Grain Nitrogen Filling Rate or Nitrogen Demand (Wheat)
The wheat nitrogen filling rate or nitrogen demand of grain is calculated during the grain filling period, specifically between the phenological stages of Anthesis and EndGrainFill. During this period, grain is the only organ that requires nitrogen for development as other organs (e.g., leaves, stems and roots) have already completed their growth and are not actively accumulating nitrogen for grain development.
Wheat grain nitrogen demand is only for structural nitrogen, which is not the same as other organs including metabolic and storage nitrogen (e.g., leaves, stems, and roots).
Overview
The demand is defined by a PhaseLookup function (NFillingRate):
- Before Anthesis (Sowing to Anthesis) as zero for no grain nitrogen demand as no grain development occurs.
- During Grain Filling (Anthesis → EndGrainFill):
\[ \text{N demand} = \frac{N_\text{kernel}}{D_\text{filling}} \times \text{ThermalTime} \times \text{GrainNumber} \]
Where:
| Term | Description |
|---|---|
| \(N_\text{kernel}\) | Potential nitrogen per kernel (default = 0.0011 g/kernel) |
| \(D_\text{filling}\) | Grain filling duration from anthesis to end of filling (°Cd) = GrainExpanding.Target + GrainFilling.Target |
| ThermalTime | Daily thermal time (°Cd), from [Phenology].ThermalTime |
| GrainNumber | Number of kernels, from [Grain].NumberFunction |
The equation above is used because it distributes the total potential nitrogen content per kernel evenly across the grain filling period, scaled by the daily thermal time and the number of grains. This approach ensures that nitrogen demand reflects both the developmental stage (timed by thermal accumulation) and the actual sink size (number of kernels), providing a biologically realistic and process-driven estimate of daily grain nitrogen requirements.
A linear nitrogen filling rate is assumed, meaning that the demand increases uniformly from anthesis to the end of grain filling. This is a simplification but is commonly used in crop models to represent nitrogen uptake during grain development.
Cultivar-Specific Parameters
| Name | Description | Default Value |
|---|---|---|
| [Grain].NFillingRate.GrainGrowthPhase.Demand.Rate.PotentialKernalN.FixedValue | Potential nitrogen per kernel (g/kernel) | 0.0011 |
Practical Example
Suppose:
- Nitrogen per kernel (\(N_\text{kernel}\)) = 0.0011 g/kernel
- Grain filling duration (\(D_\text{filling}\)) = 120 + 545 = 665 °Cd
- Daily thermal time (ThermalTime) = 20 °Cd
- Number of kernels (GrainNumber) = 18,000 per m² (typical range: 15,000–22,000 per m²)
Plug into the formula:
\[ \text{N demand} = \frac{0.0011}{665} \times 20 \times 18000 = 0.5955 \text{ g N/day} \]
So, the daily nitrogen demand for grain is 0.5955 g N/day under these conditions.