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

Abstract

Acid-sulphate sugarcane soils in the subtropics are known hot-spots for nitrous oxide (N 2 O) emissions, yet the reduction of reactive N 2 O to non-reactive dinitrogen (N 2 ) via specific pathways remains a major uncertainty for nitrogen (N) cycling and loss from these soils. This study investigated the magnitude and the N 2 O:N 2 partitioning of N 2 O and N 2 losses from a subtropical acid-sulphate soil under sugarcane production using the 15 N gas flux method, establishing the contribution of hybrid (co- and chemo-denitrification) and heterotrophic denitrification to N 2 O and N 2 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 15 N 2 O and 15 N 2 at sub-diel resolution. Peak losses of N 2 O and N 2 reached 6.5 kg N ha −1 day −1 , totalling > 50 kg of N 2 O+N 2 -N ha −1 . Emissions were dominated by N 2 , accounting for more than 57% of N 2 O+N 2 losses, demonstrating that the reduction of N 2 O to N 2 proceeded even under highly acidic conditions. Over 40% of N 2 O, but only 2% of N 2 emissions, were produced via hybrid pathways. These findings demonstrate hybrid pathways are generally limited to N 2 O 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 N 2 O emissions demonstrates the environmental, but also the potential agronomic significance, of hybrid pathways of N 2 O formation for N loss from fertilised acid-sulphate soils.

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