Thèse Effets du Pâturage et de la Sécheresse Seuls ou Combinés sur le Fonctionnement Biogéochimique et la Résilience des Tourbières Pyrénéennes H/F Doctorat.Gouv.Fr

Établissement : Institut National Polytechnique de Toulouse École doctorale : SDU2E - Sciences de l'Univers, de l'Environnement et de l'Espace Laboratoire de recherche : CRBE - Centre de Recherche sur la Biodiversité et l'Environnement Direction de la thèse : Laure GANDOIS ORCID 0000000293266830 Début de la thèse : 2027-10-01 Date limite de candidature : 2026-11-23T23:59:59 Les tourbières sont des écosystèmes clés assurant des fonctions essentielles : stockage du carbone, régulation de l'eau et conservation de la biodiversité. Bien qu'elles ne couvrent que ~3 % des terres émergées, elles stockent jusqu'à 30 % du carbone organique des sols et abritent une biodiversité unique. Elles jouent un rôle central dans la régulation de l'eau, leur fonctionnement dépendant des interactions entre conditions hydrologiques, dynamique végétale et processus biogéochimiques.

Dans les Pyrénées, les tourbières de montagne sont très sensibles au changement climatique en raison de leur dépendance à l'eau. Elles coexistent souvent avec un pâturage extensif, créant des interactions complexes entre pressions climatiques et anthropiques. Le pâturage modifie la végétation et la biomasse, tandis que le piétinement et le tassement des sols perturbent les conditions hydrologiques et les cycles du C, N et P. Parallèlement, la hausse des températures et les changements des précipitations augmentent la fréquence des sécheresses, menaçant la stabilité des bryophytes (ex. Sphagnum), essentielles pour les cycles biogéochimiques. Les effets combinés du pâturage et de la sécheresse ne peuvent être considérés indépendamment : une baisse de la nappe phréatique modifie le fonctionnement des plantes, les échanges de GES et les transformations des nutriments. Les changements de végétation et la compaction de la tourbe induits par le pâturage peuvent amplifier les effets de la sécheresse ou réduire la récupération.

Ce projet de thèse vise à étudier comment le stress hydrologique et le pâturage interagissent pour affecter les cycles du C, N et P et la résilience des tourbières. Il repose sur quatre objectifs :
1. Caractériser les cycles du C, N et P selon des gradients environnementaux, hydrologiques et anthropiques, en intégrant des données de surveillance (jusqu'à 10 ans) et de nouvelles mesures.
2. Déterminer l'impact du pâturage sur ces cycles avec un focus sur les changements de végétation, la compaction de la tourbe et l'hydrologie.
3. Évaluer les interactions sécheresse-pâturage pour comprendre leur impact sur le fonctionnement et la récupération des écosystèmes.
4. Identifier des indicateurs de vulnérabilité et de résilience en intégrant les réponses hydrologiques, biogéochimiques et végétales.

Les méthodes mises en oeuvre vont combiner:
- l'analyse de données existantes (réseau pyrénéen : France, Espagne, Andorre) incluant >10 ans de mesures hydrologiques, climatiques, végétales et de GES (CO, CH, NO).
- des campagnes de terrain pour mesurer les flux de GES et collecter des sondages de tourbe
- l'acquisition de nouvelles données sur le cycle de l'azote : quantification de la fixation biologique de N, émissions de NO et potentiel de dénitrification via campagnes de terrain et incubations en labo.
- l'évaluation des impacts de la sécheresse sur la végétation avec observations in situ, télédétection et expériences en labo.
- l'intégration des réponses via modèles statistiques et mécanistes.

Les résultats attendus sont
- Une caractérisation régionale des cycles du C, N et P des tourbières de montagne
- Une meilleure compréhension des interactions pâturage-sécheresse sur les cycles biogéochimiques
- Des indicateurs de vulnérabilité et résilience pour les tourbières pyrénéennes. Peatlands are key ecosystems, providing critical functions for carbon (C) storage, water regulation, and biodiversity conservation. Although they cover only ~3% of the global land surface, they store up to 30% of terrestrial soil organic C and support unique, highly specialized biodiversity (Yu et al. 2011). They also play a pivotal role in regulating water availability and quality, with their functioning heavily dependent on the interactions between hydrological conditions, vegetation dynamics, and biogeochemical processes.

Mountain peatlands, particularly in the Pyrenees, are highly sensitive to climate change due to their strong dependence on water availability and climatic conditions (Garisoain et al, 2024; Loisel et al., 2020; UNEP 2022). In this region, these ecosystems often coexist with extensive livestock grazing, creating a direct interaction between climate-driven and anthropogenic pressures (Pellerin et al. 2006, Noble et al. 2017). Grazing can alter vegetation composition and biomass, while trampling and soil compaction may disrupt hydrological conditions and biogeochemical processes, including C storage and nutrients (N and P) cycling (Ward et al. 2007, Olofsson & Oksanen 2002). Meanwhile, rising temperatures, shifting precipitation patterns, and changing snow dynamics are expected to increase the frequency and intensity of droughts in the Pyrenees, leading to the decay of bryophyte layers that play a key role in peatland C, N, and P biogeochemical cycling (Ma et al. 2006) .

The combined effects of these pressures cannot be considered independently. For example:
- A decline in water table depth during drought can modify plant functioning, greenhouse gas (GHG) exchanges, and nutrient transformations.
- Grazing-induced vegetation changes and peat compaction may further alter how peatlands respond to water table fluctuations, potentially amplifying drought effects or reducing ecosystem recovery capacity after extreme events.

Thus, the interaction between grazing and hydrological stress may determine not only the magnitude of ecosystem responses but also their resilience to extreme events. Despite their ecological and climatic importance, the mechanisms controlling the response and resilience of mountain peatlands to combined climatic and land-use pressures remain poorly understood. This project will investigate how hydrological stress and grazing interact to affect C and nutrient cycling in Pyrenean peatlands, and identify the processes driving their resilience to extreme drought.

The project will focus on four complementary questions:
Q1: How do hydrological and environmental gradients control C, N and P dynamics across Pyrenean peatlands? In particular, how does water table fluctuations influence C exchange, nutrient transformations, and their retention or export ?
Q2: How does grazing alter C and nutrient retention and loss pathways? How does grazing affect C storage and C exchange via C export or GHG emissions? Where are the excess nutrients retained within the peatland, and to what extent are they lost through plant uptake, denitrification, gaseous emissions, or lateral export? How do grazing-induced changes in vegetation and peat compaction influence these pathways?
Q3: Does grazing increase the sensitivity of peatland ecosystems to extreme drought? Does grazing-induced changes in vegetation and peat compaction amplify the effect of drought on C and nutrient cycling, plant functioning, and hydrological conditions?
Q4: What determines the capacity of Pyrenean peatlands to recover after extreme drought? Do peatland types, vegetation composition, and hydrological conditions determine resistance and recovery? Are some plant communities, including Sphagnum-dominated systems, more resilient than others? This project will investigate how hydrological stress and grazing interact to modify C and nutrient dynamics and ecosystem resilience in Pyrenean peatlands. To address this overarching question, the project has four interconnected objectives:
- Characterize C, N, and P cycling across Pyrenean peatlands: Establish how C, N, and P cycling varies across environmental, hydrological, and anthropogenic gradients, integrating existing monitoring data (up to 10 years) with new measurements.
- Determine how grazing affects C, N and P dynamics. Quantify how grazing modifies C and nutrients retention and loss pathways, with particular focus on vegetation changes, peat compaction, and hydrological conditions.
- Assess the interaction between drought and grazing. Determine whether grazing modifies peatland sensitivity and recovery following extreme drought events, and identify the mechanisms underlying changes in ecosystem functioning.
- Identify indicators of peatland vulnerability and resilience. Integrate hydrological, biogeochemical and vegetation responses to identify indicators of peatland vulnerability and resilience, and use these indicators to assess potential trajectories under changing climate and land-use conditions. In order to answer these scientific questions, the project will combine different approaches
- Process existing data from long-term monitoring data from a Pyrenean peatland network (Addressing Q1)(France/Spain/Andorra), covering contrasting vegetation, hydrology, and grazing pressures. Available datasets include >10-year survey of hydrological monitoring, climate observations, vegetation surveys, and GHG measurements, as well as grazing exclusion experiments at Spanish sites. These data will be used to quantify CO2, CH4 and N2O emissions, establish C budgets (Garisoain et al., 2024), and calculate C and nutrient accumulation rates across diverse Pyrenean peatland sites. For sites where key information is lacking, field campaigns will be conducted to measure GHG fluxes (ADD REF) and collect new peat cores for radiometric dating (Olid et al. 2013, 2014 2020), enabling the reconstruction of C, N, and P accumulation rates .

- Acquire new data on N cycling : Quantify biological N fixation and NO emissions via dedicated field campaigns and laboratory denitrification potential experiments (Addressing Q2). N2O emissions will be quantified through dedicated field campaigns, while N fixation will be assessed using tracer or incubation approaches (Crestey-Chury et al. 2026, Darnajoux et al. 2026). Laboratory peat incubations will be conducted under contrasting water-table conditions to quantify denitrification potential and assess the influence of hydrological conditions on N loss (Kanaan et al. 2025). Dissolved C and N export through surface water and groundwater will be quantified under contrasting hydrological conditions, including wet and dry periods. The contribution of groundwater to C and N export will be assessed using 222Rn as groundwater tracer (Olid et al. 2021, 2022).

- Assess drought impacts on vegetation functioning and recovery, with particular focus on Sphagnum. In situ and remote sensing observations will quantify drought-induced bleaching and recovery rates after drought events (Garisoain et al., 2023).Controlled Lab experiments will expose Sphagnum to simulate drought stress to assess physiological responses and recovery, allowing to quantify resistance and resilience to drought.

- Integrate hydrological, biogeochemical, and vegetation responses using statistical and process-based models to identify the main controls of C and N cycling to determine if whether grazing modifies peatland responses to drought (Adressing Q4). The results will be used to identify indicators of vulnerability and resilience and explore future trajectories under different climate and land-use scenarios.