The effects of earthworms on soil structure in an upland grassland
As Charles Darwin first noted in 1881, earthworms through their burrowing and casting activities, play an important role in the creation and maintenance of soil structure. Burrowing activity leads to the reorganisation of voids and creation of macropores within the soil. This has implications for aeration and the flow properties of water through soils. Casting activity affects the structural stability of soil through the stabilisation of aggregates. The overall aim of this research project has been to investigate the effects of earthworm activity and diversity on void space and aggregation in an upland soil. This research has been carried out as part of NERC's Thematic Programme on Soil Biodiversity. The field site was located on the Macaulay Land Use Research Institute's experimental farm at Sourhope in the Scottish Borders. Three experiments were designed to investigate the impact of earthworms on soil fabric, with each experiment representing an increased level of system complexity. The simplest experiment took place in a controlled environment and used an artificial soil and different earthworm treatments. The second level of system complexity used soil from Sourhope which had its structure removed, and then earthworm and liming treatments applied. The most complex experiment also used Sourhope soil and liming and earthworm treatments, except in this case the soil was undisturbed. The effects of earthworms and liming on void space were characterised using saturated hydraulic conductivity to measure macroporosity, and image analysis to quantify total porosity and void size distribution. Aggregation was assessed through aggregate stability and point counts of earthworm excremental features. The effect of earthworm inoculation in the simplest experiment led to the reorganisation of voids through increased abundance of voids > 2 mm2 in area, and decreases in the proportion of voids with an area < 2 mm2. No significant effects were observed on aggregate stability. The effect of liming in the experiment using disturbed soil was to increased abundance of voids > 2 mm2. No significant effects were observed on aggregation due to either liming or earthworm inoculation. In the most complex experiment, neither liming nor earthworm inoculation led to changes in void space or aggregation, except for an increase in saturated hydraulic conductivity and therefore macroporosity due to earthworm inoculation. The overall conclusions from this research were that as system complexity increased, then the effects of the treatments on void space and aggregation became more difficult to isolate. Nevertheless, it was clear that liming significantly affected void space through increased abundance of earthworms. Out of the two treatments applied to the Sourhope soil, liming had the strongest effect on both earthworm abundance and void space.