Soil Chemistry and Microbiome Determine N2O Emission Potential in Soils

Microbial nitrogen (N) transformations in soil, notably denitrification, result in the production of the potent greenhouse and ozone depleting gas nitrous oxide (N 2 O). Soil chemistry and microbiome composition impact N 2 O emission potential but the relative importance of these factors as determinants of N 2 O emission in denitrifying systems is rarely tested. In addition, previous linkages between microbiome composition and N 2 O 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 N 2 O emission potential utilizing an anoxic cell based assay system. Cells and chemistry for assays were sourced from soils with contrasting N 2 O/N 2 O+N 2 ratios, combined in various combinations and denitrification gas production was measured in response to nitrate addition. Average directionless effects of cell and chemical extract on N 2 O/N 2 O+N 2 (Cell: Δ0.16, Chemical extract: Δ0.22) and total N 2 O hypothetically emitted (Cell: Δ2.62 μmol-N, Chemical extract: Δ4.14 μmol-N) indicated chemistry is the most important determinant of N 2 O emissions. Independent pH differences of just 0.6 points impacted N 2 O/N 2 O+N 2 on par with independent chemical extract differences, supporting the dominance of this variable in previous studies. However, impacts on overall N 2 O hypothetically emitted were smaller suggesting that soil pH manipulation may not necessarily be a successful approach to mitigate emissions over a fixed time period. In addition, we observed increased N 2 O accumulation and emission potential at the end of incubations concomitant with predicted decreases in carbon availability suggesting that carbon limitation increases N 2 O emission transiently with the magnitude of emission dependent on the both chemical and microbiome controls.