Clay-sized organo-mineral complexes in a cultivation chronosequence: revisiting the concept of the 'primary organo-mineral complex'

摘要

Primary organo-mineral complexes are defined as organic matter (OM) bound to primary mineral particles, isolated after complete dispersion of soil. Organic matter present in 2 mum particle-size fractions of soils has slow turnover times and it is assumed to be stabilized mainly by interaction with minerals. We aimed to quantify how much of the organic matter in 2 mum particle-size fractions was free versus bound to minerals and to describe the nature of the association. Furthermore, we hypothesized that this bound organic matter was more resistant to biodegradation than free organic particles. We tested this by using a cultivation chronosequence on temperate silty soils and quantified free and clay-bound organic matter using density fractionation coupled with elemental analyses, as well as transmission electron microscopy (TEM) coupled with image analyses. Both methods showed that free organic matter was a minor fraction and that it was more depleted by cultivation than clay-bound organic matter. We deduced that clay-bound organic matter was more resistant to biodegradation. TEM showed that the distribution of organic matter in clay-sized fractions was patchy and that many of the so-called 2 mum 'particles' were in fact nanometre- to micrometre-sized microaggregates in which OM was encrusted by minerals or coated minerals. We conclude that true primary organo-mineral complexes do not correspond to reality and must be regarded as conceptual entities. We suggest that the very small microaggregates, which were evidenced here, are major sites of OM stabilization, both by adsorption and by entrapment of organic matter. Complexes organo-mineraux 2mum au sein d'une chronosequence de mise en culture de sols: une reevaluation du concept de "complexe organo-mineral primare" Resume Les complexes organo-mineraux primaires sont definis comme l'association entre des matieres organiques (MO) et des particules primaires minerales, que l'on separe apres une dispersion complete du sol. On attribue generalement le renouvellement lent des MO des fractions granulometriques texturales 2 mum a leur interaction avec les mineraux. Notre objectif etait de quantifier la proportion de MO de ces fractions liee aux mineraux et de decrire ces associations. De plus, nous avons fait l'hypothese que les MO liees etaient plus resistantes a la biodegradation que les MO libres 2 mum. Pour le tester, nous avons utilise une chronosequence de mise en culture de sols limoneux temperes et avons quantifie et decrit les MOlibres et liees de la fraction 2 mum par fractionnement densimetrique ainsi que par microscopie electronique a transmission et analyse d'images. Les deux methodes ont montre que les MO libres etaient tres peu abondantes et plus affectees par la mise enculture que les MO liees. Nous en avons deduit que les MO liees se biodegradent plus lentement. Les observations en microscopie electronique ont montre que la distribution des MO dans la fraction 2 mum etait tres heterogene et ont revele l'abondance de microagregats organo-argileux, de taille nanometrique a micrometrique. Nous en concluons que les complexes organo-mineraux primaires ne decrivent pas bien la realite et sont plutot des entites conceptuelles. Nous suggerons que les microagregats observes sont des sites majeurs de stabilisation des matieres organiques, par adsorption sur les mineraux comme par occlusion.