Contribution of vegetation (trees and ground vegetation) on the methane budget of a temperate forest

Methane (CH4) is one the most important greenhouse gas and is responsible for approximatively 20% of the global warming (IPCC, 2013). Soils and mainly upland forest soils where aerobic environment prevails, are one of the main global sink of methane (IPCC 2013). At the soil-atmosphere interface, the net methane efflux consists in a net balance between the production of CH4 by methanogenic bacteria mainly in deep anaerobic soil layers and the consumption by methanotrophic bacteria in the aerobic soil horizons of the methane produced in the soil or diffusing from the atmosphere into the soil. In upland forest, some episodic temporary waterlogging may exist, especially in managed forest where trafficked work on silty or clayey soils compacts the soil and then, enhanced the waterlogging (Startsev and McNabb, 2000). But the methane budget of ecosystem may be improved when considering not only soil but also plant compartments. Plants can impact the CH4 production and consumption by different pathways (enhance production, consumption, and/or gases transport). When the soil is submitted to compaction and then, to an increase of waterlogging, the ground vegetation is modified in favor of vegetation with aerenchymous tissues (Goutal-Pousse et al, 2012) and the soil can shift from a methane sink to an episodic methane source (Epron et al 2016). In the present study, our objectives were to determine (i) if vegetation emits CH4, (ii) if abiotic factors drive the seasonal CH4 flux pattern by plants (ground vegetation and trees) and (iii) to quantify the impact of the emissions by vegetation (tree and ground vegetation) on the methane budget of a forest submitted to compaction. We hypothesized that in an upland forest, vegetation (ground vegetation and tree stems) by enhancing the CH4 emission or by producing CH4 may reduce the methane sink of the forest ecosystem. This study was carried out in a 6-ha experimental site set up in 2007 in the state-owned forest of "les Hauts Bois" (north-eastern France) to assess the long-term impact of a loaded forwarder. To study this effect, the soil was compacted before afforestation. We recorded CH4 fluxes during 7 months at a 3-hour frequency using automated chambers on stem tree, bare soil and soil with vegetation, connected to a laser-based gas analyser in a forest site where the ground-vegetation consists mainly in two aerenchymous plants (glyceria striata and juncus sp) and trees in planted Quercus petraea. In contradiction with our hypothesis and previous studies, in this studied site, the presence of ground vegetation increases the methane forest ecosystem uptake compared to the bare soil but with an impact varying during the season. In addition, the increase in the methane uptake depended on the species, from 80 % to 120%. Methane emission by tree stem were low compared to methane uptake by soil (-3.6 ± 0.4 kg ha-1 and 0.90 ± 0.31 g ha-1 respectively) but methane emission by stem was enhanced when methane was produced into the soil

Reconnaissance de la signature géochimique des eaux microporales de l'aquifère de la Craie (Bassin de Paris) (origine(s) et évolution(s))

L'eau microporale de cet aquifère est difficilement extractible, et, par voie de conséquence, ses origine et caractéristiques chimiques demeurent très peu étudiées. Toutefois, l'eau microporale est extrêmement importante car elle correspond à 98 % de la teneur en eau de l'aquifère. Ce travail consiste en une étude géochimique de l'eau microporale contenue dans 600 m de Craie. Elle a permis de définir l'eau microporale comme étant la résultante d'un mélange entre 3 eaux : (i) le premier pôle consiste en une eau météorique, (ii) le second pôle qui représenterait au maximum 2 % de l'eau microporale actuelle, serait une eau marine, et (iii) le troisième pôle serait l'eau adsorbée sur la phase solide présentant une composition très enrichie (60 à 70 % plus enrichie que le reste de l'eau interstitielle). La proportion de ce troisième pôle augmente avec la profondeur, et permet d'expliquer la composition isotopique très enrichie de l'eau interstitielle. Une modélisation de l'évolution de l'eau microporale d'une eau marine à l'eau actuelle a permis de déterminer que la circulation de l'eau se faisait essentiellement par diffusion dans la microporosité. Toutefois l'eau microporale en profondeur semble également être influencée par l'eau circulant dans les fractures. Au vu de ces hypothèses, la mise en place du système actuel nécessiterait de 50 à 65 millions d'années. L'étude chimique, quant à elle, a permis de déterminer également une interaction de l'eau avec la roche encaissante. L'étude de la phase solide a permis de déterminer (i) la composition des principales phases de recristallisations et (ii) l'origine diagénétique précoce pour la formation du niveau dolomitique caractéristique du forage étudié. L'étude du rapport 234U/238U a mis en évidence des interaction eau-roche récentes entre l'eau microporale et la roche, interactions qui n'avaient pas pu être tracées par les isotopes stables des carbonates.The Chalk, which is an important groundwater source for water supply in NW Europe, has been extensively studied, mainly for the description of the macroporosity water. Previous studies describing the interstitial water are rare because this water is not easily extractible. However, it represents 98 % of the total water content of the aquifer. This paper deals with a geochemical approach of the interstitial water features. This interstitial water is a mixing between three different waters : the first one is a meteoric water, the second one a seawater which consists of 2 % of the water content and the third one is the adsorbed water on the solid phase which presents a delta-value enrichment of 60-70 % in delta18O and delta2H. The proportion of this latter end-member in the mixing increases downward due to the reduction of pore diameter and can explain the interstitial water enrichment with depth. A model of the evolution of the water from a seawater to the modem water determines a diffusion in the microporosity and an influence downward of the macroporosity water. The time required to reach the modern concentration of the interstitial water is 50 to 65 millions years. The chemical study permits to determine rock-water interaction processes. The geochemical features of the solid phase allow for the determination of the composition of the major secondary precipitation phases, and mainly for the definition of an early diagenetic origin of the dolomitic level. The study of 234U/238U highlights water-rock interaction relatively recent (< to 1 million year), this interaction having not been shown by stable isotopes.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Pulse labelling of deep soil layers in forest with 13 CH 4 : testing a new method for tracing methane in the upper horizons, understorey vegetation and tree stems using laserbased spectrometry

International audienceMethane emissions from plants in wetlands are mainly due to internal transport, from the anoxic soil layers where methane is produced, to the atmosphere. This pathway has not yet been clearly demonstrated for upland forest vegetation, where methane can be produced in deep soil layers. We developed a new method to trace methane transfer from the deep soil. We conducted a 13 CH 4 pulse labelling at 40-cm soil depth and then monitored 13 CH 4 in the upper horizons, at the soil surface (with or without understorey vegetation) and emitted by tree stems until the total disappearance of the labelled gas. Most of the injected 13 CH 4 was oxidized in the soil despite high soil water content. The understorey vegetation did not contribute to 13 CH 4 emission by the soil. We prove that tree stems can emit methane produced in an upland forest soil, even when the said soil is a net methane sink. We conclude that pulse labelling with 13 CH 4 and tracing by laser-based spectrometry is a promising tool approach to study the transport of methane from production to emission

Les sols forestiers, puits de méthane : un service écosystémique méconnu

International audienceMethane ranks second after carbon as a greenhouse gas whose concentration in the atmosphere is increasing as a result of human activity. Biological oxidization of methane due to methanotrophic bacteria that are present in soils contributes to slowing down the exponential growth in atmospheric methane concentration. Forest soils are a significant methane sink on the scale of the biosphere, and forest management can affect methane flux from soil to atmosphere. Increasing the minimal age for felling and reducing heavy vehicle movements that cause compacting would increase the methane sink of forest stand soils since this sink increases with tree age but diminishes with compaction.Le méthane est le second gaz à effet de serre, derrière le gaz carbonique, dont la concentration dans l’atmosphère augmente du fait des activités humaines. Son oxydation biologique par les bactéries méthanotrophes présentes dans les sols contribue à atténuer l’augmentation exponentielle de sa concentration dans l’atmosphère. Les sols forestiers sont un puits de méthane important à l’échelle de la biosphère, et le flux de méthane échangé entre le sol et l’atmosphère peut être affecté par la gestion. Augmenter les âges d’exploitabilité, tout en réduisant la fréquence de passage d’engins lourds responsables du tassement, permettrait d’accroître le puits de méthane des peuplements forestiers, car ce puits augmente avec l’âge mais diminue lorsque les sols sont compactés

Belowground carbon allocation in a temperate beech forest: new insight into carbon residence time using whole tree 13C labelling

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Contribution of vegetation (trees and ground vegetation) on the methane budget of a temperate forest

Methane (CH4) is one the most important greenhouse gas and is responsible for approximatively 20% of the global warming (IPCC, 2013). Soils and mainly upland forest soils where aerobic environment prevails, are one of the main global sink of methane (IPCC 2013). At the soil-atmosphere interface, the net methane efflux consists in a net balance between the production of CH4 by methanogenic bacteria mainly in deep anaerobic soil layers and the consumption by methanotrophic bacteria in the aerobic soil horizons of the methane produced in the soil or diffusing from the atmosphere into the soil. In upland forest, some episodic temporary waterlogging may exist, especially in managed forest where trafficked work on silty or clayey soils compacts the soil and then, enhanced the waterlogging (Startsev and McNabb, 2000). But the methane budget of ecosystem may be improved when considering not only soil but also plant compartments. Plants can impact the CH4 production and consumption by different pathways (enhance production, consumption, and/or gases transport). When the soil is submitted to compaction and then, to an increase of waterlogging, the ground vegetation is modified in favor of vegetation with aerenchymous tissues (Goutal-Pousse et al, 2012) and the soil can shift from a methane sink to an episodic methane source (Epron et al 2016). In the present study, our objectives were to determine (i) if vegetation emits CH4, (ii) if abiotic factors drive the seasonal CH4 flux pattern by plants (ground vegetation and trees) and (iii) to quantify the impact of the emissions by vegetation (tree and ground vegetation) on the methane budget of a forest submitted to compaction. We hypothesized that in an upland forest, vegetation (ground vegetation and tree stems) by enhancing the CH4 emission or by producing CH4 may reduce the methane sink of the forest ecosystem. This study was carried out in a 6-ha experimental site set up in 2007 in the state-owned forest of "les Hauts Bois" (north-eastern France) to assess the long-term impact of a loaded forwarder. To study this effect, the soil was compacted before afforestation. We recorded CH4 fluxes during 7 months at a 3-hour frequency using automated chambers on stem tree, bare soil and soil with vegetation, connected to a laser-based gas analyser in a forest site where the ground-vegetation consists mainly in two aerenchymous plants (glyceria striata and juncus sp) and trees in planted Quercus petraea. In contradiction with our hypothesis and previous studies, in this studied site, the presence of ground vegetation increases the methane forest ecosystem uptake compared to the bare soil but with an impact varying during the season. In addition, the increase in the methane uptake depended on the species, from 80 % to 120%. Methane emission by tree stem were low compared to methane uptake by soil (-3.6 ± 0.4 kg ha-1 and 0.90 ± 0.31 g ha-1 respectively) but methane emission by stem was enhanced when methane was produced into the soil