Nitrogen source and inhibitors, effects on nitrous oxide emissions and pasture yield

The use of different nitrogen (N) fertilisers affects nitrous oxide (N₂O) emissions. To quantify effects of different N fertilisers on N₂O emissions and pasture yield, a one-year field experiment was carried out on a volcanic grassland soil. This study involved the split application of 200 kg N/ha using either urea (46%N) or calcium ammonium nitrate (CAN, 27%N), alone or in combination with dual N transformation inhibitors. The inhibitors used were the nitrification inhibitor Dicyandiamide (DCD), applied in autumn, and the urease inhibitor N-(n-Butyl) thiophosphoric triamide (NBPT), applied in spring. A control treatment with no N fertiliser was also included. Nitrous oxide emissions were quantified using the static chamber technique (n=5). The emission factor (EF1) was calculated as the N₂O-N net losses relative to N from fertiliser (%). Measurements of N₂O were complemented with pasture yield determinations. Results were analysed using ANOVA (P≤0.05).

Emissions of N₂O were low in the control (0.41±0.03 kg N-N₂O/ha) and increased 2.4 and 1.6 times following the application of urea and CAN, respectively (1.0±0.07 and 0.7±0.05 kg N₂O-N/ha for urea and CAN, respectively; P≤0.05). The use of inhibitors reduced N₂O emissions by 46% with the application of urea only (P≤0.05), resulting in a 64% reduction of EF1, from 0.28 to 0.10%. The EF1 was low with CAN (0.13%) and did not vary with inhibitors’ inclusion.

Pasture yield increased by 38% with N application (P≤0.05), from 5.8±0.56 (control) to 8.3±0.28 and 7.9±0.40 t DM/ha for the urea and CAN treatments, respectively. The use of inhibitors did not further increase yield, independently of the N source (P>0.05).

The use of CAN offers an opportunity to reduce N₂O emissions. The absence of yield benefits when using inhibitors in a combined strategy is a significant barrier for adoption of this mitigation option for pastoral systems.