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Managed grasslands occupy a large portion of the agricultural landbase, are rich in C and N and therefore represent a risk for emissions of N₂O during landuse change. Two adjacent grassland plots, one amended with 100m³ ha⁻¹ of liquid swine manure annually since 1978 and an unamended grassland were either (i) left with vegetation intact (Control) or killed by glyphosate in the autumn. Glyphosate-treated subplots were either (ii) left as an undisturbed chemical fallow, (iii) ploughed by full inversion tillage (FIT) in the autumn, or (iv) in the spring. Cumulative emissions of N₂O over the measuring period (static chambers), soil solution NO₃-N (tension lysimeters) and soil NO₃-N (KCl extraction) were monitored following tillage for one fallow year. Emissions of N₂O decreased in the order: amended chemical fallow (3.0gNm⁻²), amended spring-FIT (2.0gNm⁻²), amended autumn-FIT and unamended chemical fallow (1.7gNm⁻², both), unamended spring-FIT (1.2gNm⁻²) and the unamended autumn-FIT subplots (0.9gNm⁻²) while grassland controls emitted 0.07 and 0.09gNm⁻² for the unamended and amended plots, respectively. Emissions of N₂O were positively correlated with soil solution NO₃-N at 30-cm depth in the chemical fallow, but negatively with soil solution NO₃-N at 45cm in FIT subplots. In chemical fallow soils, N₂O was likely produced as the NO₃ ⁻ mineralized at the soil surface moved down the soil profile, enriching denitrification sites at shallow depths during rainfall events. In contrast, FIT placed the C and N required for denitrification deeper in the soil profile, and complete reduction to N₂ was likely favoured. On these poorly drained grassland soils, FIT reduced emissions of N₂O relative to a chemical fallow by a factor of 2 to 3 for equivalent soil NO₃-N. Emission factors accounting for the interaction between soil characteristics - in this case drainage conditions - and management practices may be important in providing accurate N₂O emission estimates.
