Agriculture Reference
In-Depth Information
(drilosphere) can be measured and quantified physically, chemically, and biologically under con-
trolled conditions, the drilosphere is connected with the rest of the soil system. This means that it
can interact profoundly with other soil organisms and functional domains (e.g., rhizosphere, poro-
sphere, aggregatusphere, detritusphere, mermycosphere, termitosphere) (Brown et al. 2000). This
interconnectedness becomes even more evident as in attempts to separate the mechanisms respon-
sible for plant responses to earthworms in any given situation, soil type, or area.
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Roots, as sensitive sensors of the soil environment and the producers of many signals that
ultimately control plant shoot growth (Aiken and Smucker 1996), are the primary and immediate
receivers of the contributions of earthworms to soil functions. By controlling nutrient and water
supply to the shoots, it is the biomass, density, distribution, and activity (growth rate and
longevity) of roots within the soil profile that will largely determine plant productivity (Brown
and Scott 1984). Thus, it is the response of roots to earthworm activity that usually controls the
overall plant response.
A simple conceptual model connecting the physical, chemical, and biological effects of earth-
worms on soils with their potential effects on plant root or shoot growth and nutrition is provided
in
Figure 2.2
. The interdependence of earthworm physical activities (production of casts and
burrows) and earthworm physiological activities (excretions, secretions, and tissue death) in inter-
actions with soil properties such as organic matter (soil OM, root and residue inputs), microbial
populations, and plant production is evident. The effects of chemical substances on soil properties
and processes are based on the selection by earthworms of particular soil particles and organic
matter, the different nutrient compositions of their feces compared with uningested soil, cutaneous
mucus secretion, and excretion of metabolic products. Biological effects on soils are caused
primarily by interactions of earthworms with the rhizosphere and soil microorganisms, depending
especially on feeding and digestive habits of the earthworms; the physical effects are associated
mainly with the structural properties of the drilosphere.
The following sections in this chapter explore the various ways in which earthworms can directly
and indirectly affect plant growth, and we propose seven main mechanisms by which this is achieved.
The focus is mainly on roots, although we recognize that indirect interactions with the aboveground
plant parts and other organisms (both above- and belowground) may also be important (Wurst and
Jones 2003). Given that the latter subject is a very recent field of study and that few results are
available, we will limit the discussion primarily to belowground interactions and processes.
THE SEVEN MAIN MECHANISMS BY WHICH EARTHWORMS
AFFECT PLANTS
We define the seven main mechanisms by which earthworms affect plant growth as follows (see
1. Dispersal and changes in populations and activity of beneficial microorganisms
2. Changes in populations and impacts of plant pests, parasites, and pathogens
3. Production of plant growth-regulating (PGR) and plant growth-influencing (PGI) substances
4. Root abrasion and ingestion of living plant parts by earthworms
5. Interactions of earthworms with seeds
6. Changes in soil structure caused by earthworms
7. Changes in nutrient spatiotemporal availability caused by earthworms
Mechanisms 1 to 5 are mainly biological, operating indirectly (1 and 2) or directly (3 to 5); the
last two (6 and 7) are indirect physical (6) or chemical (7) mechanisms.
