Regeneration of compacted soil aggregates by earthworm activity

Soil compaction is a problem of modern agriculture, caused by heavy machinery when used in unsuitable, especially moist, conditions. Some regeneration processes in compacted loess soil were studied in a field experiment near Relliehausen, at the edge of the Solling mountains in Lower Saxony, Germany. Conventional tillage (CT) and conservation tillage (CS) systems were compared. The compaction was induced by the use of different wheel loads. The influence of earthworms was determined by comparing soil aggregates and casts with respect to dry and moist porosity, swelling, and water stable aggregation. For visualisation of the microstructure, a scanning electron microscope was used. The casts were obtained from two earthworm species living for 6 months in the laboratory in monoliths, taken on the plots after the wheeling procedure. The casts showed 10-20% higher values for porosity and about 50% higher swelling values than comparable soil aggregates, while the relative water stability was ca. 10% lower. We conclude that casts are looser and less stable than aggregates from the soil the earthworms ingested. To show the ecological relevance of the changes in the casts, the cast production per hectare per year was calculated. It was especially high in the most loaded soil under CS with endogeic species.