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

## TL;DR

This study examines how different herbicides affect the survival and plant growth-promoting abilities of the bacterium Nitrospirillum amazonense used in sugarcane farming.

## Contribution

The study reveals specific herbicide effects on bacterial survival and function, offering insights for sustainable herbicide use in agriculture.

## Key findings

- Some herbicides like indaziflam and metribuzin had no negative effects and even enhanced bacterial growth and function.
- Herbicides like clomazone and imazapic reduced growth but did not harm soil survival and increased plant growth-promoting activities.
- Flumioxazin and tebuthiuron increased growth and nitrogen fixation but reduced IAA production.

## Abstract

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.

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.

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.

Herbicides elicit a spectrum of responses in Nitrospirillum amazonense, ranging from the stimulation of metabolic functions (biological nitrogen fixation, IAA production) to significant growth inhibition and acute toxicity.