Morphological, physical and biochemical characteristics of burrow walls formed by earthworms

The influence of two anecic earthworm species, Lumbricus terrestris and Aporrectodea giardi, on the soil adjacent to their burrows was studied in the laboratory. The morphological, physical and biochemical characteristics of the burrow walls were compared with those of casts and surrounding soil by scanning electron microscopy and measurement of water stable aggregation (WSA), enzymatic activities and elemental content (C and N). For both species, we observed a reduction in porosity in the inner layer and to a lesser extent in the outer layer of burrow walls as compared to the surrounding soil. This is apparently due to the fine texture of subsurface casts constituting the inner walls, and to the compaction occurring during burrow formation and use. In contrast, the WSA of the burrow walls was not increased compared to the surrounding soil. Enzymatic activities and C and N content of burrow walls and casts were enhanced compared to the parent material. The behaviour of earthworms in, e.g. incorporation of litter into burrows, selection of the finest soil particles providing adsorption sites for enzymes, re-using burrows resulting in a frequent input of mucus to the burrow walls, may explain the observed differences. These differences were more or less pronounced depending on the morphoecological characteristics of the earthworms. The results indicate that earthworm burrow walls may influence the movement of water and solutes from channels to soil matrix and vice versa and may also have a great impact on microbial activities. (C) 2001 Elsevier Science B.V. All rights reserved.