DETERMINATION OF A CRITICAL NITROGEN DILUTION CURVE FOR WINTER-WHEAT CROPS

A set of N-fertilization field experiments was used to determine the ' critical nitrogen concentration', i.e. the minimal concentration of to tal N in shoots that produced the maximum aerial dry matter, at a give n time and field situation. A unique 'critical nitrogen dilution curve ' was obtained by plotting these concentrations N(et) (% DM) vs. accum ulated shoot biomass DM (t ha-1). It could be described by the equatio n: N(et) = 5.35 DM-0.442 when shoot biomass was between 1.55 and 12 t ha-1. An excellent fit was obtained between model and data (r2 = 0.98, 15 d.f.). A very close relationship was found using reduced N instead of total N, because the nitrate concentrations in shoots correspondin g to critical points were small. The critical curve was rather close t o those reported by Greenwood et al. (1990) for C3 plants. However, th is equation did not apply when shoot biomass was less than 1.55 t ha-1 . In this case, the critical N concentration was independent of shoot biomass: the constant critical value N(er) = 4.4% is suggested for red uced-N. The model was validated in all the experimental situations, in spite of large differences in growth rate, cultivar, soil and climati c conditions; shoot biomass varying from 0.2 to 14 t ha-1. Plant N con centration was found to vary by a factor of four at a given shoot biom ass level. In the heavily fertilized treatments, shoot N concentration could be 60% higher than the critical concentration. Most (on average 80%) of the extra N accumulated was in the form of reduced N. The pro portion of nitrate to total N in shoot mainly depended on the crop sta ge of development. It was independent of the nitrogen level.