Pj. Bohlen et al., EFFICACY OF METHODS FOR MANIPULATING EARTHWORM POPULATIONS IN LARGE-SCALE FIELD EXPERIMENTS IN AGROECOSYSTEMS, Soil biology & biochemistry, 27(8), 1995, pp. 993-999
We established a long-term field experiment in 1991 to investigate the
influence of earthworms on C and N cycling processes in agroecosystem
s. In a replicated field experiment we decreased earthworm populations
using electroshocking, increased them by adding field-collected worms
or left them unmanipulated. Population manipulations and sampling wer
e done twice per year in 20 m(2) field enclosures that were made from
sheets of PVC buried 45 cm deep and extending 15 cm above the soil sur
face. The experiment was established in maize (Zea mays) agroecosystem
s in which N was provided in the form of NH4NO3-N, cow-manure-N or leg
ume-cover-crop-N. The two dominant earthworm species at the site were
Lumbricus terrestris and Aporrectodea tuberculata. Electroshocking was
effective at reducing earthworm populations to about 25% of their nat
ural abundance. In the autumn of 1993, electroshocked enclosures had 7
5% fewer earthworms and 65% less earthworm biomass than plots with unm
odified populations. Electroshocking was equally effective at reducing
populations of all earthworm species and did not alter the relative s
pecies abundance. The addition of field-collected worms was not as eff
ective at increasing earthworm populations as electroshocking was at r
educing populations. Enclosures with added earthworms had 1.17-fold mo
re earthworms and 2.18-fold greater earthworm biomass than control enc
losures. The biomass of L. terrestris was significantly greater in enc
losures with increased earthworm populations than in enclosures with r
educed or unmodified populations; the biomass of A. tuberculata was no
t increased. Total earthworm biomass at the site declined from nearly
90 g m(-2) in the spring of 1991 to <30 g m(-1) in the autumn of 1993,
probably due to: (1) extreme climatic conditions, including severe dr
oughts in the summers of 1991 and 1993 and a period of excessive rain
in the summer of 1992; and (2) the conversion of the field from perenn
ial alfalfa (Medicago sativa) to cultivated maize. The manipulation of
earthworm populations in large-scale, replicated field experiments pr
ovides a unique and successful approach for investigating the effects
of earthworms on soil structure and nutrient cycling processes.