Effects of summer irrigation and trampling in dairy pastures on soil physical properties and earthworm number and species composition
Authors: Bruyn Lisa A. Lobry de , Kingston T. J. 
Published: August 2004

In 1989 a replicated split-plot trial on a Krasnozem soil was established at
Elliott Research Station (ERS) in the north-west of Tasmania, as well as 14
on-farm trials in newly irrigated pastures on 3 different soil types
(Alluvial, Podzolic, Krasnozem) in the dairy districts of Scottsdale,
Smithton, and Deloraine. There were 3 main treatments at ERS: irrigated before
grazing, irrigated after grazing, and grazed and not irrigated. Part of each
main plot was fenced to prevent trampling but still allowed grazing. Effects
of summer irrigation and trampling by dairy cows were examined for pasture
production, and soil chemical and structural properties. Summer irrigation at
ERS and on-farm trials has led to a decline in soil structure indicated by
slower ponded water in filtration rates on irrigated plots compared with the
dryland plots. The decline in ponded water in filtration rates suggests a
reduction in macroporosity, especially in the soil surface. However, other
indicators for soil structural change in the top 100 mm|percentage
water-stable aggregates (>2·5 mm) and bulk density|revealed
no significant variation between the irrigated and dryland paddocks. There
were, however, higher water in filtration rates and lower bulk densities in
the untrampled areas than the trampled areas at ERS. Pasture production at ERS
was about 50% more with irrigation in each of the 2 years of the study.
Data collected at ERS in autumn and spring on the numbers of
Aporrectodea caliginosa(Savigny) and
Lumbricus rubellus (Hoffmeister) earthworms showed that
they respond quite differently to irrigation. After 2 irrigation seasons, A.
caliginosa numbers in irrigated plots dropped by over
50%, whereas in the dryland plots densities of this earthworm have
remained around 390 earthworms/m 2. In contrast the
densities of L. rubellus at ERS rose under irrigation
practices, especially in the autumn{winter sampling period. Therefore,
with the advent of summer irrigation at ERS, there was a shift in earthworm
composition from a fauna dominated by A. caliginosa to a
fauna with an increasing proportion of L. rubellus and a
decreasing number of A. caliginosa.
The typical dairy pasture in the on-farm trials recorded 2 main species, A.
caliginosa (70%) and
L. rubellus (30%). Total earthworm densities were
highest in the north-west (Smithton) region of the State (293{351
earthworms/m 2) regardless of soil type, and the
lowest densities were recorded in the Alluvial soils of Deloraine (96
earthworms/m 2). The north-west area also had the
most diverse earthworm fauna, with 5 species recorded in one site: A.
caliginosa, A. longa,
Allolobophora chlorotica, L. rubellus, and
O. cyaneum. Summer irrigation effects after 2 seasons on
earthworm composition and abundance on dairy farms caused no significant
change in A. caliginosa numbers, but there was a
45% increase in the numbers of L. rubellus in
irrigated treatments. L. rubellus was considerably more
active over summer in irrigated paddocks (25 earthworms/m
2) than in non-irrigated paddocks (7 earthworms/m
2). In contrast the number of A.
caliginosa recorded in dryland paddocks was not
statistically different to the irrigated paddocks, but the A. caliginosa in dryland paddocks were mostly inactive 8-20 mm from the soil surface.