Nitrogen isotope fractionation in the fodder tree legume tagasaste (Chamaecytisus proliferus) and assessment of N-2 fixation inputs in deep sandy soils of Western Australia

Nitrogen (N) isotope fractionation and symbiotic N fixation were investigat ed 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 N-15 (delta(15)N) of 3.45 parts per thousand], the delta(15)N of dry matter N of fully symbiotic cultures indicated a greater isotope fractionation during distribution of N between nodules, stems, leav es and roots than for N-2 fixation itself, with whole-plant delta(15)N bein g near zero (-0.46 to 0.42 parts per thousand). Regardless of whether plant s were field-grown, pot-cultured, fixing N-2 or utilising mineral N, woody stems were depleted in N-15 relative to all other plant parts. The similar orders of ranking of delta(15)N for plant components of the nitrate- treate d and fully symbiotic plants, and a general increase in delta(15)N as plant s were exposed to increasing concentrations of nitrate, indicated that N is otope fractionation can be accounted for, and thus not undermine N-15 natur al abundance as means of measuring N-2 fixation inputs in tagasaste trees. In pot culture the percentage of plant N derived from the atmosphere (% Ndf a) by symbiotic N-2 fixation fell from 85 to 37% when the nitrate supply wa s increased from 1 to 10 mM, with evidence of nitrate N being preferentiall y allocated to roots. delta(15)N natural abundance assessments of N-2 fixat ion 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 sit e at Moora, Western Australia, over a 2-year period of shoot regrowth (copp icing). Cumulative N yields and %Ndfa were similar for trees of alley and p lantation spacing, with much less coppice N accumulation in the first compa red to the second year after cutting. Scaling values from a tree to plot ar ea basis, and using a mean %Ndfa value of 83% for all trees at the site, in puts 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 plantatio n tagasaste fixed 587 kg N ha(-1) in the second year, close to the maximum value reported in the literature for any N-2-fixing system, this should not be seen as typical where the trees are used for animal production, since g razing and cutting management will substantially reduce productivity and N- 2 fixation input.