Nitrogen isotope fractionation in the fodder tree tagasaste (Chamaecytisus proliferus) and assessment of N2 fixation inputs in deep sandy soils of Western Australia

Abstract

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.