Arbuscular mycorrhizal fungi (AMF) are belowground symbionts associated with most plant species. AMF can improve plant tolerance to stressful abiotic environments by increasing nutrient uptake and/or decreasing root respiration and water use. Both abiotic factors and herbivores can affect plant-mycorrhizal symbioses, thereby affecting both plant performance and defenses. In addition, plant-mycorrhizal associations can regulate the composition and quantity of root exudates, potentially affecting other soil organisms. However, the direct and indirect effects of AMF on plant interactions with other soil organisms, and how these effects are mediated by the plant's environment, are still poorly understood. Determining the indirect (i.e., plant-mediated) effects of AMF on nematode communities could provide important information on belowground processes in natural systems. We hypothesized that annual precipitation patterns affect AMF and soil nematode communities associated with milkweed plant species (Asclepias spp.) directly, and that the presence of AMF in milkweed roots affects the associated soil nematode community indirectly.
We collected the entire root system and associated soil of Asclepias fascicularis and Asclepias speciosa from five sites across a rainfall gradient in the state of Nevada. For all sampled individuals, we determined the presence of AMF in the roots and characterized the associated soil nematode community.
Plants with AMF had ~75% less root-feeding nematode species and individuals in the associated soil than plants without AMF in both Asclepias species. The presence of AMF in roots resulted in a 50% decrease in diversity of the nematode community in both milkweed species, independently of site and precipitation levels. Thus, contrary to prediction, precipitation did not have significant direct effects on either AMF or the nematode community, whereas AMF did affect the nematode community, as predicted. Ongoing analyses will determine whether these effects were direct or indirect.
Our results suggest that plant-mycorrhizal associations not only reduced root-nematode interactions but also decreased the diversity of nematode communities in natural systems. Furthermore, these results also reveal the potential of AMF to modify the interactions between roots and belowground organisms independently of precipitation levels. Overall, these data show that, by affecting plant-associated nematode communities, AMF can have important indirect effects on soil processes that will have further effects on the plant. Future studies are needed to determine how plant-mycorrhizal associations can affect the wider soil biota and how such changes can influence the stability of the soil communities.