Earthworm population dynamics under conservation tillage systems in south-eastern Australia

Abstract

Changes in earthworm abundance, biomass, and diversity were monitored under a range of tillage and stubble management practices in a wheat/alternative crop rotation over 5 years on a Sodosol (Alfisol) in southern New South Wales, Australia. There were 3 tillage and 2 stubble management practices in a completely randomised block design with 3 replications. The 3 tillage treatments were no-tillage (NT), 1 tillage pass (1T), and 3 tillage passes (3T). Stubble management practices were stubble retained (sr) and stubble burnt (sb). Positive responses of earthworm abundance and biomass to stubble retention (>2-fold increase) were evident in the second year and to both stubble and tillage in the third year. In the latter, abundance in NT/sr was 6.6 times that found under 3T/sb (239 v. 36/m2). Higher earthworm abundance in NT/sr compared with 3T/sb prevailed for the remaining duration of the experiment. However, a drastic decline in total population (to a mean of 31/m2) was observed in the fourth year in all the treatments and this was followed by further decline to a mean abundance of 4/m2 in the fifth year. The drastic decline in abundance was also accompanied by a shift in earthworm species composition. The earthworm population was originally dominated by the exotic Lumbrid, Aporrectodea trapezoides (Lumbricidae) (~100% in composition), but by the fifth year, Microscolex dubious (Acanthodrilidae) was the dominant species, making up 75% of the earthworm population in NT/sr. Improvement in soil quality as detected in the fifth year under a conservation tillage system compared with a conventional system included higher transmitting macropores, higher labile carbon, and water-stable aggregation. The reason for the decline in earthworm abundance was not clear but was unlikely related to changes in soil quality, wheat yield, and rainfall. Instead, we suggest that it was related to the changes in insecticide applications during the course of the experiment. The study highlights the importance of judicious use of chemicals in farming systems if earthworm presence is to be encouraged.