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See detailRestoring a worn-out pasture: what impact on N2O exchanges ?
Lognoul, Margaux ULiege; Gourlez de la Motte, Louis ULiege; Naiken, Alwin ULiege et al

Poster (2019, April 08)

A paired-flux tower experiment was set up in a 40 y-o grazed pasture managed by a local farmer. A parcel under restoration was compared to a control plot. In addition to N2O flux monitoring, soil mineral ... [more ▼]

A paired-flux tower experiment was set up in a 40 y-o grazed pasture managed by a local farmer. A parcel under restoration was compared to a control plot. In addition to N2O flux monitoring, soil mineral N content (ammonium and nitrate) was measured every 10-15 days. [less ▲]

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See detailHerd position habits can bias net CO2 ecosystem exchange estimates in free range grazed pastures.
Gourlez de la Motte, Louis ULiege; Dumortier, Pierre ULiege; Beckers, Yves ULiege et al

Conference (2019, April)

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In ... [more ▼]

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In free range grazed pastures, NEE estimations are supposed to also include cattle respiration. However, cattle respiration measurement by an EC system is challenging as animals act as moving points emitting CO2 that are more or less captured by the EC tower depending on their presence in the footprint. Often it is supposed that, over the long term, cattle distribution in the pasture is homogeneous so that fluctuations due to moving sources are averaged and NEE estimates are reasonably representative of cattle respiration. In this study, we test this hypothesis by comparing daily cow respiration rate per livestock unit (LU) estimated by postulating a homogeneous cow repartition over the whole pasture with three other estimates based on animal localization data, animal scale carbon budget and confinement experiments. We applied these methods to an intensively managed free range grassland and showed that the NEE estimate based on a homogeneous cow repartition was systematically lower than the three other estimates. Consequently, in order to allow estimating the validity of this hypothesis but also to improve inter site comparisons, we advocate to compute separately pasture NEE and grazer’s respiration. In the presentation, we will propose a method based on cattle presence detection using CH4 fluxes, elimination of data with cattle and gap filling on the basis of data without cattle. For the second we propose three independent methods (animal localization with GPS, animal scale carbon budget, confinement experiments) to estimate the cattle respiration rate and discuss their use depending on site specificities. [less ▲]

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See detailHerd position habits can bias net CO2 ecosystem exchange estimates in free range grazed pastures.
Gourlez de la Motte, Louis ULiege; Dumortier, Pierre ULiege; Beckers, Yves ULiege et al

in Agricultural and Forest Meteorology (2019), 268

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In ... [more ▼]

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In free range grazed pastures, NEE estimations are supposed to also include cattle respiration. However, cattle respiration measurement by an EC system is challenging as animals act as moving points emitting CO2 that are more or less captured by the EC tower depending on their presence in the footprint. Often it is supposed that, over the long term, cattle distribution in the pasture is homogeneous so that fluctuations due to moving sources are averaged and NEE estimates are reasonably representative of cattle respiration. In this study, we test this hypothesis by comparing daily cow respiration rate per livestock unit (LU) estimated by postulating a homogeneous cow repartition over the whole pasture with three other estimates based on animal localization data, animal scale carbon budget and confinement experiments. We applied these methods to an intensively managed free range grassland and showed that the NEE estimate based on a homogeneous cow repartition was systematically lower than the three other estimates. The bias was about 60 g C m–2 yr–1, which corresponded to around 40% of the annual NEE. The sign and the importance of this bias is site specific, as it depends on cow location habits in relation to the footprint of the EC measurements which highlight the importance of testing the hypothesis of homogeneity of cattle distribution on each site. Consequently, in order to allow estimating the validity of this hypothesis but also to improve inter site comparisons, we advocate to compute separately pasture NEE and grazer’s respiration. For the former we propose a method based on cattle presence detection using CH4 fluxes, elimination of data with cattle and gap filling on the basis of data without cattle. For the second we present and discuss three independent methods (animal localization with GPS, animal scale carbon budget, confinement experiments) to estimate the cattle respiration rate. [less ▲]

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See detailRotational and continuous grazing does not affect the total net ecosystem exchange of a pasture grazed by cattle but modifies CO2 exchange dynamics
Gourlez de la Motte, Louis ULiege; Mamadou, Ossenatou; Beckers, Yves ULiege et al

in Agriculture, Ecosystems and Environment (2018), 253

Grassland carbon budgets are known to be greatly dependent on management. In particular, grazing is known to directly affect CO2 exchange through consumption by plants, cattle respiration, natural ... [more ▼]

Grassland carbon budgets are known to be greatly dependent on management. In particular, grazing is known to directly affect CO2 exchange through consumption by plants, cattle respiration, natural fertilisation through excreta, and soil compaction. This study investigates the impact of two grazing methods on the net ecosystem exchange (NEE) dynamics and carbon balance, by measuring CO2 fluxes using eddy covariance in two adjacent pastures located in southern Belgium during a complete grazing season. Rotational (RG) grazing consists of an alternation of rest periods and short high stock density grazing periods. Continuous grazing (CG) consists of uninterrupted grazing with variable stocking rates. To our knowledge, this is the first study to assess the impact of these grazing methods on total net ecosystem exchange and CO2 exchange dynamics using eddy covariance. The results showed that NEE dynamics were greatly impacted by the grazing method. Following grazing events on the RG parcel, net CO2 uptake on the RG parcel was reduced compared to the CG parcel. During the following rest periods, this phenomenon progressively shifted towards a higher assimilation for the RG treatment. This behaviour was attributed to sharp biomass changes in the RG treatment and therefore sharp changes in plant photosynthetic capacity. We found that differences in gross primary productivity at high radiation were strongly correlated to differences in standing biomass. In terms of carbon budgets, no significant difference was observed between the two treatments, neither in cumulative NEE, or in terms of estimated biomass production. The results of our study suggest that we should not expect major benefits in terms of CO2 uptake from rotational grazing management when compared to continuous grazing management in intensively managed temperate pastures. [less ▲]

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See detailRestoring a worn-out pasture : What impact on greenhouse gas exchanges ?
Lognoul, Margaux ULiege; Gourlez de la Motte, Louis ULiege; Debacq, Alain ULiege et al

Poster (2018, September 12)

The restoration of permanent pastures is often required in order to restore a productive state and the palatability of the grass. The restoration process consists on destroying the former vegetation using ... [more ▼]

The restoration of permanent pastures is often required in order to restore a productive state and the palatability of the grass. The restoration process consists on destroying the former vegetation using herbicides followed by tillage and reseed. The short term and long term impacts of such operations on the carbon cycle and N2O emissions are not well defined for old permanent pastures. Therefore, a paired flux tower measurement campaign was started in March 2018 at the Dorinne Terrestrial Observatory in Southern Belgium, with the aim to study the impact of pasture restoration on CO2 and N2O fluxes exchanged by the ecosystem. The site is a 100-year-old intensively managed grassland which last restoration was performed more than 40 years ago. It is grazed by Belgian blue beef cattle and fertilized with around 120 kgN ha-1 per year on average, reflecting common practices in the area. A former study carried out at the same site, showed that the pasture acted as significant carbon sink before the start of experiment. Two adjacent parcels belonging to the same farm were both equipped with identical instrumentation including eddy covariance measurements of CO2 (LICOR 7000) and N2O/CH4 (Aerodyne Inc. quantum cascade laser) exchanges to allow the comparison between a control and a restored plot subject to identical pedo-climatic conditions. Preliminary results of greenhouse gas fluxes will be presented in relation to climatic conditions and management operations and the evolution of soil ammonium and nitrate. [less ▲]

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See detailN2O flux short-term response to meteorological solicitations and farming practices in a fertilized crop
Lognoul, Margaux ULiege; Debacq, Alain ULiege; Manise, Tanguy ULiege et al

Poster (2018, September 11)

Using the eddy covariance technique, half-hourly N2O fluxes were measured over a sugar beet crop (Terrestrial Observatory of Lonzée, BE, ICOS site level 2) between March and October 2016. Several ... [more ▼]

Using the eddy covariance technique, half-hourly N2O fluxes were measured over a sugar beet crop (Terrestrial Observatory of Lonzée, BE, ICOS site level 2) between March and October 2016. Several parameters of data quality control tests were modified to suit the characteristics of N2O. The u* filtering threshold was determined based on CO2 data as the procedure could not be implemented using N2O fluxes. The uncertainty on N2O fluxes was assessed for several aspects of data treatment (total random uncertainty, spectral correction, u* filtering, gap-filling), which were combined to determine the uncertainty on the budget. N2O flux variability was characterized by three peak episodes during the experiment, interspersed with background fluxes. These events were driven by several variables, depending on the time-scale. The more time had passed after fertilization, the lower the potential for high fluxes was, and by the end of the crop season, only background flux was recorded. The soil water content at 5 cm was identify as the single trigger of N2O emission bursts, while intraday oscillations were positively correlated to the variations of surface temperature. For the first time, an inhibiting effect of surface soil disturbance (seed-bed preparation) on N2O fluxes was observed, which delayed the start of the following emission peak. This observation combined to the synchronicity between surface temperature and the oscillations of N2O fluxes supports the hypothesis of a N2O producing microbial community located in the topmost soil layer. Between fertilization and harvest, the crop emitted 6520 (± 908) μmol N2O m-2 which corresponds to an EF of 1.3 % - slightly above the IPCC estimate. Our results stress the importance of measuring N2O exchanges in fertilized crops, as it weighed for 20% of the GHG budget. [less ▲]

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See detailRotational and continuous grazing does not affect the total net ecosystem exchange of a pasture grazed by cattle but modifies CO2 exchange dynamics
Gourlez de la Motte, Louis ULiege; Beckers, Yves ULiege; Bodson, Bernard ULiege et al

Conference (2018, April)

Grassland carbon budgets are known to be greatly dependent on management. In particular, grazing is known to directly affect CO2 exchange through consumption by plants, cattle respiration, natural ... [more ▼]

Grassland carbon budgets are known to be greatly dependent on management. In particular, grazing is known to directly affect CO2 exchange through consumption by plants, cattle respiration, natural fertilisation through excreta, and soil compaction. This study investigates the impact of two grazing methods on the net ecosystem exchange (NEE) dynamics and carbon balance, by measuring CO2 fluxes using eddy covariance in two adjacent pastures located in southern Belgium during a complete grazing season. Rotational (RG) grazing consists of an alternation of rest periods and short high stock density grazing periods. Continuous grazing (CG) consists of uninterrupted grazing with variable stocking rates. To our knowledge, this is the first study to assess the impact of these grazing methods on total net ecosystem exchange and CO2 exchange dynamics using eddy covariance. The results showed that NEE dynamics were greatly impacted by the grazing method. Following grazing events on the RG parcel, net CO2 uptake on the RG parcel was reduced compared to the CG parcel. During the following rest periods, this phenomenon progressively shifted towards a higher assimilation for the RG treatment. This behaviour was attributed to sharp biomass changes in the RG treatment and therefore sharp changes in plant photosynthetic capacity. We found that differences in gross primary productivity at high radiation were strongly correlated to differences in standing biomass. In terms of carbon budgets, no significant difference was observed between the two treatments, neither in cumulative NEE, or in terms of estimated biomass production. The results of our study suggest that we should not expect major benefits in terms of CO2 uptake from rotational grazing management when compared to continuous grazing management in intensively managed temperate pastures. [less ▲]

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See detailImpact of CO2 storage flux sampling uncertainty on net ecosystem exchange measured by eddy covariance
Nicolini, G.; Aubinet, Marc ULiege; Feigenwinter, C. et al

in Agricultural and Forest Meteorology (2018), 248

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained ... [more ▼]

Complying with several assumption and simplifications, most of the carbon budget studies based on eddy covariance (EC) measurements quantify the net ecosystem exchange (NEE) by summing the flux obtained by EC (FC) and the storage flux (SC). SC is the rate of change of a scalar, CO2 molar fraction in this case, within the control volume underneath the EC measurement level. It is given by the difference in the quasi-instantaneous profiles of concentration at the beginning and end of the EC averaging period, divided by the averaging period. The approaches used to estimate SC largely vary, from measurements based on a single sampling point usually located at the EC measurement height, to measurements based on profile sampling. Generally a single profile is used, although multiple profiles can be positioned within the control volume. Measurement accuracy reasonably increases with the spatial sampling intensity, however limited resources often prevent more elaborated measurement systems. In this study we use the experimental dataset collected during the ADVEX campaign in which turbulent and non-turbulent fluxes were measured in three forest sites by the simultaneous use of five towers/profiles. Our main objectives are to evaluate both the uncertainty of SC that derives from an insufficient sampling of CO2 variability, and its impact on concurrent NEE estimates.Results show that different measurement methods may produce substantially different SC flux estimates which in some cases involve a significant underestimation of the actual SC at a half-hourly time scales. A proper measuring system, that uses a single vertical profile of which the CO2 sampled at 3 points (the two closest to the ground and the one at the lower fringe of the canopy layer) is averaged with CO2 sampled at a certain distance and at the same height, improves the horizontal representativeness and reduces this (proportional) bias to 2–10% in such ecosystems. While the effect of this error is minor on long term NEE estimates, it can produce significant uncertainty on half-hourly NEE fluxes. © 2017 [less ▲]

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See detailTowards long-term standardised carbon and greenhouse gas observations for monitoring Europe´s terrestrial ecosystems: a review
Franz, D; Acosta; Altimir et al

in International Agrophysics (2018)

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See detailInter-annual variability of Net Ecosystem Productivity for a temperate mixed forest: A predominance of carry-over effects?
Aubinet, Marc ULiege; Hurdebise, Quentin ULiege; Chopin, Henri ULiege et al

in Agricultural and Forest Meteorology (2018), 262

This study presents twenty years of Net Ecosystem Productivity estimations obtained using eddy covariance in a mixed forest, dominated by beech with sparse conifers, at the Vielsalm station, in the ... [more ▼]

This study presents twenty years of Net Ecosystem Productivity estimations obtained using eddy covariance in a mixed forest, dominated by beech with sparse conifers, at the Vielsalm station, in the Belgian Ardennes. First the quality and reliability of the data set is discussed. An uncertainty analysis showed that if, on one hand, the site heterogeneity and set-up changes may strongly affect yearly NEP estimates, questioning thus the total carbon budget relevance, on the other hand, robust inter-annual anomalies may be obtained as long as a site dedicated data treatment is carefully applied. A validation of the anomalies by comparison with a growth index derived from tree ring measurements is given. The resulting anomalies (range: [−206; + 123] g C m−2 yr−1, standard deviation: 93 g C m−2 yr−1) being larger than their own uncertainty (∼30 g C m−2 yr−1), an inter-annual variability analysis is possible. This analysis shows that the sources of NEP inter-annual variability at the Vielsalm station are multiple but the most prominent causes are biotic processes driven by carry-over effects of preceding meteorological events. The lowest observed NEP, in 2000, resulted from a bark beetle attack probably prompted by an early frost event in 1998. Besides, the robust lagged correlation between NEP anomalies and mean vapor pressure deficit during the preceding vegetation season also suggests a carry-over effect of water limitation during the previous year on the beech NEP. Mechanisms driving this carry-over effect are supposedly linked to tree physiology, which is confirmed by a dependency of canopy photosynthetic capacity to previous year water limitation. Some hypotheses, involving biomass allocation and bud formation, are proposed to explain its lagged impact on canopy photosynthetic capacity. Other causes of NEP inter-annual variability are the radiation during the current vegetation season and the temperature at the end of the winter but the latter variable rather indicates an effect on the conifers interspersed in the plot. Overall, the photosynthetic capacity combined with these two factors explained about 75% of NEP inter-annual variability. © 2018 Elsevier B.V. [less ▲]

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See detailICOS eddy covariance flux-station site setup: A review
Rebmann, C.; Aubinet, Marc ULiege; Schmid, H. et al

in International Agrophysics (2018), 32(4), 471-494

The Integrated Carbon Observation System Research Infrastructure aims to provide long-Term, continuous observations of sources and sinks of greenhouse gases such as carbon dioxide, methane, nitrous oxide ... [more ▼]

The Integrated Carbon Observation System Research Infrastructure aims to provide long-Term, continuous observations of sources and sinks of greenhouse gases such as carbon dioxide, methane, nitrous oxide, and water vapour. At ICOS ecosystem stations, the principal technique for measurements of ecosystem-Atmosphere exchange of GHGs is the eddy-covariance technique. The establishment and setup of an eddy-covariance tower have to be carefully reasoned to ensure high quality flux measurements being representative of the investigated ecosystem and comparable to measurements at other stations. To fulfill the requirements needed for flux determination with the eddy-covariance technique, variations in GHG concentrations have to be measured at high frequency, simultaneously with the wind velocity, in order to fully capture turbulent fluctuations. This requires the use of high-frequency gas analysers and ultrasonic anemometers. In addition, to analyse flux data with respect to environmental conditions but also to enable corrections in the post-processing procedures, it is necessary to measure additional abiotic variables in close vicinity to the flux measurements. Here we describe the standards the ICOS ecosystem station network has adopted for GHG flux measurements with respect to the setup of instrumentation on towers to maximize measurement precision and accuracy while allowing for flexibility in order to observe specific ecosystem features. © 2018 Corinna Rebmann et al., published by Sciendo 2018. [less ▲]

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See detailMethanol exchange dynamics between a temperate cropland soil and the atmosphere
Bachy, Aurélie ULiege; Aubinet, Marc ULiege; Amelynck, Crist et al

in Atmospheric Environment (2018), 176

Soil methanol (CH3OH) exchange is often considered as several orders of magnitude smaller than plant methanol exchange. However, for some ecosystems, it is significant in regard with plant exchange and ... [more ▼]

Soil methanol (CH3OH) exchange is often considered as several orders of magnitude smaller than plant methanol exchange. However, for some ecosystems, it is significant in regard with plant exchange and worth thus better consideration. Our study sought to gain a better understanding of soil exchange. Methanol flux was measured at the ecosystem scale on a bare agricultural soil over two contrasted periods using the disjunct eddy covariance by mass scanning technique. A proton-transfer-reaction mass spectrometer was used for the methanol ambient mixing ratio measurements. Bi-directional exchange dynamics were observed. Methanol emission occurred under dry and warm conditions and correlated best with soil surface temperature, whereas methanol uptake occurred under wet and mild conditions and correlated well with the methanol ambient concentration. After having tested a physical adsorption-desorption model and by confronting our data with the literature, we propose that the exchange was ruled by both a physical adsorption/desorption mechanism and by a methanol source, which still needs to be identified. The soil emission decreased when the vegetation developed. The reasons for the decrease still need to be determined. Overall, the dynamics observed at our site were similar to those reported by other studies for both cropland and forest ecosystems. The mechanism proposed in our work can thus be possibly applied to other sites or ecosystems. In addition, the methanol exchange rate was in the upper range of the exchange rates reported by other soil studies, suggesting that cropland soils are more important methanol exchangers than those in other ecosystems and should therefore be further investigated. [less ▲]

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See detailDecrease in the photosynthetic performance of temperate grassland species does not lead to a decline in the gross primary production of the ecosystem
Digrado, Anthony ULiege; Gourlez de la Motte, Louis ULiege; Bachy, Aurélie ULiege et al

in Frontiers in Plant Science (2018), 9

Plants, under stressful conditions, can proceed to photosynthetic adjustments in order to acclimatize and alleviate the detrimental impacts on the photosynthetic apparatus. However, it is currently ... [more ▼]

Plants, under stressful conditions, can proceed to photosynthetic adjustments in order to acclimatize and alleviate the detrimental impacts on the photosynthetic apparatus. However, it is currently unclear how adjustment of photosynthetic processes under environmental constraints by plants influences CO2 gas exchange at the ecosystem-scale. Over a two-year period, photosynthetic performance of a temperate grassland ecosystem was characterized by conducting frequent chlorophyll fluorescence (ChlF) measurements on three primary grassland species (Lolium perenne L., Taraxacum sp., and Trifolium repens L.). Ecosystem photosynthetic performance was estimated from measurements performed on the three dominant grassland species weighed based on their relative abundance. In addition, monitoring CO2 fluxes was performed by eddy covariance. The highest decrease in photosynthetic performance was detected in summer, when environmental constraints were combined. Dicot species (Taraxacum sp. and T. repens) presented the strongest capacity to up-regulate PSI and exhibited the highest electron transport efficiency under stressful environmental conditions compared with L. perenne. The decline in ecosystem photosynthetic performance did not lead to a reduction in gross primary productivity, likely because increased light energy was available under these conditions. The carbon amounts fixed at light saturation were not influenced by alterations in photosynthetic processes, suggesting photosynthesis was not impaired. Decreased photosynthetic performance was associated with high respiration flux, but both were influenced by temperature. Our study revealed variation in photosynthetic performance of a grassland ecosystem responded to environmental constraints, but alterations in photosynthetic processes appeared to exhibit a negligible influence on ecosystem CO2 fluxes. [less ▲]

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See detailBiogenic volatile organic compound emissions from senescent maize leaves and a comparison with other leaf developmental stages
Mozaffar, Md Ahsan ULiege; Schoon, N.; Bachy, Aurélie ULiege et al

in Atmospheric Environment (2018), 176

Plants are the major source of Biogenic Volatile Organic Compounds (BVOCs) which have a large influence on atmospheric chemistry and the climate system. Therefore, understanding of BVOC emissions from all ... [more ▼]

Plants are the major source of Biogenic Volatile Organic Compounds (BVOCs) which have a large influence on atmospheric chemistry and the climate system. Therefore, understanding of BVOC emissions from all abundant plant species at all developmental stages is very important. Nevertheless, investigations on BVOC emissions from even the most widespread agricultural crop species are rare and mainly confined to the healthy green leaves. Senescent leaves of grain crop species could be an important source of BVOCs as almost all the leaves senesce on the field before being harvested. For these reasons, BVOC emission measurements have been performed on maize (Zea mays L.), one of the most cultivated crop species in the world, at all the leaf developmental stages. The measurements were performed in controlled environmental conditions using dynamic enclosures and proton transfer reaction mass spectrometry (PTR-MS). The main compounds emitted by senescent maize leaves were methanol (31% of the total cumulative BVOC emission on a mass of compound basis) and acetic acid (30%), followed by acetaldehyde (11%), hexenals (9%) and m/z 59 compounds (acetone/propanal) (7%). Important differences were observed in the temporal emission profiles of the compounds, and both yellow leaves during chlorosis and dry brown leaves after chlorosis were identified as important senescence-related BVOC sources. Total cumulative BVOC emissions from senescent maize leaves were found to be among the highest for senescent Poaceae plant species. BVOC emission rates varied strongly among the different leaf developmental stages, and senescent leaves showed a larger diversity of emitted compounds than leaves at earlier stages. Methanol was the compound with the highest emissions for all the leaf developmental stages and the contribution from the younggrowing, mature, and senescent stages to the total methanol emission by a typical maize leaf was 61, 13, and 26%, respectively. This study shows that BVOC emissions from senescent maize leaves cannot be neglected and further investigations in field conditions are recommended to further constrain the BVOC emissions from this important C4 crop species. [less ▲]

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See detailN2O flux response to meteorological solicitations and farming practice in a sugar beet crop
Lognoul, Margaux ULiege; Debacq, Alain ULiege; Manise, Tanguy ULiege et al

Poster (2017, October 20)

N2O fluxes were measured using eddy covariance on a sugar beet crop from fertilization to harvest. Our poster presents our results and questions about methodology and uncertainties.

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See detailEnvironmental controls of methanol emissions from a grazed grassland in Dorinne, Belgium
Michel, Colin ULiege; Heinesch, Bernard ULiege; Bachy, Aurélie ULiege et al

Poster (2017, October 20)

Despite the growing interest for oxygenated volatile organic compounds (OVOC) over the last 15 years due to their role in the atmospheric chemistry, current knowledge about OVOC exchanges by grassland and ... [more ▼]

Despite the growing interest for oxygenated volatile organic compounds (OVOC) over the last 15 years due to their role in the atmospheric chemistry, current knowledge about OVOC exchanges by grassland and the environmental factors driving them remains lacunar. However, those ecosystems represent an important part of the total earth surface (13.37%). This study conducted on a grazed grassland therefore aims to quantify OVOC exchanges over full grazing seasons in order to understand the mechanisms behind these exchanges. It took place within the activities of the CROSTVOC project (CROp Stress VOC) and therefore gives an important attention to the stress induced fluxes. BVOC flux measurements were performed with a PTR-MS for the measurements of OVOC mixing ratios on two different scales: the eddy Covariance method was used during two whole grazing seasons (2014 and 2015) and in the 2016 summer, measurements were also performed on a smaller scale by using all-teflon automated dynamic chambers. The chambers allowed measuring accurately the impact of grazing by following simultaneously undisturbed and grazed grassland patches. This study pointed out that several OVOC were exchanged in variable quantities, with methanol being by far the most important. Methanol fluxes exhibited a clear diurnal cycle with close-to-zero fluxes at night and maximum fluxes at midday. The flux was also much larger in the summer than during autumn or spring. For the eddy Covariance data, the average methanol flux in the summer (0.033 μg.m-2s-1) was in the same range of other studies, being lower than the average found by Bamberger et al. in 2010 (0.080 μg.m-2s-1) and by Ammann (0.077 μg.m-2s-1). Fluxes from the chambers were slightly lower with an average value of 0.27 μg.m-2s-1 but they were measured in the late summer. Driver analysis is still ongoing but first results showed that the flux was strongly correlated at short time scale (half hourly for the EC method and one and a half hourly for the chambers) to the photosynthetic photon flux density and the latent heat flux. We believe that their influence comes mainly from the control practiced by stomatal conductance in non-steady state conditions and expansion of cell walls in steady state conditions. The cuvette data showed that grazing resulted in significantly enhanced emissions of methanol that lasted for several days. On the contrary, the eddy Covariance method cannot distinguish between grazed and non-grazed grass and only a small fraction of the method footprint had been grazed the previous days on average. Our eddy-covariance data suggested that an increase in the stocking density did not contribute to a significant rise of methanol emission. The limited variability in stocking density during the two growing seasons and the more powerful effect of other drivers likely induced that the flux variability due to the SD was too low to be measured by the eddy Covariance method. In depth comparisons between the eddy-covariance data and the dynamic chambers data will be carried out. [less ▲]

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See detailBeech phenology and productivity at the Vielsalm Terrestrial Observatory
Hurdebise, Quentin ULiege; Vincke, Caroline; De Ligne, Anne ULiege et al

Poster (2017, October 20)

Temperate forests, as other ecosystems and oceans, mitigate the accumulation in the atmosphere of CO2, the main responsible for the current climate change. It is, therefore, a necessity to understand how ... [more ▼]

Temperate forests, as other ecosystems and oceans, mitigate the accumulation in the atmosphere of CO2, the main responsible for the current climate change. It is, therefore, a necessity to understand how these forests react and will react in a changing environment. That requires long-term simultaneous monitoring of the forest productivity and phenology as well as of climatic variables. Using the 20 years dataset from the Vielsalm Terrestrial Observatory (VTO), an eddy covariance site located in a mixed forest in east Belgium and dominated by beeches, the relations between phenological and productivity indicators were analyzed. Phenological indicators (leaf out beginning, duration and ending as well as leaf fall date) were derived from the relative light transmissivity through the canopy and from the temporal dynamic of the Net Ecosystem Productivity (NEP). No trend was observed for these indicators during the last two decades, but significant relations were found between them and between them and climatic variables. Productivity indicators were derived from eddy covariance measurements (NEP), tree ring widths (growth index) and masting intensity. Growth index was significantly related to the NEP and a reverse proportionality to the masting intensity was found. The analysis of the relation between the beech stand NEP and the phenology gives us evidences that annual NEP is significantly related to the length of the carbon uptake period. A model combining the length of the carbon uptake period and the photosynthetic capacity of the ecosystem was able to predict more than two third of the interannual variability at the VTO. This work underlines the multiplicity of the relation between phenological and productivity indicators. [less ▲]

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