Combining automated field gas sampling with 15N gas flux method: lessons from 8 site-seasons of measuring denitrification in Australian grains cropping

Measuring denitrification losses including both N2O and N2 emitted from soil remains one of the biggest challenges in closing nitrogen (N) budgets in cropping systems. Despite being considered a major N loss pathway, direct field measurements of N2 emissions are lacking. The 15N gas flux (15NGF) method is one of the few approaches to measuring in situ N2 fluxes, yet its application remains limited due to technical complexity and the need to meet key assumptions, all of which become more challenging under field conditions. Furthermore, infrequent sampling schedules may miss peak emission events, particularly when field access is limited after heavy rainfall. Here we present findings from applying the 15NGF method in 8 field campaigns over 2 seasons at 4 sites in Australian grains cropping systems.The field experiments covered summer and winter cropping systems under varying N fertiliser and irrigation treatments in subtropical to temperate climates, giving a range of environmental and management conditions. Gas samples were collected into evacuate vials using automated chamber systems consisting of 8 electronically actuated chambers connected to a control box containing a gas sampling manifold and injection actuator. Gas samples were analysed for N2O by gas chromatography and for 15N2O and 15N2 by isotope ratio mass spectrometry (IRMS).The method of calculating N2 flux by Spott et al. 2006 based on R29 and using the denitrifying pool enrichment estimated from N2O (ad) gave the greatest number of valid fluxes with >600 measurements passing quality control. The method detection limit (MDL) averaged 33 g N2-N ha-1 d-1 across all sites and seasons, but varied from 6.3 to 290 g N2-N ha-1 d-1 with IRMS precision and as ad declined after fertiliser application. Average N2 fluxes ranged from