Partitioning of carbon dioxide exchange in rapidly and slowly changing ecosystems

Im Hinblick auf den aktuellen Klimawandel besteht die Frage, wie die Biosphäre auf den Globalen Wandel und die daraus hervorgehende lokale Landnutzungsänderung bezüglich ihres Kohlenstoffkreislaufes reagiert. Die Landoberfläche ist zum gegenwärtigen Zeitpunkt eine Senke für anthropogene Emissionen von Kohlenstoffdioxid (CO2), jedoch wird gleich-zeitig durch Landnutzungsänderungen zusätzliches CO2 freigesetzt. Nach wie vor ist nicht eindeutig geklärt, wie sensitiv die photosynthetische CO2-Aufnahme und die atmungsbedingte CO2-Freisetzung eines Ökosystems gegenüber Umweltparametern reagieren. Eine Möglichkeit, den vertikalen Fluss der Treibhausgase in ihre Quellen und Senken aufzuspalten, bietet das sogenannte Source-Partitioning. Hierbei werden z. B. vertikale CO2-Flüsse in Photosynthese und Respiration oder im Fall von Wasserdampfflüssen (H2O) in Evaporation und Transpiration aufgetrennt. Derzeitig existieren mehrere Ansätze und Verfahren für das Source-Partitioning, jedoch besitzt jede Methode auch gewisse Nachteile und läßt Raum für Erweiterungen und Verbesserungen. In dieser Arbeit wird zum einen ein Ansatz getestet, der sogenannte Zusatzmessungen benötigt und zum anderen eine analytische Partitionierungsmethode aufgegriffen. Beide Ansätze werden anhand von Fallbeispielen in Agrar- und Waldökosystemen demonstriert und untersucht. Zuerst wird der Prototyp einer mobilen Liftanlage präsentiert, mit der zeitlich sowie räumlich hochaufgelöste Messung von CO2-, H2O-, Temperatur- und Windgeschwindigkeitsprofilen zwischen der Bodenoberfläche und der erdnahen atmosphärischen Grenzschicht über der Pflanzenoberfläche eines Ackers durchgeführt wurden. Die vertikale Verteilung der Konzentrationen von CO2 und H2O kann somit qualitativ für einen dichten Pflanzenbestand bestimmt werden. Dafür wurden zwischen Frühjahr 2015 und Herbst 2016 Kampagnenmessungen in Winterweizen, Wintergerste und einer Zwischenfruchtmischung während verschiedenen Stadien der Pflanzenentwicklung und zu unterschiedlichen Tageszeiten durchgeführt. Mit Hilfe eines Gasanalysators wurden kontinuierlich über eine Profilhöhe von 2 m die Konzentrationen mit einer Frequenz von 20 s-1 aufgezeichnet. Wir demonstrieren die Nachbearbeitung der Messungen (z. B. die Korrektur von Zeitverzögerungen) und zeigen die resultierenden vertikalen Profile als 30-minütige Mittel mit einer Auflösung von 0.025 m. Die Profile zeigen innerhalb des Planzenbestandes deutlich die Effekte der Bodenatmung und der photosynthetischen Kohlenstoffaufnahme, die sowohl innerhalb der Tageszeiten als auch während der Vegetationsperiode variieren. Mit Hilfe der Monin-Obukhov'schen Ähnlichkeitstheorie wurden Messungen über unbewachsenem Boden und einer niedrigen Pflanzendecke analysiert, um die Validität der Profilmessungen und der Rohdatenverarbeitung zu überprüfen. Die abgeleiteten Flüsse von CO2, latente und sensible Wärme und Impuls zeigen eine gute Übereinstimmung zu den parallel durchgeführten Eddy-Kovarianz-Messungen. Während die Kohlenstoffbilanz einer Ackerfläche im Laufe einer Vegetationsperiode zwischen Quelle und Senke wechselt, dauert dieser Prozess in bewirtschafteten Waldökosystemen meist Jahrzehnte. Im Allgemeinen nehmen Wälder in Mitteleuropa im Jahresmittel mehr CO2 auf, als sie abgeben und stellen somit eine Senke für atmosphärisches CO2 dar. Diese Situation kann sich ändern, sobald ein Eingriff in das Waldökosystem stattfindet. Ein Extrembeispiel eines solchen Eingriffs sind flächenhafte Kahlschläge, die den Wald nach der Abholzung von einer ehemaligen Senke zu einer Quelle für CO2 umwandeln. In dieser Arbeit präsentiert werden sieben Jahre CO2-Flussmessungen über einer rund 70 Jahre alten Fichten-Monokulturfläche im Nationalpark Eifel, von der rund 20% drei Jahre nach Beginn der Messung abgeholzt wurden. Ein Eddy-Kovarianz-System, das auf einem 37.8 m hohen Turm innerhalb des Waldes montiert wurde, erfasste kontinuierlich Flüsse sensibler und latenter Wärme, CO2 und Impuls. Nach der teilweisen Entfichtung wurde eine zweite EC-Station innerhalb der Entfichtungsfläche installiert und parallel zur Waldstation betrieben. Komplette Zeitreihen und jährliche Kohlenstoffbilanzen des Netto-Ökosystemaustauschs von CO2 (NEE) und seiner Komponenten Brutto-Primärproduktion (GPP) und Ökosystematmung (Reco) wurden mit Hilfe von Gapfilling- und Source-Partition-ing Methoden berechnet. Daneben wird die gemessene Bodenatmung berücksichtigt und dich sich gegenüberstehenden Klimaeffekte der durch die Entfichtung veränderten CO2-Sequestrierung und dem biophysikalischen Effekt der geänderten Albedo betrachtet. Im Gegensatz zur abgeholzten Fläche zeigten die über dem Wald gemessenen jährlichen NEE-Summen eine starke Kohlenstoffsenke mit geringer zwischenjährlicher Variabilität. Ein Jahr nach der Entfichtung bestand die Vegetation auf der abgeholzten Fläche hauptsächlich aus Gräsern und Sträuchern; ab dem zweiten Jahr konnte ein vermehrter Zuwachs neuer Bäume (vorwiegend Eberesche) beobachtet werden. Die wiederaufkommende Vegetation spiegelte sich in den jährlichen Summen des NEE wieder, so entwickelte sich die Entfichtungsfläche von einer Kohlenstoffquelle (ca. 500 g C m-2 y-1) innerhalb der betrachteten vier Jahre aufgrund der Zunahme photosynthetischer Aktivitäten zunehmend zu einem CO2 neutralen Zustand. Im anschließenden Kapitel wird die Kohlenstoffbilanz eines Ackers über eine drei Jahre andauernde Fruchtwechselfolge untersucht. Der Versuchsstandort Selhausen befindet sich in einer landwirtschaftlich intensiv genutzen Region innerhalb der Niederrheinischen Bucht. Rund 34% der Fläche Deutschlands war im Jahr 2015 durch Landwirtschaft genutzt (FAO, 2015). Die Fähigkeit von landwirtschaftlichen Flächen Kohlenstoff zu binden, aber auch zu emittieren, ist von großer Bedeutung für den lokalen und globalen Kohlenstoffkreislauf. Um eine lokale Kohlenstoffbilanz für ein Agrarökosystem aufstellen und modellieren zu können, benötigt man neben dem gemessenen vertikalen Netto-Ökosystemaustausch zusätzliche Informationen bezüglich seiner Zusammensetzung aus Brutto-Primärproduktion und Ökosystematmung. Die in Ökosystemstudien am häufigsten genutzten Partitionierung-Methoden sind die sogenannten datenbasierenden nichtlinearen Funktionen (NLR). Sie beschreiben den nichtlinearen Zusammenhang zwischen dem gemessenen NEE und Umgebungsvariablen wie Lufttemperatur oder solare Strahlung, die maßgeblich Atmungs- und Photosyntheseprozesse steuern. In der hier vorgestellten Studie wird für die Aufteilung der gemessenen NEE über einer 3-jährigen Fruchtwechselfolge, bestehend aus Winterweizen / Wintergerste / Zwischenfrucht und Zuckerrübe, der Ansatz einer reinen Nacht- (NT) und einer größtenteils Tagdaten (DT) basierenden NLR benutzt. Zusätzlich wurde ein eigener Algorithmus entwickelt und implementiert, der NLR ohne eine vorangehende Aufteilung in Tag- und Nachtdaten berechnet. Der Verlauf der saisonalen und zwischenjährlichen Flüsse von NEE, GPP und Reco zeigten typische Muster und Größenordnungen einer landwirtschaftlich genutzen Fläche innerhalb Mitteleuropas. Die kumulierten Tagessummen der NEE variierten je nach angebauter Frucht und Jahreszeit zwischen +10 und -14 g C m-2 d-1. Die höchste CO2-Aufnahme fand zwischen Mai und Juni im Winterweizen statt. Die höchsten Emissionen wurden nach der Ernte von Wintergerste beobachtet, wobei vermutlich untergepflügte Erntereste im Boden einen Anstieg der Bodenatmung durch Dekompositionsvorgänge begünstigt haben. Über die komplette Fruchtwechselfolge und bei reiner Betrachtung des vertikalen CO2-Flusses zeigte das Ökosystem, je nach verwendetem Partitionierungsmodell, eine Netto-CO2-Aufnahme von -1.3 bis -1,6 kg C m-2 und stellte somit eine Senke für Kohlenstoff dar. Werden zusätzlich zum NEE der Kohlenstoffeintrag und -austrag durch Sähen und Ernte, sowie die Emissionen aus Feldbewirtschaftungsmaßnahmen in der Kohlenstoffbilanzierung berücksichtigt, wird der Acker eine Kohlenstoffquelle (0.7 bis 1.0 kg C m-2). Beim Vergleich der unterschiedlichen NLR fiel auf, dass die Anwendung, die ausschließlich auf Nachtdaten basiert, grundsätzlich höhere Werte der Ökosystematmung ermittelt, als die anderen verwendeten Methoden. So kam es in den kummulierten Flüssen zu Abweichungen von 16%, 6% und 15% zwischen NEE, GPP und Reco im Vergleich zwischen NT und DT. Geringer fielen die Unterschiede zwischen NT und der eigenen Methode aus. Auch andere Studien berichten von Diskrepanzen in der Partitionierung von NEE bei der Verwendung der oben beschriebenen Methoden. Diese und auch unsere Arbeit zeigen, dass weiterhin Forschungsbedarf hinsichtlich der Anwendung von Source-Partitioning besteht.A key question in times of climate change is, how the biosphere responds to global change and the local land use management in regard to its carbon cycle. At the present time, the land surface acts as a sink for anthropogenic carbon dioxide (CO2) emissions. However, additional CO2 is released simultaneously by land use change. There is still no clear understanding of the sensitivity of photosynthetic CO2 uptake and respiratory CO2 release to environmental parameters. One possible way to disentangle the flux of greenhouse gases is source-partitioning, e.g. into photosynthesis and respiration (CO2) or into evaporation and transpiration (H2O). Currently, there are a number of procedures for source-partitioning, however, each method has its disadvantages and allows for extensions and improvements. In this thesis, one instrumental and a data-driven partitioning approaches are taken up and demonstrated by examples of an agro- and forest ecosystem. First, we present the prototype of a portable elevator based device for measuring temporal and spatial high-resolution profiles of CO2, H2O, temperature and wind velocity between the soil surface and the atmospheric surface layer above crop canopies. The vertical distribution of CO2 and H2O concentrations can thus be determined qualitatively for dense crop stands. Between spring 2015 and autumn 2016, campaign measurements were carried out in winter wheat, winter barley, and in an intercrop mixture during different plant development stage and at different times of day. A gas analyzer continuously records the concentrations at a frequency of 20 s-1 over a 2 m profile height. We present a post-processing technique of the measurements (e.g. the correction of time lags) and show the resulting vertical profiles as 30-minute averages over height steps of 0.025 m. The profiles clearly show the effects of soil respiration and photosynthetic carbon uptake within the plant stand, which vary both during the time of day and during the vegetation period. Using the Monin-Obukhov similarity theory, measurements over bare soil and a short plant canopy were analyzed to check the validity of the elevator measurements and the raw data processing. It was found that the derived fluxes of CO2, latent and sensible heat, and momentum correlated well with eddy-covariance (EC) measurements. While the carbon balance of an agricultural area alternates between source and sink during a vegetation period, this process usually requires decades for the management of forest ecosystems. In general, forests in Central Europe assimilate more CO2 on an annual average than they emit and thus are a sink for atmospheric CO2. This may change as soon as the forest ecosystem is intervened. An extreme example of such an intervention is clear cutting. After deforestation, the forest changes from a former sink to a source of CO2. We present seven years of CO2 flux measurements over a 70 year old spruce monoculture in the Eifel National Park, from which about 20% were deforested three-years after beginning of the observation period. An EC system mounted on top of a 37.8 m high tower within the forest, continuously collects fluxes of sensible and latent heat, CO2 and momentum. After partial deforestation, a second EC station was installed within the deforested area and was running parallel to the forest station. Complete time series and annual carbon budgets of the net ecosystem exchange (NEE) of CO2 and its components, gross primary production (GPP) and ecosystem respiration (Reco), were calculated using gap-filling and source-partitioning methods. In addition, local chamber measurements of soil respiration are taken into account and the climatic effects of the changed CO2 sequestration and the biophysical effect of changed albedo are compared. In contrast to the deforested area, the annual sums of NEE measured above the forest show a strong carbon sink with low inter-annual variability. One year after deforestation, the vegetation on the deforested area consisted mainly of grasses and shrubs; from the second year onwards, an increased growth of new trees (mainly mountain ash) could be observed. The recovering vegetation is reflected in the annual sums of NEE, which decreased from a carbon source (500 g C m-2 y-1) towards neutral over the past four years, due to an increase in the photosynthetic activities. In the last chapter, the carbon balance of a three-year crop rotation cycle was examined. The study site Selhausen is located in an intensively managed agricultural region within the Lower Rhine Embayment. About 34% of the area of Germany was covered by agriculture in 2015 (FAO, 2015). The ability of agricultural areas to sequester or also to emit carbon gives them an important role in the local and global carbon cycle. In order to calculate or to model the local carbon balance for an agroecosystem, information about the measured NEE and its components GPP and Reco are needed. The most frequently used partitioning methods in ecosystem studies are the so-called data-based nonlinear regression functions (NLR). NLR describes the nonlinear relationship between the measured NEE and environmental variables, such as air temperature or solar radiation, which are the main drivers of respiration and photosynthetic processes. The study presented here uses a nighttime (NT) and daytime data based (DT) NLR approach for the partitioning of measured NEE in a 3-year crop rotation cycle, consisting of winter wheat / winter barley / catch-crop and sugar beet. In addition, an own algorithm was developed and implemented that calculates NLR without a previous separation of the dataset into day- and nighttime data. The seasonal and inter-annual fluxes of NEE, GPP and Reco showed typical patterns and orders of magnitude of an agroecosystem within Central Europe. The cumulated daily sums of the NEE varied between +10 and -14 g C m-2 d-1 depending on the cultivated crop and season. The highest CO2 uptake took place between May and June in winter wheat. The highest emissions were observed after harvest of winter barley, when crop residues in the soil favoring an increase in soil respiration due to decomposition processes. Over the 3-year crop rotation, the ecosystem acted as a carbon source with a release of 0.7 to 1.0 kg C m-2, depending on the used source-partitioning model. Comparing the different NLR methods, it became apparent that the NT based application overestimated Reco compared to the other methods, resulting in deviations in NT vs. DT of 16%, 6% and 15% between the cumulated fluxes of NEE, GPP and Reco. The differences between NT and the own method were in general smaller. Other studies also reported discrepancies in the partitioning of NEE using the methods described above. Their and our work shows that there is still a need for further investigation regarding source-partitioning strategies

COSMOS-Europe: a European network of cosmic-ray neutron soil moisture sensors

[EN] Climate change increases the occurrence and severity of droughts due to increasing temperatures, altered circulation patterns, and reduced snow occurrence. While Europe has suffered from drought events in the last decade unlike ever seen since the beginning of weather recordings, harmonized long-term datasets across the continent are needed to monitor change and support predictions. Here we present soil moisture data from 66 cosmic-ray neutron sensors (CRNSs) in Europe (COSMOS-Europe for short) covering recent drought events. The CRNS sites are distributed across Europe and cover all major land use types and climate zones in Europe. The raw neutron count data from the CRNS stations were provided by 24 research institutions and processed using state-of-the-art methods. The harmonized processing included correction of the raw neutron counts and a harmonized methodology for the conversion into soil moisture based on available in situ information. In addition, the uncertainty estimate is provided with the dataset, information that is particularly useful for remote sensing and modeling applications. This paper presents the current spatiotemporal coverage of CRNS stations in Europe and describes the protocols for data processing from raw measurements to consistent soil moisture products. The data of the presented COSMOS-Europe network open up a manifold of potential applications for environmental research, such as remote sensing data validation, trend analysis, or model assimilation The dataset could be of particular importance for the analysis of extreme climatic events at the continental scale. Due its timely relevance in the scope of climate change in the recent years, we demonstrate this potential application with a brief analysis on the spatiotemporal soil moisture variability. The dataset, entitled "Dataset of COSMOS-Europe: A European network of Cosmic-Ray Neutron Soil Moisture Sensors", is shared via Forschungszentrum Julich: https://doi.org/10.34731/x9s3-kr48 (Bogena and Ney, 2021).We thank TERENO (Terrestrial Environmental Observatories), funded by the Helmholtz-Gemeinschaft for the financing and maintenance of CRNS stations. We acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) of the research unit FOR 2694 Cosmic Sense (grant no. 357874777) and by the German Federal Ministry of Education of the Research BiookonomieREVIER, Digitales Geosystem -Rheinisches Revier project (grant no. 031B0918A). COSMOS-UK has been supported financially by the UK's Natural Environment Research Council (grant no. NE/R016429/1). The Olocau experimental watershed is partially supported by the Spanish Ministry of Science and Innovation through the research project TETISCHANGE (grant no. RTI2018-093717-BI00). The Calderona experimental site is partially supported by the Spanish Ministry of Science and Innovation through the research projects CEHYRFO-MED (grant no. CGL2017-86839C3-2-R) and SILVADAPT.NET (grant no. RED2018-102719-T) and the LIFE project RESILIENT FORESTS (grant no. LIFE17 CCA/ES/000063). The University of Bristol's Sheepdrove sites have been supported by the UK's Natural Environment Research Council through a number of projects (grant nos. NE/M003086/1, NE/R004897/1, and NE/T005645/1) and by the International Atomic Energy Agency of the United Nations (grant no. CRP D12014).Bogena, HR.; Schrön, M.; Jakobi, J.; Ney, P.; Zacharias, S.; Andreasen, M.; Baatz, R.... (2022). COSMOS-Europe: a European network of cosmic-ray neutron soil moisture sensors. Earth System Science Data. 14(3):1125-1151. https://doi.org/10.5194/essd-14-1125-20221125115114

COSMOS-Europe : a European network of cosmic-ray neutron soil moisture sensors

We thank TERENO (Terrestrial Environmental Observatories), funded by the Helmholtz-Gemeinschaft for the financing and maintenance of CRNS stations. We acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) of the research unit FOR 2694 Cosmic Sense (grant no. 357874777) and by the German Federal Ministry of Education of the Research BioökonomieREVIER, Digitales Geosystem – Rheinisches Revier project (grant no. 031B0918A). COSMOS-UK has been supported financially by the UK’s Natural Environment Research Council (grant no. NE/R016429/1). The Olocau experimental watershed is partially supported by the Spanish Ministry of Science and Innovation through the research project TETISCHANGE (grant no. RTI2018-093717-BI00). The Calderona experimental site is partially supported by the Spanish Ministry of Science and Innovation through the research projects CEHYRFO-MED (grant no. CGL2017-86839- C3-2-R) and SILVADAPT.NET (grant no. RED2018-102719-T) and the LIFE project RESILIENT FORESTS (grant no. LIFE17 CCA/ES/000063). The University of Bristol’s Sheepdrove sites have been supported by the UK’s Natural Environment Research Council through a number of projects (grant nos. NE/M003086/1, NE/R004897/1, and NE/T005645/1) and by the International Atomic Energy Agency of the United Nations (grant no. CRP D12014). Acknowledgements. We thank Peter Strauss and Gerhab Rab from the Institute for Land and Water Management Research, Federal Agency for Water Management Austria, Petzenkirchen, Austria. We thank Trenton Franz from the School of Natural Resources, University of Nebraska–Lincoln, Lincoln, NE, United States. We also thank Carmen Zengerle, Mandy Kasner, Felix Pohl, and Solveig Landmark, UFZ Leipzig, for supporting field calibration, lab analysis, and data processing. We furthermore thank Daniel Dolfus, Marius Schmidt, Ansgar Weuthen, and Bernd Schilling, Forschungszentrum Jülich, Germany. The COSMOS-UK project team is thanked for making its data available to COSMOS-Europe. Luca Stevanato is thanked for the technical details about the Finapp sensor. The stations at Cunnersdorf, Lindenberg, and Harzgerode have been supported by Falk Böttcher, Frank Beyrich, and Petra Fude, German Weather Service (DWD). The Zerbst site has been supported by Getec Green Energy GmbH and Jörg Kachelmann (Meteologix AG). The CESBIO sites have been supported by the CNES TOSCA program. The ERA5-Land data are provided by ECMWF (Muñoz Sabater, 2021). The Jena dataset was retrieved at the site of The Jena Experiment, operated by DFG research unit FOR 1451.Peer reviewedPublisher PD

Altered energy partitioning across terrestrial ecosystems in the European drought year 2018

Drought and heat events, such as the 2018 European drought, interact with the exchange of energy between the land surface and the atmosphere, potentially affecting albedo, sensible and latent heat fluxes, as well as CO(2)exchange. Each of these quantities may aggravate or mitigate the drought, heat, their side effects on productivity, water scarcity and global warming. We used measurements of 56 eddy covariance sites across Europe to examine the response of fluxes to extreme drought prevailing most of the year 2018 and how the response differed across various ecosystem types (forests, grasslands, croplands and peatlands). Each component of the surface radiation and energy balance observed in 2018 was compared to available data per site during a reference period 2004-2017. Based on anomalies in precipitation and reference evapotranspiration, we classified 46 sites as drought affected. These received on average 9% more solar radiation and released 32% more sensible heat to the atmosphere compared to the mean of the reference period. In general, drought decreased net CO(2)uptake by 17.8%, but did not significantly change net evapotranspiration. The response of these fluxes differed characteristically between ecosystems; in particular, the general increase in the evaporative index was strongest in peatlands and weakest in croplands. This article is part of the theme issue 'Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale'

Partitioning of carbon dioxide exchange in rapidly and slowly changing ecosystems

A key question in times of climate change is, how the biosphere responds to global change and the local land use management in regard to its carbon cycle. At the present time, the land surface acts as a sink for an thropogenic carbon dioxide (CO$_{2}$) emissions. However, additional CO$_{2}$ recarried out in winter wheat, winter barley, and in an intercrop mixture during different plant development stage and at diffrent times of day. A gas analyzer continuously records the concentrations at a frequency of 20 s$^{−1}$ over a 2 m profile height. We present a post-processing technique of the measurements (e.g. the correction of timelags) and show the resulting vertical profiles as 30-minute averages over height steps of 0.025m. The profiles clearly show the effects of soil respiration and photosynthetic carbon uptake within the plant stand, which vary both during the time of day and during the vegetation period. Using the Monin-Obukhov similarity theory, measurements over bare soil and a short plant canopy were analyzed to check the validity of the elevator measurements and the raw data processing. It was found that the derived fluxes of CO$_{2}$, latent and sensible heat, and momentum correlated well with eddy-covariance (EC) measurements [...

Determination of fluxes and their source partitioning from high-resolution profile measurements of wind speed and scalars within and above short canopies

A continuously moving elevator-based system is described to measure vertical profiles of wind speed, temperature, CO2 and H2O within and above short plant canopies with a vertical resolution in the centimeter range. On sample days in 2015 to 2017, we measured profiles from the soil surface to 2 m a.g.l. in a crop rotation including wheat, barley, bare soil, winter catch crops and sugarbeet, with canopy heights of up to 1 m.Profiles over bare soil or very short canopies could be described well by fitting Monin-Obukhov-like profiles, and the derived fluxes of momentum and all three scalars matched well those of a nearby eddy-covariance station. In green canopies during the day, CO2 profiles clearly indicated the plant sink and soil source by a local minimum in the canopy and a maximum at the soil surface. H2O profiles, indicating sources both in the canopy and at the soil surface, did or did not show a local minimum between both, depending on canopy structure and turbulence. Temperature profiles showed various shapes including solar incident angle effects, and often the expected opposing signs of thermal stability between the subcanopy and the roughness sublayer.Finally, we test different existing parametrizations to estimate the vertical source / sink distribution from the measured profiles, compare the resulting vertically integrated fluxes to eddy-covariance based net fluxes, and discuss limitations and needed improvements to quantify subcanopy soil respiration and evaporation from such approaches

High-Resolution Vertical Profile Measurements for Carbon Dioxide and Water Vapour Concentrations Within and Above Crop Canopies

We present a portable elevator-based facility for measuring CO2, water vapour, temperature and wind-speed profiles between the soil surface and the atmospheric surface layer above crop canopies. The end of a tube connected to a closed-path gas analyzer is continuously moved up and down over the profile range (in our case, approximately 2 m) while concentrations are logged at a frequency of 20 s−1. Using campaign measurements in winter wheat, winter barley and a catch crop mixture (spring 2015 to autumn 2016) during different stages of crop development and different times of the day, we demonstrate a simple approach to correct for time lags, and the resulting profiles of 30-min mean mole fractions of CO2 and H2O over height increments of 0.025 m. The profiles clearly show the effects of soil respiration and photosynthetic carbon assimilation, varying both during the diurnal cycle and during the growing season. Profiles of temperature and wind speed are based on a ventilated finewire thermocouple and a hot-wire anemometer, respectively. Measurements over bare soil and a short plant canopy were analyzed in the framework of Monin–Obukhov similarity theory to check the validity of the measurements and raw-data-processing approach. Derived fluxes of CO2, latent and sensible heat and momentum show good agreement with eddy-covariance measurements

Instrumental Approaches to Source Partitioning of CO2_{2} and H2_{2}O Fluxes

How does the biosphere react on global change and local land use management? The land surface currently acts as a sink for anthropogenic emissions from fossil fuels, but an additional CO2 release is caused by land use change. The sensitivities of photosynthetic CO2 uptake and respiratory CO2 release to environmental parameters remain uncertain. One possible way to disentangle the flux of greenhouse gases is source partitioning, e.g. into photosynthesis and respiration (CO2) or into evaporation and transpiration (H2O).The BMBF-funded project IDAS-GHG (Instrumental and Data-driven Approaches to Source-Partitioning of Greenhouse Gas Fluxes: Comparison, Combination, Advancement) aims at comparing and improving existing methods for partitioning of CO2 and H2O fluxes into their respective raw components. Data-driven approaches use existing (raw or processed) data of typical eddy-covariance stations. Instrumental approaches of source partitioning require additional measurements at different parts of ecosystems and different methods, e.g. soil-flux chamber measurements, profile measurements or tracer measurements (isotopes).We present preliminary results of a profile measurement system involving a small elevator continuously moving up and down. It measures changes in the concentration of CO2 and H2O at a high vertical and temporal resolution between the soil surface, the plant canopy and the atmosphere. Tests were carried out at the TERENO research site of Selhausen (Lower Rhine Embayment in the river Rur catchment (50°52'09’’N, 06°27'01’’E, 104.5 m MSL, Germany) on a winter wheat field for a growing season from seeding to harvest (April - August 2015).The half hourly mean profiles of CO2 and H2O show the effects of soil respiration and photosynthetic carbon assimilation very clearly, varying both during the daily cycle and during the growing season.An additional way to partition CO2 and H2O fluxes is through measurements of concentration profiles of their stable isotopologues (13CO2, 12C18O16O, 1H2H16O, and 1H218O). Following controlled-conditions experiments in the laboratory on soil columns in autumn and winter 2015, a quantum-cascade dual isotope laser will be deployed at the Selhausen test site in a low-flow (i.e., soil atmosphere and chamber measurements) and high flow (i.e., Eddy-Covariance measurements) configurations for comparison with the above-mentioned profile measurement system