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

Anecic earthworms (those that build semipermanent vertical burrows) are kno wn to alter the biological activity and physical structure of soils through their burrowing and casting. Information on how earthworms change the phys ical structure of soil may provide clues about the mechanisms by which eart hworms affect microbial processes such as nutrient mineralization. We evalu ated the pore structure of bulk soil and of the soil in burrows and casts f ormed by an anecic species of earthworm (Lumbricus terrestris) in a fallow field. Differences in pore structure (specific pore volume, V-sp, and media n 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) an d specific C mineralization rate (C-sp = C-min/C content of soil). Mean val ues 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 associ ated 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 in versely proportional to V-sp and D. C-min and C-sp followed the order: burr ow >cast > bulk soil. Both C-min and C-sp decreased inversely with V-sp. By contrast, no consistent relationship was observed between either measure o f C mineralization and D. Our results suggest that the changes in soil pore structure produced by anecic. earthworms cause a shift towards smaller por e volume and smaller pore neck diameters. These changes in turn affect phys ical (e.g. water retention) and microbial (e.g. C mineralization) processes in soil. (C) 2001 Elsevier Science Ltd. All rights reserved.