Agriculture Reference
In-Depth Information
documented changes from podzolic moder soils to mull-structured soils within 3 to 4 years. In
the laboratory, C. Shaw and Pawluk (1986) observed a complete mixing of organic and mineral
soil materials if both anecic and endogeic earthworm species were present. Scheu and Parkinson
(1994b) showed that endogeic earthworm species may entirely mix the F and H material of aspen
forest soil into the mineral soil, thereby transforming moder to mull humus. Alban and Berry
(1994) documented that the organic layers of an aspen forest in Minnesota were completely
incorporated into the mineral soil, thereby transforming the former E horizon into an A horizon.
This process was caused mainly by an invasion of endogeic earthworm species (
Aporrectodea
tuberculata
), but the epi-endogeic species
L. rubellus
, which also invaded this forest although
in lower populations, presumably also contributed to the incorporation of the L-layer material
into the mineral soil. However, detailed analyses of the time course of this mixing process and
the interactions between earthworm species responsible for the soil transformations under field
conditions are still lacking.
C
HANGES
IN
S
OIL
B
IOTA
C
AUSED
BY
C
OLONIZING
E
ARTHWORMS
The profound changes in soil structure resulting from the invasion of forest soils by earthworms
modify the habitat and distribution of resources for the diverse soil microorganisms and soil
invertebrates and therefore the whole decomposer food web. Unfortunately, information about these
changes is fragmentary at best.
Soil Microorganisms
Microbial biomass in the L and F layers appeared to be quite resistant to earthworm-mediated
changes in soil structure and soil processes, although microbial respiration decreased with increasing
earthworm biomass in pine forest soils (McLean and Parkinson 1997). However, significant differ-
ences in the fungal community structure and species composition because of earthworm activities
were observed in both the laboratory and field. In a 6-month laboratory mesocosm experiment, the
activities of the epigeic earthworm species
D. octaedra
caused changes in the numbers of fungal
isolates per soil particle, which reflects the density of different fungal taxa on an individual soil
particle and is an indication of the degree of competition between fungi. In this experiment,
increasing earthworm activities resulted in an initial increase in the number of fungal isolates per
particle followed by a decrease in the number of isolates per particle (McLean and Parkinson
1998b). This may be because of the following factors: (1) in the absence of earthworms, competition
between fungi for nutrients may limit the density of fungi on each particle; (2) low levels of
earthworm activities decrease fungal competition (and therefore increase the number of isolates
per particle) through increases in spatial heterogeneity or nutrient release; and (3) high levels of
earthworm activities decrease numbers of fungal isolates per particle by decreasing their spatial
heterogeneity (i.e., complete homogenization of the organic layers) or by repeated disruption of
fungal hyphae (McLean and Parkinson 1998a). In addition, the fungal communities in the soil
organic layers became less similar over time at low earthworm population densities and more
similar at high earthworm densities, probably reflecting increasing then decreasing spatial hetero-
geneity or the stimulatory effects of low levels of fungal disruption and the destructive effects of
high levels of fungal disruption (McLean and Parkinson 1998b).
Data from a 2-year field experiment supported the observations of the effects of earthworms
on the fungal community in mesocosms (McLean and Parkinson 2000a). The fungal community
in the FH layer, the layer of highest earthworm activity, became more similar to that in the A
h
horizon over time and less similar to that in the L layer with high earthworm activity (McLean and
Parkinson 2000a). In the FH layer, the high earthworm populations were correlated positively with
fungal dominance and negatively with fungal species richness and diversity (McLean and Parkinson
2000a). Two pieces of evidence suggesting that the source of these effects on the fungal community
