LAUSR

Aims The effects of increasing atmospheric carbon dioxide (CO 2 ) concentrations on beneficial soil fauna, such as entomopathogenic nematodes (EPNs), are poorly understood. We hence aimed to characterize how elevated CO 2 (eCO 2 ) affects maize plant ( Zea mays ) growth, root morphology and the effectiveness of the EPN Heterorhabditis bacteriophora . Methods We grew plants under ambient CO 2 (aCO 2 ; 400 μmol mol -1 ) and eCO 2 (640 μmol mol -1 ) and quantified shoot growth and six root traits. We simultaneously quantified the effectiveness of EPNs (mortality of insect hosts ( Galleria mellonella) and EPN density within hosts) when foraging in planted and plant-free environments. Structural equation modeling (SEM) was used to model direct and indirect relationships between atmospheric CO 2, root morphology and EPN effectiveness. Results Root systems of plants grown under eCO 2 grew faster, longer, denser, and larger than plants grown under aCO 2 . This in turn reduced EPN effectiveness as, despite no significant difference between aCO 2 and eCO 2 in host mortality, significantly more nematodes were recovered from hosts in the vicinity of plants grown in aCO 2 environment. The SEM model revealed that this impact was indirect and mediated by the increased root morphological traits. Conclusions We provide the first example of how changes in atmospheric CO 2 indirectly reduce the effectiveness of an EPN used globally for crop protection. Other factors (e.g. plant volatile emissions) may moderate or exacerbate these patterns but our findings suggest that modifications in root traits at eCO 2 negatively impact EPN effectiveness and therefore soil-dwelling insect pest management.