Earthworm burrowing modes and rates depend on earthworm species and soil mechanical resistance

Earthworms drive multiple soil processes, but their specific impact on soil functions differs between earthworm species and ecological categories. A key challenge in modern agriculture is soil compaction due to heavy machinery, but we have limited quantitative knowledge about how the burrowing activity of different earthworm species is affected by compaction. Here, we address this question in a laboratory experiment with 2-D terraria, where we used Aporrectodea caliginosa (Savigny, 1826) and Aporrectodea longa (Ude, 1885) as representatives of two different ecological categories. We exposed both species to four different soil mechanical resistance levels and monitored their burrowing activity for three days. We quantified burrowing rates and cast production, assessed the burrowing mode, and estimated energy requirements as a function of soil mechanical resistance. The results showed that the burrowing rates of both earthworm species significantly decreased with increasing soil mechanical resistance, but that the impact was species-dependent and lower for A. longa. Earthworms changed their burrowing mode towards ingestion when soil mechanical resistance increased, and this shift was more prominent for A. caliginosa that primarily burrowed via cavity expansion (i.e. by pushing soil aside) at low soil mechanical resistance. We further show that energy requirement and cast produced per unit burrow length increased with soil mechanical resistance. Our study revealed significant and species-dependent adverse effects of soil mechanical resistance on earthworm burrowing, which in turn has consequences for many soil processes mediated by earthworms, such as water infiltration, soil aeration, nutrient cycling and soil organic matter turnover.