Influence of soil aeration status on denitrifier gene expression and denitrification gas fluxes (N2O and N2) from repacked pasture soils

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Pasture soils are major sources of nitrous oxide (N₂O), a potent greenhouse gas mainly produced through microbial denitrification. However, interactions between soil aeration status and microbial responses involved in regulating N₂O emissions remain poorly understood. We investigated denitrifier gene expression together with N₂O and dinitrogen (N₂) emissions using repacked cores of three pasture soils with contrasting bulk densities incubated across ten matric potentials (− 1 to − 10 kPa). Peak N₂O fluxes occurred at matric potentials of − 1 to − 2 kPa and were strongly related to relative gas diffusivity (Dp/Do) and water-filled pore space (WFPS). In contrast, the expression of nirS, norB, and nosZ was more closely associated with matric potential and volumetric water content than with Dp/Do and WFPS. However, denitrifier gene transcription alone did not explain the observed gaseous N emissions, indicating that microbial transcriptional responses and measured N fluxes were decoupled under changing soil aeration conditions. These findings suggest bulk soil aeration indices (e.g. Dp/Do or WFPS) do not fully represent the local conditions regulating microbial response, whereas measured gaseous N emissions are additionally constrained by soil physical processes governing gas transport. Overall, these results highlight the importance of integrating microbial responses with soil physical processes to improve our mechanistic understanding of N₂O emissions from grazed pasture soils.