Comparisons of biological nitrogen fixation in association with white clover (Trifolium repens L.) under four fertiliser nitrogen inputs as measured using two 15N techniques
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The objectives of this study were to compare techniques for measuring biological nitrogen fixation (BNF) and to assess how fertiliser N input affects the balance between BNF and sustainable herbage production on perennial ryegrass (Lolium perenne L.)/white clover (Trifolium repens L.) grassland.
Biological N fixation and herbage production by white clover based grassland was measured in 2011 and 2012 under four nominal annual fertiliser N inputs: 0 (0N), 86 (86N), 140 (140N) and 280 kg ha−1 (280N). Biological N fixation was measured using the 15N isotope dilution and 15N natural abundance techniques under all fertiliser N inputs and also using the nitrogen difference technique under 0N.
The two 15N techniques produced similar annual estimates of above-ground BNF across the range of fertiliser N inputs. Fertiliser N input resulted in higher herbage dry matter yield, but reduced annual BNF which averaged 80, 64, 66 and 47 kg ha−1 on 0N, 86N, 140N and 280N, respectively, across both techniques and years.
The two 15N techniques were comparable in estimating BNF. Intermediate fertiliser N inputs achieved a balance between minimising detrimental impact on BNF and potential N loss while producing more herbage DM yield than clover swards receiving no fertiliser N.
Keywords15N techniques Isotope dilution Natural abundance Fertiliser nitrogen White clover N fixation
This work was funded by the EU Framework Programme 7 project Legume Futures. The author thanks Laura Lopez del Egido and the Stable Isotope Facility at the James Hutton Institute for assistance with the B s -value analyses and Kevin McNamara, internship students and the staff of Solohead Research Farm for assistance with field sampling.
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