Comparison between APSIM and NZ-DNDC models when describing N-dynamics under urine patches

Nitrous oxide (N ₂O) emissions from soil are the result of complex interactions between physical, chemical and biological processes. We compared two process-based models (APSIM and NZ-DNDC) with measurements of N ₂O emissions, soil and content (0–75 mm) and water-filled pore space from a series of field campaigns where known amounts of animal urine-N were applied to four soil types under permanent pastures, in two regions within New Zealand, at different times of the year. We also compared cumulative N ₂O emissions with an N ₂O inventory emission factor approach (EF ₃ method). Overall, the two process-based models performed less well than the EF ₃ method for simulating cumulative N ₂O emissions over the complete data set. However, in winter, the APSIM model correlated well with measurements (r = 0.97), while NZ-DNDC performed well on the Otago soils (r = 0.83 and 0.92 for Wingatui and Otokia, respectively). The process-based models have the potential to account for the effect of weather conditions and soil type on N ₂O emissions that are not accounted for by the EF ₃ method. However, further improvements are currently needed. The fractions of N lost to different processes within the complex soil–plant atmosphere system differed between the two models. The size of the predicted plant uptake, leaching and NH ₃ volatilisation fluxes are large compared with N ₂O emissions and could affect the simulated soil N pools and thus the predicted N ₂O fluxes. To simulate N ₂O fluxes accurately, it is therefore important to ensure these processes are well modelled and further validation studies are needed.