Artificial light at night alters earthworm communities and soil aggregation

Abstract The global expansion of artificial light at night (ALAN) threatens many plants and animals. However, potentially important predicted impacts on earthworm communities, both directly (dALAN) and through plant‐mediated pathways (plant‐mediated ALAN), remain little explored. Also, whether and how any impacts extend to changes in key ecosystem processes, such as soil aggregation, are not known. Here, we sampled earthworm assemblages and soil near and between streetlights that have been nocturnally lit for 9 years. Using streetlight illuminance and correlated colour temperature (CCT) as ALAN proxies, we explored the impacts of dALAN and plant‐mediated ALAN on earthworm abundance and biomass across ecological categories (total, anecic, epigeic and endogeic) and age classes (total, adult and juvenile), and subsequently, on earthworm‐driven soil aggregation (large macro‐aggregates, small macro‐aggregates and micro‐aggregates). We found that increasing CCT (‘bluer’ light) decreased both total earthworm abundance and biomass, whilst increasing illuminance promoted total earthworm biomass; the magnitude of positive effects of illuminance reduced as CCT increased. However, such impacts differed across earthworm ecological categories and age classes. Our results also suggested that illuminance and CCT can shape earthworm communities contingent on plant cover. The pathway of ALAN‐earthworm communities‐soil aggregation was, in part, confirmed. Specifically, ALAN, via juvenile endogeic earthworms enhanced the formation of large macro‐aggregates whilst inhibiting that of small macro‐aggregates and micro‐aggregates. Synthesis and applications . Our results provide evidence that continuous night‐time street lighting can affect earthworm communities and earthworm‐driven soil aggregation. The nature of these effects differed across earthworm ecological categories and age classes and appeared to depend on illuminance, light spectrum and/or plant cover. This highlights that, rather than a one‐size‐fits‐all reduction in light, effective ALAN management should involve adjusting illuminance and spectral composition in relation to local ecological conditions—including variations in habitat characteristics—to support community diversity and maximise earthworm‐driven soil functionality.