Age matters: Dynamics of earthworm casts and burrows produced by the anecic Amynthas khami and their effects on soil water infiltration

International audience; By creating vertical and continuous burrows, anecic earthworms accelerate the transfer of water in soils. However, the degradation mechanisms and lifespan of burrows and the consequence of changes in burrow characteristics for water infiltration remain poorly known. In this study, the dynamics of the degradation and hydraulic properties of burrows made by the anecic earthworm Amynthas khami in a clayey soil were investigated in a meadow and in a woodland in North Vietnam. We selected three categories of surface casts, namely, (i) fresh (a few days old), (ii) dry (>1 month old) and (iii) degraded by rain (older than the dry casts), as proxies of the age of burrows. The physical and chemical properties of casts were measured and compared to the surrounding soil aggregates without visible earthworm activity (control). Soil cores were sampled below casts and control and the 3D structure of burrows was characterized using X-ray tomography. Then, water infiltration was measured in the saturated soil cores. Fresh and degraded casts had a lower water stability than control aggregates, whereas higher values were found in dry casts. Water infiltration was twice higher in columns below fresh and dry casts than in the control. However, below degraded casts, the positive effect on water infiltration was reduced or disappeared in some cases. The degradation of burrows led to significant increase in the specific surface area, decrease in their minimum diameter and increase in the abundance of cracks connected to burrows. Our results indicate that anecic burrows persist at least for months below degraded casts but that aging due mainly to physical processes reduces water infiltration. This study highlights the importance of taking into account the lifetime of burrows in the soil when assessing the effect of earthworms on soil structure and water transfer.