Root uptake of mineral nitrogen
Root uptake of nitrate and ammonium is driven by the plant demand, NDemand. The plant will take up nitrate and ammonium according to their relative abundance in the soil determined by a ratio, rN, defined as:
(6.31)
The primary plant uptake of nitrate is calculated as:
where fNUpt is a parameter and ∆z is the layer thickness.
The primary plant uptake of ammonium is calculated similarly:
If the total uptake of nitrogen is less than the plant demand, a secondary compensatory uptake due to flexible roots takes place. The nitrogen demand of the second uptake is governed by the parameter nUptflex, which is the flexibility degree of the roots:
where NTotUpt is the total mineral nitrogen uptake, i.e. primary nitrate and ammonium uptake.
The secondary uptake of nitrate is calculated as:
where rcompfact is the compensatory root factor i.e. the ratio of the total root system and the fraction of the total root system where a surplus occurred during the primary uptake.
Similarly the secondary uptake of ammonium is calculated as:
The total flows of nitrate and ammonium to the plant roots are the sums of the primary and the secondary root uptake for nitrate and ammonium respectively.
If the demand of nitrogen is not satisfied by the uptake of mineral nitrogen, an uptake of organic nitrogen can optionally be included in the simulation (see Root uptake of organic nitrogen). However, if organic nitrogen uptake is not simulated, any nitrogen that has been taken up by the leaf directly from atmospheric deposition (see section External inputs), is also finally added to the total plant nitrogen uptake.