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 o r arable management) on soil organic C and microbial biomass C content and on the size and composition of earthworm populations were investi gated on a range of mixed cropping (arable in rotation with pasture) f arms. Some 105 fields located at 24 different commercial farms across the Canterbury Plains of New Zealand were sampled during spring. No na tive megascolecid earthworms were recorded. Up to five introduced Euro pean species were identified at sample sites. Overall, about 80% of ea rthworms were Aporrectodea caliginosa, 10% were Aporrectodea trapezoid es and 5% were Lumbricus rubellus. The remaining 5% were Octolasion cy aneum and Aporrectodea rosea. All five species were found under long-t erm pasture. As the length of time under arable production increased, the number of species declined until, under continuous arable producti on, only A. caliginosa and A. trapezoides were recovered. Although soi l organic C content was not greatly affected by cropping history, micr obial biomass C content generally increased with time under pasture an d decreased with time under arable. Earthworm numbers and biomass show ed a similar but more pronounced trend to that of microbial biomass. T his trend is reflected in the increased organic matter inputs under pa sture and decreased inputs and increased decomposition under arable. I n mixed cropping rotations, earthworm populations varied greatly with cropping history. Populations reached their maximum (over 800 m(-2)) a fter about 3 years under pasture whilst with more than 3 years under a rable they were less than 400 m(-2). With continuous arable production for more than 9 years, populations fell to below 100 m(-2). Similar t rends were found for earthworm biomass.