Residue-C effects on denitrification vary with soil depth

A stable isotope (¹³C-residue, ¹⁵N-NO₃ ^– fertiliser) approach combined with measurements of soil pore space gas concentrations was used to investigate spatial and temporal mechanisms of residue carbon (C) affecting denitrification. Whilst relationships between residue addition and N₂O fluxes have previously been well characterised, the influence of residues on production and reduction of N₂O at depth is less well understood. Here we investigated the relationship between residue-¹³C addition (0, 1 and 2 mg C g⁻¹ soil) and denitrification (¹⁵N-N₂O and ¹⁵N-N₂ production) at 2, 5 and 8 cm soil depths and also fluxes from the soil surface. Hydrophobic probes that equilibrate with the soil gas phase were used to extract gases at soil depth, followed by analysis for ¹⁵N-N₂O, ¹⁵N-N₂, ¹³C-CO₂ and O₂ concentrations. ¹⁵N-N₂O and CO₂ surface fluxes peaked one day after ¹⁴NH₄ ¹⁵NO₃ application (1 mg N g⁻¹ soil), with residue application resulting in a more than 20-fold greater ¹⁵N-N₂O emission rate compared to the non-amended control. Eight days after N application ¹⁵N-N₂O pore space concentrations had significantly increased 20-fold at 8 cm depth below the residue layer compared to no residue application. However, simultaneous increases in ¹⁵N-N₂ surface fluxes and profile concentrations showed efficient reduction of the N₂O at shallow depth (3–10 cm depth) resulting in surface emission of N₂ rather than N₂O. Our results have implications for management to lower emissions as denitrifier activity at greater depth, and the greater reduction of N₂O to N₂, appeared to be indirectly driven by residue addition via the depletion of O₂ during aerobic heterotrophic respiration in the surface layer. In contrast, net surface fluxes of N₂O were more directly related to the residue addition through substrate provision for denitrification.