Impact of fine litter chemistry on lignocellulolytic enzyme efficiency during decomposition of maize leaf and root in soil
Résumé
Residue recalcitrance controls decomposition and soil organic matter turnover. We hypothesized that the complexity of the cell wall network regulates enzyme production, activity and access to polysaccharides. Enzyme efficiency, defined as the relationship between cumulative litter decomposition and enzyme activities over time, was used to relate these concepts. The impact of two contrasting types of cell walls on xylanase, cellulase and laccase efficiencies was assessed in relation to the corresponding changes in residue chemical composition (xylan, glucan, lignin) during a 43-day incubation period. The selected residues were maize roots, which are rich in secondary cell walls that contain lignin and covalent bridges between heteroxylans and lignin, and maize leaves having mostly non-lignified primary cell walls thus making the cellulose and hemicelluloses less resistant to enzymes. Relationships between C mineralization and change in residue quality through decomposition indicated that the level of substitution of arabinoxylans (arabinan to xylan ratio) provides a good explanation of the decomposition process. In leaves enriched in primary cell walls, arabinose substitution of xylan controlled C mineralization rate but hampered polysaccharide decomposition, but to a lesser extent than in roots in which arabinoxylans were mostly cross-linked with lignin. Enzyme activity was higher in leaf than root amended soils while enzyme efficiency was systematically higher in the presence of roots. This apparent paradox suggests that residue quality could preselect the microbial community. Indeed, we found that microorganisms exhibited an initial rapid growth in the presence of a high quality litter and produced enzymes that are not efficient in degrading recalcitrant cell walls while, in the presence of the more recalcitrant maize roots, microbial biomass grew more slowly but produced enzymes of higher efficiency. This high enzyme efficiency could be explained by the synergistic action of hydrolytic and oxidative enzymes even in the early stage of decomposition.