Influence of earthworms, Aporrectodea spp. (Lumbricidae), on pasture production in south-eastern Australia

Abstract

Most of the earthworms now found in agricultural fields in south-eastern Australia are exotic, patchily distributed, mineral soil dwellers (endogeic species). The influence of two of the most common endogeic species, Aporrectodea caliginosa and A. trapezoides, on pasture production was compared with that of another exotic, surface-feeding, deep-burrowing (anecic) species, A. longa, which is essentially restricted in its Australian distribution to Tasmania. Comparisons were made by introducing 4 different densities of the worms in cages at 10 field sites and measuring pasture production for approximately 5 months. All 3 species increased pasture production at most of the 10 sites. Increases in pasture production were greatest where original pasture production was low. Pasture production increased with earthworm density. Averaged across sites, earthworms increased pasture production by up to 61%. Significant increases in yield were detected with ≥214 A. longa/m ² and ≥429 A. caliginosa or A. trapezoides/m ². A. longa is a much bigger worm than A. caliginosa and A. trapezoides. If the initial number of worms introduced to the cages was taken as the measure of earthworm ‘abundance’, then A. longa increased pasture production more than the 2 endogeic species. If the biomass of introduced worms was taken as the measure of abundance, then the reverse was the case. A. longa reduced nitrogen levels in some soils. Phosphorus levels were not affected. No residual effects on pasture production were detected when soil that had been improved by earthworms was potted and resown with ryegrass. Broad scale introduction of A. caliginosa, A. longa, or A. trapezoides to sites lacking them in south-eastern Australia is likely to improve pasture production significantly. Addition of A. longa to existing communities of endogeic species should further increase pasture yield by enhancing functional diversity.