Ecosystem cycles and plant uptake of N

A central assumption of ecosystem N cycling has been that organic nitrogen (N) must be converted to inorganic N (ammonium and nitrate) to be available for plant uptake, and thus N mineralization has been viewed as the bottleneck in plant N nutrition. We are investigating the new hypothesis that plants can bypass the mineralization bottleneck by taking up organic N. This project is using stable isotopes of N and C to investigate uptake of a variety of different forms of organic N (several amino acids, soluble proteins) in a selection of Australian native plants.  We are testing the hypothesis that plants from habitats where N mineralization is slow are better able to take up organic N than plants from areas where N mineralization is fast.

Examination of uptake of N by plants is being accompanied by extensive studies of fluxes and pools of N in soil. We are using and developing analytical technqiues fro rapid measurments of different froms of N, e.g. amino acids by capillary eelctrophoresis with laser-induced fluorescence, nitrate and ammonium by microplate methods.

Location: Field experiments in Tasmania (Warra, long-term ecological research site), Victoria (Eucalyptus spp. trials in Gippsland) and the Snowy Mountains

Prelimary results indicate that species from Tasmanian wet scelerophyll forest "prefer" glycine and ammonium. Mean rate of uptake of glycine, ammonium and nitrate from equimolar (100 µmol L^-1) solutions. Uptake was determined in situ on attached roots of Nothofagus cunninghamii, Eucalyptus obliqua and Phyllocladus asplenifolius. Rates of uptake did not vary between species (P>0.05), and species did not vary in their “preference” for N forms (species * N form interaction, P>0.05). Data are the mean of 5-8 replicate measurements per species at each of three sites.

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