Agriculture Reference
In-Depth Information
processes other than faunal activity (Pawluk 1985). In compact soil fabrics, root development (with
subsequent decomposition) and earthworm activities may allow the development of Ñchannel
fabrics.Ò Again, these effects on soil structure can enhance microbial and faunal development, and
it could be hypothesized that this would include increases in their species diversity.
Another major impact of earthworm activities is the incorporation of organic debris from the
soil surface into the subsurface mineral soil layers. As Doube and Brown (see
Chapter 12
, this
volume) have stated, the subsequent increases in the organic matter content in these mineral soil
layers may allow changed soil structure, increased microbial biomass and activity, changed patterns
of colonization by saprotrophic microorganisms on the decomposing organic matter, and increased
diversity of microhabitats for soil invertebrates. The importance of earthworm casts in adding
ÑprocessedÒ organic materials, surface organic materials, or mineral soil and thus affecting other
soil biota is considered later.
Anecic species of earthworms (e.g.,
) produce vertical burrows and remove litter
from the forest floor and either take it down into the soil or effectively mix the litter material with
large numbers of casts on the soil surface, thereby constructing middens, which are conspicuous
structures on the forest floor. In these middens, there is increased microbial and faunal activity,
which causes increased litter palatability for ingestion by earthworms and other soil invertebrates
(Brown et al. 2000). Maraun et al. (1999) reviewed work on the effects of the middens of
L. terrestris
L.
on other micro- and mesofauna in a beech forest. Also, they presented the results of a
detailed ambitious study that assessed the importance of middens to other groups of soil inverte-
brates. At the outset of their study, they hypothesized that, as a result of higher microbial biomass
in middens, there would be an increase in microbivorous micro- and mesoinvertebrates plus the
occurrence of invertebrate species specific to middens. The values they reported for carbon and
nitrogen, microbial biomass, and basal respiration were significantly higher in midden than in
nonmidden material, and middens became, at least temporarily, a preferred habitat for several soil
invertebrate species. However, other species either avoided middens or were affected detrimentally
by their environmental conditions. Many species of organisms coexist in middens, and conditions
there, including high microbial biomass, allow various potential prey species (e.g., Nematoda,
Collembola) to achieve high population densities. This benefits predaceous groups (e.g., Gamasina,
Uropodina). Finally, their initial speculation that midden microhabitats would allow increased niche
diversity and hence high faunal diversity was not supported. Almost all midden species also occurred
in nonmidden materials, but as mentioned, populations above the densities of individual species
were higher in middens.
terrestris
EFFECTS OF EARTHWORM GRAZING, GUT TRANSIT, AND
CASTS ON SOIL ORGANISMS
The effects of earthworm grazing on soil organisms are investigated by Brown (1995) and Doube
and Brown (see Chapter 12 this volume); hence, the following comments are brief and attempt, if
possible, to deal particularly with forest ecosystems. There is evidence from feeding choice studies
that earthworms prefer softer rather than harder textured leaves (Heath and Arnold 1966; Wright
1972), small organic fragments rather than large ones (Judas 1992), and leaves with high nitrogen
content and low amounts of secondary metabolites (Hendriksen 1990; Edwards and Bohlen 1996).
It has been considered for some time that leaf decomposer microorganisms, particularly fungi, may
be an important food source for many earthworm species (see reviews by
Edwards and Fletcher
that fragmented leaf material, supporting a high microbial biomass, would be preferred food for
earthworms. When studying the impacts of
during colonization of a lodge-
pole pine forest McLean et al. (1996) reported that, in comparison with material from other litter
Dendrobaena octaedra
