Effects of nitrogen fertilization on soil N2O reducing bacteria and their importance for mitigating N2O emissions

EABIOmECT3Arable soils are a major source of nitrous oxide (N2O). The only known sink in Earth’s biosphere is the reduction of N2O to N2 via the N2O reductase encoded by the nosZ gene, identified as nosZ clade I and II. The nosZI gene is mainly found among denitrifying bacteria, whereas >50% of microorganisms with nosZII lack other denitrification genes. The abundance and phylogenetic diversity of nosZII was previously shown to correlate with a soils N2O sink capacity. To provide a proof of principle, we manipulated 11 indigenous soil microbial communities by increasing the abundance of a non-denitrifying N2O-reducing strain capable of growth with N2O as the only electron acceptor. Consequently, potential N2O emissions were lowered in 1/3 of the soils by 51% in average and the magnitude was significantly influenced by the soil pH and C/N ratio. These results provide direct evidence that non-denitrifying N2O-reducing bacteria can contribute to lowering the net N2O production in soil, which indicates that net N2O emissions are ultimately determined by the relative abundance of specific bacterial taxa with and without nosZ. The relative abundance of these bacterial taxa is known to be influenced by fertilization. Hence, we investigated predictable effects of N fertilizer on microbial communities across 14 Swedish longterm field trials, each with fertilized and non-fertilized treatments. The 16S rRNA and nosZ genes were sequenced, the abundance of denitrification genes quantified and the potential denitrification and N2O production rates determined. The genetic potential for denitrification correlated with the potential activity and high nosZ/nir gene ratios coincided with lower N2O:N2 emission ratios. The nosZII, but not nosZI, community was significantly affected by fertilization and the phylogenetic diversity of nosZII correlated negatively with the N2O:N2 emission ratio. The community data remains to be further analysed to identify factors influencing the overall bacterial and N2O reducing communities