Morphological characterisation of weathered earthworm casts by 2D-image analysis

Earthworm casts weathered by rainfall are difficult to differentiate from surrounding soil aggregates. We tested the hypothesis that morphological characterisation of soil aggregates by 2D-image analysis helps to perform this differentiation. Compact surface casts produced by Amynthas khami and surrounding aggregates lacking visible signs of biological activity (control) were left to disaggregate by natural rainfall and were separated into five size classes (10-5, 5-2, 2-0.5, 0.5-0.25 and < 0.25 mm). The external shape and pore-space characteristics of aggregates were measured by image analysis. Casts > 2 mm were characterised by less surface roughness and similar round shape than control aggregates. However, casts and control aggregates < 2 mm had a similar external shape. The structural porosity of casts > 0.25 mm was lower than that of control aggregates, whereas their textural porosity was similar. Therefore, this study demonstrates that measuring the porosity of soil aggregates by 2D-image analysis is a better indicator of earthworm-aggregate origin than external shapes.