Interaction of straw amendment and soil NO₃−content controls fungal denitrification and denitrification product stoichiometry in a sandy soil

The return of agricultural crop residues are vital to maintain or even enhance soil fertility. However, the influence of application rate of crop residues on denitrification and its related gaseous N emissions is not fully understood. We conducted a fully robotized continuous flow incubation experiment using a Helium/Oxygen atmosphere over 30 days to examine the effect of maize straw application rate on: i) the rate of denitrification, ii) denitrification product stoichiometry N₂O/(N₂O+N₂), and iii) the contribution of fungal denitrification to N₂O fluxes. Five treatments were established using sieved, repacked sandy textured soil; i) non-amended control, ii) nitrate only, iii) low rate of straw + nitrate, iv) medium rate of straw + nitrate, and iv) high rate of straw + nitrate (n = 3). We simultaneously measured NO, N₂O as well as direct N₂ emissions and used the N₂O ¹⁵N site preference signatures of soil-emitted N₂O to distinguish N₂O production from fungal and bacterial denitrification. Uniquely, soil NO₃⁻ measurements were also made throughout the incubation. Emissions of N₂O during the initial phase of the experiment (0–13 days) increased almost linearly with increasing rate of straw incorporation and with (almost) no N₂ production. However, the rate of straw amendment was negatively correlated with N₂O, but positively correlated with N₂ fluxes later in the experimental period (13–30 days). Soil NO₃⁻ content, in all treatments, was identified as the main factor responsible for the shift from N₂O production to N₂O reduction. Straw amendment immediately lowered the proportion of N₂O from bacterial denitrification, thus implying that more of the N₂O emitted was derived from fungi (18 ± 0.7% in control and up to 40 ± 3.0% in high straw treatments during the first 13 days). However, after day 15 when soil NO₃⁻ content decreased to<40 mg NO₃ ⁻-N kg⁻¹ soil, the N₂O ¹⁵N site preference values of the N₂O produced in the medium straw rate treatment showed a sharp declining trend 15 days after onset of experiment thereby indicating a clear shift towards a more dominant bacterial source of N₂O. Our study singularly highlights the complex interrelationship between soil NO₃ ⁻ kinetics, crop residue incorporation, fungal denitrification and N₂O/(N₂O + N₂) ratio. Overall we found that the effect of crop residue applications on soil N₂O and N₂ emissions depends mainly on soil NO₃ ⁻content, as NO₃ ⁻ was the primary regulator of the N₂O/(N₂O + N₂) product ratio of denitrification. Furthermore, the application of straw residue enhanced fungal enitrification, but only when the soil NO₃ ⁻ content was sufficient to supply enough electron acceptors to the denitrifiers.