Performance and microbiota of Lupinus polyphyllus for Plant and Soil

Purpose: In cold climates, glyphosate residues may linger in
soils, with effects on plant-microbe interactions and, consequently, plant
performance. Here, we explore the influence of glyphosate residues on the
endophytic microbiota (bacteria and fungi) and performance of the
perennial nitrogen-fixing weed Lupinus polyphyllus. Methods: In a common
garden, we grew plants from six populations of L. polyphyllus in
glyphosate-treated or untreated control soils, with or without additional
phosphorus. We sampled plant microbiota (leaves, roots, nodules) and
assessed plant performance based on six traits: height, retrogression
probability (i.e. shrinkage), biomass, root:shoot ratio, nodule number,
and nodule viability. Results: The richness of plant endophytic microbial
communities was determined by soil phosphorus level rather than by
glyphosate treatment. However, for bacteria, the composition of these
communities differed between glyphosate-treated and control soils across
plant tissue types; no difference was observed for fungi. The plant
bacterial communities in both soil types were dominated by potential
nitrogen-fixing bacteria belonging to family Bradyrhizobiaceae, and
particularly so in glyphosate-treated soils. Overall, though, these
changes in plant bacterial communities had a minor effect on plant
performance: the only difference we detected was that the probability of
retrogression was occasionally higher in glyphosate-treated soils than in
control soils. Conclusion: Our findings indicate that glyphosate-based
herbicides, when applied at the recommended frequency and concentration,
may not have critical effects on the growth of short-lived weeds after the
safety period has passed; however, the endophytic microbiota of such weeds
may experience longer-lasting shifts in community structure.