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A central assumption of ecosystem N cycling has been that organic N must be converted to inorganic N to be available for plant uptake, but this has been questioned by recent studies. We examined uptake of nitrate, ammonium and the amino acid glycine in three species from Eucalyptus obliqua L'Her. wet forest in Tasmania, south-eastern Australia, to test the hypothesis that all three species can take up glycine, and to compare rates of glycine uptake with rates of uptake of nitrate and ammonium uptake. The alternative hypothesis that species vary in their preference for nitrate, ammonium and glycine ("niche differentiation") was also examined. Measurements were made on the canopy dominant Eucalyptus obliqua, and two rain forest tree species found in the understory or as sub-dominants of the canopy, Nothofagus cunninghamii (Hook.) Oerst. and Phyllocladus aspleniifolius (Labill.) Hook.f. Nitrogen uptake was examined in situ with attached roots placed in uptake solutions containing equimolar concentrations (100 micromol l(-1)) of (15)N-nitrate, (15)N-ammonium and 2-(13)C(2) (15)N-glycine. Species did not differ in their preference for different forms of N (species x N form interaction, P > 0.05), and thus there was no evidence of niche differentiation. In all species, rates of uptake were highest for ammonium (11 +/- 5 micromol g(DM) (-1) h(-1); mean +/- SD, n = 108), uptake of glycine occurred at less than half this rate (4.4 +/- 2.6 micromol g(DM) (-1) h(-1)), whereas uptake of nitrate occurred at one-tenth of this rate (0.9 +/- 1.2 micromol g(DM) (-1) h(-1)). The strong positive relationship between (15)N and (13)C uptake indicated that at least 72% of glycine-N was taken up intact. These findings indicate the potential for considerable uptake of organic N in the field.
Warren et al. (Thu,) studied this question.