Effects of summer irrigation and trampling in dairy pastures on soil physical properties and earthworm number and species composition

Abstract

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 infiltration rates on irrigated plots compared with the dryland plots. The decline in ponded water infiltration 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 infiltration 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/m². 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/m²) regardless of soil type, and the lowest densities were recorded in the Alluvial soils of Deloraine (96 earthworms/m²). 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/m²) than in non-irrigated paddocks (7 earthworms/m²). 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.