Earthworm sampling
To compare earthworm abundance and biomass for specific
treatment combinations, one soil area of 50 × 50 cm was manually
excavated in each field plot to a depth of 25 cm from the end
of April to the beginning of May 2011. Due to dry weather conditions
that spring, 1 m2 of each plot was irrigated twice with 20 mm
of water, two weeks prior to soil excavation. The central 0.25 m2
of this irrigated field section were sampled. Extraction of earthworms
with mustard solution, which we used in combination with
hand sorting as standard procedure in arable soils, was not done
since previous field tests did not show significant improvement in
extraction efficiency in this clay soil. The high clay content of the
soil prevents mustard solution from infiltration to sufficient depth.
Thus, four soil samples per treatment were carefully hand sorted
twice and finally earthworms including cocoons were preserved in
4% formaldehyde solution. Earthworms were counted and weighed
in the lab, classified into ecological groups applying the description
by Bouché (1972). They were subsequently sorted according
to their age state (adult, juvenile) and the species of adult earthworms.
For biomass calculation a time-related correction of weight
loss due to preservation in formaldehyde was made according to
Cuendet (2009). Species identification was performed according to
taxonomic keys of Bouché (1972), Christian and Zisci (1999) and
Cuendet (2009).