Lignin Decomposition by Saprotrophs

As cellulose decomposition proceeds, the concentration of the more recalcitrant lignin increases (Figure 1; Berg et al., 1982). At later stages of decomposition, decay correlate well with lignin concentration in the litter (McClaugherty and Berg, 1987). Polyphenolic compounds, either tannins present in the fresh litter or products of lignin decomposition, form recalcitrant complexes with nitrogen-containing compounds, such as proteins and chitin (Kelley and Stevenson, 1995). As a result, nitrogen progressively becomes incorporated into the highly recalcitrant, polyphenolic litter fraction during decomposition (Berg, 1988). In highly decomposed coniferous forest humus, more than half of the nitrogen was found in the acid insoluble (i.e. polyphenolic) fraction (Johnsson et al., 1999). Bas-idiomycetes have been highlighted as the main organisms responsible for lignin degradation, using elaborate oxidative enzyme systems (Rayner and Boddy, 1988; Chapter 2). There are, however, large energy costs associated with the synthesis and activation of these enzymes. No organism has been found to use macromo-lecular lignin as a sole carbon source, and lignin degradation is believed to be a co-metabolic process requiring other substrates, such as cellulose, as energy sources (Kirk and Farrel, 1987; Hatakka, 2001). Most literature claims that the major benefit of lignin decomposition is increased access to ligno-cellulose. However, the well-developed ligninolytic enzyme systems of litter fungi may also be used to decompose humified polyphenolic compounds (Steffen et al., 2002), increasing the availability of nitrogen rather than carbohydrates. Given a resource characterised by low nitrogen availability, such as coniferous litter, the energy investment associated with the production of ligninolytic enzymes could be rewarded by increased nitrogen availability. Hypothetically, litter fungi could thus translocate carbohydrates from fresh litter to forage for polyphenolic-bound nitrogen in well-decomposed litter. This hypothesis is supported by the observations that nitrogen additions decreased the activity of ligninolytic enzymes both in pure cultures of P. chrysosporium in the lab (Keyser et al., 1978; Kirk and Fenn, 1982) and in field trials (Carreiro et al., 2000; Frey et al., 2004; Sinsabaugh et al., 2005). In line with the reduced enzyme activities, increasing exogenous nitrogen hampered decomposition of more recalcitrant material, that is high in lignin and polyphenolic compounds (Fog, 1988; Berg and Matzner, 1997). The decomposition-stimulating effect of nitrogen during early decomposition stages thus contrasts with the inhibition of degradation at later stages (McClaugherty and Berg, 1987).

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