Source partitioning using N 2 O isotopomers and soil WFPS to establish dominant N 2 O production pathways from different pasture sward compositions.

Nitrous oxide (N ₂ O) 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 N ₂ O 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 N ₂ O fluxes and associated isotopomers from eight experimental plots (7.8 m ² ) 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. N ₂ O 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 = δ ¹⁵ N ^α - δ ¹⁵ N ^β ) and δ ¹⁵ N ^bulk ((δ ¹⁵ N ^α + δ ¹⁵ N ^β ) / 2) values were used to attribute N ₂ O production to nitrification, denitrification or a mixture of both nitrification and denitrification over a range of soil WFPS (%). Daily N ₂ O fluxes ranged from 8.26 to 86.86 g N ₂ O-N ha ^-1 d ^-1 . Overall, 34.2% of daily N ₂ O 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 δ ¹⁵ N ^bulk indicated that the inclusion of ribwort plantain may decrease N ₂ O 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 N ₂ O 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 N ₂ O mitigation strategies.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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