EFFICACY OF METHODS FOR MANIPULATING EARTHWORM POPULATIONS IN LARGE-SCALE FIELD EXPERIMENTS IN AGROECOSYSTEMS

We established a long-term field experiment in 1991 to investigate the influence of earthworms on C and N cycling processes in agroecosystems. 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 were 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 surface. The experiment was established in maize (Zea mays) agroecosystems in which N was provided in the form of NH4NO3-N, cow-manure-N or legume-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 natural abundance. In the autumn of 1993, electroshocked enclosures had 75% fewer earthworms and 65% less earthworm biomass than plots with unmodified populations. Electroshocking was equally effective at reducing populations of all earthworm species and did not alter the relative species abundance. The addition of field-collected worms was not as effective at increasing earthworm populations as electroshocking was at reducing populations. Enclosures with added earthworms had 1.17-fold more earthworms and 2.18-fold greater earthworm biomass than control enclosures. The biomass of L. terrestris was significantly greater in enclosures with increased earthworm populations than in enclosures with reduced or unmodified populations; the biomass of A. tuberculata was not 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 droughts 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 perennial alfalfa (Medicago sativa) to cultivated maize. The manipulation of earthworm populations in large-scale, replicated field experiments provides a unique and successful approach for investigating the effects of earthworms on soil structure and nutrient cycling processes.