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

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.