Amendment of soil with coal fly ash modified the burrowing habits of two earthworm species

Abstract

A good understanding of how soil biota responds to amendment of agricultural soils with coal fly ash is imperative to developing protocols for routine use of this industrial by-product for soil management. We used X-ray computed tomography (CT) images to determined key structural characteristics of burrows created by earthworms of native megascolecid and exotic 'Aporrectodea trapezoides' in intact soil cores (150 mm ID by 0.3 m deep) that were treated with coal fly ash at 0, 5 or 25 Mg ha⁻¹ mixed into the top 50 mm of the cores. The cores were inoculated at a rate equivalent to 850 worms m⁻² and after 6 weeks we found that fly ash reduced the total volume of the burrow system (Vs) by up to 39% for the native species and 29% for the exotic species due mostly to fewer and smaller burrows; these reductions averaged 33% with addition of ash at 5 Mg ha⁻¹ and 39% at 25 Mg ha⁻¹. While the native earthworms responded to treatment by burrowing deeper into the soil core and away from the ash-tainted surface soil, the exotic species reduced the depth of burrowing and remained close to the surface. Fly ash addition did not have significant effect on tortuosity (T) of the burrows for either earthworm species. 'A. trapezoides' created predominantly vertical burrows, while the native megascolecid worms produced more horizontally oriented burrows in addition to vertical ones. These modifications of earthworm behavior by fly ash addition to soil, along with previous experience with plant growth, suggest that an ash application rate of 5 Mg ha⁻¹ is close to optimum for routine agronomic applications. Structural analysis of the burrows as presented in this paper provide more useful information on the response of earthworm behaviour to fly ash that may not be apparent from an assessment of population and growth of these important soil biota.