Structure and Activity of Denitrifier Communi- ties in Biochar-Amended Soil and Their Impact on N2O Emissions

Nitrous oxide is a greenhouse gas with a global warming potential about 300 times higher than CO2. The main sources of N2O are microbial-mediated nitrogen transformation reactions in soils. Denitrification represents one of the major N2Oproducing pathways in oxygen-limited zones. Soil biochar amendment has been demonstrated to reduce N2O emissions in microcosms and in the field. Although N2O emission mitigation due to soil biochar amendment has frequently been reported for different soils and biochars it remains unclear how biochar affects the structure and activity of the denitrifying microbial community in soils. We setup soil microcosms containing wood-derived biochar and quantified NO3-derived N2O and N2 emission rates. Genes and transcripts of functional marker genes for microbial denitrification were quantified by qPCR and analyzed by Illumina sequencing. Soil biochar amendment did not alter N2O sources, but decreased NO3-derived N2O emission rates by increasing the quantity of N2 entrapped in the soil matrix. Furthermore biochar addition promoted the expression of functional marker genes for complete microbial denitrification. Sequence analyses of the N-cycling marker genes and their transcripts revealed biochar induced community shifts. While 16S rRNA gene diversity was only slightly affect by biochar, functional gene abundance and transcription levels varied among taxonomic groups over time suggesting the contribution of rare but active taxa to soil N2O emissions. Our findings further the mechanistic understanding of the complex coupling between nitrogen pools, nitrogen-transforming microorganisms and nitrogen gas fluxes in pyrogenic carbon amended soils. Implications of the presented research for the development of biofilter for the removal of nitrate from water will be discussed.