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. ... mehrThe 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.