Informing APSIM using 15N recovery data to establish fertiliser N budgets in grain systems

Abstract Simulation models are an important tool to predict how farming practices influence utilisation and loss of nitrogen (N). However, many simulation exercises lack sufficient validation of N dynamics from both soil and fertiliser sources and rely on single or a few measurable N pools, potentially shifting bias from one pool to another. This study evaluated the capacity of Agricultural Production Systems sIMulator (APSIM) to simulate N fertiliser budgets in dryland sorghum and wheat systems of eastern Australia using an extensive 15 N dataset from 18 field trials with up to four N rates. Key metrics included 15 N fertiliser recoveries at harvest in soil and plant, and 15 N fertiliser losses. Fertiliser N in APSIM was calculated as the balance between simulated N in the non-fertilised control and the respective applied N rate, and compared to relevant 15 N data. APSIM’s performance for fertiliser N budget simulation was tested with (i) conventional calibration fitting plant N uptake and (ii) improved calibration including the denitrification and mineralisation parameters to fit fertiliser N loss. Simulations with conventional calibration showed good agreement with phenology, soil water and plant N uptake data but largely underestimated the measured fertiliser N loss (RMSE: 23 kg N ha −1 , PBIAS: − 75%). The simulated fertiliser N loss with improved calibration agreed better with the observation (RMSE: 18 kg N ha −1 , PBIAS: -34%). The identified inconsistencies in the default model’s representation of N cycling highlight opportunities for combined efforts of experimental research and model development to improve the reliability and utility of the model.