Physical protection of soil organic matter : scales and questions

Organic matter is stabilized in soils for years to centuries or millennia because of its chemical recalcitrance, stabilization by interaction with the mineral matrix or physical stabilization. Physical protection or physical stabilization of soil organic matter comes from the limited access that microbial decomposers have to organic substrates, water and oxygen in the complex three dimensional architecture of soils. Physical protection is though to be a major process at temporal scales from decades to centuries. Much work has been performed on this process, usually based on the physical fractionation of soil aggregates of different sizes and stabilities and of their associated organic matter. Its importance has been demonstrated in soils under various land uses. Here we focus on what we perceive as standing questions regarding this stabilization process: - What are the spatial scalesof physical protection? - Which approach should be preferred: the aggregate-based approach or a pore systembased approach? - What is the relative importance of physical protection and physico-chemical protection? Physicochemical protection results from the adsorption of organic matter to minerals. - How to model physical protection? To address these questions we will use results obtained on soils from long term experiments as well as from microcosms experiments.