On the complementary relationship between marginal nitrogen and water-use efficiencies among Pinus taeda leaves grown under ambient and CO2-enriched environments

Background and Aims Water and nitrogen (N) are two limiting resources for biomass production of terrestrial vegetation. Water losses in transpiration (E) can be decreased by reducing leaf stomatal conductance (g(s)) at the expense of lowering CO2 uptake (A), resulting in increased water-use efficiency. However, with more N available, higher allocation of N to photosynthetic proteins improves A so that N-use efficiency is reduced when g(s) declines. Hence, a trade-off is expected between these two resource-use efficiencies. In this study it is hypothesized that when foliar concentration (N) varies on time scales much longer than g(s), an explicit complementary relationship between the marginal water- and N-use efficiency emerges. Furthermore, a shift in this relationship is anticipated with increasing atmospheric CO2 concentration (c(a)). Methods Optimization theory is employed to quantify interactions between resource-use efficiencies under elevated c(a) and soil N amendments. The analyses are based on marginal water- and N-use efficiencies, lambda = (partial derivative A/partial derivative g(s))/(partial derivative E/partial derivative g(s)) and eta = partial derivative A/partial derivative N, respectively. The relationship between the two efficiencies and related variation in intercellular CO2 concentration (c(i)) were examined using A/c(i) curves and foliar N measured on Pinus taeda needles collected at various canopy locations at the Duke Forest Free Air CO2 Enrichment experiment (North Carolina, USA). Key Results Optimality theory allowed the definition of a novel, explicit relationship between two intrinsic leaf-scale properties where eta is complementary to the square-root of lambda. The data support the model predictions that elevated c(a) increased eta and lambda, and at given c(a) and needle age-class, the two quantities varied among needles in an approximately complementary manner. Conclusions The derived analytical expressions can be employed in scaling-up carbon, water and N fluxes from leaf to ecosystem, but also to derive transpiration estimates from those of eta, and assist in predicting how increasing c(a) influences ecosystem water use.