Nitrous oxide emissions after renovation of festulolium, and mitigation potential of 3,4-dimethyl pyrazole phosphate (DMPP)

High-yielding perennial grass can, when compared to classical grain crop production, considerably increase biomass yields per unit area in Northern European agriculture, which is important to meet the demand for biomass in a growing bioeconomy. Potential benefits for the carbon footprint will, however, depend on greenhouse gas (GHG) emissions during the entire biomass production cycle, which includes grassland renovation at regular intervals to maintain high biomass yields. The renovation phase may accelerate nitrous oxide (N2O) emissions associated with residue decomposition. This study examined the effect of renovating a six-year old festulolium (x Festulolium braunii L.) crop on N2O emissions. As a secondary objective, the study evaluated the potential for mitigating N2O emissions in spring by spraying the sward with a nitrification inhibitor containing 3,4-dimethylpyrazole phosphate (DMPP) prior to cultivation. A replicated split-plot design was used, where one half of each main plot was rotovated and seeded with spring barley (Hordeum vulgare L.) as a catch crop during spring, followed by ploughing and reseeding of festulolium in the autumn. In the other half of each main plot, festulolium was left without cultivation as reference. Four subplots were defined within both cultivated plots and reference plots with (" + DMPP") or without (" - DMPP") DMPP spraying of festulolium before cultivation, and with ("F") or without ("NF") fertilisation with 119, 425 and 50 kg N ha(-1) in spring barley, festulolium and re-established festulolium, respectively. All four combinations of DMPP treatment and fertilisation were represented, i.e., F + DMPP, F-DMPP, NF + DMPP and NF-DMPP. Monitoring of N2O emissions occurred in two periods, April-June ("spring") and August-October ("autumn"). In the autumn, where festulolium was reestablished, N2O emissions were only monitored in the plots without DMPP treatment in spring, since potential legacy effects of DMPP were not part of this study. Cultivation increased N2O emissions 2.5-fold in spring, and 2-fold in autumn, compared to uncultivated plots. The N2O emissions induced by fertilisation were similar from cultivated and reference plots, and emission factors for spring barley (Apr-Jun), re-sown festulolium (Aug-Oct) and uncultivated festulolium (reference, Apr-Oct) during the monitoring periods were, respectively, 0.40, 0.42 and 0.12%. Spraying festulolium with DMPP delayed the transformation of ammonium to nitrate during spring. DMPP did not reduce N2O emissions significantly in this study. In contrast, there was an apparent interaction between decomposing residues and mineral fertiliser with respect to emissions of N2O, which is a potential GHG mitigation target.