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

• Published in: Biology and fertility of soils
• Main Authors: Kirkby, Robert, Friedl, Johannes, De Rosa, Daniele, Clough, Timothy J., Rowlings, David W., Grace, Peter R.
• Format: Journal Article
• Language: English
• Published: 03-01-2024
• ISSN: 0178-2762; 1432-0789
• DOI: 10.1007/s00374-023-01783-9

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.