Calculation of theoretical nitrogen rate for simple nitrogen recommendations in intensive cropping systems: A case study on the North China Plain

Nitrogen (N) is a crucial nutrient that requires careful management in intensive cropping systems because of its diverse beneficial and detrimental effects. Here we propose the concept of theoretical N rate (TNR) to answer the important question of how much fertilizer N should be applied to intensive systems based on the N fluxes due to transformation processes in the soil–crop–environment continuum. We define TNR as the theoretically calculated fertilizer N rate with the quantitative relationships of the core N fluxes among fertilizer N, soil N and crop uptake N in the crop root zone to obtain high target yield, maintain soil N balance and minimize environmental risk. We deduced one basic mathematical expression (Nfert = Nuptake − Nstraw + Nfert3) and two simplified expressions [Nfert = (Nuptake − Nstraw)/(1 − Coeff); Nfert ≅ Nuptake] for calculating the TNR. These expressions do not need much field experimentation or elaborate soil and plant testing to obtain information on crop N demand and soil N supply, and are simple to implement in farming practice to provide a very cost-effective approach. We consider this scheme to be a useful contribution to rational fertilizer practice, especially in developing countries where other N recommendation systems are usually not available and agricultural extension services are poorly developed or absent.