Agriculture Reference
In-Depth Information
Again, there is the possibility of earthworms as vectors not only of saprotrophic fungi, but also
of plant root pathogens and of beneficial mycorrhizal fungi. Toyota and Kimura (1994) showed
that, although the numbers of viable spores of the pathogen
Fusarium oxysporum
f. sp.
Raphini
decreased during passage through
sp. guts, there was effective spread of this fungus via
the earthworm casts. Earthworms have been considered potential vectors of mycorrhizae, particu-
larly vesicular arbuscular mycorrhizae (VAM) fungi. Pattinson et al. (1997) provided a brief review
of this issue and considered that, although earthworms have the potential for this dispersal, they
found little evidence for it. They not only found no evidence for VAM spore dispersal by
Pheretima
Aporrec-
todea trapezoides,
but also reported that this earthworm reduced the VAM colonization of
Trifolium
subterraneum
roots. Lawrence et al. (2003) found a similar situation with VAM colonization rates
of sugar-maple roots and suggested, as did Pattinson et al. (1997), that disruption of the extramatrical
hyphae and hyphal networks of the VAM fungus was a strong possibility for the cause for this
reduction in colonization. However, Rabatin and Stinner (1988) and Cavendar et al. (2003) reported
that soil invertebrates, particularly springtails and, to a lesser extent, earthworms, had important
positive impacts on the transmission of VAM fungi.
Very little work seems to have been done on the effects of earthworms on ectomycorrhizal
fungi. Reddell and Spain (1991a) reported live spores and hyphae of hypogeous ectomycorrhizal
fungi were present at low frequencies in earthworm casts. Obviously, the interactions between
earthworms and ectomycorrhizal fungi are a topic requiring more research.
ROLES OF THE SOIL BIOTA IN DEVELOPMENT OF
FOREST HUMUS FORMS
In forest ecosystems, the importance of soil invertebrates, usually with emphasis on the roles
of earthworms, in the development and maintenance of soil structure has been recognized for
more than 100 years. In temperate forests, organic matter (humus in the broad sense) accumu-
lates on the soil surface, and there may be an organic-enriched upper mineral horizon. In
classifying forest and heathland humus forms, Mller (1878) introduced the terms
mull
and
mor,
. Green et al. (1993) provided an account of the
tortuous subsequent history of the development of terminology for humus forms and provided
a proposal for their classification. In this chapter, a simplified view is taken, and only mor,
mull, and moder forms are considered.
The mor litter form is one in which organic matter accumulates on the soil surface and in which
definable L, F, and H horizons can be distinguished. There is very little or no mixing of organic
materials with the upper mineral soil. In the mull litter form, there is little accumulation of organic
matter on the soil surface because there is incorporation of this material into the upper mineral
soil. This humus form is generally found in forests on base-rich soils and in grasslands. In forests
with the moder humus form, there is accumulation of organic materials on the soil surface, but the
distinction between the F and H layers is frequently unclear, and in these layers, there may be
incorporation of mineral particles. Also, there may be limited infiltration of humus material into
the uppermost mineral soil layer. Moders have been considered intermediate in the complex gradient
from mor to mull humus forms (Green et al. 1993).
The type of humus form in a particular ecosystem is determined by a range of climatic, edaphic,
and vegetation factors. However, it is well known that the soil biota play important, interactive
roles in the development and maintenance of specific humus forms (e.g., Rusek 1985).
In low pH, base-poor, high C:N conditions in mor humus forms, there is a large fungal biomass
with frequent production of hyphal mats and hyphal strands, large arthropod and nematode popu-
lations, and a rarity or absence of earthworms; therefore, the mixing of the organic layers or mixing
of organic matter into the mineral soil does not occur often. The L, F, and H layers formed in mor
and Ramann (1911) added the term
moder
