Bacillus velezensis SQR9 inhibition to fungal denitrification responsible for decreased N2O emissions from acidic soils

Tropical and subtropical acidic soils are hotspots of global terrestrial nitrous oxide (N2O) emissions, with N2O pro-duced primarily through denitrification. Plant growth-promoting microbes (PGPMs) may effectively mitigate soil N2O emissions from acidic soils, achieved through differential responses of bacterial and fungal denitrification to PGPMs. To test this hypothesis, we conducted a pot experiment and the associated laboratory trials to gain the under-lying insights into the PGPM Bacillus velezensis strain SQR9 effects on N2O emissions from acidic soils. SQR9 inocula-tion significantly reduced soil N2O emissions by 22.6-33.5 %, dependent on inoculation dose, and increased the bacterial AOB, nirK and nosZ genes abundance, facilitating the reduction of N2O to N2 in denitrification. The relative contribution of fungi to the soil denitrification rate was 58.4-77.1 %, suggesting that the N2O emissions derived mainly from fungal denitrification. The SQR9 inoculation significantly inhibited the fungal denitrification and down-regulated fungal nirK gene transcript, dependent on the SQR9 sfp gene, which was necessary for secondary me-tabolite synthesis. Therefore, our study provides new evidence that decreased N2O emissions from acidic soils can be due to fungal denitrification inhibited by PGPM SQR9 inoculation.

Automated summary

Tropical and subtropical acidic soils are major contributors to global nitrous oxide (N2O) emissions, primarily through denitrification. Plant growth-promoting microbes (PGPMs) have the potential to mitigate N2O emissions from acidic soils by affecting bacterial and fungal denitrification differently. This study found that the PGPM Bacillus velezensis strain SQR9 significantly reduced N2O emissions from acidic soils by 22.6-33.5%, depending on the inoculation dose. The reduction was achieved by increasing bacterial denitrification genes abundance and inhibiting fungal denitrification, indicating the important role of fungi in N2O emissions from these soils.