Agriculture Reference
In-Depth Information
TABLE 8.1
Effects of Inoculation of the Earthworm
Millsonia anomala
into Cropping Systems
on Bulk Density and Aggregation of Soil
Bulk Density
% Aggregates Larger than 2 mm
Maize
0-10 cm
Maize
0-10 cm
Yam Mounds,
Sieved Soil
Maize 0-10 cm,
Undisturbed Soil
Maize 0-10 cm,
Sieved Soil
Time (months)
3
36
34
36
36
Inoculated
1.31
1.48
53.5
42.5
42.2
Noninoculated
1.24
1.37
29.8
38.8
24.6
p
0.06
<0.001
<0.05
n.s.
<0.01
n.s., not significant
from 35.4 to 27.4%. Such changes in soil aggregation resulted in a slight increase of bulk density
(significant during the first three cropping cycles) and a significant decrease in infiltration rates
and sorptivity, the latter decreasing from 0.34 cm sec
−
1
in non-earthwormÏinoculated soils to
0.15 cm sec
fresh biomass of earthworms (Alegre et
al. 1996). This transformation in soil physical properties resulted in eventual changes in the soil
water regime because soil tended to become drier during dry periods and wetter in periods of
heavy rainfall than in the non-earthwormÏinoculated treatment.
Other endogeic earthworm species have been reported to have opposite effects because they
tend to break down large (>0.5 mm) aggregates and split them into smaller ones (Derouard et al.
1997; Blanchart et al. 1999). For example, in western African savannahs, small species of the
Eudrilidae family possess such abilities, and it has been hypothesized that soil aggregation is
regulated by the opposite effects of large compacting species such as
−
1/2
in treatments inoculated with 36 g m
−
2
Millsonia anomala
and
decompacting species like the common eudrilid
In the absence of such
decompacting earthworms or other earthworm species, the activity of compacting earthworms
may create severe soil problems. This was the case in a pasture derived from primary forest near
Manaus (Brazil). At this site, the peregrine earthworm
Hyperiodrilus africanus.
had increased to rather
large and active populations; at the same time, deforestation had eliminated 75% of other
macroinvertebrate species, especially from the decompacting group. The accumulation of casts
of
P. corethrurus
at the soil surface in very moist soil conditions resulted in the formation of a
continuous muddy layer of earthworm casts. When droughts occurred, this layer turned into a
compact 3-cm thick crust that prevented plants from growing, leaving large patches of bare soil.
These results contrast with a rather broad set of other results suggesting that earthworm activities
improve the aeration of soil and infiltration of water (Lee 1985; Edwards and Bohlen 1996), and
P. corethrurus
P. corethrurus
has been reported to repair a compacted oxisol, showing that interactions with
other invertebrates and soil characteristics may be the ultimate determinants of the effects of
earthworms on soils (Zund et al. 1997). Three hypotheses may explain such discrepancies:
1.Most studies on the relationships between earthworm activities and soil physical param-
eters have been based on Lumbricidae. This family, unlike most tropical families, com-
prises a large proportion of species that create semipermanent burrows that influence
water infiltration significantly.
2.In natural ecosystems, the association of compacting and decompacting earthworm
species may regulate soil physical properties and, in the end, favor infiltration and
aeration. It is important to consider that decompacting species may belong to other taxa
such as Enchytraeidae (Didden 1990; Van Vliet et al. 1993), ants (Decans et al. 2001),
termites (Isoptera), or millipedes (Diplopoda) (Tajovsky et al. 1991).
