Researchers from the Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences have clarified how the herbicide chlorimuron-ethyl affects nitrogen cycling in soybean rhizosphere soils and demonstrated that a chlorimuron-ethyl-degrading bacterial strain, Chenggangzhangella methanolivorans CHL1, can help restore ecological stability in contaminated soils.

Published in Geoderma, the study addresses concerns over the long-term accumulation of herbicide residues in agricultural soils.

Herbicides that remain in the soil after application can disrupt the microbial communities and nutrient cycling processes necessary for crop growth. Nitrogen cycling is a series of biological processes that convert nitrogen into forms that are available to plants and microorganisms. It plays an important role in maintaining soil fertility and agricultural productivity. Microbial remediation, which uses microorganisms to break down pollutants, has become an important approach for managing contaminated farmland.

The study was led by XU Mingkai of the Innovation Group on Environmental Pollution Processes and Effects at IAE. Using field experiments and controlled microcosm systems, the researchers investigated how chlorimuron-ethyl, a widely used sulfonylurea herbicide in soybean production, affects rhizosphere ecosystems. They combined soil analyses with metagenomic approaches to evaluate changes in microorganisms related to nitrogen cycling and to assess the contribution of CHL1 to soil recovery.

The researchers found that exposure to chlorimuron-ethyl significantly inhibited soybean growth and root nodule formation while altering the soil's nitrogen balance and reducing its pH. Root nodules host nitrogen-fixing microorganisms that help plants obtain usable nitrogen and are essential for soybean nutrient acquisition. The herbicide also caused noticeable changes in rhizosphere microbial communities and nitrogen cycling processes.

Further analyses showed that chlorimuron-ethyl reduced the abundance and functional contribution of microorganisms involved in nitrogen fixation and denitrification, while temporarily increasing the abundance of some microorganisms associated with nitrification. The abundance of key nitrogen fixation genes was strongly suppressed under herbicide exposure. These microbial changes were consistent with shifts in soil nitrogen forms, including reduced ammonium nitrogen and increased nitrate nitrogen.

Importantly, the researchers found that the CHL1 strain could rapidly degrade residual chlorimuron-ethyl in soil and reduce its ecological impact. The bacterium shortened the herbicide's persistence in soil by more than half and alleviated the inhibition of soybean growth and root nodule formation. It also helped restore nitrogen cycling-related microbial groups and functional genes, improving soil nitrogen conditions and helping normalize soil pH.

According to the researchers, these findings provide scientific support for the potential use of degrading bacteria in environmentally friendly soil remediation.