COMPUTER-ASSISTED TOMOGRAPHY OF MACROPOROSITY AND ITS APPLICATION TO STUDY THE ACTIVITY OF THE EARTHWORM APORRECTODEA-NOCTURNA

Tn a pre-alpine meadow in the Toggenburg (Switzerland), casts of the e arthworm Aporrectodea nocturna, initially detected 20 years ago around a newly planted hedge, now extend 170 m from the hedge. The abundance of A. nocturna decreased between 140 and 170 m from the hedge from 23 7 (site: An+) to 0 (site: An-) individuals m(-2). The worm's activity and the macroporosity it has created have been studied using CAT scann ing followed by automated image analysis of pores from 1.5 to 9 mm dia meter. Brightness values in images from CAT scanning showed distinct l ocal frequency maxima for stones, soil matrix and macropores. Measured diameters of pores were strongly linearly correlated with diameters o f needles used to produce artificial pores. Pores from 1.5 to 3 mm wer e most abundant. The maximum number of pores from 1.5 to 9 mm (about 1 600 m(-2)) was larger than the maximum number of all earthworms (about 480 m(-2)). Modelling the cast production of A. nocturna indicated th at 23.8 kg m(-2) were deposited on the soil's surface within 3 months. The measured macroporosity (1.5-6 mm) was one-quarter to one-sixth of the pore volume corresponding to the removed casts. Thus, net product ion of pores by earthworms was a result both of the burrowing activity and of the refilling with eroded cast material. Depth distributions o f bulk density, total porosity and pores from 1.5 to 9 mm were differe nt at An+ from those at An-. However, diffusion of butane was equal at An+ and An-. Three-dimensional reconstruction suggested that the disp osition (continuity, interconnectivity) of pores was more important fo r gas diffusion than the structure (size distribution, frequency) and bulk soil parameters.