Source partitioning using NO isotopomers and soil WFPS to establish dominant NO production pathways from different pasture sward compositions.

Nitrous oxide (NO) 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 NO 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 NO fluxes and associated isotopomers from eight experimental plots (7.8 m) post a urea-N fertiliser application (40 kg N ha). 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. NO 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 = δN - δN) and δN ((δN + δN) / 2) values were used to attribute NO production to nitrification, denitrification or a mixture of both nitrification and denitrification over a range of soil WFPS (%). Daily NO fluxes ranged from 8.26 to 86.86 g NO-N ha d. Overall, 34.2% of daily NO 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 δN indicated that the inclusion of ribwort plantain may decrease NO 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 NO 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 NO mitigation strategies.