A Simple Procedure to Model N Uptake Based on the Single Root Concept.
Søren Hansen, Royal Veterinary and Agricultural Univ, Dept of Agricultural Science, Agrovej 10, Dk-2630, Taastrup, Denmark
The nitrogen uptake model is based on the concept of a potential nitrogen demand simulated by a crop model, and the availability of nitrogen in the soil for plant uptake, i.e. the rate at which nitrogen can be made available at the root surfaces. Potential nitrogen demand: The transport of nitrogen from the bulk soil to the root surfaces is based on the following assumptions: i) each root exploits an average effective volume of soil which is assumed to a cylinder around each root; ii) the diameter of the considered cylinder corresponds to the average distance between roots as calculated by root length density; iii) nitrogen flux to the root surface by both mass flow and diffusion; iv) the concentration-distance profile develops in time in a stepwise manner and at each timestep (1 hour) it approximates to a steady state profile; v) the nitrogen concentration at the root surface act as a driving force and the plant can lower this concentration to a predefined concentration characterizing the plant; vi) this concentration is assumed to assume a common value along the entire root system; vii) soil layers with lower average nitrogen concentrations than the common nitrogen concentration at the root surface do not contribute to the uptake; ix) the uptake by the plant equals the either the nitrogen flux to the root surface or an maximum absorption characterizing the root system; and x) nitrogen can be taken up as either nitrate or ammonium and ammonium has preference over nitrate. The volumetric water content and the uptake of water by plant roots can be estimated from a water uptake model based on similar assumptions as the nitrogen uptake model. The model is fairly robust and has been applied with success at scales ranging from field scale to catchment scale.