Agriculture Reference
In-Depth Information
Tillage can reduce earthworm populations in a variety of ways. Earthworms may be damaged
directly by machinery or exposed to predation by birds or adverse weather, their burrow systems
may be disrupted, or the availability of suitable food may be reduced (Edwards and Lofty 1982a;
Springett 1983; Lee 1985). Baker et al. (1999c) inoculated earthworms into undisturbed and
disturbed soil (tilled to a fine tilth) in a pasture in S.A. and recorded the survival of the earthworms.
Although the disturbance greatly reduced the survival of
A. longa
and
A. trapezoides
, it had no
effect on the survival of
. Baker et al. (1999c) speculated that the tillage
could have induced a relative shortage of food (Andrewartha and Browning 1961) for earthworms
that prefer to feed at microsites rich in organic matter. However, the biomass of the surviving
earthworms did not differ significantly between treatments, and when Dalby (1996) distributed
dung throughout the same soil evenly and in patches, he reported no differences in the survival
and biomass of these same earthworm species.
Waterlogging of soils is a significant problem in some high-rainfall zones of southeastern
Australia (Reed and Cocks 1982). Underground drainage is expensive to install but can increase
lucerne production significantly (Chin 1990). Drainage can increase earthworm populations signif-
icantly (Baker 1998a). Such increases in earthworm populations may contribute, at least in part,
to observed increases in plant productivity under drainage.
Without irrigation, pasture growth usually ceases during the hot, dry summer in southern
Australia, and no earthworms are active in the root zone (Baker et al. 1992a, 1993c,d). Baker et
al. (1993c) suggested that earthworm activity ceases in soils above approximately 150 kPa of water
suction potential. With irrigation, several species can remain active during summer (e.g.,
A. caliginosa
and
A. rosea
A. trap-
ezoides
,
A. caliginosa
,
A. rosea
). Irrigated pastures in the Mt. Lofty Ranges of S.A. are dominated
by
A. caliginosa
in winter, whereas dryland pastures are dominated by
A. trapezoides
. These
differences reflect the greater dependence of
on moist soil (and its more northerly
distribution in the European distributions of two earthworm species).
A. caliginosa
occurs
in small numbers in irrigated pastures in S.A. but has never been found in dryland fields. In eastern
Australia, where soil moisture levels may be higher,
Lumbricus rubellus
is occasionally very abundant.
Lobry de Bruyn and Kingston (1997) also demonstrated changes in earthworm community structure
resulting from irrigation in northern Tasmania.
Noble and Mills (1974) reported that populations of
L. rubellus
increase with irrigation
in pastures but decline over time under heavy irrigation. They attributed this population decline
to increased earthworm surface activity and greater predation by birds. In Tasmania, Kingston
(1989) and Lobry de Bruyn (1993) recorded decreases in populations of
A. caliginosa
A. caliginosa
after
irrigation but increases in numbers of
. These authors explained their results in terms
of trampling-induced mortality for both species and increased parasitism by Diptera, which are
overcome for
L. rubellus
by greatly enhanced summer survival and reproduction.
Although earthworms avoid freshly limed soil (Doube et al. 1995), several authors (Edwards
and Lofty 1977) have shown that liming an acid soil can increase earthworm abundance in the
longer term. Springett and Syers (1984) argued that changes in pH
L. rubellus
influence earthworms
rather than the availability of calcium. Edwards and Lofty (1977) concluded that population
responses to lime are not likely to occur if the initial pH of the soil is above 4.5 to 5.0, a level
above which most species are insensitive. Mixed results have been recorded in response to liming
pastures in southeastern Australia. Baker (1992) found that liming a pasture on a clay loam soil in
western Victoria had no overall impact on total earthworm populations 9 years later (rates of 0 to
10 t ha
per se
, pH range 4.5 to 5.6 at the time of earthworm sampling), but at the species level, there
were increases in abundance of
−
1
Octolasion cyaneum
and
M. dubius
, decreases in
Heteroporodrilus
sp. and
Spenceriella
sp. with increased pH, and no significant changes in
A. trapezoides
and
A.
rosea
. By contrast, Buckerfield (1994) reported that liming a pasture in S.A. and increasing the
soil pH over a range similar to that of Baker (1992) increased populations of
A. trapezoides
.
