Altered surface behaviour in earthworms (Lumbricus terrestris) under artificial light at night

Artificial light at night (ALAN) has been spreading rapidly globally, disrupting animal physiology, behaviour and associated ecosystem processes. However, impacts on soil-dwelling animals that are nocturnally active aboveground remain underexplored. Here, we examined these for the earthworm Lumbricus terrestris L., an ecosystem engineer that frequently surfaces at night. We recorded four types of L. terrestris nighttime surface behaviour, including overall surface activity, and three finer categories: risk exploration before surfacing, foraging and over-surface exploration. One experiment (controlled-temperature-room-based) tested these behavioural responses to seven ALAN levels, whilst another (field-based) investigated whether behavioural responses can consequently affect litter removal and soil respiration. In the controlled-temperature room, compared to darkness, L. terrestris significantly decreased their overall nighttime foraging at ALAN levels of 10 lx and above, with these negative ALAN effect sizes increasing with light intensity. With more risk exploration before surfacing, L. terrestris still showed greatly reduced foraging activity at the 10 lx ALAN level in the field. However, such altered behaviour did not cause clear patterns in either litter removal or soil respiration. Our results confirm that 10 lx ALAN (or potentially lower) is effective in disrupting L. terrestris nighttime surface behaviour, but this may have limited impacts on short-term C cycling.