Fungal Degradation of Lignin: Chapter 2

Of all naturally produced organic chemicals, lignin is probably the most recalcitrant. This is consistent with its biological functions, which are to give vascular plants the rigidity they need to stand upright and to protect their structural polysaccharides (cellulose and hemicelluloses) from attack by other organisms. Lignin is the most abundant aromatic compound on earth, and is second only to cellulose in its contribution to living terrestrial biomass (Crawford, 1981). When vascular plants die or drop litter, lignified organic carbon is incorporated into the top layer of the soil. This recalcitrant material has to be broken down and recycled by microorganisms to maintain the earth’s carbon cycle. Were this not so, all carbon would eventually be irreversibly sequestered as lignocellulose. Lignin biodegradation has diverse effects on soil quality. The microbial degradation of litter results in the formation of humus, and ligninolysis probably facilitates this process by promoting the release of aromatic humus precursors from the litter. These precursors include incompletely degraded lignin, flavanoids, terpenses, lignans, condensed tannins, and uberins (Hudson, 1986). Undegraded lignocellulose, e.g. in the form of straw, has a deleterious effect on soil fertility because decomposing (as opposed to already decomposed) lignocellulose supports high populations of microorganisms that may produce phytotoxic metabolites. High microbial populations in undecomposed litter also compete with crop plants for soil nitrogen and other nutrients (Lynch and Harper, 1985). By breaking down the most refractory component of litter, ligninolysis thus contributes to the removal of conditions that inhibit crop productivity. Conditions that disfavour the biological breakdown of lignocellulose lead to soils with pronounced accumulations of litter. For example, the soils of coniferous forests in the northwest United States may contain 50 years of accumulated litterfall, because the low pH of the litter and the lack of summer rainfall inhibit microbial activity. In mature forests of this type, woody material such as dead trunks and branches can constitute 50–60% of the litter. By contrast, the soils under broadleaf forests in the eastern United States accumulate only a few years’ worth of litter, and soils in some tropical rain forests accumulate virtually none, because conditions are more favourable for decomposition (Spurr and Barnes, 1980). Warm temperature, high moisture content, high oxygen availability, and high palatability of the litter to microorganisms all favour decomposition. The more highly lignified litter is, the less digestible it is, and the more its decomposition depends on the unique organisms that can degrade lignocellulose.