Hybrid pathways of denitrification drive N2O but not N2 emissions from an acid-sulphate sugarcane soil

Acid-sulphate sugarcane soils in the subtropics are known hot-spots for nitrous oxide (N2O) emissions, yet the reduction of reactive N2O to non-reactive dinitrogen (N2) via specific pathways remains a major uncertainty for nitrogen (N) cycling and loss from these soils. This study investigated the magnitude and the N2O:N2 partitioning of N2O and N2 losses from a subtropical acid-sulphate soil under sugarcane production using the 15N gas flux method, establishing the contribution of hybrid (co- and chemo-denitrification) and heterotrophic denitrification to N2O and N2 losses. Soils were fertilised with potassium nitrate, equivalent to 25 and 50 kg N ha−1, watered close to saturation then incubated over 30 days. An innovative, fully automated incubation system coupled to an isotope-ratio mass-spectrometer enabled real time analysis of 15N2O and 15N2 at sub-diel resolution. Peak losses of N2O and N2 reached 6.5 kg N ha−1 day−1, totalling > 50 kg of N2O+N2-N ha−1. Emissions were dominated by N2, accounting for more than 57% of N2O+N2 losses, demonstrating that the reduction of N2O to N2 proceeded even under highly acidic conditions. Over 40% of N2O, but only 2% of N2 emissions, were produced via hybrid pathways. These findings demonstrate hybrid pathways are generally limited to N2O production, likely driven by high organic matter content and low soil pH, promoting both biotic, and abiotic nitrosation. Regardless of the underlying process, the magnitude of the N2O emissions demonstrates the environmental, but also the potential agronomic significance, of hybrid pathways of N2O formation for N loss from fertilised acid-sulphate soils.