M. Bernard LONGDOZ (Rapporteurs) : Professeur, Université de Liège, Gembloux Agro-Bio Tech TERRA Teaching & Research Center (BIODYNE Team). 

M. Laurent Saint ANDRE (Directeur de thèse) : Directeur de Recherche INRAE, BEFE, INRAE.

 

 

 

This study is a continuation of our previous geochemical monitoring finding at the injection wells of Rousse 1 ( Total CCS pilot, Lacq- Rousse, France) where it was identified that the soil CO2 mole fraction (χc) evolution in subsoil was negatively correlated with the level of the water table and the CO2 sources were attributed to the CO2-rich aquifers. However, it is still unclear whether this relationship exists in the forest ecosystem, representing a significant proportion of the CO2 atmospheric budget. For this reason, this thesis focuses on monitoring the gas exchange and its main driver of the transport process between soil (-1 m), subsoil (-6 m), and biosphere.

We developed and implemented an in-situ geochemical monitoring system for continuous monitoring of CO2 mole fraction in the subsoil coupled with a micrometeorological monitoring system using a pre-established flux tower in the forest Ecosystem (Montiers, Lorraine Region, France). This soil gas measurement infrastructure combining borehole measurement with micrometeorological measurement offers great possibilities for long-term in-situ and continuous gas monitoring to derive the vertical distribution of CO2. Thus, this infrastructure allowed the observation of the temporal dynamics in soil-gas CO2 research. During the study periods, the ecosystem acted as a net carbon sink with a mean annual NEE, GPP, and Reco of -453±122 gC m-2y-1, -1468 ±109  gC m-2y-1, and  1052 ±88   gC m-2y-1 consecutively. The Carbon exchange, climate, and environmental drivers during the drought episodes were compared with long-term reference data recorded from 2014 to 2017. In contrast with some previous research where GPP and Reco parallelly decreased during the drought episodes, our site showed Reco is more sensitive to drought than GPP, resulting in a significant increase in Net Ecosystem exchange. Reco decreased by 20%, and 26% were found in Summer and Autumn (2018-2019) relative to the reference years (2014-2017).

This study shows strong empirical shreds of evidence that wind turbulence plays a significant role in driving the deep soil CO2 concentration. We hypothesize that this could be due to pressure pumping effects where it decreases the CO2 molar fraction in the soil during high turbulence and increasing the CO2 storage in deep soil during low turbulence. This study also demonstrates that permeability significantly reduced during wet periods diminishing molecular diffusion and advection. This study also revealed a strong biotic influence on CO2 production. The δ13CCO2 values in our site subsoil can be attributed to respiration and decomposition of the C3 plants. These biological origins of soil CO2 are highly likely to increase air density resulting in gravitational percolation that leads the CO2 stored in a deeper layer of soil. The relationship of subsoil gases also emphasizes that biogenic components dominate the origins and controlling process of subsoil CO2 while the geochemical process plays an insignificant role.