A potato model built using the APSIM Plant.net Framework

Abstract

The APSIM potato model is a comprehensive daily time step, deterministic potato crop model build in the APSIM Plant.net Framework which integrates with the APSIM soil, management and user interface components to provide a robust and user friendly potato crop model. This paper provides a brief description of the potato models mechanisms with a particular focus on the arbitrator module that was developed to enable explicit linkages between the partitioning of dry matter (DM) and nitrogen (N). Potential DM production is estimated from the product of solar radiation, radiation interception, radiation use efficiency and stress factors (water, temperature and CO 2). Radiation interception is calculated from predictions of leaf area index (LAI) and an extinction coefficient. The estimation of LAI is created using a phytomer type canopy model that uses inputs of tuber planting density and the number of stems per tuber to determine the population density of primary stem units. It then predicts the appearance, expansion, size and duration of individual leaves on primary stems and the occurrence of branching to give a canopy model that can explicitly account for the effects of planting arrangement and seed tuber characteristics on the development of the crops canopy. One of the most important elements of Plant.NET models is the arbitrator which determines how much of potential DM production can be assimilated and the partitioning of DM and N between competing organs. An alternative to the existing arbitrator module was written for the potato model, based on the concepts of partitioning used in the SIRIUS wheat model. This arbitrator may be used for other crop models built in Plant.NET. The new arbitrator gets each organs DM supplies and demands and then allocates potential DM accordingly. If DM supply is greater than demand it will partition excess to sink organs and if DM supply is less than demand it will remobilize non-structural DM from organs that have labile DM components. Potential DM allocations and accumulated N deficit are used to calculate each organs N demand and then the arbitrator steps through a series of N allocation routines. The first is the reallocation of N from senescing organs, followed by the allocation of N available for uptake from the soil and the last is the remobilization of N from non-structural pools in organs. Once N allocation is set the arbitrator then determines if this is enough to assimilate the potential DM allocation and if not DM allocation is reduced so each organ maintains it minimum N concentration. Testing and parameterization is ongoing but with current progress the model has been tested against a number of datasets and has performed well. The most extensive testing has occurred on N uptake and response. The model gives realistic reproduction of the effects of N shortage on both tuber yield and the time course of tuber DM accumulation. In particular it predicts substantial plasticity in the N concentration of tubers such that large differences in tuber N uptake have little impact on tuber growth. It predicts that stem growth will be the first thing to stop under N shortage and the plant will draw down stem N concentrations to maintain leaf area and tuber growth. Leaf growth is next to be reduced to ensure tuber N supply is maintained and finally tuber growth will slow if N shortage persists long enough for tubers to fall to their minimum N concentration. Having a comprehensive potato model operating in the APSIM platform will be valuable to a wide range of model users interested in the physiology and agronomy of potato. The model is currently available to advanced users and will be available in general release, once all release criteria have been satisfied.