Modelling nitrification inhibitor effects on N ₂O emissions after fall- and spring-applied slurry by reducing nitrifier NH ₄⁺ oxidation rate

Reductions in inline-formulaN₂O emissions from nitrification inhibitors (NI) are substantial but remain uncertain because measurements of inline-formulaN₂O emissions are highly variable and discontinuous. Mathematical modelling may offer an opportunity to estimate these reductions if the processes causing variability in inline-formulaN₂O emissions can be accurately simulated. In this study, the effect of NI was simulated with a simple, time-dependent algorithm to slow inline-formula $M7inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg3226c502fdca30fe88bf9305df4b3716 bg-17-2021-2020-ie00003.svg24pt15ptbg-17-2021-2020-ie00003.png oxidation in the ecosystem model ecosys. Slower nitrification modelled with NI caused increases in soil inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgcc9fcbc3808c845ed7a1b90adb40bd46 bg-17-2021-2020-ie00004.svg24pt15ptbg-17-2021-2020-ie00004.png concentrations and reductions in soil inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgb3512ed4eb493ff037a5c39221523c47 bg-17-2021-2020-ie00005.svg25pt16ptbg-17-2021-2020-ie00005.png concentrations and in inline-formulaN₂O fluxes that were consistent with those measured following fall and spring applications of slurry over 2 years from 2014 to 2016. The model was then used to estimate direct and indirect effects of NI on seasonal and annual emissions. After spring slurry applications, NI reduced inline-formulaN₂O emissions modelled and measured during the drier spring of 2015 (35 % and 45 %) less than during the wetter spring of 2016 (53 % and 72 %). After fall slurry applications, NI reduced modelled inline-formulaN₂O emissions by 58 % and 56 % during late fall in 2014 and 2015 and by 8 % and 33 % during subsequent spring thaw in 2015 and 2016. Modelled reductions were consistent with those from meta-analyses of other NI studies. Simulated NI activity declined over time so that reductions in inline-formulaN₂O emissions modelled with NI at an annual timescale were relatively smaller than those during emission events. These reductions were accompanied by increases in inline-formulaNH₃ emissions and reductions in inline-formula $M15inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b bg-17-2021-2020-ie00006.svg25pt16ptbg-17-2021-2020-ie00006.png losses with NI that caused changes in indirect inline-formulaN₂O emissions. With further parameter evaluation, the addition of this algorithm for NI to ecosys may allow emission factors for different NI products to be derived from annual inline-formulaN₂O emissions modelled under diverse site, soil, land use and weather.