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

Abstract

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.

Keywords

• nitrous oxide
• green house gas emissions
• climate change
• denitrification
• nitrification
• pastures