Agriculture Reference
In-Depth Information
Earthworms form an important group of soil invertebrates and, although they show different
ecological strategies, all facets of their life in organic and mineral soil layers can potentially have
major impacts on other groups of soil organisms. (These are discussed, with particular reference
to the effects of earthworms in temperate and subtropical agricultural systems, in
Chapter 12
this
volume.)
The aim of this chapter is to provide information and comments on the influences of earthworms
on the structures of microbial and faunal communities in forest soils, particularly in the organic
layers of northern montane forests. At the outset, it must be admitted that, although substantial
data are available on the effects of earthworms on various soil processes and on microbial biomass
dynamics, accumulation of relevant detailed data on their effects on other soil biota has begun only
relatively recently.
EFFECTS OF CHANNELING, COMMINUTION, AND MIXING
Although different earthworm species differ in size and behavior, their activities have great conse-
quences for the physical and chemical characteristics of organic and mineral soil (Lee and Foster
1991). These activities include the ingestion of soil and organic material (plus comminution of
organic matter), the production of mucus and urine, and the intermixing of these materials; ejection
of gut contents as casts; and the formation of earthworm burrow systems. Comminution of organic
matter has important effects on decomposition rates (Swift et al. 1979) and on microbial growth
(Gunnarsson et al. 1988). It is often considered an important result of earthworm activity despite
the paucity of quantitative evidence. Schulmann and Tiunov (1999) found that
Lumbricus terrestris
selectively ingested sand from a mixture of sand and litter and hypothesized this increased organic
matter fragmentation during passage through the gut.
Earthworms are well known for their abilities to improve soil conditions, such as increased
stability through cast production and the production of macropores (burrows), which enhance the
movement of water and gases and root penetration through soil (Devliegher and Verstraete 1997;
Francis and Fraser 1998). Shipitalo and LeBayon and Kretzschmar (see
Chapter 10
and
Chapter
11
, respectively, this volume) provide detailed comments on earthworm-originated soil porosity
and the effects of earthworm burrows on soil structures. This increase in soil macroporosity also
enables smaller animals such as springtails (Marinissen and Bok 1988) and mites (Walter and
Proctor 1999) to penetrate to deeper soil horizons.
Earthworm burrowing activities range from those of species (epigeic earthworms) that inhabit
surface organic layers and do not produce well-defined burrows, to species (anecic earthworms)
that produce deep vertical burrows into the mineral soil, and finally to endogeic species that construct
networks of burrows in the upper mineral soil. Although earthworm activities are generally con-
sidered beneficial for soil structure, it must be remembered that, in some soils, they can have
adverse effects (e.g., Puttarudriah and Shivashankara Sastry 1961; Ester and van Rozen 2002).
Earthworm burrows are lined by the drilosphere soil zone (Bouch 1975). This is described by
Doube and Brown (see Chapter 12, this volume) and in detail by Brown et al. (2000). It is a zone
generally richer in nitrogen, phosphorus, and humified organic material than the surrounding soil
(Beare et al. 1994; Tiunov and Scheu 1999); moreover, burrow walls usually have a higher pH,
presumably because of excretion of earthworm subcutaneous mucus (Schrader 1994). However,
the detailed conditions in the drilosphere depend on the ecological group of the burrowing species
and the type of organic material eaten, the amount of organic fragments or humified material in
the drilosphere wall, and materials ejected in casts. The drilosphere appears to be a zone in which
significant increases in populations of some important physiological groups of soil bacteria occur,
such as nitrogen transformers (Bhatnagar 1975) and cellulolytic species (Tiunov and Kuznetsova
2000; Tiunov et al. 2001); also, increased enzyme activities and CO
efflux were recorded by Loquet
2
(1978).
