The use of 15N techniques allows for the quantitative evaluation of N2 fixation and distribution and their impact on the N balance in various soil‐plant systems. The A‐value approach was used in this investigation to assess N2 fixed at various growth stages in fieldgrown soybean [Glycine max (L.) Merrill] cv. Chippewa in a Typic Eutrocrepts soil. At physiological maturity (R7), the amount of N derived from fixation (Ndfa) was 102 kg/ha, equivalent to 47% of total N assimilated, while the contributions from soil (Ndfs) and 15N‐labelled fertilizer (Ndff) accounted for 50 and 3%, respectively. Up to growth stage V6, which occupied half of the total duration of growth, Ndfa was less than 5% of N2 fixed by physiological maturity. A rapid increase in Ndfa occurred from R1 onwards, and during the reproductive stages (R1‐R4), which spanned less than one‐third of the total duration of growth, this represented about 45% of total Ndfa. An almost equal portion of N (approximately 43%) was fixed from pod‐filling (R5) to physiological maturity (R7), a period slightly more than one‐fifth of the total duration of growth. Therefore, substantial N2 fixation occurred during periods of active sink development and contributed more than 65% of the plant's N accrued during pod fill (R3‐R7). Nitrogen assimilated between R3 and R7 (when N2 fixation was high) seemed to be the predominant source of N for pod development. Thus there was a greater contribution from fixed N (55%) than soil N (43%) in pods and seeds at the end of R7. After grain removal, it was estimated that the growth of cv. Chippewa in this soil led to a net soil depletion of 54 kg N/ha.

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Summary Field experiments were conducted to determine the effects of the amount, time and method of fertilizer N application on the efficiency of N uptake, N 2 fixation and yield of soybean.Soil and foliar fertilizer N, applied during the pod-filling stage were absorbed by plants with equal and high efficiency, compared to an appreciably lower utilization efficiency for N applied before seedling emergence. These results reveal that the soybean roots were active in N uptake during these late stages of growth. Nitrogen fertilization during pod-filling resulted in significant yield increases over the control treatment which received an early application of 20 Kg N/ha. Seed yield increases were, however, more pronounced than total dry matter yield, and virtually all of the late-applied N was translocated into the pods. Nitrogen fixation in soybean was not influenced by the application of 40 kg N/ha to plants as soil or foliar N during the pod-filling stage. However, 80 kg N/ha supplied during pod-filling as 40 kg soil plus 40 kg foliar N/ha significantly reduced the amount of N 2 fixed.The results obtained in these studies suggest that inadequate N supply during pod-filling limited soybean yields, and that by the judicious application of fertilizer N during the late stages of growth, it was possible to enhance soybean yields without necessarily inhibiting N 2 fixation.

The accuracy of 15N estimates of N2 fixation may be influenced by the choice of the reference crop, especially in mixed legume‐grass swards, where transfer of fixed N2 from the legume to the reference crop may occur. To investigate this, a 2‐yr field experiment was conducted in a Typic Eutrocrepts soil in Austria to measure N2 fixed in alfalfa (Medicago sativa L.) grown alone or in two seeding ratios with ryegrass (Lolium perenne L.), using pure or mixed sward ryegrass as the reference crop. Alfalfa, on the average, derived over 80% of its N from fixation over the 2‐yr period, equivalent to about 415 kg N/ha. Compared to N2 fixation in pure alfalfa sward, the % N derived from atmospheric N2 (% Ndfa) in alfalfa increased significantly in the mixed sward and even more by increasing the seeding ratio of ryegrass in the mixture. Except for the first harvest, the atom % 15N excess in mixed ryegrass was slightly lower than in the pure grass. These differences in 15SN enrichment were insignificant, suggesting that the possible release of N from alfalfa and subsequent uptake by ryegrass may have been small. The highest proportion of ryegrass N that could have been derived from the legume was estimated to be 16% in the second harvest, equivalent to only 4 kg N/ ha and <10 kg N/ha over the 2 yr. These differences in 15SN enrichments of pure and mixed ryegrass reference plants did not significantly affect the estimates of % Ndfa in alfalfa.

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Biological nitrogen fixation of leguminous crops is becoming increasingly important in attempts to develop sustainable agricultural production. However, these crops are quite variable in their effectiveness in fixing nitrogen. By the use of the ~SN isotope dilution method some species have been found to fix large proportions of their nitrogen, while others like common bean have been considered rather inefficient. Methods for increasing N 2 fixation are therefore of great importance in any legume work. Attempts to enhance nitrogen fixation of grain legumes has been mainly the domain of microbiologists who have selected rhizobial strains with superior effectiveness or competitive ability. Few projects have focused on the plant symbiont with the objective of improving N 2 fixation as done in the F A O / I A E A Co-ordinated Research Programme which is being reported in this volume. The objective of the present paper is to discuss some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes. Examples will be presented on work performed using agronomic methods, as well as work on the plant and microbial symbionts. There are several methods available to scientists working on enhancement of N 2 fixation. No one approach is better than the others; rather work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes should be pursued.