The new CoupModel release introduces a feature to scale plant nitrogen demand (N-demand). In the standard model, nitrogen demand is proportional to the carbon input to the plant, determined by photosynthesis and allocation functions. This new feature allows users to reduce nitrogen demand by applying scaling coefficients to different plant compartments.
This means that the N demand is propostional to the C input to the plants as determined by the photosynthesis and the allocation functions. To reduce the demand for N to the plant the new swith will allow the user to scale the demand to lower values by new coefficients related to the different components. The new parameters are called CN Scale Root, CN Scale Stem and CN Scale Leaf all with a range from 1 to 4, where 1 means the the minimum CN ratio will remain as the unscaled parameter values. Default value are suggested to be 1.5, 2.0 and 2.0 for root, stem and Leaf.
The scaling becomes active when the switch is turned on and the GrowthStageIndex exceeds 2 (start of grain filling). Users can now choose when and how the scaling should take effect:
If the On ad GSI 3 is chosen the demand will be a step function when GrowthStageIndex exceed 3. The other 2 option will make a linear interpolation in a range from 2 to 3 ( Grain filling completed) or from 2 to 4 (ready for harvest).
An example of the impact of this feature is shown in the figure. The green line represents the default model behavior (switch off), and the blue line represents simulations using the linear 2 → 4 option. This example is based on a simulation by David Nimblad Svensson (SLU), who also suggested this improvement.
Note: Nitrogen demand may differ from previous versions even when the switch is off. This is due to a bug in previous versions where the value for CN ratio min Leaf was mixed with parameter values associated with CN LOpt, CN LTh, and FixN supply. With the previous default value for FixN supply (0.6), nitrogen demand was underestimated compared to the corrected model in the current release.