Impact of Selected Herbicides on the Bacterial Community in Pristine Soil

Introduction: Herbicides are employed in agriculture to control weeds. However, their ecological impact is highly debated among researchers. This study aimed to determine the impact of commonly used herbicides on bacterial community in pristine soil from Makurdi.

Methods: Ten soil samples were collected from each of undisturbed wetland soil (UWS) and pristine loam-textured soil (PLS) at a depth of 15 cm and reduced to two composites. These were spiked with Force Up, PropaForce Plus and ButaForce at the recommended field rate and twice the recommended field rate of application. Bacteriological analysis was done on a 7 days’ interval using conventional culture and metagenomics. DNA was isolated from the soil samples using the Meta-G-Nome DNA Isolation Kit. Libraries were prepared with Nextera® XT DNA Library Prep Kit, sequenced with the NextSeq 500 System, and analyzed using the MG-RAST metagenomics analysis server. Molecular studies, Gas Chromatography and Mass Spectroscopy analysis were done at the beginning and the last day of the study.

Results: Of the nine genera of bacteria culturally isolated and identified, Pseudomonas had the highest significant occurrence (p<0.05). ButaForce was most utilized by the genera: Brevibacterium, Pseudomonas, Bacillus, Staphylococcus, Paracoccus, and Flavobacterium, and Force Up by: Pseudomonas, Bacillus, Flavobacterium, and Phenilobacterium, while Pseudomonas, Bacillus, Flavobacterium, and Staphylococcus utilized Propaforce Plus. Molecular analysis showed that Betaproteobacteria, Verrucomicrobia and Gammaproteobacteria dominated the UWS, while for the PLS, Betaproteobacteria, Alphaproteobacteria and Gammaproteobacteria dominated. Bacteria belonging to Proteobacteria, Verrucomicrobiota, and Bacteroidota dominated soils spiked with PropaForce Plus. Proteobacteria, Bacteroidota, and Actinobacteriota dominated in ButaForce treated soils, while Proteobacteria, Firmicutes and Patescibacteria predominated soils spiked with Force Up. GC – MS identified several compounds associated with bacterial transformation of the herbicides and other compounds of uncertain origin. For Force Up, the major compounds detected were: 2,4 – Di-tert-butylphenol; Hexahydropyridine; 1,2,5-Oxadiazol-3-amine other ester derivatives. The degradation of PropaForce Plus was associated predominantly with aromatic amines and phenoxy derivatives particularly 3,4-dichloroaniline; 2,4-dichloroaniline, benzanamine and dichlorophenoxy derivatives. For ButaForce, the main products included: 2-Chloro-N-(2,6-diethylphenyl) – acetamide; 2,6-Diethylaniline; and 2-Sec-butyl-6-ethylaniline.

Conclusion: Although the herbicides affected the soil bacterial community structure, indigenous bacterial transformation of the herbicides was established. Moderation in the use of herbicides is advocated to preserve soil health.