Soil structure and earthworm activity in a marine silt loam under pasture versus arable land

Abstract.

Agricultural management influences soil organic matter (SOM) and earthworm activity which interact with soil structure. We aimed to describe the change in earthworm activity and related soil (micro)structure and SOM in a loamy Eutrodept as affected by permanent pasture (PP) and conventional arable (CA). Thin sections were studied and biogenic calcite spheroids, worm casts, infillings and groundmass coatings were quantified. In both soils, sedimentary stratification was absent up till 50 cm depth, and equal amounts of biogenic calcite spheroids were counted, suggesting similar earthworm activity in the past. Currently the percentage volume of optically recognizable biologically influenced groundmass was 54% in PP and 10% in CA. The Ah of PP had an organic matter content of 66 mg kg^–1 whereas the content was 22 mg kg^–1 in the Ah of the CA soil. Low earthworm activity in the CA soil has led to the formation of a physicogenetic soil structure with mainly angular blocky aggregates, fissure and angular blocky microstructures. The percentage volume of unsorted coatings counted in thin sections was 5%, indicating soil structure deterioration. SOM was evenly distributed through the groundmass. In contrast, high earthworm activity in the PP soil has caused a biogenic structure consisting of granular and subangular blocky aggregates and spongy and granular microstructures with abundant channels. SOM was incorporated as fine (10–100 µm) particulate organic matter in worm casts and infillings and intimately mixed with clay material. Such encapsulated SOM indicates the presence of microaggregates within biogenic macroaggregates, in which SOM may be physically protected against rapid decomposition.