Soil micropore structure and carbon mineralization in burrows and casts of an anecic earthworm (Lumbricus terrestris)

Anecic earthworms (those that build semipermanent vertical burrows) are known to alter the biological activity and physical structure of soils through their burrowing and casting. Information on how earthworms change the physical structure of soil may provide clues about the mechanisms by which earthworms affect microbial processes such as nutrient mineralization. We evaluated the pore structure of bulk soil and of the soil in burrows and casts formed by an anecic species of earthworm (Lumbricus terrestris) in a fallow field. Differences in pore structure (specific pore volume, V-sp, and median pore neck dia., D) were assessed using mercury intrusion porosimetry. We also examined the relationship of these physical properties with mass water content at field capacity (theta (m)), rate of C mineralization (C-min) and specific C mineralization rate (C-sp = C-min/C content of soil). Mean values of Vp (+/- SD) for bulk, cast and burrows were 242 +/- 35, 213 +/- 13, and 197 +/-4 d g(-1), respectively. Values for D were ( SD) 10.8 +/-2.5, 7.9 +/-3.3, and 5.5 +/-2.9 mum for bulk, burrow, and cast soil, respectively. A smaller proportion of the pore volume in cast and burrow soil was associated with pore diameters in the 3-30 and 30-100 mum range than in bulk soil. theta (m) was higher in burrow and cast soil than in bulk soil and was inversely proportional to V-sp and D. C-min and C-sp followed the order: burrow >cast > bulk soil. Both C-min and C-sp decreased inversely with V-sp. By contrast, no consistent relationship was observed between either measure of C mineralization and D. Our results suggest that the changes in soil pore structure produced by anecic. earthworms cause a shift towards smaller pore volume and smaller pore neck diameters. These changes in turn affect physical (e.g. water retention) and microbial (e.g. C mineralization) processes in soil. (C) 2001 Elsevier Science Ltd. All rights reserved.