Soil Chemistry and Microbiome Determine N2O Emission Potential in Soils Metagenome

Microbial nitrogen (N) transformations in soil, notably denitrification, result in the production of the potent greenhouse and ozone depleting gas nitrous oxide (N2O). Soil chemistry and microbiome composition impact N2O emission potential but the relative importance of these factors as determinants of N2O emission in denitrifying systems is rarely tested. In addition, previous linkages between microbiome composition and N2O emission potential rarely demonstrate causality. Here, we determined the relative impact of microbiome composition (i.e. soil extracted cells) and chemistry (i.e. water extractable chemicals) on N2O emission potential utilizing an anoxic cell based assay system. Cells and chemistry for assays were sourced from soils with contrasting N2O/N2O+N2 ratios, combined in various combinations and denitrification gas production was measured in response to nitrate addition. Average directionless effects of cell and extract origin changes on N2O/N2O+N2 (Cell: delta0.16, Extract: delta0.22) and total N2O hypothetically emitted (Cell: delta2.62 micromol-N, Extract: delta4.14 micromol-N) indicated chemistry is the most important determinant of N2O emissions. Independent pH differences of just 0.6 points impacted N2O/N2O+N2 on par with independent extract differences, supporting the dominance of this variable in previous studies. However, impacts on overall N2O hypothetically emitted were smaller suggesting that soil pH manipulation may not necessarily be a successful approach to mitigate emissions. In addition, we observed increased N2O accumulation and emission potential at the end of incubations concomitant with predicted decreases in carbon availability suggesting that carbon limitation increases N2O emission transiently with the magnitude of emission dependent on the both chemical and microbiome controls.