We found evidence for a combined effect of soil temperature and transpiration that influenced the diel changes in soil CO2 production. A possible explanation for this pattern could be that a significant part of CO2 produced in the soil may be transported across soil layers via the xylem.

Measurements of diurnal variations and eddy covariance (EC) fluxes of glyoxal in the tropical marine boundary layer: description of the Fast LED-CE-DOAS instrument

Abstract. Here we present first eddy covariance (EC) measurements of fluxes of glyoxal, the smallest α-dicarbonyl product of hydrocarbon oxidation, and a precursor for secondary organic aerosol (SOA). The unique physical and chemical properties of glyoxal – i.e., high solubility in water (effective Henry's law constant, KH = 4.2 × 105 M atm−1) and short atmospheric lifetime (~2 h at solar noon) – make it a unique indicator species for organic carbon oxidation in the marine atmosphere. Previous reports of elevated glyoxal over oceans remain unexplained by atmospheric models. Here we describe a Fast Light-Emitting Diode Cavity-Enhanced Differential Optical Absorption Spectroscopy (Fast LED-CE-DOAS) instrument to measure diurnal variations and EC fluxes of glyoxal and inform about its unknown sources. The fast in situ sensor is described, and first results are presented from a cruise deployment over the eastern tropical Pacific Ocean (20° N to 10° S; 133 to 85° W) as part of the Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOCs (TORERO) field experiment (January to March 2012). The Fast LED-CE-DOAS is a multispectral sensor that selectively and simultaneously measures glyoxal (CHOCHO), nitrogen dioxide (NO2), oxygen dimers (O4), and water vapor (H2O) with ~2 Hz time resolution (Nyquist frequency ~1 Hz) and a precision of ~40 pptv Hz−0.5 for glyoxal. The instrument is demonstrated to be a "white-noise" sensor suitable for EC flux measurements. Fluxes of glyoxal are calculated, along with fluxes of NO2, H2O, and O4, which are used to aid the interpretation of the glyoxal fluxes. Further, highly sensitive and inherently calibrated glyoxal measurements are obtained from temporal averaging of data (e.g., detection limit smaller than 2.5 pptv in an hour). The campaign average mixing ratio in the Southern Hemisphere (SH) is found to be 43 ± 9 pptv glyoxal, which is higher than the Northern Hemisphere (NH) average of 32 ± 6 pptv (error reflects variability over multiple days). The diurnal variation of glyoxal in the marine boundary layer (MBL) is measured for the first time, and mixing ratios vary by ~8 pptv (NH) and ~12 pptv (SH) over the course of 24 h. Consistently, maxima are observed at sunrise (NH: 35 ± 5 pptv; SH: 47 ± 7 pptv), and minima at dusk (NH: 27 ± 5 pptv; SH: 35 ± 8 pptv). In both hemispheres, the daytime flux was directed from the atmosphere into the ocean, indicating that the ocean is a net sink for glyoxal during the day. After sunset the ocean was a source for glyoxal to the atmosphere (positive flux) in the SH; this primary ocean source was operative throughout the night. In the NH, the nighttime flux was positive only shortly after sunset and negative during most of the night. Positive EC fluxes of soluble glyoxal over oceans indicate the presence of an ocean surface organic microlayer (SML) and locate a glyoxal source within the SML. The origin of most atmospheric glyoxal, and possibly other oxygenated hydrocarbons over tropical oceans, remains unexplained and warrants further investigation.

Changes in fluxes of heat, H2O, and CO2 caused by a large wind farm

The Crop Wind-Energy Experiment (CWEX) provides a platform to investigate the effect of wind turbines and large wind farms on surface fluxes of momentum, heat, moisture, and carbon dioxide (CO2). In 2010 and 2011, eddy covariance flux stations were installed between two lines of turbines at the southwest edge of a large Iowa wind farm from late June to early September. We report changes in fluxes of momentum, sensible heat, latent heat, and CO2 above a corn canopy after surface air had passed through a single line of turbines. In 2010, our flux stations were placed within a field with homogeneous land management practices (same tillage, cultivar, chemical treatments). We stratify the data according to wind direction, diurnal condition, and turbine operational status. Within these categories, the downwind–upwind flux differences quantify turbine influences at the crop surface. Flux differences were negligible in both westerly wind conditions and when the turbines were non operational. When the flow is perpendicular (southerly) or slightly oblique (southwesterly) to the row of turbines during the day, fluxes of CO2 and water (H2O) are enhanced by a factor of five in the lee of the turbines (from three to five turbine diameter distances downwind from the tower) as compared to a west wind. However, we observe a smaller CO2 flux increase of 30–40% for these same wind directions when the turbines are off. In the nighttime, there is strong statistical significance that turbine wakes enhance upward CO2 fluxes and entrain sensible heat toward the crop. The direction of the scalar flux perturbation seems closely associated to the differences in canopy friction velocity. Spectra and co-spectra of momentum components and co-spectra of heat also demonstrate nighttime influence of the wind turbine turbulence at the downwind station.

Field Evaluation of Open System Chambers for Measuring Whole Canopy Gas Exchanges

The ability to monitor whole canopy CO2 and H2O fluxes of crop plants in the field is needed for many research efforts ranging from plant breeding to the study of climate change effects on crops. Four portable, transparent, open system chambers for measuring canopy gas exchanges were field tested on well‐watered and fertilized cotton (Gossypium hirsutum L.) over 6 d at Lubbock, TX, in 2010. Our objective was to (i) characterize changes in canopy microclimate variables due to chamber enclosure and (ii) evaluate chamber‐to‐chamber variability in canopy gas‐exchange parameters on ground as well as leaf area basis. Chamber wall materials reduced photosynthetically active radiation (PAR) by about 13%. Programmable data loggers controlled variable speed fans and, with one minor exception, limited heat buildup in the chambers to a maximum of <1.0°C above ambient air temperature. Differentials between incoming and outgoing atmospheric CO2 and H2O concentrations were used to calculate canopy net assimilation (A) and transpiration (E) at 10‐s intervals using solenoid valve actuated sample lines connected to an infrared gas analyzer. Water use efficiency (WUE = A/E) was then calculated and these data were averaged over both 5‐min and hourly intervals. Coefficient of variation (CV) for midday A, E, and WUE averaged 12.3, 9.7, and 5.8%, respectively. Expressing these parameters on a canopy leaf area basis produced similar CV results. These results will help guide experimental design for future research using these or similar chambers.