Benefits from enhanced-efficiency nitrogen fertilisers in rainfed temperate pastures are seasonally driven

Context Enhanced-efficiency nitrogen (N) fertilisers (EEFs) can reduce N loss, but productivity and environmental benefits vary. A field experiment on rainfed ryegrass dominant pastures in southern Australia examined the effect of EEFs combined with reduced N rate on agronomic N efficiency (AE) (additional biomass per unit N applied) and apparent NUE (ApNUE) (additional plant N uptake per unit N applied) relative to urea. Methods EEFs examined were granular urea coated with (i) urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT), (ii) nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) or (iii) polymer coating (PCU). Nitrogen was applied five times over 9months at 0–84kgNha−1 per application. Key results AE varied with season from –0.5to 22.3kgDMkgN−1 in late (dry conditions) and early spring (ideal conditions), respectively. AE of PCU was significantly lower than urea for the first harvest due to slow N release. Using DMPP led to higher AE than equivalent urea rates in August (at 34 and 50kgNha−1) and early October (34kgNha−1). At the lowest rate (17kgNha−1), NBPT generally led to higher AE than at higher N rates. Using NBPT at 17kgNha−1 led to higher AE than urea at the same rate in July, August and February. The ApNUE ranged from 3.6% (summer) to 68% (early spring), and decreased with increasing N rate. Using DMPP did not significantly change cumulative ApNUE (entire experiment) (34%) due to limited leaching and denitrifying conditions. Cumulative ApNUE was lowest with PCU (23%) due to initial slow N release. Conclusion Seasonal variations in pasture responses to fertiliser N affected the efficiency outcomes from using EEFs. Implications Benefits in ApNUE and AE from using EEFs rely on their seasonal strategic use combined with reduced N inputs. The initial slow release of N from the PCU could be overcome by strategies such as combining the PCU with uncoated urea, to improve ApNUE.