Modelling biomass, soil water content and mineral nitrogen in dairy pastures: a comparison of DairyMod and APSIM

Realistic simulation of soil nitrogen cycling is important for quantifying nitrogen loss pathways to the environment, as well as the influence of N on pasture productivity. Although several models have been evaluated for their ability to simulate pasture growth, few studies have compared the models APSIM and DairyMod. Here, our objectives were to examine the capability of each model in simulating field measurements of pasture biomass, soil water content, mineral nitrogen and N₂O emissions. For site one, DairyMod generally simulated mineral N, cumulative N₂O and soil water with lower residual error than that from APSIM, but APSIM produced better estimates of pasture biomass. At site two, DairyMod produced more precise estimates of mineral N, but APSIM simulations were more reliable in terms of cumulative N₂O. Overall this study demonstrated that both models produced satisfactory estimates of pasture biomass and soil water dynamics, but further research is necessary to diagnose reasons for the sometimes large discrepancies between simulated and measured mineral N and cumulative N₂O emissions. Part of this discrepancy is likely to be caused by heterogeneity of soil N in the field, spatially and temporally. Although both models produce temporal estimates of mineral N and N₂O, quantification of parameter uncertainty associated with spatial variation in mineral N would help improve model evaluation such as performed in this study.