POPULATION-DYNAMICS OF EARTHWORMS IN A SILT LOAM SOIL UNDER CONVENTIONAL AND INTEGRATED ARABLE FARMING DURING 2 YEARS WITH DIFFERENT WEATHER PATTERNS

Regular field estimates of earthworm populations were made for two consecutive years in two fields of an experimental farm in the ''Noord Oost Polder'' in The Netherlands. The fields represent different agricultural management systems: conventional management and ''integrated'' management (i.e. reduced cultivation and lower inputs of inorganic fertilizers and biocides as compared with conventional management). Practically no earthworms were present in the conventionally managed field. In the ''integrated'' field, however, earthworms were observed on almost all occasions. Three species of earthworms were found, namely Aporrectodea caliginosa, Lumbricus rubellus and A. rosea. During the sampling period numbers ranged between 0 and 400 individuals m-2 representing a biomass of 0.100 g fresh biomass m-2. Tillage (rotary hoeing) reduced populations by >50%, but under favorable food and weather conditions populations recovered within 5 months. A. caliginosa and L. rubellus showed different life strategies: the former migrated to deeper layers to survive winter cold, the latter survived frost periods as cocoons. Potential influences of earthworms on soil structure are the protection of the soil surface to slaking by the production of stable casts, and the perforation of deeper soil layers and plough pans, opening up the subsurface soil for plant roots. However, in the integrated system used in this study, cast were buried in the soil at the time they were needed most on the surface. Perforation of deeper layers occurred mainly when populations were high, which was in autumn. Early in the season, presence of earthworms could not prevent ponding during heavy rainfall.