Nitrogen (N) isotope fractionation and symbiotic N fixation were investigated
in the shrub legume tagasaste, growing in the glasshouse and field. In a pot
study of effectively nodulated plants supplied with 0, 1, 5 and 10 mM nitrate
[stable isotope 15N
(δ15N) of 3.45‰], the
δ15N of dry matter N of fully symbiotic cultures
indicated a greater isotope fractionation during distribution of N between
nodules, stems, leaves and roots than for N2 fixation
itself, with whole-plant δ15N being near zero
(–0.46 to 0.42‰). Regardless of whether plants were field-grown,
pot-cultured, fixing N2 or utilising mineral N, woody
stems were depleted in 15N relative to all other plant
parts. The similar orders of ranking of δ15N for
plant components of the nitrate-treated and fully symbiotic plants, and a
general increase in δ15N as plants were exposed to
increasing concentrations of nitrate, indicated that N isotope fractionation
can be accounted for, and thus not undermine 15N natural
abundance as means of measuring N2 fixation inputs in
tagasaste trees. In pot culture the percentage of plant N derived from the
atmosphere (%Ndfa) by symbiotic N2 fixation fell
from 85 to 37% when the nitrate supply was increased from 1 to 10 mM,
with evidence of nitrate N being preferentially allocated to roots.
δ15N natural abundance assessments of
N2 fixation of 4-year-old trees of field-grown tagasaste
in alley (550 trees ha-1) or plantation (2330 trees
ha-1) spacing were undertaken at a study site at Moora,
Western Australia, over a 2-year period of shoot regrowth (coppicing).
Cumulative N yields and %Ndfa were similar for trees of alley and
plantation spacing, with much less coppice N accumulation in the first
compared to the second year after cutting. Scaling values from a tree to plot
area basis, and using a mean %Ndfa value of 83% for all trees at
the site, inputs of fixed N into current biomass plus fallen litter over the 2
years of coppicing were calculated to be 83 kg N ha-1
year-1 for the alley and 390 kg N
ha-1 year-1 for the plantation
spacing. Although the plantation tagasaste fixed 587 kg N
ha-1 in the second year, close to the maximum value
reported in the literature for any N2-fixing system,
this should not be seen as typical where the trees are used for animal
production, since grazing and cutting management will substantially reduce
productivity and N2 fixation input.
Nitrogen isotope analysis of aqueous ammonium and nitrate by membrane inlet isotope ratio mass spectrometry (MIRMS) at natural abundance levels
