Impact of manganese concentration on litter decomposition

(eng) Litter quality controls organic matter decomposition in temperate forest ecosystems and thereby, storage and cycling of nutrients and organic carbon. The litter quality is traditionally linked to lignin concentration, an indicator of litter recalcitrance. Manganese ion (Mn2+) may play a key role in the catalytic cycle of the manganese peroxidase, an enzyme involved in the lignin degrading system and secreted by the white-rot fungi. Previous studies reported positive correlations between Mn concentration and litter decay rate. However, the role of Mn on litter decomposition has not been directly measured so far. This thesis was dedicated to the experimental study of the Mn impact on beech (Fagus sylvatica L.) litter decomposition. For this purpose, we performed incubations of organic materials with contrasting Mn concentrations (i) in laboratory under controlled conditions and (ii) in situ using a litterbag method. The contrasting Mn concentrations were obtained by aspersion of a MnSO4 solution and by experimental Mn-enrichment of leaves. We determined the decay rate, the carbon and nitrogen release, the chemical composition and, the lignin parameters (from lignin-derived phenolic compounds) of the decomposing organic materials. We observed a positive correlation between the decay rate and the initial Mn concentration in green litter. Also, our results showed that Mn had a positive influence on C-mineralization by enhancing microbial activity and/or by enhancing ligninolysis; in the case of brown litters, this influence depended on the forest floor type and on the organic decomposition stage. The computed lignin alteration degrees partly supported the hypothesis of ligninolysis enhancement by Mn and underlined the importance of the biotic environment in which decomposition took place. Finally, we suggest that the polyphenolic compounds released during ligninolytic process might have a key influence in this issue.