Dear all,

I am thrilled to invite you to my Ph.D. defense entitled: “Trade-off between increased soil carbon sequestration and nitrous oxide emissions in managed cropland/grassland systems”.

The defense will be held on 22 May 2024 at 2 pm (Wednesday) at the Agro Paris-Saclay campus (22 Place de l’Agronomie, 91120 Palaiseau), C1.0.06 amphitheater in the C building. It will also be possible to attend online to the defense (a link for the videoconference will be shared soon).

The jury members are: 

Reporter : Laurent PHILIPPOT, Research director, INRAE, Dijon

Reporter: Davey JONES, Professor, Bangor University, UK

Examinator : Cornelia RUMPEL, Research director, CNRS, Paris

Examinator : Caroline PETITJEAN, Associate professor, AgroParisTech, Nancy

Examinator : Laurent CANER, Professor, Poitiers University

thesis director : Abad CHABBI, Directeur de recherche, INRAE, Lusignan

Abstract of the thesis:

Agricultural soils play an important role in nitrous oxide (N₂O) emissions into the atmosphere. However, predicting and understanding these emissions remains a challenge, along with the relationship between soil organic carbon (SOC) storage and N₂O fluxes. This relationship is crucial for the adoption of sustainable land management practices.

This study focuses on the impact of different grassland management practices on SOC storage and in situ N₂O emissions. The management systems under study include a continuous crop rotation (maize, wheat, barley), temporary grassland (Tall fescue, Cocksfoot, Ryegrass) with various fertilization regimes alternating with crops, and a fertilized mowed permanent grassland. The aim is also to identify the factors that influence N₂O emissions in both bulk soil and aggregate size fractions and to assess the effectiveness of the CERES-EGC model in predicting N₂O emissions from these crop/grassland systems. To achieve this, field data collected alongside laboratory analysis are used to refine the model’s site-specific parameters, with the goal of improving its ability to predict N₂O emissions.

The results highlight that despite fluctuations in SOC stocks caused by different management practices, no significant trade-off was observed between SOC storage at different depths (10 and 30 cm) and N₂O emissions. Additionally, the inclusion of temporary grassland in crop rotations had a positive impact on SOC stocks but simultaneously led to increased N₂O emissions. Interestingly, the chemical composition of SOC, soil physical properties, and denitrifier activity appear to play a more dominant role in on-site N₂O emissions than SOC content and denitrifier abundance. This underscores the importance of environmental conditions and denitrifier activity rather than mere abundance in influencing N₂O emissions at the bulk soil level. However, although the results indicate that bulk and aggregate size fractions reveal similar determinants of N₂O emissions, the characteristics of aggregate size fractions better explain the variability of on-site N₂O emissions. Larger aggregates (>2 mm) mainly influenced on-site N₂O emissions via the abundance of N₂O producers regulated by carbon quality, while N₂O reducers were influenced by the pH and moisture of the same fine aggregate. Additionally, the finest aggregates played a role in N₂O consumption.

From a modeling perspective, utilizing the CERES-EGC model, we successfully simulated in situ N₂O emissions from both temporary grassland and continuous cropping systems. This suggests that accurate simulation of N₂O emissions from both arable and grassland systems is feasible using field-collected data. We propose refining the site-specific parameters of the N₂O emission module based on soil chemical properties identified as key drivers of N₂O emissions in previous studies, and enhancing the model to accommodate small-scale soil variability. These enhancements have the potential to significantly improve the accuracy of N₂O simulations, thereby advancing the development of more effective and sustainable soil management practices.

Keywords: Nitrous oxide (N₂O), cropland, grassland, soil aggregates, soil carbon quality, carbon stock, lignin, denitrifiers, CERES-EGC, modelling.

It will be a pleasure seeing you all!

See you very soon!

Asma