Root uptake of organic nitrogen

In some ecosystems e.g. forests, plants are known to be supplemented with nitrogen, with the aid of ectomycorrhiza with proteolytic capacity. In the model this could therefore optionally be considered by including a plant nitrogen uptake directly from an organic source (see switch Organic Uptake). The functioning of mycorrhiza and its symbiosis with plant roots show complexities and uncertainties that concern both basic mechanisms as well as quantities of mass flows. Mycorrhiza takes up both mineral and organic nitrogen. It is assumed that the infected roots always take up mineral nitrogen in preference to organic nitrogen and that the organic nitrogen uptake is in the form of amino acids. The pool of amino acids is never explicitly calculated; instead organic nitrogen is transferred directly from the litter and humus pools to the roots. Hence, mycorrhiza is not explicitly represented but is instead expressed as a part of the infected roots.

When conditions for uptake of nitrogen and carbon directly from the organic pools are favorable (high plant demand and low mineral nitrogen levels), the uptake is limited by a maximum organic nitrogen uptake rate, o_L or o_H. However, organic nitrogen uptake occurs only if the amount taken up from the mineral pools, N_{Mineral→Plant}, is lower than the plant demand, N_Demand. This is expressed by setting the uptake proportional to the deficiency in supply of mineral, f_Def. The uptake, N_{Litter→Plant} and N_{Humus→Plant} is then estimated as:

                                                  (6.22)

          (6.23)

where the deficiency fraction, f_Def, is determined by the smallest of two values; the organic nitrogen demand in relation to total potential organic uptake, or the fraction of organic nitrogen from each pool that is available for uptake also in relation to total potential uptake:

                           (6.24)

 

(and using N_Humuso_H in the second term for humus).

o_Def is the maximum uptake of nitrogen from the litter and humus pool:

                                                 (6.25)

where o_L and o_H are the maximum uptake rates for litter and humus respectively.

If direct nitrogen deposition to the leaf (see section External inputs), this amount of nitrogen is also finally added to the total plant nitrogen uptake.