Development of a lucerne model in APSIM next generation: 4 N dynamics and forage quality of genotypes with different fall dormancies

Nitrogen (N), and metabolizable energy (ME) content are important characteristics of forage crops. This paper describes the development of mechanisms in the Agricultural Production Systems sIMulator (APSIM) next generation (APSIM NextGen) lucerne model to predict how these characteristics change seasonally in response to genotype and defoliation management. The supply of N for lucerne growth comes from a mixture of uptake of mineral N in the soil, remobilization of stored N from the taproots and crown and fixation of atmospheric N in the root nodules. In this model, N fixation was simulated as the amount required by growing organs less soil N uptake up to a potential fixation of calculated as 2% of daily biomass assimilation. Nitrogen supply is then allocated in proportion to the relative demand of each organ estimated from potential organ growth and empirical N dilution curves. Specifically, leaf and stem N concentrations decreased as a function of increasing organ biomass. Root N demand is zero while N is being remobilized and reaches its maximum during periods when storage is occurring. Once the structure of the model had been assembled, a model optimization approach was used to fit required parameters. The model showed overall fair (Nash-Sutcliffe efficiency (NSE) ranged from 0.11 to 0.27) estimates of lucerne seasonal N dynamics and mean observed values of N concentration and content among each organ. However, fair to poor predictions of these variables were found within individual defoliation events across the different defoliation treatments for each FD class. Simulation accuracy for leaf and stem crude protein (CP) and ME showed no difference among the three FD classes with fair to good agreement (NSE ranged from 0.19 to 0.68) across all defoliation treatments and FD classes. The APSIM NextGen lucerne model provided a mechanistic framework to model N dynamics among leaf, stem and root, root maintenance respiration, N remobilization in spring, partitioning in autumn, and leaf and stem quality. More information is needed to account changes in N concentration and content among organs associated with shoot population decay patterns caused by different defoliation regimes and FD class combinations.