Uptake of organic nitrogen and preference for inorganic nitrogen by two Australian native Araucariaceae species

Background: Demonstration of the uptake of organic nitrogen (N) by many boreal plant species and the identification of the transporters mediating amino acid uptake have attracted increasing attention, focusing on organic N uptake by plant species worldwide, particularly by species in tropical and subtropical regions. Due to the rapid decomposition of organic N by plants and the dilution of its isotopes, the capacity of the methods used to test plant uptake of organic N are controversial. The ecological significance of organic N uptake is also under debate. Aims: To assess whether two subtropical species, Araucaria bidwillii and A. cunninghamii, have the ability to take up intact organic N, and whether organic N is their preference. Methods: We used a dual-isotope-labelling method that explicitly took into account isotope dilution, in order to minimise the possibility of differential isotope dilution among different nitrogen forms. Results: Both study species showed the ability to take up glycine intact, which was indicated by the non-significant difference between the C-13:N-15 ratio in fine roots and that of glycine (i.e. 1: 1). However, the discrepancy in the C-13: N-15 ratios after 72h indicated that the absorbed glycine was being metabolised quickly. In addition, the profound preference of these two plant species for NO3--N relative to organic N suggests that organic N may only become an important N source for these subtropical plants during times when soil NO3--N is scarce.