Embankment as a carbon sink: a study on carbon sequestration pathways and mechanisms in topsoil and exposed subsoil

Carbon (C) sequestration is receiving increasing scientific and political attention in a framework of greenhouse gasses mitigation. However, geotechnical soils have been neglected for their C sequestration potential, with the global attention focusing on agricultural and natural soils. In the present thesis project we aim to assess the potential of geotechnical embankments as C sink, and, through the study of plant species and soils showing contrasting features, shed light on C sequestration mechanisms and the role of the different actor involved. We aim not only to quantify the C gained and lost in soil, but even its origin (fresh new C input or old preexistent C) and how it is partitioned in different C pools characterized by different C stability (quality of stored C).
First, we evaluated the C storage in different pools under soil sowed with 12 different herbaceous species in a 10 months experiment. Assessing different root traits allowed understanding the influence of root economic spectrum on C storage. We showed how traits linked to high labile C are linked to a higher C increase in the stable SILT+CLAY pool (<20µm). Root traits related to a low input of recalcitrant, instead, favor accumulation in the unstable POM fraction. Thanks to a 183 days stable isotope labelling experiment (CO2 constantly enriched with 13C) we were able to study the C dynamics in different C pools under two species (Lolium perenne and Medicago sativa) sowed on two soil (topsoil, 0-30cm depth and subsoil brought to the surface, 110-140 cm depth) showing contrasting characteristics. We evidenced the great interest of bridging C origin and C pools when studying soil C fates, allowing unveiling processes those more traditional methods would hide. New C and old C showed synergetic covariation, with lower old C losses associated to higher new C inputs. This is in good accordance with the Preferential Substrate Utilization hypothesis.
The Preferential Substrate Utilization hypothesis was also validated with the study of priming effect and soil respiration, that showed higher plant derived C in respired CO2 when plant C input were high, while increasing old C mineralization when plant C input were low, i.e. in subsoil. We observed significant plant derived new C input in the SILT+CLAY fraction (<20µm, highly stable) supporting evidence of the in vivo entombing effect in the soil Microbial Carbon Pump hypothesis. The species effect mainly occurred on new C input, but it was overpowered by the soil effect, with lower C storage in low quality soil (low nitrogen and microbial biomass and activity). In general, microbiological conditions were the main driver for new C accumulation and old C loss, and helped to explain why no effect of soil C saturation – a central theory in recent studies on C sequestration - was find in the protected carbon. Such fundamental understanding of plant-soil interactions help us to better optimize soil and vegetation management for road embankment revegetation.

Composition du jury :
Dr Alexia STOKES, DR, HDR, INRA Montpellier, Examinateur
Dr Giacomo RUSSO, Associate Prof., UNICAS Cassino, Examinateur
Dr Catherine PICON-COCHARD, DR, HDR, INRA Clermont-Ferrand, Rapporteur
Dr Isabelle BASILE-DOELSCH, DR, HDR, CEREGE Aix-en-Provence, Rapporteur
Dr Wilma POLINI, Associate Prof., UNICAS Cassino, Examinateur
Dr Sandra LUQUE, DR, HDR, IRSTEA Montpellier, Examinateur
Dr Mark BAKKER, IR, HDR, INRA Bordeaux, Examinateur
Dr Paul HALLETT, Full Prof., Univ. Aberdeen, Examinateur
Dr Zhun MAO, CR, INRA Montpellier, Invitée
Dr Tiphaine CHEVALLIER, CR, HDR, IRD Montpellier, Invitée