The fungal community of composts
Composts can be made from any type of decompos-
able organic matter, stacked into a heap (or long
heaps called windrows in the case of mushroom com-
posts) and supplied with adequate mineral nutrients
(especially nitrogen), moisture, and aeration. They are
used on a vast scale for the commercial production of
mushrooms (Agaricus), where the compost normally
consists of a mixture of cereal straw and animal man-
ure. They are also used extensively for the processing
of horticultural and urban wastes.
Provided that the basic requirements are met, the
decomposition process follows an essentially standard
pattern, shown in Fig. 11.11 for an experimental
compost made from wheat straw supplemented with
nitrogen in the form of ammonium. Within a few days
the temperature in the bulk of the compost rises to
70 – 80°C, caused by the heat generated when micro-
organisms grow on the more readily utilizable nutri-
ents. Bacteria are primarily responsible for this initial
heating phase. They include the thermophilic actino-
mycetes that cause farmer’s lung disease (Chapter 10),
and members of the bacterial genus Thermus, which
used to be considered a typical inhabitant of thermal
springs but has recently been found to be almost
universally present in composts and other self-heating
materials. Fungi play only a minor role in the early
stages of composting because they are slower to initi-
ate growth, and the maximum temperature for growth
of any fungus is about 62–65°C (Chapter 8).
After “peak-heating” the compost starts to cool
because most of the microorganisms are killed in the
center of the heap, but some Bacillusspp. can survive
as spores, and other bacteria can recolonize from the
surface, giving rise to second and third smaller
temperature peaks. Then the temperature gradually
declines to ambient temperature over 20 –30 days.
Bacterial populations remain high throughout this
time, but fungi are considered to play the most import-
ant role when they recolonize after peak-heating and
degrade much of the organic matter as the temperat-
ure slowly declines.
Chang & Hudson (1967) recognized several beha-
vioral groups of fungi, occurring at different stages of
the composting process. They are modified here for ease
of presentation.1 A mixture of weak parasites and pioneer sapro-
trophic fungiare found in the first few days. Many
of them were present on the original material,
including the leaf-surface fungi (Cladosporium, etc.)
and Fusariumspp. which often grow on cereal straw.
A few thermophilic fungi also grow in the first
few days, notably Thermomucor pusillusand Mucor
(or Thermomucor)miehei. These are pioneer sapro-
trophic fungi, which have maximum growth tem-
peratures of 57–60°C. They are inactivated or killed
by the peak-heating and they do not reappear.
2 Cellulolyticspecies of Ascomycota and mitosporic
fungi colonize after peak-heating and grow over
the next 10 –20 days. High temperature might activ-
ate the dormant ascospores of some of these fungi
(Chapter 10). Their incidence can differ between
different types of compost, but these cellulolytic
fungi commonly includeChaetomium thermophile,
Humicola insolens, Thermoascus aurantiacus(Fig. 11.12),
Scytalidium thermophilum, and Aspergillus fumigatus
(see Fig. 8.4). The temperature range of each fungus
is likely to determine how quickly it colonizes
after peak-heating (see Fig. 8.2). For example, C.FUNGAL ECOLOGY: SAPROTROPHS 223
Fig. 11.11Changes in temperature (thick line), populations of mesophilic fungi (broken line), and populations of
thermophilic fungi (narrow line) in a wheat straw compost. (Based on Chang & Hudson 1967)