Effect of Microplastic Contamination and Bradyrhizobium japonicum Inoculation on Glycine max Growth and Soil Bacterial Community

Microplastic soil contamination is an emerging global ecological problem due to potential impacts on plant growth, soil microbial communities, and ecofunctionality. This study investigated the effect of low-concentration high-density polyethylene microplastic contamination (Treatments 1:0.01%, 2: 0.02%, and 3: 0.04%) and inoculation with Bradyrhizobium japonicum strains (USDA4 and USDA110) on soybean germination and plant growth. Changes in soil bacterial communities due to the microplastic contamination were also assessed. The soybean germination period varied (3–5 days), and 63.89, 72.22, 75.0, and 72.22% of the seeds germinated for the control, Treatment 1, Treatment 2, and Treatment 3 respectively. The microplastic treatments had no significant effect on soybean growth and nodulation. Bradyrhizobium japonicum inoculation did not significantly affect soybean growth (p > 0.05). The soil bacterial diversity and relative abundance in the microplastic-contaminated soils varied compared to the control. Actinobacteriota, Proteobacteria, and Firmicutes were among the top ten phyla with varying relative abundance across the treatments. There was a selective proliferation of Sphingomonas and suppression of Exiguobacterium in the microplastic treatment groups compared to the control. Functional annotation of prokaryotic taxa revealed that chemoheterotrophy was the most dominant biochemical process across the control and microplastic-contaminated soils. Also, nitrate reduction, nitrate respiration, denitrification, aromatic compound degradation, and manganese oxidation were among the major processes in the microplastic-contaminated soils. In contrast, photoautotropy and sulphate respiration were dominant in the control soil. Understanding the ecological implication of the effect of long-term microplastic contamination on plants, soil microorganisms including bioinoculants and their interactions is imperative for achieving sustainable agriculture.