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It is essential that the 15 N source and method of labeling should not introduce significant errors into the estimation of N2 fixed in plants. This study, which was conducted in 1984 and continued in 198S, compares N 2 fixation measurements based on the repeated or rotated application of 15 N-Iabeled (NH 4 ) 2 S0 4 on plots of white clover (Trifolium repens cv. Zapican)-fescue (Festuca arundinacea cv. Tacuabe) sward with values obtained from plants that relied solely on residual ' 5 N uptake. The soil used was a Typic Hapludoll, pH 6.9. The percentage of N fixed assessed by the different ' 5 N application methods averaged 86% in 1984 and 93% in 198S. In 1984, when white clover was predominant in the sward, 109 kg N/ha was fixed in 26 wk, compared to only 13.3 kg N/ha in the 22-wk period of 198S during which fescue dominated the sward. The yield-independent estimates of percentage of clover N derived from N 2 fixation were more precise than the yield-dependent values of total N 2 fixed. In general, 15 N applications made at the harvest preceding the N 2fixation measurement period gave similar percentages of N fixed in the 2 yr, compared to the higher values obtained from estimations based solely on residual 15 N uptake. In all cases, however, estimations of total N 2 fixed, in contrast to percentage N fixed, were not significantly influenced by method of 15 N supply. Additional

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