Nitrogen-use efficiency from leaf to stand level: Clarifying the concept

The concept of nitrogen-use efficiency (NUE) is widely used in the fields of plant physiology and ecology. However, the use of the same terminology across different scales of space and time, and the lack of homogeneity among the various definitions of NUE have sometimes led to confusion. In the first part of this chapter, we present a simple model of nitrogen cycling in a biological system (leaf, whole plant or stand), which then allows us to express the various existing definitions of NUE within a common mathematical framework. In agreement with several previous authors, we conclude that NUE should be expressed as the ratio between the biomass produced by, and the flux of nitrogen through, a system over a given period of time (unit: kg biomass [mol N](-1)). The flux of nitrogen considered is usually that which is lost by the system. In the second part, we examine: (1) whether current experimental evidence support the hypothesis of a trade-off between the two components of NUE, i.e., the mean residence time of nitrogen in a system (MRT, the time a molecule of nitrogen remains in the system) and its mean annual nitrogen productivity (aNP, the biomass increment per unit time and nitrogen), and (2) the relative impact of these two components on NUE. For whole plants, the four experiments conducted to date which have specifically addressed these questions have all yielded contradictory results. Therefore it is not yet possible to draw conclusions on either of these points. At the stand level (above-ground parts only), comparisons between evergreen and deciduous forests show that MRT is higher in the former, while the reverse is true for aNP. This results in a slightly higher NLTE in evergreen dominated formations. Finally, the underlying components of MRT and aNP are presented, and their impact on each of these two variables are discussed. We conclude that, once clarified and standardised as proposed in the present chapter, the NUE concept could become a powerful tool for evaluating a number of critical phenomena in evolutionary biology and ecology.