DIFFERENCES IN NITROGEN-FIXATION AMONG FIELD-GROWN RED-CLOVER STRAINS AT DIFFERENT LEVELS OF N-15 FERTILIZATION

Genetic variation in fixed nitrogen (N) yield of red clover (Trifolium pratense L.) strains and cultivars was investigated using the N-15 isotope dilution method under three regimes of N fertilization: 0.5, 30, and 60 N (kg N ha(-1) per cut). The yield of fixed N per cut (the mean of eight cuts over 2 production years) varied among the strains (progenies of crosses between inbred parents) from 148 to 443 mg per plant at 0.5 N, from 76 to 324 mg at 30 N, and from 69 to 300 mg at 60 N. There were significant and consistent strain differences in the percentage of clover N derived from the atmosphere (% N-dfa) However, %N-dfa was positively correlated with dry mass yield. Consequently, ranking of the strains according to fixed N yield reflected that of dry mass yield. There were only minor strain x N fertilizer interactions, suggesting that selection for enhanced N fixation can be carried out at a single rate of fertilizer N. For a selected pair of strains, the difference in yield of fixed N was confirmed in an Italian ryegrass-red clover mixture, both without and with the addition of N fertilizer (50 kg N ha(-1) per cut). Results with 7-week-old seedling plants in a growth chamber, although obtained in the presence of mineral N and with the isotope dilution method, did not adequately predict field performance. It is concluded that selection for dry matter or total N yield is likely to result in an enhanced yield of fixed N at any level of mineral N availability.