FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 494

and root litter (Spain and Hodgen, 1994). This is consistent with the attraction of
P. corethrurus to roots where it may feed and release nutrients that are readily used by plants.

4.4 Influence of the drilosphere on plant growth

Positive effects of earthworms on plant growth have been observed with many species
and in many environments. Most of these studies have been 'pot-experiments' in which
earthworms and plants are maintained together in a relatively small volume of soil.
Suitable conditions are provided for both the plant and earthworms, and earthworms
are forced to conduct their activities in the rhizosphere itself. Under these conditions,
spectacular increases in plant production may occur over short-time periods. For example,
plant biomass increases of up to 300 % over a three-month period followed the intro-
duction of the mesohumic endogeic earthworm Millsonia anomala into pots containing
the tropical grass Panicum maximum (Spain et al ., 1992). The N and P contents of plant
material (especially roots) were significantly increased in treatments with earthworms.
The increase in production was proportional to the earthworm biomass added up to a
maximum biomass equivalent to ca. above this, the soil became compacted

and water infiltration was impeded.
The responses of tropical tree seedlings to inoculation with P. corethrurus were highly
variable: Bixa orellana (Bixaceae) which has a fine, dense root system responded vigor-
ously leading to biomass increases of 14-24 times the control value after 120 days.
Eugenia stipitata (Myrtaceae) responded less (+1.6 to 2.5 times) and the palm tree
Bactris gasipaes was negatively affected by the inoculation of earthworms (-1.8 to 2.7
times). In a similar experiment conducted with the epi-anecic species Lumbricus
rubellus in a soil taken from a Finnish coniferous forest, above-ground production of
seedlings of Betula pendula was enhanced by ca. 30 % whereas root production was
slightly decreased, probably as a result of a better soil nutrient availability (Haimi et al.,
1992). Further experimentation has confirmed the existence of species-specific responses
by plants to earthworm presence.
In a series of 240 experiments conducted in pots or intermediate scale field designs
in tropical and subtropical areas, with 30 different earthworm species or associations
of species and 15 plant species, Blakemore (1995) and Brown et al. (1999) found
positive effects of earthworm inoculation in 72 % cases and measured an average
increase of 63 % in production (Figure IV.60). Increases were most marked in soils that
had low organic contents, high sand concentrations and were of intermediate pH. Not all
plants responded in the same way, and there was some evidence that not all earthworms
had the same impact. Response depended on the amount of earthworm biomass: average
effects on plant growth were significant at biomass levels greater than
Graminaceous species such as rice ( Oryza sativa ) and maize ( Zea mays ), and trees such
as Bixa orellana, tea ( Camellia sinensis ) or Eugenia stipitata are highly responsive
whereas legumes and the palm tree Bactris gasipaes do not seem to benefit from earth-
worm activities.
The carrying capacity of soils for earthworms is highly variable. The availability of
assimilable organic residues in the soil appears to be one of the major factors that sup-