Agriculture Reference
In-Depth Information
volume). For example, Gilot-Vilienave (1995), Baker et al. (1995a,b), and Blakemore (1995) all
reported greater plant productivity or nutrient uptake in soils when earthworms were present
compared with control soils without earthworms. However, Devliegher and Verstraete (1995)
reported decreased plant growth in the presence of
Lumbricus terrestris,
and Baker et al. (1995)
recorded no effect of
Aporrectodea rosea
on clover growth. Lavelle et al. (
Chapter 8
, this volume)
reported that legumes generally did not seem to respond to earthworm inoculations or additions,
whereas maize and rice and other crops did. Several possible mechanisms were proposed to account
for increased plant growth in the presence of earthworms, including reduced incidence of plant
diseases (i.e., earthworms may consume disease fungi, promote antagonistic microorganisms, or
make soil inhibitory to them); increased availability of nutrients (e.g., through stimulation of
mineralization); enhanced soil structure (e.g., reduced bulk density and greater porosity), thereby
creating a more favorable environment for root growth; or combinations of these factors.
Despite all the various beneficial effects (well documented and hypothesized) of earthworms
on nutrient dynamics, soil structure, and plant growth, Parmelee et al. (1998) reviewed some aspects
of earthworm activities that may be considered undesirable in agricultural systems. These include
the removal or burial of surface organic residues that would otherwise protect soil surfaces from
erosion; increasing soil erosion and surface sealing via freshly produced, unstable casts that are
susceptible to impact of raindrops; perforating the walls of irrigation ditches, making them less
able to carry water; increasing nitrogen losses through leaching and denitrification; and increasing
carbon losses through enhanced microbial respiration. Bohlen et al. (
Chapter 9
, this volume) suggest
that it is the net balance of the positive and negative effects of earthworms that is important in
determining whether they may have any detrimental impacts in agroecosystems, although, overall,
their contributions are certainly beneficial.
As pointed out by Curry (1994) (
Chapter 6
, this volume), any management practices applied
to soils are likely to have some effects on earthworm populations. The well-used diagram of Edwards
and Lofty (1969) and Wallwork (1976) is modified in
Figure 15.1
to illustrate this point. Thus,
agricultural management practices can have positive or negative effects on the abundance and
diversity of earthworms. Apart from the direct toxicity of some agricultural chemicals and fertilizers
or substances contained in soil amendments (e.g., heavy metals in sewage sludge), these effects
are primarily the result of changes in soil temperature, soil moisture, and organic matter quantity
or quality, which are the principal driving variables of soil biological activity.
Finally, an interesting trend comes from Australia, which has historically had a strong interest
in earthworms: scientists studying earthworm ecology are developing a tradition of interacting
closely with farmers and with the public (see
Chapter 14
, this volume). The ÑEarthworms Down-
underÒ program (Baker et al. 1995a), organized involving young people in the collection, identifi-
cation, and census of earthworm species nationwide, not only accumulated useful information for
agricultural purposes, but also stimulated public interest in earthworm research and may serve to
recruit new scientists into agricultural research.
FUTURE RESEARCH NEEDS
Priorities for earthworm research relevant to agriculture include research into the influences of
earthworms on soil processes and plant growth and the impacts of agricultural practices on earth-
worm populations (Hendrix 1995). With respect to the effects of earthworms on biogeochemical
cycles, Blair et al. (1995) identified several key topics. They considered that a better understanding
of the effects of earthworms on soil nutrient and organic matter dynamics should come from studies
over a range of spatial scales, ranging from short-term physiological processes within the earthworm
gut to long-term patterns of nutrient storage and loss in ecosystems (see Chapter 9, this volume).
Equally important is the need for a clear linkage between results derived from short-term laboratory
experiments and long-term field studies. In particular, means of translating microsite phenomena
