Soil Bulk Density and Moisture Content Influence Relative Gas Diffusivity and the Reduction of Nitrogen-15 Nitrous Oxide

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 (rho(b)) 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 rb (0.94, 1.00, 1.07 Mg m(-3)). Over 19 d, N-15-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 rb and WFPS interacted to affect the recovery of N2O-N-15 and the antecedent inorganic-N contribution to surface fluxes. Reduction rates of N2O-N-15 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 N-15 enrichment (r = 0.80) and were affected by a soil WFPS x soil rb 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.