Earthworm population responses to cultivation and irrigation in a South Australian soil

Abstract

Summary The changes in the population of earthworms, predominantly Aporrectodea trapezoides , induced by cultivation and cropping of Brussels sprouts, followed by a return to pasture, were monitored for a period of more than 2 years in a South Australian soil. Worms were hand-sorted from 0.5 m 2 areas at monthly intervals during 3 winter/spring periods, and during irrigation of the crop throughout one summer. Peak numbers in pasture reached 240 m −2 (biomass 66 g m −2 ) during winter/spring of 1999, were reduced to a very few individuals (13 m −2 ) by cultivation in late September 1999, but increased to 578 m −2 (biomass 62 g m −2 ) from January to April 2000 (the warm summer months) under irrigation in a similar manner to a normal winter and spring period (June to October). The loose matrix of the cultivated soil and abundant organic matter provided excellent conditions for reproduction, hatching and growth of earthworms. Ploughing in crop residues reduced the earthworm population to 32 m −2 , then there was a steady increase in the number and size of the earthworms during the following winter to 168 worms m −2 in October, with a mean weight of 450 mg and a biomass of 75 g m −2 . No earthworm activity occurred during the very hot dry summer of 2000–2001. However, in June 2001 after opening rains, the number of earthworms found was very similar to that of the previous late spring (178 vs. 168), but the average weight had practically halved (451 mg vs. 238 mg). Nearly all the earthworms had survived the summer months despite a loss of almost 50% of their body weight. During the winter of 2001, the population increased in average weight, with frequent appearance of large breeding adults, indicative of a mature population.