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

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 15 N‐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 N 2 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 N 2 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 N 2 (84 v . 79% respectively).