untitled

2 During growth on a carbon substrate, fungi convert
approximately one-third of the substrate carbon into
cellular material (a substrate conversion efficiency
of about 33% as explained in Chapter 4). The other
two-thirds is respired to CO 2. These values are only
approximate, but they serve as guidelines.

It follows that a material with a C : N ratio of about
30 : 1 is a “balanced” substrate – it can be degraded
rapidly and completely because:

10 of the carbon “units” (units are unspecified) are

incorporated into fungal biomass;

20 of the carbon units are released as CO 2 ;

1 unit of nitrogen is incorporated into fungal

biomass.

Now, consider a material of C : N ratio 100 : 1 (wheat
straw is roughly 80 : 1, sawdust ranges from 350 : 1 to
1250 : 1, and newsprint has essentially no nitrogen).
The fungus starts to grow on this material, but the
available nitrogen is depleted long before all the
organic carbon has been used. Viewed in simple
terms, decomposition will stop at the point where:

10 C units are combined with 1 N unit in the

mycelium;

20 C units have been respired to CO 2 ;

70 C units are left in the residual substrate.

In effect the fungus is now starved because there is no

nitrogen available for growth. At this point several

things can happen. Some fungi seem to recycle their

cellular nitrogen, perhaps by controlled autolysis of

the older hyphae. Other fungi preferentially allocate

nitrogen to essential metabolic processes – for example,

wood-decay fungi such as Coriolus versicolorseem to

do this (discussed later). A third possibility is to recruit

extra nitrogen from soil, but this is not possible in a

“closed” system such as a compost. In practice, many

of the fungal hyphae will die when nitrogen becomes

limiting, and the nitrogen can then be reused, either

by the remaining cells of the same species or by other

species. The same points apply to any essential mineral

nutrient that is in short supply.

Can nitrogen-depletion drivea

decomposition sequence?

This question arises from the comments above: if

one fungus depletes the available nitrogen and cannot

recycle it, then a proportion of the cells will die and

their products might be used by another fungus later

in the succession.

This possibility has been tested experimentally in

laboratory conditions (Fig. 11.13), with flasks contain-

ing 7 g sterile filter paper (almost pure cellulose) plus

nitrate as the nitrogen source (C : N, 200 : 1) and

FUNGAL ECOLOGY: SAPROTROPHS 225

Fig. 11.13A laboratory experiment simulating
a succession of thermophilic fungi growing on
cellulose (Deacon 1985; see text for details). A.
fum =Aspergillus fumigatus; N =nitrogen; Scyt =
Scytalidium thermophilum; Thermo = Thermoascus
aurantiacus; T.l. =Thermomyces lanuginosus.