Chemical and physical properties of earthworm casts as compared to bulk soil under a range of different land-use systems in Vietnam

Earthworms are known to significantly influence soil aggregate stability and hence to regulate underlying ecosystem functions such as soil organic matter (SOM) decomposition and soil erosion. However, the available research information on this aspect in the tropics, and especially in Asia, is rather scanty. The aims of this paper are therefore to study the influence of Amynthas khami, an anecic earthworm species, on soil structural properties on one hand, and to determine the factors involved and the consequences for the soil matrix properties on the other hands. Investigations were carried out in different management systems ranging from low to high above-ground casts density. We measured the physical and chemical properties and water behavior (mean weight diameter: MWD, and water drop penetration time: WDPT) of casts and those of the surrounding soil aggregates (control). Pore space was quantified using a morphological approach and 2D image analysis.Soil aggregate stability was primarily specific for each-land use type and afterwards affected by earthworm activity within each ecosystem. Earthworm casts were always more stable than control surrounding soil aggregates. Casts were characterized by higher pH, C, polysaccharides, Ca2+, Mg2+ and K+ contents. Conversely, the particle size fraction and the content of oxides (Alcbd and Fecbd) were not significantly affected by earthworm activity. Soil porosity was sharply decreased in casts as compared to the surrounding soil aggregates and casts were characterized by the absence of packing void poroids and large pores > 50,000 µm2. Regressions between soil physico-chemical properties and MWD underlined the relationships between SOM (C and polysaccharide content), pH, Ca2+ content, soil porosity, and the structural stability of cast aggregates. Surprisingly, the abundance of casts on the soil surface was not related to the bulk density of the soil matrix, suggesting that cast aggregates are not integrated into it or that they are integrated with different soil structural characteristics.