Nitrous oxide respiring bacteria in biogas digestates for reduced agricultural emissions

Inoculating agricultural soils with nitrous oxide respiring bacteria (NRB) can reduce N₂O-emission, but would be impractical as a standalone operation. Here we demonstrate that digestates obtained after biogas production are suitable substrates and vectors for NRB. We show that indigenous NRB in digestates grew to high abundance during anaerobic enrichment under N₂O. Gas-kinetics and meta-omic analyses showed that these NRB's, recovered as metagenome-assembled genomes (MAGs), grew by harvesting fermentation intermediates of the methanogenic consortium. Three NRB's were isolated, one of which matched the recovered MAG of a *Dechloromonas*, deemed by proteomics to be the dominant producer of N₂O-reductase in the enrichment. While the isolates harbored genes required for a full denitrification pathway and could thus both produce and sequester N₂O, their regulatory traits predicted that they act as N₂O sinks in soil, which was confirmed experimentally. The isolates were grown by aerobic respiration in digestates, and fertilization with these NRB-enriched digestates reduced N₂O emissions from soil. Our use of digestates for low-cost and large-scale inoculation with NRB in soil can be taken as a blueprint for future applications of this powerful instrument to engineer the soil microbiome, be it for enhancing plant growth, bioremediation, or any other desirable function.

Kjell Rune Jonassen, Live H. Hagen, Silas H. W. Vick, Magnus Ø. Arntzen, Vincent G. H. Eijsink, Åsa Frostegård, Pawel Lycus, Lars Molstad, Phillip B. Pope, Lars R. Bakken