Interaction between clay microstructure, decomposition of plant residues and humification

We have studied microstructural changes in montmorillonite and kaolinite, w hich were amended with organic debris (leaves of chestnut or beech), microb ial inoculum and nutrients and subjected, for 30 days at 25 degrees C, to a lternate drying and wetting (D-W) or kept continuously moist at water-holdi ng capacity (WHC). The objective was to evaluate the interactions between t he decomposition of plant residues and clay microstructural organization in different pore-water regimes. The microstructure was studied by Hg porosim etry, N adsorption at 77 K, and scanning electron microscopy. Decomposition was assessed by measuring residual C and N, the amount of humified materia l, the relative contents of humic (HA) and fulvic (FA) acids, and their mol ecular weight distribution. The structural organization at the end of the e xperiments was different for the two clays, and new classes of pores were f ound as the result of the microbial metabolism. The decomposition was also significantly affected by clay type. Decomposition of the organic matter (O M) was rapid in the montmorillonite. The humified material occurred mainly as low molecular weight FAs. By contrast, transformation was slow in kaolin ite, leaving much residual C, and more of the humified material consisted o f HAs. Alternate D-W had little effect on microstructure and decomposition Samples that were kept moist and amended with chestnut leaves produced the greatest amount of humified substances. The dynamics of the decomposition w as significantly affected by the nature of the leaves as indicated by the d ifferences in residual C, humified material and C/N ratio. By contrast, cha nges in pore size distribution (PSD) were little influenced by the type of organic debris.