Soil structure and characteristics of organic matter in two orchards differing in earthworm activity

Abstract

By consuming plant remains and soil, earthworms incorporate organic matter (OM) into the soil and form biogenic soil structures, which can affect OM dynamics. We carried out a (micro)morphological study of soil structure development and OM distribution in two orchards (45 year) in a Dutch calcareous marine loam: RI − without, and KR + with high earthworm activity, the result of different levels of heavy metal contamination from fungicides. In both soils, sedimentary–stratification was absent to 60 cm depth and equal amounts of biogenic calcite spheroids were counted, suggesting similar earthworm activity in the past. In RI − the current vol.% of worm-worked groundmass in thin sections was 6% in the Ah and 7% in the Bw horizon compared with 51% in the Ah and 16% in the Bw horizon of KR +. Disappearance of earthworms with time in RI − gave rise to a compacted physicogenic soil structure with angular and prismatic aggregates and the absence of earthworm biopores. Due to restricted fragmentation and incorporation of OM fragments in casts, a litter layer formed at the soil surface. OM coatings were present in fissures and root pores of the Ah horizon, indicating the absence of mixing of organic and mineral soil materials. OM fragments were relatively coarse (>50 μm) and heterogeneously distributed through the Ah groundmass. Stronger decalcification in RI − than in KR + could be ascribed to higher production of organic acids in the litter layer of RI − and the absence of soil homogenisation by earthworms. In KR + earthworm activity was high, which has resulted in a biogenic structure with granular and subangular blocky aggregates and many worm casts and biopores. Particulate OM was relatively fine (<50 μm) and encapsulated with clayey material in casts and micro-aggregates. The organic C content was not significantly ( P<0.05) higher in the Ah horizon of KR +, than in the Ah horizon of RI − (15.7 and 13.7 g kg −1, respectively). The lower C mineralization rates in KR + below 6 cm depth, however, might be an indication of higher microbial substrate-use efficiency or physical protection of OM against decomposition. The latter explanation would accord with the observed encapsulation of OM in micro-aggregates, and with studies on other management systems that favour biogenic aggregate formation. The quality of the soil macro- and microstructures, degree of soil compaction and decalcification and soil OM dynamics were strongly determined by the occurrence of earthworms in soils.