Agriculture Reference
In-Depth Information
humus represent a chronosequence in litter decomposition. Consequently, many studies of fungal
ÑsuccessionÒ during this process have been made (Kj¾ller and Struwe 1982; Widden 1986; Hansen
1989). Similarly, there have been many studies of arthropod communities in this humus type (e.g.,
Hgvar and Abrahamsen 1980; Petersen 1982; Kaneko 1985; Seastedt et al. 1989), including studies
of the feeding preferences of various invertebrate species (Dash and Cragg 1972; Moore et al. 1987;
Shaw 1988; Klironomos et al. 1992) and of their interactions with the litter fungi. As stated,
earthworms are generally considered relatively rare in mor humus, but some data on their occurrence
in this humus form have been given by Piearce (1972), Huhta and Koskenniemi (1975), Muys and
Lust (1992), and McLean and Parkinson (1997).
In the mull humus forms with higher pH, base-rich and lower C:N conditions, fungal biomass
is usually lower and bacterial biomass higher than in mor humus forms. Arthropod populations are
low, and earthworm populations are high (Schaefer and Schauermann 1991). The activities of anecic
and endogeic earthworms are major factors in removing organic materials from the soil surface
and in maintaining the mixing of these materials into the mineral soil with subsequent, stimulatory
effects on soil microorganisms.
The nature of the moder humus form results, at least in part, from the activities of epigeic species
of earthworms (Rusek 1985). Their activities, which cause mixing of organic materials and the
incorporation of earthworm casts, create major changes in the surface organic layers that may have
considerable effects on the communities of other litter-inhabiting invertebrates and microorganisms.
From the foregoing comments and those by Doube and Brown (see
Chapter 12
, this volume),
it can be seen that earthworms can affect significantly the physical and chemical conditions in
soils and different humus forms; this may affect the structure of the communities of other soil
organisms. However, detailed studies of these biological effects are rare because of experimental
difficulties.
EXPERIMENTAL APPROACHES TO INTERACTIONS
Just as in many grassland and agricultural ecosystems, in mull and moder forests, changes in
populations and activities of earthworms in forests have been occurring over long time periods. As
a result, a total complex litter/soil biota has evolved in coexistence with earthworms. Consequently,
there appear to be relatively few laboratory and field approaches possible for assessing impacts of
earthworms on other groups of soil organisms.
L
S
I
ABORATORY
TUDIES
OF
NTERACTIONS
The laboratory experimental approach involves the use of soil systems (micro- or mesocosms)
of varying degrees of complexity; this allows experiments to be done under controlled temperature
and moisture conditions. Thus, the variable field conditions of soil microclimate and the like are
avoided. However, if laboratory experiments are oversimplified, it is difficult to interpret them
in terms of actual field situations and conditions. In research into the effects of earthworms on
soil processes and soil communities, serious attempts have been made to use laboratory micro-
cosms or systems that reflect the realities of field situations. Nevertheless, it is often very difficult
to use this approach for studies of litter/soil systems taken from locations in which earthworms
have been active for long periods and have already affected soil structure, processes, and the rest
of the soil biota. In a laboratory study of the litter/soil profile of a cool, temperate aspen forest,
in the early stages of colonization by an epigeic earthworm species (
), Scheu and
Parkinson (1994b) showed that the presence of these earthworms produced microbial biomass
changes.
D. octaedra
