On the link between nitrogen productivity and residence time: Two opposite nitrogen use strategies?

NUE, i.e. the net primary production (NPP) per unit of nitrogen absorbed, can be calculated as the product between nitrogen productivity (NP) and residence time of nitrogen in the plant (RT). NP is the ratio between NPP and the amount of nitrogen in biomass, and RT is the ratio between such amount of N in the biomass and the amount of N absorbed. It has been hypothesized that plants maximize NUE by two different strategies: plants of nitrogen-rich sites have a high NP, and plants of nitrogen-poor sites have a high RT. However, the information about the variation of NP and RT along gradients of N availability is often conflictive with such hypothesis. We show that NP is directly correlated with biomass turnover rate (BTR), but RT is inversely correlated with BTR, and that other variables have also influence on both terms of NUE: plant N concentration (PNC), N resorption efficiency (NRE), and relative N concentration of leaves respect to the entire plant (RLNC). As a consequence, when comparing species with very different BTR, the RT vs. NP opposition becomes evident, as in adaptive comparisons between species of highly contrasting habitats. Instead, the negative association between NP and RT may disappear when comparing species of similar BTR, as when studying plastic responses within a single species, because the differences in PNC, NRE and/or RLNC may overshadow the differences in BTR.