Pore structure, stability and water repellency of earthworm casts and natural aggregates in loess soil

Abstract

Earthworms are important aggregate-forming managers in the soil owing to the vast production of casts. This study attempts to determine changes in the pore size distribution (PSD), stability and water repellency of one-week old compound casts produced by endogeic earthworms vs. surrounding natural aggregates in loess soil exhibiting unstable structure. Mercury porosimeter was used to determine the PSD. The results were presented in the form of cumulative pore volume curve and logarithmic differential pore volume curve as a function of pore radius. Stability of the air dried was assessed by measurements of tensile strength and water stability. Water drop penetration time was used to assess water repellency. The results showed that median pore radius (area) and average pore radius were greater in casts than in soil aggregates, whereas total porosity was not different (P<0.05). The volume of pores >2μm radius was greater and that of pores 0.2–2μm radius tended to be lower in casts than in natural soil aggregates. The differential PSD curves were uni-modal and bi-modal for casts and soil aggregates, respectively. The peak in the casts at equivalent pore radius of approximately 3μm was much narrower and of larger magnitude 0.4cm3g−1 than each of two respective peaks at pore radii 1.8 and 4.3μm, and magnitudes 0.19 and 0.16cm3g−1 for soil aggregates. The tensile strength of the casts being 0.065MPa was greater by 41.3% compared to soil aggregates. In the case of water resistance index, the corresponding difference was 12.9% (0.90 vs. 0.80). The casts were more repellent than natural aggregates. It seems that greater stability of the top soil cast aggregates will better protect C from microbial decomposition and reduce susceptibility to erosion and compaction of the soil.