To what extent do ageing and soil properties influence Amynthas khami cast properties? Evidence from a small watershed in northern Vietnam

Abstract Understanding the variables explaining earthworm cast properties remains a key challenge in soil ecology. For this purpose, this study aimed to determine the relationships between the physical and chemical properties of earthworm casts and those of their surrounding soil environment. Surface earthworm casts (recently emitted or degraded) produced by the anecic earthworm Amynthas khami (Thai 1984) and adjacent topsoil aggregates without traces of earthworm activity were sampled in 50 different locations covering a small watershed in northern Vietnam. We determined the organic carbon content, the spectral signatures of aggregates and the humic index of organic matter via mid-infrared spectroscopy and analysis of pH, hydrophobicity, wet aggregate stability and bulk density of soil aggregates. While the physical properties of casts were not related to those of topsoil aggregates, correlations were measured between cast and topsoil aggregate chemical properties. The values of hydrophobicity, pH, bulk density and organic carbon were higher in casts than in topsoil aggregates, whereas the humic index values were lower in casts than in topsoil aggregates. No difference was measured between recent and degraded casts, with the exception of their spectral signatures. Interestingly, the effect of A. khami was more important in soils presenting a lower organic carbon content or higher humic index, whereas its influence on soil pH was constant (i.e., the same regardless of the pH of the topsoil). In conclusion, this study suggests a discrepancy between the impact of A. khami on soil physical and chemical properties. The results also show that the spatial variability of cast physical properties cannot be related with that of topsoil environment physical properties.