Exploring the potential of plant-growth promoting rhizobacteria to mitigate yield decline in wheat rotations

Winter wheat is one of the most important crops worldwide. Consequently, farmers have increased the proportion of wheat in their crop rotations. However, self-succession of WW leads to significant yield decline, which is attributed to the soil-borne fungus Gaeumannomyces tritici (Gt; take-all). This yield decline is also observed in years without pronounced Gt presence in the soil, suggesting a moderating role of the soil microbial community in plant-soil feedbacks. At the same time, there is growing interest in harnessing the beneficial properties of plant growth-promoting bacteria to enhance plant health and productivity. The potential of using such beneficial rhizobacteria to alleviate biomass reduction in successive wheat rotations is substantial. In this experiment, we explored this management option by seed-inoculating wheat plants with Bacillus pumilus. Wheat was grown in soil after oilseed rape (W1) and soil after one season of wheat (W2). We measured soil mineral N, microbial diversity and community composition, as well as, microbial activity. Special focus was placed on root plastic responses as a function of the microbial inoculant and wheat rotational position. W1 produced more biomass and had a higher yield than W2. Successively grown wheat had a much lower root growth, compared to wheat grown after oilseed rape. Bacillus pumilus inoculation did not mitigate the yield reduction in W2. Differences in catalytic efficiency of β-glucosidase and leucine aminopeptidase were observed between W2 and W1, with higher and lower efficiencies, respectively, in W2; these effects were mainly driven by Bacillus pumilus inoculation. We discuss potential mechanisms that moderate these effects.