Comparisons of biological nitrogen fixation in association with white clover (Trifolium repens L.) under four fertiliser nitrogen inputs as measured using two ¹⁵N techniques

Aims: 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.

Methods: 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.

Results: 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.

Conclusions: 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.