Uptake of inorganic and amino acid nitrogen from soil by Eucalyptus regnans and Eucalyptus pauciflora seedlings

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Abstract
This study examined whether two species of Eucalyptus can take up the amino acid glycine from soil and compared the uptake rate of glycine with the uptake rates of nitrate and ammonium. Ectomycorrhizal seedlings of two ecologically disparate species were studied: Eucalyptus regnans F. Muell., a fast-growing forest tree from low altitudes; and Eucalyptus pauciflora Sieber ex Spreng., a slow-growing tree that forms the alpine treeline. Seedlings were grown from seeds in field soil. When seedlings were 4–5 months old, soils were injected with equimolar mixtures of isotope-labeled glycine, ammonium and nitrate. Seedlings and soil were harvested 4 and 48 h later. Isotope ratio mass spectrometry analysis of 13C and 15N enrichment in plants receiving glycine indicated uptake of 1.5 13C for every 15N at the 4-h harvest (versus 2:1 13C:15N in labeled glycine), suggesting intact uptake of around 75% of glycine. Gas chromatography–mass spectrometry analysis detected intact 13C2,15N-glycine in roots, but the pool of 13C2,15N-glycine was 10–500 times smaller than 13C and 15N excess, and no 13C2,15N-glycine was detected in shoots. This is consistent with glycine being taken up as an intact molecule that is subsequently metabolized rapidly. Both species took up more nitrate than ammonium, and glycine was the least preferred form of nitrogen (N). Microbes took up more N than seedlings, and their preference for N forms was the mirror image of the plant preferences. These data suggest that patterns of microbial uptake may determine plant preference for forms of N.

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