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

Abstract 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 N 2 O/(N 2 O+N 2 ), and iii) the contribution of fungal denitrification to N 2 O 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, N 2 O as well as direct N 2 emissions and used the N 2 O 15 N site preference signatures of soil-emitted N 2 O to distinguish N 2 O production from fungal and bacterial denitrification. Uniquely, soil NO 3 − measurements were also made throughout the incubation. Emissions of N 2 O during the initial phase of the experiment (0–13 days) increased almost linearly with increasing rate of straw incorporation and with (almost) no N 2 production. However, the rate of straw amendment was negatively correlated with N 2 O, but positively correlated with N 2 fluxes later in the experimental period (13–30 days). Soil NO 3 − content, in all treatments, was identified as the main factor responsible for the shift from N 2 O production to N 2 O reduction. Straw amendment immediately lowered the proportion of N 2 O from bacterial denitrification, thus implying that more of the N 2 O 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 NO 3 − content decreased to 3 − -N kg −1 soil, the N 2 O 15 N site preference values of the N 2 O 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 N 2 O. Our study singularly highlights the complex interrelationship between soil NO 3 − kinetics, crop residue incorporation, fungal denitrification and N 2 O/(N 2 O + N 2 ) ratio. Overall we found that the effect of crop residue applications on soil N 2 O and N 2 emissions depends mainly on soil NO 3 − content, as NO 3 − was the primary regulator of the N 2 O/(N 2 O + N 2 ) product ratio of denitrification. Furthermore, the application of straw residue enhanced fungal denitrification, but only when the soil NO 3 − content was sufficient to supply enough electron acceptors to the denitrifiers.