Production and nitrogen fixation by ‘Grasslands Kopu’ and ‘Grasslands Huia’ white clovers under different nitrogen regimes

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Abstract
The production and nitrogen (N) fixation by established ‘Grasslands Kopu’ and ‘Grasslands Huia’ white clovers grown with ryegrass were measured in spring 1986 after application of 15N-labelled ammonium sulphate at 1, 5, 10, 25, and 50 kg N/ha, to plots with nil or regular N fertiliser use in the past. Application of 25 and 50 kg N/ha produced a large increase in grass growth (18 - 28 kg dry matter (DM)/kg N). However, there was little effect of recent N application on growth or N2 fixation by Kopu or Huia white clovers although a maximum was recorded at 5 kg N/ha which was most prominent with Kopu. Past N use reduced growth and N2 fixation by about 40070 for both clovers. Kopu and Huia absorbed similar amounts of N from the soil. However, Kopu produced 15% more DM and fixed 35% more N than Huia. It also fixed a significantly greater proportion of its total N from atmospheric N2 (84 v. 79% respectively).