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

Agricultural management influences soil organic matter (SOM) and earthworm activity which interact with soil structure. We aimed to describe the chang e in earthworm activity and related soil (micro)structure and SOM in a loam y 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, sedimen tary stratification was absent up till 50 cm depth, and equal amounts of bi ogenic calcite spheroids were counted, suggesting similar earthworm activit y in the past. Currently the percentage volume of optically recognizable bi ologically 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 forma tion of a physicogenetic soil structure with mainly angul ar blocky aggregates, fissure and angular blocky microstructures. The perce ntage volume of unsorted coatings counted in thin sections was 5%, indicati ng soil structure deterioration. SOM was evenly distributed through the gro undmass. In contrast, high earthworm activity in the PP soil has caused a b iogenic structure consisting of granular and subangular blocky aggregates a nd spongy and granular microstructures with abundant channels. SOM was inco rporated as fine (10-100 mum) particulate organic matter in worm casts and infillings and intimately mixed with clay material. Such encapsulated SOM i ndicates the presence of microaggregates within biogenic macroaggregates, i n which SOM may be physically protected against rapid decomposition.