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 (a_d) 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 a_d declined after fertiliser application. Average N₂ fluxes ranged from <30 to 3300 g N₂-N ha^-1 d^-1, although many fluxes were lower, not exceeding 400 g N₂-N ha^-1 d^-1 in multiple seasons. Most N₂ flux measurements occurred within 100 days after applying fertiliser, however in some seasons split fertiliser applications lengthened the measurement period.

Experiences from the sampling campaigns highlighted the challenges of applying the ¹⁵N gas flux method across diverse systems and environments. Nonetheless, the aggregated data set represents a significant contribution to in situ denitrification measurements, supporting both direct quantification of seasonal gaseous N losses, and incorporation into modelling approaches for estimating denitrification at wider scales.