The significance of organic separates to carbon dynamics and its modelling in some cultivated soils

Les modeles de la dynamique du carbon des sols repartissent le carbone du sol en compartiments fonctionnels, de durees de vie tres differentes. On a cherche a separer materiellement les composes de durees de vie differentes, afin de proposer des estimateurs de ces compartiments. Les vitesses de renouvellement sont mesurees par la methode du marquage naturel en l3 C, dans des sols en culture cerealiere (maos) provenant d'experimentations de longue duree. Le modele de Rothamsted donne une bonne representation de la dynamique observee. Les fractions granulometriques grossieres permettent de quantifier le compartiment structural d'origine vegetale des modeles. Les composes extractibles a l'eau sont enrichis en composes jeunes, mais ne sont pas identifiables aux compartiments labiles. Concernant les matieres organiques <50 μm, aucune des methodes classiques evaluees (hydrolyses acides, oxydations humides, oxydations thermiques, pyrolyses, extraction alcalines) ne permettent de separer des composes de vitesses de renouvellement differentes. L'analyse permet de rejeter l'hypothese de formation des acides humiques par condensation des acides fulviques. La localization et la protection physique des matieres organiques apparaot comme plus determinante pour leur vitesse de biodegradation que leur appartenance a une famille chimique.

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