Agriculture Reference
In-Depth Information
However, despite the abundant literature on the responses of plants to earthworms and the
identification of a number of soil, environmental, or earthworm factors associated with particular
plant responses, rarely has the question of how these effects occur (i.e., what the mechanisms
behind the observed effects are) been addressed properly (Blakemore and Temple-Smith 1995;
Edwards and Bohlen 1996; Brussaard 1999). In most papers, mechanisms were alluded to only
briefly, and in several instances, the possible reasons for the observed effects of earthworms were
not even mentioned. Furthermore, the proposed mechanism often cannot be confirmed or validated.
The reason for this apparent lack of focus on the mechanisms behind the effects of earthworms on
plants may be partly because of the following:
¤The predominant paradigms driving agricultural development from Liebig (1840) up to
the Ñgreen revolutionÒ period (ending in the 1970s), with research focusing mainly on
alleviating physical and chemical constraints to plant production through the use of
artificial inorganic inputs and improved (often hybrid) crop varieties (Snchez 1994)
¤Production (yield-oriented) research that has concentrated mainly on aboveground plant
responses and rarely has studied changes in root growth, morphology, distribution, and
the belowground interactions (e.g., of earthworms with microorganisms)
¤Inadequate experimental designs or insufficient criteria on parameters measured to assess
the possible mechanisms involved
¤The very complex nature of indirect and direct biological interactions that occur in soils,
particularly between earthworms, soil properties and processes, and other organisms in
soils
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Many aspects of the effects and management of earthworms in tropical agroecosystems were
reviewed by Lavelle et al. (1999). In particular, Brown et al. (1999) summarized the results of 28
experiments in the greenhouse and at the field level that identified the various soil properties and
processes affected by earthworm activities and their impacts on plant production. The experiments
were done in 8 tropical countries and involved at least 34 earthworm species and 19 plant species
and were tested in 23 soil types belonging to 8 soil groups. An analysis of 246 studies of the effects
of earthworms on plant shoot production (
Figure 2.1
) and 88 studies of the effects of earthworms
on grain yields demonstrated clearly that earthworms usually have positive effects on plant growth
(75% of all studies resulted in plant growth increases) and biomass. A mean 57% increase was
observed in plant shoot mass, and a 36% increase was found for grain yields. Important negative
effects occurred only rarely, usually because of some dysfunction in the soil created or induced by
earthworm activities. They also observed that root production, contrary to that of the aboveground
parts, was usually affected less by earthworm activity, possibly because of difficulties in studying
this parameter or because plants growing in more healthy soils (presumably the case in earthworm-
worked soils) tend to invest more energy in growing the aboveground plant parts, producing fewer
roots per unit shoot biomass, resulting in higher shoot:root ratios.
The factors that seemed to affect the ultimate responses of plants to earthworms were the
following:
¤
The part of the plant harvested, with greater effects of earthworms on biomass (positive)
of shoots than grains and with the smallest effects on root growth.
¤
The species of plant involved, with greater effects of earthworms on the shoot growth
of perennial plants (trees and bushes) and larger effects on yields of gramineous grain
crops compared with legumes.
¤
The species of earthworms involved, with the pantropical endogeic species
Pontoscolex
corethrurus
producing the greatest yield increases and the widespread Indian Dichogastrini
