Sources of nitrous oxide from intensively managed pasture soils: the hole in the pipe

• Published in: Environmental research letters Vol. 16; no. 6; pp. 65004 - 65017
• Main Authors: Friedl, Johannes, Scheer, Clemens, De Rosa, Daniele, Müller, Christoph, Grace, Peter R, Rowlings, David W
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01-06-2021
• Subjects: Ammonium; Ammonium nitrate; climate change; Denitrification; Emissions; green house gas emissions; Greenhouse gases; Irrigation; Irrigation pipes; Microcosms; Nitrification; Nitrous oxide; Oxidation; Pasture; Pasture management; pastures; Rainfall; Soil fertility; Soil management; Soils; Substrates; Wetting
• ISSN: 1748-9326; 1748-9326
• DOI: 10.1088/1748-9326/abfde7

Rainfall and irrigation trigger large pulses of the powerful greenhouse gas N
2
O from intensively managed pastures, produced via multiple, simultaneously occurring pathways. These N
2
O pulses can account for a large fraction of total N
2
O losses, demonstrating the importance to determine magnitude and source partitioning of N
2
O under these conditions. This study investigated the response of different pathways of N
2
O production to wetting across three different textured pasture soils. Soil microcosms were fertilised with an ammonium nitrate (NH
4
NO
3
) solution which was either single or double
15
N labelled, wetted to four different water-filled pore space (WFPS) levels, and incubated over two days. The use of a
15
N pool mixing model together with soil N gross transformations enabled the attribution of N
2
O to specific pathways, and to express N
2
O emissions as a fraction of the underlying N transformation. Denitrification and nitrification mediated pathways contributed to the production of N
2
O in all soils, regardless of WFPS. Denitrification was the main pathway of N
2
O production accounting for >50% of cumulative N
2
O emissions even at low WFPS. The contribution of autotrophic nitrification to N
2
O emissions decreased with the amount of wetting, while the contribution of heterotrophic nitrification remained stable or increased. Following the hole-in-the-pipe model, 0.1%–4% of nitrified N was lost as N
2
O, increasing exponentially with WFPS, while the percentage of denitrified N emitted as N
2
O decreased, providing critical information for the representation of N
2
O/WFPS relationships in simulation models. Our findings demonstrate that the wetting of pasture soils promotes N
2
O production via denitrification and via the oxidation of organic N substrates driven by high carbon and N availability upon wetting. The large contribution of heterotrophic nitrification to N
2
O emissions should be considered when developing N
2
O abatement strategies, seeking to reduce N
2
O emissions in response to rainfall and irrigation from intensively managed pastures.