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 N2O from intensively managed pastures, produced via multiple, simultaneously occurring pathways. These N2O pulses can account for a large fraction of total N2O losses, demonstrating the importance to determine magnitude and source partitioning of N2O under these conditions. This study investigated the response of different pathways of N2O production to wetting across three different textured pasture soils. Soil microcosms were fertilised with an ammonium nitrate (NH4NO3) solution which was either single or double N-15 labelled, wetted to four different water-filled pore space (WFPS) levels, and incubated over two days. The use of a N-15 pool mixing model together with soil N gross transformations enabled the attribution of N2O to specific pathways, and to express N2O emissions as a fraction of the underlying N transformation. Denitrification and nitrification mediated pathways contributed to the production of N2O in all soils, regardless of WFPS. Denitrification was the main pathway of N2O production accounting for >50% of cumulative N2O emissions even at low WFPS. The contribution of autotrophic nitrification to N2O 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 N2O, increasing exponentially with WFPS, while the percentage of denitrified N emitted as N2O decreased, providing critical information for the representation of N2O/WFPS relationships in simulation models. Our findings demonstrate that the wetting of pasture soils promotes N2O 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 N2O emissions should be considered when developing N2O abatement strategies, seeking to reduce N2O emissions in response to rainfall and irrigation from intensively managed pastures.

Automated summary

The study highlights that rainfall and irrigation can lead to large pulses of N2O emissions from intensively managed pastures, emphasizing the need to determine the magnitude and sources of N2O under such conditions. Different pathways of N2O production respond differently to wetting, with denitrification being the main pathway even at low water-filled pore space levels. Heterotrophic nitrification plays a significant role in N2O emissions, especially as wetting increases.