Effects of Sorgoleone on Soil Microbial Communities and Soybean Nodulation

Bioinputs offer a promising alternative to synthetic herbicides, reducing environmental impacts, but their effects on soil microbial communities are not well understood. This study assessed the effects of aqueous sorghum extract on soil microbial communities and nodulation in soybean cultivated on sorghum and maize crop residues. The experiment was conducted in a completely randomized split-plot design with five replications, with sorghum or maize crop residues in the plots, and weed control with or without aqueous sorghum extract application in the subplots. Microbial biomass carbon (MBC), basal soil respiration (BSR), microbial quotient (qMIC), metabolic quotient (qCO2), mycorrhizal colonization, and number of viable nodules were measured. Aqueous sorghum extract application reduced MBC (77.92 mg C kg⁻¹ soil) and BSR (40.58 mg C-CO2 kg⁻¹ soil day⁻¹) under sorghum residue treatments, increased qCO2 (indicating higher microbial stress), and reduced qMIC, suggesting lower carbon use efficiency. Soybean mycorrhizal colonization was unaffected, but nodulation was significantly reduced under sorghum residue treatments (37 viable nodules per plant), suggesting an inhibitory effect on soybean-rhizobium symbiosis. These findings indicate that phenolic compounds and quinones in sorghum alter soil microbial activity and impair biological nitrogen fixation, particularly when combined with sorghum crop residues.