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Moor- und weitere organische Böden speichern große Mengen an organischem Bodenkohlenstoff, die bei Entwässerung als Kohlendioxid (CO2) freigesetzt werden. Da in Deutschland ein Großteil dieser Böden entwässert ist, tragen sie nach derzeitigen Berechnungen mit 54 Mio. t CO2-Äqivalenten pro Jahr 7,5% zu den gesamten deutschen Treibhausgasemissionen bei. Zunehmende Anstrengungen bei der Umsetzung von Minderungsmaßnahmen erfordern sowohl adäquate Methoden in der Emissionsberichterstattung als auch ein konsistentes Monitoring. Das Projekt 'Aufbau eines deutschlandweiten Moorbodenmonitorings für den Klimaschutz (MoMoK) - Teil 1: Offenland' (2020 bis 2025) stellt sich der Herausforderung, ein langfristiges deutschlandweites repräsentatives Monitoringnetz für Moorböden aufzubauen. Dieses Messnetz wird ca. 200 Standorte umfassen. Dabei werden sowohl Standorte unter entwässerungsbasierter landwirtschaftlicher Nutzung als auch naturnahe Standorte und wiedervernässte ungenutzte oder als Paludikultur genutzte Moor- und weitere organische Böden untersucht. Eine Langzeitmessung von Treibhausgasflüssen ist nur an wenigen Standorten möglich. Entsprechend fokussiert das Moorbodenmonitoring auf die Erfassung von Geländehöhen, da Geländehöhenänderungen als Proxy für Änderungen der Vorräte an organischem Bodenkohlenstoff dienen können. Daneben wird an allen Standorten der Vorrat an organischem Bodenkohlenstoff gemessen, um die Grundlage für eine mögliche Wiederholungsinventur zu legen. Standorte anderer Monitoringprogramme (Langzeitmessstandorte für Treibhausgase, Bodendauerbeobachtung) werden nach Möglichkeit eingebunden. Das hier vorgelegte Methodenhandbuch beschreibt das generelle Messdesign des Moorbodenmonitorings, das Vorgehen für die Einrichtung von Untersuchungsflächen, die Probenahme im Gelände und die Labormethoden. Aufgenommene Größen umfassen Geländehöhen (jährliche Erfassung der gesamten Untersuchungsfläche und kontinuierliche Aufnahme an deren Zentralpunkt), Gesamtvorräte an organischem Bodenkohlenstoff und Stickstoff, horizontbezogene bodenchemische und -physikalische Eigenschaften, Moor- und Oberflächenwasserstände sowie die Vegetation und Bewirtschaftung. Neben praktischen Anleitungen wird der wissenschaftliche Hintergrund für methodische Entscheidungen erläutert. Parallel zum Aufbau des Messnetzes werden die derzeit in der Emissionsberichterstattung genutzten Regionalisierungsansätze für Treibhausgasemissionen bzw. deren Steuergrößen verbessert. Dies umfasst die Aktualisierung der Kulisse organischer Böden, die Verbesserung der Regionalisierung der Moorwasserstände und die Ableitung der Treibhausgasemissionen. Diese Arbeiten sind jedoch nicht Teil des Methodenhandbuchs. Ziel des Methodenhandbuchs ist es, die im Rahmen des Moorbodenmonitorings (Teil 1: Offenland) im Gelände und Labor angewandten Methoden umfassend und transparent zu dokumentieren. Daneben sollen diese detaillierten Informationen, Anleitungen und Protokolle all jenen als Anregung zur Verfügung stehen, die ein Monitoring von Moorstandorten planen oder einzelne Größen konsistent zum Moorbodenmonitoring aufnehmen möchten. Peat and further organic soils store large amounts of soil organic carbon, which is released as carbon dioxide (CO2) upon drainage. In Germany, most of these soils are drained, and they thus contribute with 54 Mio. t CO2-equivalents per year 7.5% to the total German greenhouse gas emissions. Increasing efforts in implementing mitigation measures call for both adequate methods in emission reporting and consistent monitoring. The project 'Establishment of a German peatland monitoring programme for climate protection (MoMoK) - Part 1: Open land' (2020-2025) rises to the challenge of building a long-term representative monitoring network for peatlands all over Germany. This network will comprise around 200 sites. It encompasses drained grassland and cropland, paludiculture as well as unutilized semi-natural and rewetted sites. As long-term monitoring of greenhouse gas exchange will only be feasible at a few sites, the peatland monitoring programme focusses on the measurement of soil motion, which may be used as proxy for changes in the stocks of soil organic carbon. Beside soil motion, total stocks of organic carbon are determined at all sites as a baseline for possible future inventories. Sites of other monitoring programmes (long-term greenhouse gas flux sites, soil monitoring) are integrated into the network wherever possible. This handbook describes the general design of the peatland monitoring programme, the site setup and field sampling as well as laboratory methods. The monitoring programme collects data on surface heights (annual survey of the monitoring sites, continuous point measurements of surface motion), total soil organic carbon and nitrogen stocks, soil chemical and physical properties of the individual soil horizons, peat and surface water levels as well as on vegetation and management. Besides giving practical guidance, the handbook explains the scientific background of methodological choices. Parallel to the establishment of the monitoring network, the regionalisation methods for greenhouse gasses and their drivers as currently used in emission reporting are improved. This comprises an update of the map of organic soils as well as the improvement of the regionalisation of water table depths in organic soils and of the greenhouse gas emissions. These parts of the project, however, are not part of this handbook. With the publication of the handbook, we aim at a comprehensive and transparent documentation of the field and laboratory methods of the German peatland monitoring programme (Part 1: Open land). Furthermore, these detailed information, guidelines and protocols are available as inspiration to anyone who is planning monitoring of peatlands or wishes to measure single parameters consistently to the peatland monitoring programme.

Two important threats to the sustainable functioning of seminatural grasslands in temperate zones are (1) nutrient loading due to agricultural fertilization and pollution, and (2) the increase of extreme drought events due to climate change. These threats may cause substantial shifts in species diversity and abundance and considerably affect the carbon and water balance of ecosystems. The synergistic effects between those two threats, however, can be complex and are poorly understood. Here, we experimentally investigated the effects of nitrogen addition and extreme drought (separately and in combination) on a seminatural temperate grassland, located in Freiburg (South Germany). To study the grassland response, we combined eddy-covariance techniques with open gas exchange systems. Open gas exchange chambers were connected to an infrared gas analyzer and water isotope spectrometer, which allowed the partitioning of net ecosystem exchange and evapotranspiration. Vegetation parameters were described by species richness, species abundance, and leaf area index. Our results suggest that grassland communities, strongly weakened in their stress response by nitrogen loading, can substantially lose their carbon sink function during drought. While nitrogen addition caused a significant loss in forb species (−25%), precipitation reduction promoted a strong dominance of grass species at season start. Consequently, the grass-dominated and species-poor community suffered from a strong above-ground dieback during the dry summer months, likely caused by lower water use efficiency and weaker drought adaptations of the species community. Over the growing season (April-September), the carbon sequestration of the studied grassland was reduced by more than 60% as a consequence of nitrogen addition. Nitrogen addition in combination with precipitation reduction decreased carbon sequestration by 73%. Eutrophication can severely threaten the resilient functioning of grasslands, in particular when drought periods will increase as predicted by future climate scenarios. Our findings emphasize the importance of preserving high diversity of grasslands to strengthen their resistance against extreme events such as droughts.

International audience

International audience

International audience; The EGU General Assembly 2015 will be held at the ACV centre in Vienna, Austria, from 12 to 17 April 2015. The event will bring together geoscientists from all over the world to one meeting covering all disciplines of the Earth, Planetary and Space Sciences. The EGU aims to provide a forum where scientists, especially early career researchers, can present their work and discuss their ideas with experts in all fields of geoscienc.

International audience; Semi-arid ecosystems contribute about 40% to global net primary production (GPP) even though water is a major factor limiting carbon uptake. Evapotranspiration (ET) accounts for up to 95% of the water loss and in addition, vegetation can also mitigate drought effects by altering soil water distribution. Hence, partitioning of carbon and water fluxes between the soil and vegetation components is crucial to gain mechanistic understanding of vegetation effects on carbon and water cycling. However, the possible impact of herbaceous vegetation in savanna type ecosystems is often overlooked. Therefore, we aimed at quantifying understory vegetation effects on the water balance and productivity of a Mediterranean oak savanna. ET and net ecosystem CO2 exchange (NEE) were partitioned based on flux and stable oxygen isotope measurements and also rain infiltration was estimated. The understory vegetation contributed importantly to total ecosystem ET and GPP with a maximum of 43 and 51%, respectively. It reached water-use efficiencies (WUE; ratio of carbon gain by water loss) similar to cork-oak trees. The understory vegetation inhibited soil evaporation (E) and, although E was large during wet periods, it did not diminish WUE during water-limited times. The understory strongly increased soil water infiltration, specifically following major rain events. At the same time, the understory itself was vulnerable to drought, which led to an earlier senescence of the understory growing under trees as compared to open areas, due to competition for water. Thus, beneficial understory effects are dominant and contribute to the resilience of this ecosystem. At the same time the vulnerability of the understory to drought suggests that future climate change scenarios for the Mediterranean basin threaten understory development. This in turn will very likely diminish beneficial understory effects like infiltration and ground water recharge and therefore ecosystem resilience to drought.

International audience

International audience

The presence of vegetation alters hydrological cycles of ecosystems. Complex plant-soil interactions govern the fate of precipitation input and water transitions through ecosystem compartments. Disentangling these interactions is a major challenge in the field of ecohydrology and pivotal foundation for understanding the carbon cycle of semi- arid ecosystems. Stable water isotopes can be used in this context as tracer to quantify water movement through soil-vegetation-atmosphere interfaces. The aim of this study is to disentangle vegetation effects on soil water infiltration and distribution as well as dynamics of soil evaporation and grassland water-use in a Mediterranean cork-oak woodland during dry conditions. An irrigation experiment using δ 18 O-labeled water was carried out in order to quantify distinct effects of tree and herbaceous vegetation on infiltration and distribution of event water in the soil profile. Dynamic responses of soil and herbaceous vegetation fluxes to precipitation regarding event water-use, water uptake depth plasticity and contribution to ecosystem evapotranspiration were quantified. Total water loss to the atmosphere from bare soil was as high as from vegetated soil, utilizing large amounts of unproductive water loss for biomass production, carbon sequestration and nitrogen fixation. During the experiment no adjustments of main root water uptake depth to changes of water availability could be observed, rendering light to medium precipitation events under dry conditions useless. This forces understory plants to compete with adjacent trees for soil water in deeper soil layers. Thus understory plants are faster subject to chronic drought, leading to premature senescence at the onset of drought. Despite this water competition, the presence of Cork oak trees fosters infiltration to large degrees. That reduces drought stress, caused by evapotranspiration, due to favourable micro climatic conditions under tree crown shading. This study highlights complex soil-plant-atmosphere and inter-species interactions in both space and time controlling the fate of rain pulse transitions through a typical Mediterranean savannah ecosystem, disentangled by the use of stable water isotopes