Ploughing a poorly drained grassland reduced N2O 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 N2O during landuse change. Two adjacent grassland plots, one amended with 100 m3 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 (3.0 g N m−2), amended spring-FIT (2.0 g N m−2), amended autumn-FIT and unamended chemical fallow (1.7 g N m−2, both), unamended spring-FIT (1.2 g N m−2) and the unamended autumn-FIT subplots (0.9 g N m−2) while grassland controls emitted 0.07 and 0.09 g N m−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 45 cm 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.