Ploughing a poorly drained grassland reduced N sub(2)O emissions compared to chemical fallow

Managed grasslands occupy a large portion of the agricultural landbase, are rich in C and N and therefore represent a risk for emissions of N sub(2)O during landuse change. Two adjacent grassland plots, one amended with 100 m super(3) ha super(-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 N sub(2)O over the measuring period (static chambers), soil solution NO sub(3)-N (tension lysimeters) and soil NO sub(3)-N (KCl extraction) were monitored following tillage for one fallow year. Emissions of N sub(2)O decreased in the order: amended chemical fallow (3.0 g N m super(-2)), amended spring-FIT (2.0 g N m super(-2)), amended autumn-FIT and unamended chemical fallow (1.7 g N m super(-2), both), unamended spring-FIT (1.2 g N m super(-2)) and the unamended autumn-FIT subplots (0.9 g N m super(-2)) while grassland controls emitted 0.07 and 0.09 g N m super(-2) for the unamended and amended plots, respectively. Emissions of N sub(2)O were positively correlated with soil solution NO sub(3)-N at 30-cm depth in the chemical fallow, but negatively with soil solution NO sub(3)-N at 45 cm in FIT subplots. In chemical fallow soils, N sub(2)O was likely produced as the NO sub(3) super(-) 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 N sub(2)was likely favoured. On these poorly drained grassland soils, FIT reduced emissions of N sub(2)O relative to a chemical fallow by a factor of 2 to 3 for equivalent soil NO sub(3)-N. Emission factors accounting for the interaction between soil characteristics - in this case drainage conditions - and management practices may be important in providing accurate N sub(2)O emission estimates.