
TY  - JOUR
A1  - Unkovich, Murray J. 
A1  - Pate, John S. 
A1  - Lefroy, Edward C. 
A1  - Arthur, David J. 
T1  - Nitrogen isotope fractionation in the fodder tree tagasaste (<emph type="2">Chamaecytisus proliferus</emph>) and assessment of N<emph type="8">2</emph> fixation inputs in deep sandy soils of Western Australia
JF  - Functional Plant Biology
JO  - Functional Plant Biol.
Y1  - 2000
VL  - 27
IS  - 10
SP  - 921
EP  - 929
KW  - agroforestry, soil N cycling, nitrate, 15N natural abundance.
UR  - https://doi.org/10.1071/PP99201 
N2  - 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 &#91;stable isotope 15N (&#948;15N) of 3.45&#8240;&#93;, the &#948;15N 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 &#948;15N being near zero (&#8211;0.46 to 0.42&#8240;). 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 &#948;15N for plant components of the nitrate-treated and fully symbiotic plants, and a general increase in &#948;15N 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. &#948;15N 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. 
ER  -

