Nitrogen stress and assimilation reduction

When the N concentration in aboveground biomass falls below the critical N (depending on crop growth and nitrogen uptake), nitrogen stress is accounted for. Its value can range from 0 to 1, where 0 is the maximum stress the crop can sense, when N concentration reaches its minimum value (set at 0.54 * critical N for rice). In the range between minimum and critical concentration, N stress is defined as a linear function of N concentration in aboveground biomass:

Since -at least for rice- the main source of energy and nutrients up to the second fully expanded leaf comes from seed reserves (Stansel, 1975; Hoshikawa, 1993), no nitrogen stress is simulated before the crop has reached this stage. At this stage, crop N concentration is initialized equal to the critical concentration.

The N stress index is used as a multiplier to reduce daily gross assimilation rate.

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