Using adaptive and aggressive N₂O-reducing bacteria to augment digestate fertilizer for mitigating N₂O emissions from...

Nitrous oxide (N₂O) emitted from agricultural soils destroys stratospheric ozone and contributes to global warming. A promising approach to reduce emissions is fertilizing the soil using organic wastes augmented by non-denitrifying N₂O-reducing bacteria (NNRB). To realize this potential, we need a suite of NNRB strains that fulfill several criteria: efficient reduction of N₂O, ability to grow in organic waste, and ability to survive in farmland soil. In this study, we enriched such organisms by sequential anaerobic batch incubations with N₂O and reciprocating inoculation between the sterilized substrates of anaerobic manure digestate and soils. 16S rDNA amplicon sequencing and metagenomics analysis showed that a cluster of bacteria containing nosZ genes encoding N₂O-reductase, was enriched during the incubation process. Strains of several dominant members were then isolated and characterized, and three of them were found to harbor the nosZ gene but none of the other denitrifying genes, thus qualifying as NNRB. The selected isolates were tested for their capacities to reduce N₂O emissions from three different typical Chinese farmland soils. The results indicated the significant mitigation effect of these isolates, even in very acidic red soil. In conclusion, this study demonstrated a strategy to engineer the soil microbiome with promising NNRB with high adaptability to livestock manure digestate as well as different agricultural soils, which would be suitable for developing novel fertilizer for farmland application to efficiently mitigate the N₂O emissions from agricultural soils.