CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2006, 33, 653–660 Potential organic and inorganic N uptake by six Eucalyptus species

Abstract. There are no published studies of organicN uptake by species of south-easternAustralia (e.g.Eucalyptus) despite several studies of ecosystem N cycling. This study examines uptake of nitrate, ammonium and glycine (an amino acid) by six species of 16-year-old Eucalyptus growing at two plantations (‘common gardens’). By using two plantations, one xeric / oligotrophic and one mesic / eutrophic, I was able to disentangle genotypic from phenotypic differences in preference for N forms. Measurements were made on three separate occasions during spring. N uptake was examined in situ with attached roots placed in uptake solutions containing equimolar 100µmol L−1 concentrations of 15N-nitrate, 15N-ammonium and 2-13C215N-glycine. Water and KCl extracts were used to determine the relative abundances of nitrate, ammonium and amino acids at the two plantations. Nitrate dominated at the eutrophic site, butwas nearly absent at the oligotrophic site. N at the oligotrophic sitewas dominated by ammonium and amino acids which were present in similar concentrations. The rate of uptake of ammonium (6.3 ± 0.4µmol g h−1; mean ± s.e., n = 108), was faster than glycine (3.4 ± 0.2), which was faster than nitrate (0.62 ± 0.07). Plant ‘preference ’ for N forms did not vary between sites despite large differences in the relative abundances of N forms (nitrate v. ammonium v. amino acids). Hence, there was little evidence for acclimation of Eucalyptus species to differences in the relative availability of N forms. This study suggests the possibility for considerable organic N uptake in the field. Previous studies of ecosystem N cycling in south-eastern Australia have only examined inorganic N. The N cycle in south-eastern Australia needs to be revisited with a new perspective, one that considers inorganic N and organic N