Relations between Agronomic Practice and Earthworms in Norwegian Arable Soils
This paper presents Norwegian studies of earthworms (density, biomass, burrows density, species, juvenile to adult ratios) in arable soil in Norway conducted during the last 20 years.
The effects of crop rotations, fertilization, soil tillage and compaction on earthworms are presented, based on various field experiments.
Geophagous (soil eating) species such as Aporrectodea caliginosa and A. rosea dominate the earthworm fauna in Norwegian arable soil. Lumbricus terrestris is also present; in our studies even frequently found in an all-arable crop rotation with annual ploughing. In southern Norway, L. rubellus, and A. longa are commonly found.Earthworm density, recorded in autumn varied between 30 and 350 individuals m-2 in different studies, with the lowest values found in conventional all arable farming systems. One year of ley in the crop rotation increased earthworm burrow density, earthworm density and biomass. Even short-term leys for green manure had a positive effect, likely due to high clover content. Application of animal manure increased earthworm density and biomass. Because geophagous species prefer the upper soil layer, shallow ploughing (15 cm depth) was expected to be detrimental.
However, the earthworm density and biomass was not lower with shallow as compared to deep ploughing (25 cm depth). With earthworm densities such as those found in farming systems with ley and animal manure, an estimated amount of 221 tonne of topsoil per hectare passes through the earthworm digestion system within one year. Field-collected geophagous earthworm casts had considerably higher concentrations of plant nutrients than bulk soil (28% higher concentration of Tot-N, 36-53% for PAL, 40%-59% for KAL).
Earthworm casts are significant natural sources of plant nutrients, also in soils with high dominance of geophagous earthworm species.