Influence of introduced arbuscular mycorrhizal fungi and phosphorus sources on plant traits, soil properties, and rhizosphere microbial communities in organic legume-flax rotation

We identify P management strategies combining arbuscular mycorrhizal fungal (AMF) inoculation with rock phosphate or composted manure for intensive organic grain-production systems. We measured the response of plants traits and soil properties to the factorial combination of three rates of organic-approved P sources applied in rotation phase-1 of legume–flax cropping systems, and of granular AMF inoculant applied in the first, second, or both rotation phases, or not applied. Treatment combinations effects on the rhizosphere communities of AMF, fungi, and bacteria were tested by amplicon sequencing, in two pedoclimates. Inoculation had limited effects in both environments. Composted manure decreased lentil yield, but increased lentil N and P concentrations and soil P fertility on the Chernozem, while increasing pea productivity on the Luvisol. Composted manure applied in rotation phase-1 had a residual effect on flax productivity, N and P concentrations, and soil P fertility in both environments. Rock phosphate reduced soil P fertility and flax productivity on the Gray Luvisol. The β diversity of the rhizosphere communities was unaffected by treatments, while the α diversity of bacteria and AMF was altered by AMF inoculation and fertilization only in the Gray Luvisol. Correlations between microbial species and plant traits or soil properties were inconsistent, reflecting the complex relationships among microbial community, plant identity, and environmental conditions. Here, composted manure was more influential than AMF inoculation and rock phosphate. Given the influence of environmental conditions, small field trials are recommended before wide-scale adoption of their use.