Agriculture Reference
In-Depth Information
of a specific plant response to earthworms in an experiment is not easy; more often than not, several
mechanisms rather than a single mechanism are probably operating simultaneously.
The main aim of this chapter is to seek and identify possible mechanisms by which earthworms
can promote or suppress plant growth. Furthermore, we dig further into some of these mechanisms
and provide both conceptual diagrams of how they may be functioning and a few case studies
dealing with each of the seven main mechanisms we propose. Finally, we end with some suggestions
on how advancement will occur in this biologically complex area of research. Our basic premise
is that through better understanding of the ways by which earthworms affect plant growth and
production, plant and soil management techniques and practices can be adapted, improved, or
implemented to prevent the occurrence of negative effects of earthworms on soils and plants and
to maximize their positive effects on crops for the benefit of farmers, gardeners, ranchers, foresters,
and other land users.
THE MECHANISMS BY WHICH EARTHWORMS AFFECT PLANT
GROWTH: A CONCEPTUAL BACKGROUND
T
M
I
YPES
AND
ODES
OF
NTERACTION
The effects of earthworms on soils can take three main forms: effects on biological, physical, or
chemical soil properties and processes. Furthermore, because earthworms share the soil environment
with roots, their effects on plant growth and root development can be either direct or indirect. Thus,
the mechanisms of how earthworms influence plant productivity can be divided into three main
types: physical, chemical, and biological. These can operate either directly or indirectly.
Indirect
effects
mean that the plant is affected by earthworm activities through changes in the physical,
chemical, or biological soil or rooting environment produced by earthworms; the
of
action means that the earthworms or their activities lead to direct changes in root growth and
productivity.
direct mode
S
T
S
E
A
PATIAL
AND
EMPORAL
CALES
OF
ARTHWORM
CTION
(Lavelle 1988);
it constitutes one of the main soil functional domains (Beare et al. 1995; Lavelle 2002) that have
significance in regulating major soil processes and functions, such as structure, organic matter
(OM) decomposition, nutrient cycling, microbial and invertebrate populations, and plant growth.
The soil volume affected by earthworm activities has been termed the
drilosphere
Because earthworm burrows and casts may outlive the earthworms themselves, and regulate the
soil as an environment for other organisms (including plant roots) by controlling its physical
structure, nutrient fluxes, and energetic status (resource availability), they have been termed
eco-
system engineers
(Jones et al. 1994; Lavelle et al. 1997).
It is important to note that the drilosphere and the engineering effects of earthworms are very
variable and depend on biological factors such as the type of vegetation and the characteristics and
composition of the earthworm community at a particular location (species, abundance, biomass,
age structure, ecological strategy) and abiotic regulating factors, including climate, soil type, and
imposed anthropic (management) factors.
Furthermore, the earthworm drilosphere is a dynamic zone of action that is constantly changing
in both space and time as the earthworms ingest and reingest soil, burrow, and cast at different rates
and in different locations in the soil. Therefore, the drilosphere can affect soil functions (including
plant productivity) at different spatiotemporal scales, manifesting its effects at levels that range from
the earthworm gut up to the soil profile (Lavelle 1997); these ideas are explored in
Chapter 12
.
The effects of earthworms on plants in a given situation and the mechanisms involved are
difficult to assess because, although earthworms and their structures (burrows, casts) are often easily
identifiable or separable from the edaphosphere and their sphere of influence on the soil
