Fungus Olpidium brassicae dominates in canola-only rotations

The fungal microbiome responded more strongly to crop rotation than did bacterial communities, this study found, and researchers identified a fungus, Olpidium brassicae, that was particularly dominant in the canola-only rotations.

Study objectives were to determine the effect of crop rotation on the nutrient fluxes experienced by canola roots during flowering, examine the organic acids produced by canola that shape the composition of the root-associated microbial communities, and characterize the resulting microbial communities.

This project leveraged a long-established crop rotation study comparing continuous canola, canola-wheat and canola-pea-barley at Scott, Saskatchewan (Dark Brown soil zone), Swift Current, Saskatchewan (Brown soil zone) and Lacombe, Alberta (Black soil zone). Samples were taken from the canola years of these well-established rotations. Researchers used commercially available Plant Root Simulator probes from WesternAg to examine the flux of nutrients available to canola roots, and ion exchange chromatography to examine the organic acids produced by canola plants. Bacterial and fungal communities were characterized using DNA sequencing of the soil, rhizosphere and roots. In this way, researchers examined the long-term impact of crop rotation on soil processes and the canola microbiome to better understand how canola cropping frequency impacts crop performance.

Results

Researchers found minimal impact of rotation on root nutrient availability. Any rotation effects on nutrient availability were transient and site-dependent, likely due to the use of soil-test-based fertilization according to normal agronomic practices, which provided adequate nutrients for crop growth.

Crop rotation strategy had a stronger impact on the fungal microbiota at all sites and years. Crop rotation affected the abundance of fungi associated with both detrimental and beneficial effects on crop production, and this effect was most pronounced in and near canola roots. In particular, continuous canola resulted in a predominance of a fungus identified as Olpidium brassicae, a poorly understood root colonizer with a life cycle that is similar to that of the clubroot pathogen, Plasmodiophora brassicae. This fungus was present in and near the roots at all sites, and was particularly dominant in short-rotation and continuous canola. This provides a direction for future studies.

These findings highlight the importance of considering the impacts of crop rotation strategies on the microbiota associated with canola roots, and demonstrate that the fungal biome is more affected by continuous canola than the bacterial community.