best-studied lignin-degrading fungi, Phanerochaete
chrysosporium, the production of lignin-degrading
enzymes is strongly stimulated by a critical shortage of
nitrogen. This could explain why these fungi occur in
the later stages of decomposition – they might need to
wait until other fungi have sequestered most of the avail-
able nitrogen.Secondary (opportunistic) invadersAt many stages in the decomposition of natural mater-
ials there are opportunities for secondary invaders to
grow. Some of these opportunists might grow in close
association with polymer degraders, using some of
the breakdown products released by enzyme action.
As discussed later, Thermomyces lanuginosusseems to
behave in this way in composts. Other secondary
invaders are known to parasitize other fungi in culture(e.g. Pythium oligandrum, Chapter 12) or they grow on
dead hyphal remains; yet others (e.g.Mortierellaspp.)
might grow on the fecal pellets or shed exoskeletons
of microarthropods that are abundant in decomposing
materials. The range of potential behavior patterns of
these fungi makes it difficult to generalize, but their
features commonly include: (i) nutritional oppor-
tunism, because they can scavenge low levels of nutri-
ents, and (ii) the ability to tolerate the metabolic
byproducts of other fungi.
Eventually all the decomposable materials will be
utilized, leaving a residue of humic substances (soil
humus). These are heterogeneous polymers consisting
of a framework of aromatic and aliphatic molecules
complexed with proteins and sugars. They are essen-
tially nondegradable and they play an essential role in
soil fertility by enhancing soil structure and water
retention, and by providing ion-exchange sites for
micronutrients in the soil.222 CHAPTER 11
Fig. 11.10Part of a fairy ring caused by Marasmius oreades(a), and part of the turf lifted to show the mass of white
fungal hyphae just below the soil surface (b). A ring of small toadstools of Marasmius(c) develops near the killing zone
in autumn. (d) Close-up of two fruitbodies of Marasmius, showing the typical widely spaced gills. (e) Toadstools of
another fairy-ring fungus – the waxy, scarlet-colored Hygrocybesp. These toadstools form rings of fruitbodies but they
do not kill the turf.
(a) (b) (c)(d) (e)