Mitigation of N2O-emissions from soils is needed to reduce climate forcing by food production. Inoculating soils with N2O-reducing bacteria would be effective, but costly and impractical as a standalone operation. Here we demonstrate that digestates obtained after biogas production may provide a low-cost and widely applicable solution. Firstly, we show that indigenous N2O-reducing bacteria in digestates grow to high levels during anaerobic enrichment under N2O. Gas kinetics and meta-omic analysis show that the N2O-respiring organisms, recovered as metagenome-assembled genomes (MAGs), grow by harvesting fermentation intermediates of the methanogenic consortium. Three digestate-derived denitrifying, N2O-reducing bacteria were obtained through isolation, one of which matched the recovered MAG of a dominant Dechloromonas-affiliated N2O reducer. While the identified N2O-reducers encoded 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. Secondly, moving towards practical application, we show that these isolates grow by aerobic respiration in digestates, and that fertilization with these enriched digestates reduces N2O emissions. This shows that the ongoing implementation of biogas production in agriculture opens a new avenue for cheap and effective reduction of N2O emissions from food production.

Manipulating soil metabolism by heavy inoculation with microbes is deemed realistic if waste from anaerobic digestion (digestate) is utilized as substrate and vector, but requires organisms that can grow both in digestate and soil (=generalist). We designed a strategy to enrich and isolate such generalist N2O-respiring bacteria (NRB) in soil and digestate, to provide inoculum for reducing N2O-emissions from agricultural soil. Sequential anaerobic enrichment cultures were provided with a small dose of O2 and unlimited N2O, alternating between sterilized digestate and soil as substrates. The cultures were monitored for gas kinetics and community composition (16SrDNA), and cluster-analysis identified generalist-OTUs which became dominant, digestate/soil-specialists which did not, and a majority that were diluted out. Several NRBs circumscribed by generalist-OTU’s were isolated, genome sequenced to screen for catabolic capacity, and phenotyped, to assess their capacity as N2O-sinks in soil. The two isolates Cloacibacterium sp., carrying only N2O-reductase (Clade-II) and Pseudomonas sp., with full-fledged denitrification-pathway, were both very effective N2O-sinks in soil, with Pseudomonas sp., showing a long-lasting sink effect, suggesting better survival in soil. This avenue for utilizing waste to bioengineer the soil microbiota holds promise to effectively combat N2O-emissions but could also be utilized for enhancing other metabolic functions in soil.Graphical abstract