Predatory Nematodes Decline with Long-Term Carbon Dioxide Enrichment.

Changing resource availability either above- or belowground can alter the structure and function of soil food webs. Understanding how soil food webs respond to long-term carbon dioxide (CO₂) and nitrogen (N) enrichment either directly, or indirectly through plant community change, is important for predicting ecosystem responses to these global change factors. Nematode communities can be an excellent indicator of soil food web responses to resource enrichment because community members feed across many trophic levels. We studied responses of nematode feeding guilds to CO₂ and N enrichment in a Free Air CO₂ Enrichment (FACE) experiment at Cedar Creek Ecosystem Science Reserve, MN, USA. Nematode communities were assessed from field samples collected after nine years of treatment with all factorial combinations of ambient and elevated CO₂ (360 or 560 ppm) and ambient or elevated N (0 or 4 g N m^-2 y^-1). The overall community structure and abundance of nematodes was not affected by CO₂ or N enrichment. Likewise, no changes were found in the abundance of bacterial feeders, fungal feeders, plant feeders or omnivores. However, there were 50% fewer predators in the elevated CO₂ treatment (p<0.0001). This unexpected result may indicate that either the abundance or quality of preferred prey is impacted by CO₂ enrichment. Vector analysis revealed that the non-metric multidimensional scaling (NMS) axis most strongly related to predator abundance (r=0.45) was aligned with the abundance of arbuscular mycorrhizal fungal (AMF) hyphae (r=0.34), as well as the amount of organic matter (r=-0.21) and N (r=0.21) in the soil. Vectors corresponding to other NMS axes revealed that the nematode trophic community structure was also affected by AMF spore abundance (r=0.27), as well as organic matter (r=-0.24) and soil texture (r=-0.29). Changes in these factors under elevated CO₂ could alter the feeding patterns of nematodes in lower trophic levels and the abundance of particular species within a trophic level, resulting in a non-additive effect on predator abundance. A causal model relating measured biotic and abiotic variables to predatory nematode abundance will be presented. These results demonstrate that CO₂ enrichment can generate indirect effects which influence the structure of soil food webs such that the abundance of predatory nematodes is reduced. Future studies should examine how this change in trophic structure may impact the stability and functioning of soil food webs and consequently ecosystem function.