Abstract Background and aims Residual imidazolinone herbicides are widely recognised for constraining the growth of rotational legume crops; however, little is known about how individual active ingredients including imazapyr and imazamox influence rhizosphere microbial communities. Methods An alkaline sandy soil was treated with field-equivalent residues of either imazamox or imazapyr and planted with imidazolinone-tolerant and susceptible lentil genotypes. Rhizosphere and bulk soil samples were collected at early and late growth stages and analysed using 16S rRNA amplicon sequencing. Results The rhizosphere bacteriome was more strongly influenced by herbicide chemistry than by lentil genotype or plant growth stage. Imazapyr caused a pronounced shift in rhizosphere bacterial community composition and reduced bacterial alpha diversity during early plant development, whereas imazamox produced comparatively minor microbiome changes. Imazapyr residues significantly reduced the relative abundance of symbiotic rhizobia (0.22%) in the susceptible genotype during early growth, while imazamox had minimal effects on rhizobial abundance across both genotypes and sampling times (0.31%). Despite weaker microbiome effects, imazamox residues caused greater reductions in lentil shoot biomass at the later growth stage (67.1%) than imazapyr (6.9%). Conclusion These findings demonstrate that individual active ingredients within dual-active imidazolinone herbicides exert contrasting effects on crop performance and rhizosphere microbial assembly and can have significant implications to farming systems that rely on dual active imidazolinone herbicides to manage problematic weeds.
Dzoma et al. (2026) studied this question.