Long-term persistence of arbuscular mycorrhizal fungi in the rhizosphere and bulk soils of non-host Brassica napus and their networks of co-occurring microbes

Arbuscular mycorrhizal fungi (AMF) are obligate plant symbionts that improve the nutrition and health of their host. Most, but not all crops form a symbiosis with AMF. It is the case for canola (Brasica napus), an important crop in the Canadian prairies that is known to not form this association. From 2008 to 2018, an experiment was replicated at three locations of the Canadian prairies and it was used to assess the impact of canola on the community of AMF naturally occurring in three cropping systems, canola monoculture, canola in a two rotation systems (two-years, canola-wheat and three-years barley-pea-canola). We sampled canola rhizosphere and bulk soils to (i) determine diversity and community structure of AMF, and (ii) assess how these AMF communities interact with other fungi and bacteria. We detected 49 AMF Amplicon Sequence Variant (ASV) in canola rhizosphere and bulk soils, confirming the persistence of a diversified AMF community in canola-planted soil, even after 10 years of canola monoculture, which was unexpected considering that canola is among non-mycorrhizal plants. Network analysis revealed a broad range of potential interactions between canola-associated AMF and some fungal and bacterial taxa. We report for the first time that two AMF, Funneliformis mosseae and Rhizophagus iranicus, shared their bacterial cohort almost entirely in bulk soil. Our results suggest the existence of non species-specific AMF-bacteria or AMF-fungi relationships that could benefit AMF in absence of host plants. The persistence of an AMF community in canola rhizosphere and bulk soils bring a new light on AMF ecology and lead to new perspectives for further studies about AMF and soil microbes interactions and AMF subsistence without mycotrophic host plants.