Contribution Of Saprotrophic Basidiomycetes To Nutrient Cycling And Soil Structure

The main input into grassland decomposition systems is lignocellulose. As described by Baldrian (Chapter 2), saprotrophic basidiomycetes are able to secrete batteries of extracellular enzymes but our knowledge of lignocellulose decay in soil and the organisms involved is less detailed than for larger woody resources. While there have been detailed studies of decomposition in woodland systems (Frankland, 1982; Steffen et al., 2000, 2002), the only comparable studies in grasslands have focused on fairy rings (see above).

It is the decomposition of lignin that is generally accepted to be the rate-limiting stage in carbon and nutrient cycling in terrestrial ecosystems. In addition to containing less lignin, the composition of grass lignin contains 10-20% phenolic units, a higher proportion than in wood (Lapierre et al., 1989). This may allow easier catabolism by laccase and manganese peroxidase that directly degrade only phenolic units (Camarero et al., 1994). Grass lignins are also more extensively cross-linked with polysaccharides cell wall polymers (via p-coumaryl subunits to hemicelluloses) than are wood lignins (Iiyama et al., 1990; Lam et al., 1992). These factors make grass lignins more readily degradable (Lapierre et al., 1989). As is the case for woodland litter/soil, most lignolytic basidiomycetes in grasslands belong to the Agaricales, though as noted above Aphyllophorales are also present. There is evidence that the role of ascomycete fungi in lignin degradation may be relatively more important (Kluczek-Turpeinen et al., 2003; Deacon et al., 2006), with several soil-inhabiting species having been shown to be able to mineralize grass lignin more rapidly than wood lignin (Rodriguez et al., 1996).

Lignins in soil are a major source material for the formation of humic compounds. There is a correlation between the lignin content of organic inputs and the amount of humus formed (Hammel, 1997; Heal et al., 1997) but it is difficult to assess the degree to which plant lignins are transformed through humification. The enzymes involved in ligninolysis can also mediate formation and degradation of humic compounds (Gramss et al., 1999; Scheel et al., 1999; Steffen et al., 2002). These phenoloxidases can mediate covalent binding of aromatic compounds and it is suggested that humic compounds are the partially oxidized products of phenoloxidase activity in soil (quinones condensed with peptides, amino sugars and aromatics; Gramss et al., 1999). While the energetic benefits of degrading complex aromatic polymers are considered to be marginal, humic compounds (unlike lignin) contain N (much soil N is present in this form), so for basidiomycetes in oligotrophic grassland such sources may be important. However, the mobilization of the recalcitrant organic N pool in soil is a poorly understood process (O'Connor, 1983).

Through mucilage secretion and mycelial entanglement of soil particles, fungi are considered to be important in the formation of water-stable aggregates by binding microaggregates (50-250 p.m) into macroaggregates (> 250 p.m) (Tisdall and Oades, 1982). The role of the glycoprotein glomalin, secreted by AM fungi, in this process is well established (Rillig and Mummey, 2006) but basidiomycetes including R. solani also contribute to aggregate stabilization (Tisdall et al., 1997). An unidentified grassland basidiomycete, closely related to Peniophora, has also been shown to secrete large quantities of a polysaccharide with significant soil-binding properties (Caesar-TonThat and Cochran, 2000; Caesar-TonThat et al., 2001). Antibodies raised against cell walls of this fungus reacted strongly with larger (> 2 mm) soil aggregates from dry grassland soils and to a lesser extent in adjacent arable soils. A less desirable effect of basidiomycetes on soil texture is due to the water repellent properties of their hyphae (White et al., 2000), thought to be associated with the secretion of hydrophobin proteins (Rillig and Mummey, 2006).

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