Morphological evolution of beech litter (Fagus sylvatica L.) and biopolymer transformation (lignin, polysaccharides) in a mull and a moder, under temperate climate (Fougeres forest, Britany, France).

This study was conducted on a mull and a moder from two beech stands, 27 and 87 yr old, respectively, in the Fougeres forest. In each stand, five profiles were subdivided in OL, OF and OH (moder only) layers and A(1) subhorizons. In the mull OL-OF layer, the organic matter was quickly degraded by white-rot fungi together with bacteria and,earthworms. A fungal attack occurred in the moder OL and OF layers, whereas bacteria seemed efficient in the OF layer and especially so in the OH layer, where they appeared responsible for lignin and structural polysaccharide degradation. Lignin degradation was indicated by: (i) the decrease of all phenolic monomers, (ii) methoxyl group loss and (iii) an increase in the vanillic acid/aldhehyde ratio. The production of microbial exo-polysaccharides at the expense of the structural polysaccharides, revealed by an increase of the mannose/xylose ratio, was also supported by transmission electron microscopy observations. The decrease in the lignin phenols and structural polysaccharides continues in the underlying A(11) and A(12) layers. In the mull, earthworm activity results in a complex organo-mineral association, whereas in the moder, enchytraeid worm activity is responsible for mixing of inherited organic aggregates associated with minerals.