Source partitioning using N₂O isotopomers and soil WFPS to establish dominant N₂O production pathways from different...

Nitrous oxide (N₂O) is a potent greenhouse gas (GHG) emitted from agricultural soils and is influenced by nitrogen (N) fertiliser management and weather and soil conditions. Source partitioning N₂O emissions related to management practices and soil conditions could suggest effective mitigation strategies. Multispecies swards can maintain herbage yields at reduced N fertiliser rates compared to grass monocultures and may reduce N losses to the wider environment. A restricted-simplex centroid experiment was used to measure daily N₂O fluxes and associated isotopomers from eight experimental plots (7.8 m²) post a urea-N fertiliser application (40 kg N ha^-1). Experimental pastures consisted of differing proportions of grass, legume and forage herb represented by perennial ryegrass (Lolium perenne), white clover (Trifolium repens) and ribwort plantain (Plantago lanceolata), respectively. N₂O isotopomers were measured using a cavity ring down spectroscopy (CRDS) instrument adapted with a small sample isotope module (SSIM) for the analysis of gas samples ≤20 mL. Site preference (SP = δ¹⁵N^α - δ¹⁵N^β) and δ¹⁵N^bulk ((δ¹⁵N^α + δ¹⁵N^β) / 2) values were used to attribute N₂O production to nitrification, denitrification or a mixture of both nitrification and denitrification over a range of soil WFPS (%). Daily N₂O fluxes ranged from 8.26 to 86.86 g N₂O-N ha^-1 d^-1. Overall, 34.2% of daily N₂O fluxes were attributed to nitrification, 29.0% to denitrification and 36.8% to a mixture of both. A significant diversity effect of white clover and ribwort plantain on predicted SP and δ¹⁵N^bulk indicated that the inclusion of ribwort plantain may decrease N₂O emission through biological nitrification inhibition under drier soil conditions (31%-75% WFPS). Likewise, a sharp decline in predicted SP indicates that increased white clover content could increase N₂O emissions associated with denitrification under elevated soil moisture conditions (43%-77% WFPS). Biological nitrification inhibition from ribwort plantain inclusion in grassland swards and management of N fertiliser source and application timing to match soil moisture conditions could be useful N₂O mitigation strategies.