Herbicide toxicity to Nitrospirillum amazonense: assessing bacterial survival and microbial functionality.

BACKGROUND: Herbicides can exert significant toxicity on nontarget organisms, including plant growth-promoting bacteria. Nitrospirillum amazonense has been shown to be highly effective in enhancing sugarcane growth through biological nitrogen fixation (BNF) and the production of the phytohormone indole-3-acetic acid (IAA). However, the combined effects of herbicide application and this microbial technology remain poorly understood, representing a critical knowledge gap for sustainable sugarcane management. Therefore, the objective of this study was to evaluate, through four in vitro assays, the impact of nine pre-emergent herbicides on the survival and physiological functions of N. amazonense. RESULTS: Herbicides exerted differential and molecule-specific effects on N. amazonense. Indaziflam, metribuzin, S-metolachlor and sulfentrazone caused no detrimental impacts on bacterial performance and, in some cases, even enhanced cell growth, IAA production and/or BNF. Isoxaflutole promoted growth at sublethal concentrations but became toxic at higher doses, while leaving IAA production and BNF unaffected. Clomazone and imazapic reduced bacterial growth yet did not compromise soil survival and stimulated both IAA and BNF. Flumioxazin and tebuthiuron enhanced growth and BNF but reduced IAA production. CONCLUSION: This study provides novel evidence that herbicides induce complex and specific modulations in N. amazonense, influencing not only bacterial survival but also the physiological mechanisms underlying plant growth promotion. Such specificity underscores the importance of herbicide selection in integrated management systems. The identification of compatible herbicides enables strategic combinations that sustain weed control while preserving microbial benefits, thereby advancing sustainable sugarcane production. © 2025 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.