Importance et devenir des biopolymères (lignines et polysaccharides) dans les sols d'une chronoséquence de hêtraies (Fagus sylvatica), en forêt de Fougères (France)

Abstract

Four beech stands of various ages were selected in 1997 to study the evolution of lisnins and structural polysaccharides (cellulose, hemicelluloses) in the soil cover. The 10-year-old station has a mull type humus. In the 27-year-old stand there is a mull-moder mosaic whereas in the 87 and 145-year-old stands humus is a moder. Twenty-one soil profiles were sampled by separating the different humus layers, i.e., OL and OF, present in mull and moder, the OH layer, present only in the moder. Organo-mineral A11 and A12 horizons were also sampled with some A13 horizons in the mull stations. Lignins are abruptly degraded in mull where they represent 52‰ of the total organic carbon (TOC) in OL and only 12‰ in A1 horizons. Similarly, polysaccharides undergo degradation from 236‰ (OL) to 105‰ (A1) of TOC. The fast decrease in the concentration of these components over a small depth interval is indicative of a strong biological activity. The lignin alteration is evidenced by the decrease of the ratio of syringic compounds over vanillic compounds that reveals methoxyl group losses and the increase of vanillic acid over vanillic aldehyde which indicates oxidative depolymerization of lignins. In old stands (87, 145-y.) where humus was of an uniform moder type, the decrease of the xylose/mannose ratio in the OF and OH layers reveals the production of microbial sugars at the expense of the phyto-inherited polysaccharides, like cellulose and hemicelluloses which decrease, whereas lignins are also strongly degraded. The decrease of the structural polysaccharides continues in the underlying A1 horizon, similar to the evolution observed in the transition from mull to moder although at a less degradation rate.