Evolution of carbon stabilization mechanisms in a podzolic soil chronosequence

Predicting the evolution of the stock of soil organic matter (SOM) requires a process-based understanding of its decomposition and stabilization pathways. The stabilization of SOM results from biological and physico-chemical mechanisms: (1) the recalcitrance of SOM due to its molecular properties; (2) the inhibition of decomposers activity; (3) the interaction between SOM, mineral surfaces and metal ions (organo-mineral associations, OMA). The understanding of SOM dynamics thus requires an integrated and multidisciplinary approach, considering the interactions in soils between minerals, micro-organisms, SOM and the resulting physico-chemical environment. The PhD thesis is aimed at: (1) developing a multidisciplinary soil conceptual diagram integrating soil properties involved in carbon (C) stabilization; (2) applying this model to a podzolic soil chronosequence (0 to 530 years old soil profiles), and evaluating the short and long term (from day to century) evolution of SOM protection mechanisms. In the podzolic soil chronosequence, pedogenesis started with early vegetation development and C input from the litter, as observed in the youngest profiles P1 and P2 (120, 175 yrs). The biological activity led to a soil pH decrease in the surface horizon, as measured in P3 E (270 yrs). Net acidification, incipient in P1 and P2, induced mineralogical modifications, as clearly observed in the well-developed podzols P4 and P5 (330, 530 yrs). These modifications are: mineral weathering in the topsoil, precipitation of secondary minerals, associated with OM, in the illuvial B horizon. The accumulation of OMA led to a cementation in B, and thereby to distinct physico-chemical conditions. SOM protection mechanisms were both site and horizon-specific. Indeed, SOM protection was limited in P1 and P2; recalcitrant C compounds were selectively preserved in the topsoil of P3, P4 and P5; eventually, the largest protection of SOM occurred in the B horizons of P4 and P5 through the formation of OMA and inhibition of the decomposers activity due to cementation. The microbial populations were mainly fungi-dominated in the youngest P1, P2 and P3, and in the topsoil of P4 and P5. They were, however, mainly bacteria-dominated in the cemented B horizons of P4 and P5. Our results confirm that minerals, SOM and microbial populations evolve interdependently, resulting in SOM protection mechanisms changing over short-time scales during soil formation. Our conceptual diagram could best be applied to different soil types in order to better assess SOM dynamics in contrasted soil conditions.