Micromorphic observation of soil alteration by earthworms

• Published in: Agriculture, ecosystems & environment Vol. 34; no. 1; pp. 363 - 370
• Main Authors: West, L.T., Hendrix, P.F., Bruce, R.R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.1991
• Subjects: clay soils; Lumbricus rubellus; particle size; sandy loam soils; sandy soils; soil compaction; soil micromorphology; soil morphological features; soil texture; worm casts
• ISSN: 0167-8809; 1873-2305
• DOI: 10.1016/0167-8809(91)90121-D

Thin section methodology was used to examine soils from laboratory experiments and from experimental field plots for effects of earthworms on soil microfabrics. Samples of loamy sand or sandy clay loam were incubated with the earthworm Lumbricus rubellus. Field samples of similar soil texture were collected from experimental plots treated with conventional or no-tillage management for 5 years. All samples were subjected to thin sectioning and microscopic examination for fabric and porosity characteristics. In both sandy and clayey soil, fewer and smaller voids were present adjacent to earthworm channels compared with bulk soil. Additionally, clay domains in the clayey soil were oriented into a porostriated b-fabric adjacent to the earthworm channel, suggesting that compaction and reorientation of the soil occurred as the channels were formed. In the sandy soil, plasma (clay, organic matter, and clay-organic complexes) coatings were observed along the channel walls, and two forms of earthworm castings were observed; (1) castings deposited within channels and composed of coarse sand grains that had been depleted of plasma; and (2) castings deposited on the surface which appeared t to have fewer coarse sand grains and more plasma bridging between sand grains, compared with bulk soil. These grain size and plasma relationships between channel and surface castings suggest that earthworms passed coarse sand grains through their gut and deposited them in the channel. Plasma removed from the coarse grains appeared to be incorporated with plasma associated with the finer sand and deposited along the channel wall or as castings at the soil surface. Similar features were observed in and around macropores from field soils, suggesting similar effects of earthworms in relatively undisturbed soils from no-tillage agroecosystems. Particle reorientation along channel walls, compaction of particles adjacent to channel walls, and organic linings may limit water movement from earthworm burrows into the bulk soil, with possible impacts on solute movement and retention in the soil.