Assessment of nutrient use in annual and perennial crops: a functional concept for analyzing nitrogen use efficiency

The use of more nutrient-efficient crops is important for maintaining yields while enhancing environmental sustainability. Various approaches are being applied to evaluate aspects of plant nutrient use efficiency, among them ecological concepts based on accumulation and losses of biomass and nutrients, agronomic concepts with a major focus on agricultural crops and harvested products, and physiological approaches assessing single physiological processes important for nutrient use. Unfortunately, the various approaches are often not compatible. Here we propose, with the example of nitrogen (N) use efficiency (NUE) of cereals, to integrate the functionally important components of NUE in a common conceptual framework. We link productivity to N in crops and seeds and consider the whole life-cycle of the crop (including seeds). Three major components of NUE are separated: The N uptake efficiency, grain-specific N efficiency and grain N concentration. The three components combine to a measure of overall NUE in terms of the N yield in harvested grain per unit of N in seed grain or soil N. The concept can be applied for both annual and perennial plants, which is demonstrated with the examples of winter wheat and a perennial energy crop (Salix) grown in Central Sweden