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Other studies have shown that the microbial species favored by earthworm activity differ with
environmental conditions and the earthworm community present. For example, feeding by O.
lacteum on partly decomposed OM in peat-humus forests in Russia appeared to benefit spore-
forming bacteria (e.g., Bacillus spp. that decompose recalcitrant substances), to the detriment of
fluorescing bacteria, which generally use fresh and easily decomposed OM (Kozlovskaya and
Zhdannikova 1961). In contrast, when L. rubellus fed on fresh substrates, fluorescing bacteria were
favored over Bacillus spp. in podzolized soils, O. lacteum had the same effect on microbial
development as L. rubellus because in these soils O. lacteum came to the surface and fed on newer
materials. In the peat-humus soils later in the season (summer), the absence of fresh litter residues
caused L. rubellus to feed on older residues and have an effect similar to that of O. lacteum in the
same soil (Kozlovskaya and Zhdannikova 1961).
Microbial and invertebrate successions on decomposing plant debris and other substrates are
modified by earthworm-induced changes in the quality (physicochemical nature) of these resources.
This occurs through ingestion and comminution of organic residues by the earthworms changing
its C:N ratio and physicochemical characteristics, by creating heterogeneous microsites within the
soil, and by grazing selectively on and dispersing particular organisms. For instance, in apple
orchards (Raw 1962) and temperate deciduous forests (Knollenberg et al. 1985), it was reported
that L. terrestris may bury more than 90% of the annual litterfall. Thus, in apple orchards, up to
22 species of fungi and 5 species of insects inhabiting the surface litter were buried and their
populations reduced (Mills 1976; Niklas and Kennel 1981; Laing et al. 1986; Kennel 1990).
Under these conditions, distinct changes in the microfloral succession are also likely, not only
because some species are buried and their populations reduced, but also because the comminution
of litter by anecic earthworm species (and also epigeic and some surface-feeding endogeic species)
favors the development of r-selected (fast growing) fungi such as Phycomycetes (e.g., Mortierella
and Mucor spp.), Ascomycetes, and Deuteromycetes (e.g., Phoma and Trichoderma spp.) capable
of rapid exploitation of the easily assimilable materials found in earthworm casts (Visser 1985;
Tiunov and Scheu 2000; Orazova et al. 2003). In particular, middens created by anecic species of
earthworms are important sites of enhanced (hot spots) microbial and faunal activity and popula-
tions, and the presence of a large anecic population may enhance soil surface and subsurface
heterogeneity (the spatiotemporal distribution of resources), thereby altering the spatial and tem-
poral organization of soil communities (Brown 1995; Maraun et al. 1999; Tiunov and Kuznetsova
2000; Shuster et al. 2001).
Nevertheless, selective grazing (or burial) on fast-growing fungi by earthworms (see previous
sections) may reduce their competitive ability and allow slower-growing (k-selected) fungi such as
Basidiomycetes to gain a competitive advantage. For instance, Scheu (1992) reported that lignin-
decomposing fungi (which occur later in the succession) decomposed lignin in earthworm feces
only after a lag phase of 3 months. In a later experiment, Scheu (1993) observed an overall increase
in lignin mineralization by a factor of 1.1 in soil columns with O. lacteum and 1.2 for those with
L. castaneus . Selective grazing may also lead to an increase in populations of the selected microbial
species in earthworm structures and, occasionally, in total fungal diversity as well (Tiwari and
Mishra 1993). Similar processes may also apply for algae and protozoa. For instance, Gupta and
Doube (unpublished data) recovered over 30 species of protozoa from the casts of field-collected
A. trapezoides , a substantially increased level of diversity compared with surrounding soil on a
gram-for-gram basis.
In a mesocosm study, using a simulated forest floor and a combination of various soil inverte-
brates, Huhta et al. (1991) reported greater N mineralization in the presence of L. rubellus than by
complex fauna in the absence of earthworms. Similarly, in lysimeters with different combinations
of animals, Anderson et al. (1983) observed increased losses of Na, K, Ca, and mineral N from
oak leaf litter and up to 60 times greater NH 4 losses in the presence of L. rubellus than in its
absence. These responses were presumed to be caused by earthworm-induced changes in microbial
activity, although the organisms responsible were not characterized.
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