The effect of nitrogen fertiliser on clover nitrogen fixation in an intensively grazed Manawatu pasture

Nitrogen fixation (acetylene reduction) rates were monitored for 2 years in pastures receiving 100–150 kg N/ha per year as urea and in a control pasture. Applying 100 kg of fertiliser N in 2 applications reduced annual nitrogen fixation from 100 kg N/ha to 70 kg N/ha. Clover yields were reduced less — on average by 8% when only 100 kg N was applied preceding and during measurement, and by 17% averaged over all farmlets. Hence the major effect of nitrogen fertiliser on clovers was to substitute mineral nitrogen for nitrogen fixation. There was a high level of utilisation of the pastures, which would have minimised the reduction in clover yields expected from increased grass competition. An annual nitrogen fertiliser input of 150 kg N/ha reduced clover yields the following summer more than did an input of 100 kg N/ha. Nitrogen fixation was similar in both nitrogen treatments, because of an increase in nitrogen fixation per unit clover grown at the higher nitrogen level.