Soil bulk density and moisture content influence relative gas diffusivity and the reduction of nitrogen-15 nitrous oxide

Abstract

Nitrous oxide is a greenhouse gas and contributes to stratospheric ozone depletion. Soil physical conditions may influence N₂O reduction and subsequent N₂O emissions. We studied how soil water-filled pore space (WFPS) and soil bulk density (ρb) affect N₂O reduction and surface fluxes. Columns were repacked with soil and arranged in a factorial design at three levels of WFPS (60, 75, and 90%) and three levels of soil ρb (0.94, 1.00, 1.07 Mg m⁻³). Over 19 d, ¹⁵N-enriched N₂O was introduced at the base of the soil columns and N₂O fluxes were measured. Relative gas diffusivities (Dp/Do) were also calculated. Soil ρb and WFPS interacted to affect the recovery of N₂O-¹⁵N and the antecedent inorganic-N contribution to surface fluxes. Reduction rates of N₂O-¹⁵N ranged from 0.15 to 0.47 mg N₂O-N g⁻¹ soil d⁻¹. Calculated Dp/Do values correlated (P < 0.01) with soil NH₄⁺-N (r = − 0.73), NO₃⁻-N (r = 0.93), cumulative N₂O-N flux (r = 0.76), and N₂O-N ¹⁵N enrichment (r = 0.80) and were affected by a soil WFPS × soil ρb interaction. Soil N transformations and the net surface N₂O flux is dependent on the soil’s Dp/Do, and WFPS alone does not suffice to discriminate between N₂O emission sources. Consequently, the soil surface N₂O flux may be comprised of N₂O originating from deeper soil layers transported upward and/or from production in the topsoil.