Earthworm population size and composition, and microbial biomass: Effect — of pastoral and arable management in Canterbury, New Zealand

Abstract

Two studies were carried out to examine the effects of soil management practice on size and composition of earthworm populations and size of the microbial biomass. In the first study a survey was made of paddocks with differing cropping history (varying periods of pasture or arable management) on farmers properties. Results showed that with > 3y of pasture, earthworm numbers were large (>800m^-2) whilst with > 3y of arable, populations were less than 400m^-2. With continuous arable cultivation for > 9y populations fell to below 200m^-2. Aporrectodea caliginosa was the dominant species under all cropping histories. However, the proportion of A. caliginosa decreased with increasing time under pasture and there was a concomitant increase in the proportion of litterfeeding Lumbricus rubellus, and to a lesser extent A. trapezoides, present. A. rosea and Octolasion cyaneum made up a small proportion of the population under most cropping histories. L, rubellus and A. rosea were not present under long-term (>9y) arable cultivation. The magnitude of the effects of soil management on the size of the diverse microbial biomass were less marked than those on the earthworm population which was made up of only 5 species. With > 3y arable cultivation, microbial biomass was less than 400 mg C kg^-1 and with > 6y under pasture it was greater than 500 mg C kg^-1.

In the second study the effects of long-term treatments (wilderness, 11y arable cultivation and 38y of grazed improved pasture with 3 rates of superphosphate — 0,188 and 376 kg ha^-1 y^-1) were compared. Earthworm numbers and microbial biomass C increased in the order arable < wilderness> control < 188 < 376. This reflects the very low organic matter returns under continuous arable production and the increasing dry matter production and C cycling under improved pasture with increasing superphosphate rates. A. caliginosa was the only earthworm species present at the arable site and was the dominant species at the others. However, with increasing superphosphate application under pasture, L. rubellus made up an increasing proportion of the population.