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 14NH415NO3 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.