Sources of nitrous oxide from intensively managed pasture soils: the hole in the pipe
Johannes Friedl, Clemens Scheer, Daniele De Rosa, Christoph Müller, Peter Grace, David Rowlings
Abstract
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.