Enchytraeids: Small but important ecosystem engineers

• Enchytraeids influenced pore space in function of the soil texture and bulk density. • Their burrowing activity made pore size distribution more uniform. • Enchytraeids formed a highly connected mesopore network that matched their body size. • Enchytraeids have a greater effect in low-density soils and in clay-rich soils. • Their activity mainly influences the top 8 mm of soil without altering the pore volume. Enchytraeids (Annelida Oligochaeta), small burrowing organisms found worldwide, are known to influence soil structure, though their specific effects on pore space are not well quantified. In this study, we evaluated how the burrowing activities of Enchytraeus albidus and Enchytraeus crypticus affected the X-ray imaged porosity of soil over a 40- day period using two different soils (loamy and silty-clay-loamy soil) sieved to 2 mm and packed at two bulk densities (0.8 and 1 g cm −3 ). Our findings revealed that while enchytraeids had minimal impact on X-ray imaged porosity, they played a key role in reshaping the soil’s internal structure, increasing pore connectivity and homogenizing pore size distribution. This was evident through a reduction in the number of smaller pores and a shift toward larger pore sizes. The overall pore structure became more uniform, with enchytraeids promoting a shift in the dominant pore sizes. These structural changes were particularly pronounced in loosely compacted soils, where enchytraeids contributed to greater network complexity, as well as in the soil with a higher clay content, which is more conducive to aggregation. This suggests that enchytraeids have a significant role in modifying soil physical properties, especially in conditions where the soil is loosely compacted. X-ray microtomography is a promising tool for studying at the mesopore scale, and further studies are needed to better characterize the bioturbation activity of enchytraeids.