Investigating a method for estimating direct nitrous oxide emissions from grazed pasture soils in New Zealand using NZ-DNDC

• The NZ-DNDC model was used to create look-up tables of direct N2O emission factors (EFs).
• EFs were calculated by soil type and climate zone for New Zealand grazed pastures.
• The direct N2O emission from NZ grazed pastures in 2010 was 12.7 ± 12.1 Gg N2O–N.
• EFs were found to be highly sensitive to uncertainty in the soil properties.
• The high uncertainties were largely due to uncertainties in the soil properties.

In this study, we developed emission factor (EF) look-up tables for calculating the direct nitrous oxide (N2O) emissions from grazed pasture soils in New Zealand. Look-up tables of long-term average direct emission factors (and their associated uncertainties) were generated using multiple simulations of the NZ-DNDC model over a representative range of major soil, climate and management conditions occurring in New Zealand using 20 years of climate data. These EFs were then combined with national activity data maps to estimate direct N2O emissions from grazed pasture in New Zealand using 2010 activity data. The total direct N2O emissions using look-up tables were 12.7 ± 12.1 Gg N2O–N (equivalent to using a national average EF of 0.70 ± 0.67%). This agreed with the amount calculated using the New Zealand specific EFs (95% confidence interval 7.7–23.1 Gg N2O–N), although the relative uncertainty increased. The high uncertainties in the look-up table EFs were primarily due to the high uncertainty of the soil parameters within the selected soil categories. Uncertainty analyses revealed that the uncertainty in soil parameters contributed much more to the uncertainty in N2O emissions than the inter-annual weather variability. The effect of changes to fertiliser applications was also examined and it was found that for fertiliser application rates of 0–50 kg N/ha for sheep and beef and 60–240 kg N/ha for dairy the modelled EF was within ± 10% of the value simulated using annual fertiliser application rates of 15 kg N/ha and 140 kg N/ha respectively.