Improving crop nitrogen uptake prediction in OverseerFM through iterative soil-crop coupling and recalibration

Abstract

OverseerFM is widely used for farm nutrient budgeting in New Zealand. Earlier versions under-predicted crop
nitrogen (N) uptake for some crops under moderate to high soil N supply, leaving more residual N in the simulated soil profile and limiting the model’s ability to represent the environmental benefits of catch crops and other cropping systems. To address this, field data from 37 experiments across a range of crop types and regions were used to recalibrate crop parameters. Two updates were introduced: a luxury N uptake factor to represent additional uptake when N supply exceeded baseline requirements, and an iterative monthly coupling between crop uptake and soil mineral N supply. Together, these changes reduced under-prediction of above-ground N uptake and improved agreement with
observations across diverse crops, including reduced prediction error in an independent validation dataset. Soil-mineral-N prediction error also decreased in one rotation dataset, although substantial discrepancies remained. The revised model therefore provides a stronger basis for representing N capture by catch crops, while winter-period uptake and nitrate leaching require further validation.