Indexed on: 01 Jan '99Published on: 01 Jan '99Published in: Plant and soil
Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using 15N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing 15N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which 15N fertilizer is added after sward establishment. Immobilization of 15N in the soil organic matter has not previously been used in studies of N2 fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the 15N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the applied 15N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N2 fixation. In pure stand clover, 75–94% of the N was derived from N2 fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N2 fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha−1 day−1 in autumn to more than 2.6 kg N ha−1 day−1 in June. For clover in pure stand the average N2 fixation rate was greater and varied between 0.5 and 3.3 kg N ha−1 day−1, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha−1 in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha−1 in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha−1 to the grass in the first and second production year, respectively.