Allocation of Nitrogen
Allocation of nitrogen to different components of the plant follows to a large extent the patterns for carbon. At emergence the carbon contents in the stem, leaf and root pools are divided by the parameterised CN ratios cnMinRoot, cnMinStem and cnMinLeaf to determine the nitrogen content before the assimilation starts.
As the plant starts to grow the carbon assimilation of the plant generates a nitrogen demand in the plant according to the parameterised CN ratio (see Root uptake demand), which acts as a driving force for uptake of nitrogen from the soil (see Root uptake of mineral nitrogen in Mineral N Processes and Root uptake of organic nitrogen in Soil Organic Processes). This uptake is transferred to a mobile nitrogen storage pool, Na. From this pool the nitrogen is allocated first to roots, secondly, if any nitrogen remains in the mobile pool, to the stem, and finally also to the leaves:
Allocation to the grain pool during grain development is analogous to the carbon allocation, eq.(5.56). In order to calculate the amounts of nitrogen allocated to grain, NRoot→Grain, NLeaf→Grain and NStem→Grain, the parameters aC,rg, aC,lg and aC,sg are therefore exchanged to aN,rg, aN,lg and aN,sg respectively.
The allocation of nitrogen at harvest is handled similarly to carbon using the same equation, i.e. eq.(5.57).
Nitrogen litter fall is analogous to carbon litter fall (see eqs. (5.59) -(5.64)) and allocation to and from the mobile pool is also analogous to carbon allocation (see eqs. (5.65) and (5.66)).
Every run the CN ratios for the leaf, stem, grain and root pools are calculated from the amounts of carbon and nitrogen in each pool. In perennial plants the CN ratios are based on the amounts of carbon and nitrogen in the new and the old pools. If the nitrogen content is less than 0.1 g the CN ratio for that pool is automatically set to 20. CN ratios are used to estimate nitrogen transfer when correspondent carbon transfers or carbon storages are known.