Earthworm species, population size and biomass under different cropping systems across the Canterbury Plains, New Zealand

The effects of previous cropping history (varying periods of pasture or arable management) on soil organic C and microbial biomass C content and on the size and composition of earthworm populations were investigated on a range of mixed cropping (arable in rotation with pasture) farms. Some 105 fields located at 24 different commercial farms across the Canterbury Plains of New Zealand were sampled during spring. No native megascolecid earthworms were recorded. Up to five introduced European species were identified at sample sites. Overall, about 80% of earthworms were Aporrectodea caliginosa, 10% were Aporrectodea trapezoides and 5% were Lumbricus rubellus. The remaining 5% were Octolasion cyaneum and Aporrectodea rosea. All five species were found under long-term pasture. As the length of time under arable production increased, the number of species declined until, under continuous arable production, only A. caliginosa and A. trapezoides were recovered. Although soil organic C content was not greatly affected by cropping history, microbial biomass C content generally increased with time under pasture and decreased with time under arable. Earthworm numbers and biomass showed a similar but more pronounced trend to that of microbial biomass. This trend is reflected in the increased organic matter inputs under pasture and decreased inputs and increased decomposition under arable. In mixed cropping rotations, earthworm populations varied greatly with cropping history. Populations reached their maximum (over 800 m(-2)) after about 3 years under pasture whilst with more than 3 years under arable they were less than 400 m(-2). With continuous arable production for more than 9 years, populations fell to below 100 m(-2). Similar trends were found for earthworm biomass.