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1.
Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N2 fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by 15N dilution using 15N enrichment of soil NH inf4 sup+ -N and of Echinochloa crusgalli as the non-N2-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the 15N into the soil organic N fraction. The 15N enrichment of soil NH inf4 sup+ -N in the second crop declined slowly. The extractability ratio (15N enrichment of extractable soil N to 15N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH inf4 sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH inf4 sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using 15N enrichment of soil NH inf4 sup+ -N as a non-N2-fixing reference for reliable BNF estimates of crops in lowland puddled soil.  相似文献   

2.
Summary We used 15N technology to investigate N2 fixation by Sesbania speciosa and Sesbania rostrata and its transfer to a lowland rice crop after incorporation of the Sesbania spp. into soil as green manure. During the first 50 days after establishment in November–December 1989, S. speciosa and S. rostrata produced 1126 and 923 kg dry matter ha-1 respectively. They gathered 31 and 23 kg N ha-1 respectively, of which 62%±5% and 55%±3% respectively, came from N2 fixation. Both these species produced a greater biomass during September–October 1989, with S. rostrata producing more than S. speciosa. These results reflected differential responses by the plants to different day lengths at different times of the year. Furthermore, the dry matter yield and %N of 15N-labelled S. speciosa were smaller than those of the unlabelled plants, possibly due to inhibition of N2 fixation in root nodules by the chemical N fertilizers added during labelling. These differences were not so pronounced in the stem-nodulated S. rostrata. The increased grain yield of rice fertilized with N in the form of chemical fertilizer or green manure was a result of an increased number of panicles per hill. The rice crop manured with S. speciosa produced a lower grain yield, with a lower grain weight than that manured with S. rostrata. This was due to a low uptake of soil N by rice manured with S. speciosa. Recovery of N from the green manure in rice straw with S. speciosa was significantly higher than from rice manured with S. rostrata, because of the higher applied N uptake by rice manured with the former.  相似文献   

3.
A field experiment was conducted to study the N2 fixation efficiency of Sesbania rostrata and S. cannabina as affected by agronomic practices in semi-arid subtropical climate, Sowing seeds resulted in smaller numbers of nodules, lower dry weight, lower total biomass, less N uptake, and less N2 fixation for S. rostrata than S. cannabina, while cut-stem planting improved the symbiotic efficiency. Flooding the soil increased the relative humidity of the crop micro-environment by 4–11% and induced early appearance of stem nodules in S. rostrata. Only 67 kg N ha-1 was fixed by S. rostrata compared to 160 kg N ha-1 by S. cannabina when normal agronomic practices (sowing and non-flooding) were followed. In contrast, planting stem cuttings and flooding resulted in greater biological N2 fixation, 307 and 209 kg N ha-1 by S. rostrata and S. cannabina, respectively. Therefore, S. rostrata can be successfully exploited as a green manure when stem cuttings are planted under flooded conditions.  相似文献   

4.
Summary Biological N2 fixation was estimated in a field experiment following the addition of NH4Cl or KNO3 to unconfined microplots (1.5 m2) at 2.5 g N m-2 (10 atom% 15N). A model of total N and 15N accumulation in lupins and decreasing 15N enrichment in the KCl-extractable soil-N pool (0–0.15 m depth) was used to estimate the proportion of N in lupins derived from biological N2 fixation. Estimates of N2 fixation derived from the model were compared with 15N isotope-dilution estimates obtained using canola, annual ryegrass, and wheat as nonfixing reference plants. Biomass, total N accumulation, or 15N enrichment in the lupin and reference crops did not differ whether NH inf4 sup+ or NO inf3 sup- was added as the labelled inorganic-N source. The decrease in soil 15N enrichment was described by first-order kinetics, whereas total N and 15N accumulation in the lupins were described by logistical equations. Using these equations, the uptake of soil N by lupins was estimated and was then used to calculate fixed N2. Estimates of N2 fixation derived from the model increased from 0 at 50 days after sowing to a maximum of 0.79 at 190 days after sowing. Those based on the 15N enrichment of the NO inf3 sup- pool were 10% higher than those based on the mineral-N pool. 15N isotope-dilution estimates of N2 fixation ranged from 0.37 to 0.55 at 68 days after sowing and from 0.71 to 0.77 at 190 days after sowing. Reference plant-derived values of N2 fixation were all higher than modelled estimates during the early states of growth, but were similar to modelled estimates at physiological maturity. The use of the model to estimate N2 derived from the atmosphere has the intrinsic advantage that the need for a non-fixing reference plant is avoided.  相似文献   

5.
Summary We studied the effect of three successive cuttings on N uptake and fixation and N distribution in Leucaena leucocephala. Two isolines, uninoculated or inoculated with three different Rhizobium strains, were grown for 36 weeks and cut every 12 weeks. The soil was labelled with 50 ppm KNO3 enriched with 10 atom % 15N excess soon after the first cutting. Except for the atom % 15N excess in branches of K28 at the second cutting, both the L. leucocephala isolines showed similar patterns of total N, fixed N2, and N from fertilizer distribution in different parts of the plant at each cutting. The Rhizobium strain did not influence the partitioning of 15N among the different plant parts. Significant differences in 15N enrichment occurred in different parts. Live nodules of both isolines showed the lowest atom % 15N excess values (0.087), followed by leaves (0.492), branches (0.552), stems (0.591), and roots (0.857). The roots contained about 60% of the total plant N and about 70% of the total N derived from fertilizer over the successive cuttings. The total N2 fixed in the roots was about 60% of that fixed in the whole plant, while the shoots contained only 20% of the fixed N2. We conclude that N reserves in roots and nodules constitute another N source that must be taken into account when estimating fixed N2 or the N balance after pruning or cutting plants. 15N enrichment declined up to about fivefold in the reference and the N2-fixing plants over 24 weeks following the 15N application. The proportion and the amounts of N derived from fertilizer decreased, while the amount derived from N2 fixation increased with time although its proportion remained constant.  相似文献   

6.
Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NH inf4 sup+ , total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NO inf3 sup- levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH inf4 sup+ but not with available soil water, pH, electrical conductivity, NO inf3 sup- , or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.Contribution No. R754 of the Saskatchewan Center of Soil Research  相似文献   

7.
Appropriate 15N-labeling methods are crucial for estimating N2-fixation in trees used in agroforestry systems. A 4-year field experiment was conducted on an Alfisol in Southwestern Nigeria to compare the estimates of N2 fixed in Leucaena leucocephala, using two non-N2-fixing leguminous trees, Senna siamea and S. spectabilis, as reference plants and three different methods of introducing 15N into soil. The atom % 15N uptake pattern (as reflected in the leaves) was identical in both N2- and non-N2-fixing tree species irrespective of the 15N-application method. There was a significant decline in atom % 15N excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sampling 12 months after planting and the second sampling 18 months after sampling. From the second harvest in 1991 until the end of the experiment (fifth) harvest in 1993, however, the atom 15N % excess decline in leaves of the three species was less pronounced and depended on the method of 15N application. In those plants to which the tracer was applied once at planting, the 15N decline was steady between the second and the last prunings. In the split-application treatment, the atom 15N % excess increased slightly at the third pruning and decreased during the subsequent two prunings. The reference tree and the method of 15N application influenced the estimated proportion of N derived from atmospheric N2 by L. leucocephala, calculated as 73 and 64%, corresponding to 119 and 98 kg N ha-1 of N2 fixed per 6 months, when S. spectabilis and S. siamea were used as reference trees, respectively. The approach by which 15N-labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atmosphere but this was of little agronomic significance because total N2 fixed was similar for all methods.  相似文献   

8.
Summary The common bean (Phaseolus vulgaris L.) is generally regarded as a poor N2 fixer. This study assessed the sources of N (fertilizer, soil, and fixed N), N partitioning and mobilization, and soil N balance under field conditions in an indeterminate-type climbing bean (P. vulgaris L. cv. Cipro) at the vegetative, early pod-filling, and physiological maturity stages, using the A-value approach. This involved the application of 10 and 100 kg N ha-1 of 15N-labelled ammonium sulphate to the climbing bean and a reference crop, maize (Zea mays L.). At the late pod-filling stage (75 days after planting) the climbing bean had accumulated 119 kg N ha-1, 84% being derived from fixation, 16% from soil, and only 0.2% from the 15N fertilizer. N2 fixation was generally high at all stages of plant growth, but the maximum fixation (74% of the total N2 fixed) occurred during the interval between early (55 days after planting) and late podfilling. The N2 fixed between 55 and 75 days after planting bas a major source (88%) of the N demand of the developing pod, and only about 11% was contributed from the soil. There was essentially no mobilization of N from the shoots or roots for pod development. The cultivation of common bean cultivars that maintain a high N2-fixing capacity especially during pod filling, satisfying almost all the N needs of the developing pod and thus requiring little or no mobilization of N from the shoots for pod development, may lead to a net positive soil N balance.  相似文献   

9.
Two methods of N transfer between plants—by litter decomposition and root-to-root exchange—were examined in mixed plantations of N-fixing and non-fixing trees. Nitrogen transfers from decaying litters were measured by placing 15N-labelled litters from four actinorhizal tree species around shoots of containerized Prunus avium. Nitrogen transfers by root-to-root exchanges were measured after foliar NO3-15N fertilization of Alnus subcordata and Elaeagnus angustifolia growing in containers in association with P. avium. During the first 2 years of litter decomposition, from 5–20% of the N, depending on the litter identity, was released and taken up by P. avium. N availability in the different litters was strongly correlated with the amount of water-soluble N, which was highest in leaves of E. angustifolia. In the association between fixing and non-fixing plants, 7.5% of the A. subcordata N and 25% of E. angustifolia N was transferred to P. avium by root exchange. These results showed that the magnitude of N transfers by root exchange depended on the associated N2-fixing species. Among the species investigated, E. angustifolia displayed the highest capacity for exudating N from roots as well as for releasing N from litters. These qualities make this tree a promising species for enhancing wood yields in mixed stands.  相似文献   

10.
Summary We examined the suitability of four reference crops, i.e., two non-fixing trees,Cassia siamea andEucalyptus grandis, and two uninoculated fixing trees,Leucaena leucocephala andAcacia albida, for measuring fixed N2 fixed in inoculatedL. leucocephala andA. albida grown for 36 weeks in pots. The15N isotope-dilution (involving the addition of equal amounts of labelled N fertilizer to the non-fixing and the fixing plants) and theA-value (with different amounts of labelled N fertilizer added to the fixing and the non-fixing crops) methods were used. The isotope dilution approach gave several large negative estimates of fixed N2 inA. albida. Positive and similar values of fixed N2 were measured in all four reference crops using theA-value approach. ForL. leucocephala the isotope-dilution approach gave different estimates of fixed N2, with the different reference crops; the uninoculated N2-fixing crops indicated significantly less fixed N2 than the non-fixing reference crops. Similar values for N2 fixed inL. leucocephala were obtained using the two non-fixing trees, either by the isotope-dilution or theA-value method. On average,A. albida derived about twice as much N from fertilizer asL. leucocephala. In both species, the atom %15N excess declined by about 50% in successive harvests.  相似文献   

11.
Abstract

The enrichment of 15N in the nodules of some N2-fixing leguminous plants is an interesting finding (Shearer et al. 1982). The extent of 15N enrichment differed depending on the plant species (Shearer et al. 1982; Yoneyama 1987) and bacterial strains (Steele et al. 1983), and in soybeans it was apparently related to the nitrogen fixation efficiency (Shearer et al. 1984)  相似文献   

12.
Abstract

The popular and widely used 15nitrogen (N)–isotope dilution method for estimating biological N fixation (BNF) of pasture and tree legumes relies largely on the ability to overcome the principal source of error due to the problem of selecting appropriate reference plants. A field experiment was conducted to evaluate the suitability of 12 non‐N2‐fixing plants (i.e., nonlegumes) as reference plants for estimating the BNF of three pasture legumes (white clover, Trifolium repens L.; lucerne, Medicago sativa; and red clover, Trifolium pratense L.) in standard ryegrass–white clover (RWC) and multispecies pastures (MSP) under dry‐land and irrigation systems, over four seasons in Canterbury, New Zealand. The 15N‐isotope dilution method involving field 15N‐microplots was used to estimate BNF. Non‐N2‐fixing plants were used either singly or in combination as reference plants to estimate the BNF of the three legumes. Results obtained showed that, on the whole, 15N‐enrichment values of legumes and nonlegumes varied significantly according to plant species, season, and irrigation. Grasses and herb species showed higher 15N‐enrichment than those of legumes. Highest 15N‐enrichment values of all plants occurred during late summer under dry‐land and irrigation conditions. Based on single or combined non‐N2‐fixing plants as reference plants, the proportion of N derived from the atmosphere (% Ndfa) values were high (50 to 90%) and differed between most reference plants in the MSP pastures, especially chicory (Cichorium intybus), probably because it is different in phenology, rooting depth, and N‐uptake patterns compared to those of legumes. The percent Ndfa values of all plants studied also varied according to plant species, season, and irrigation in the MSP pastures. Estimated daily amounts of BNF varied according to pasture type, time of plant harvest, and irrigation, similar to those shown by percent Ndfa results as expected. Irrigation increased daily BNF more than 10‐fold, probably due to increased dry‐matter yield of pasture under irrigation compared to dry‐land conditions. Seasonal and irrigation effects were more important in affecting estimates of legume BNF than those due to the appropriate matching of N2‐fixing and non‐N2‐fixing reference plants.  相似文献   

13.
The proportional contribution of atmospheric N2 to the N nutrition of lupin (P atm) was estimated in a field experiment following addition of NH4Cl of KNO3 to unconfined microplots (1.5 m2) at 2.5 g N m-2 (10 atom% 15N). The integrated 15N enrichment, or mean pool abundance, of nitrate extracted from 0- to 15-cm samples taken under the lupin crop on eight occasion between 28 and 190 days after sowing was used as the reference criterion to estimate P atm by the 15N-isotope dilution technique. Estimates of P atm were similar to those obtained using canola as a non-fixing reference plant, but were higher than estimates obtained using a yield-dependent model. Use of mean pool abundance obviates the need for a non-fixing reference plant, and the frequent sampling and isotope-ratio analysis of the legume biomass required with the yield-dependent model is unnecessary. However, further work is needed to validate a sampling strategy commensurate with the growth of the legume roots.  相似文献   

14.
Summary Leptochloa fusca (L.) Kunth (kallar grass) has previously been found to exhibit high rates of nitrogen fixation. A series of experiments to determine the level of biological nitrogen fixation using 15N isotopic dilution were carried out in nutrient solution and saline soil. In the nutrient solution, E. coli inoculated plants were taken as non-nitrogen-fixing control. It was observed that nearly 60%–80% of the plant N was derived from atmospheric fixation. Estimations based on the N difference method gave much lower values (18%–35%). In experiments with saline soil which was initially sterilized with chloroform fumigation, a mixed culture of N2-fixing rhizospheric isolates from kallar grass roots was inoculated and planted to kallar grass. Uninoculated treatments were regarded as controls. The soil was previously labelled with 15N by adding cellulose and (15NH4)2SO4. The results of these studies showed fixation values of 6%–32% when estimated by 15N dilution, whereas by the N difference method 54% of the plant N was estimated to be derived from fixation. This discrepancy is due to the increase in root proliferation due to inoculation, which results in greater uptake of soil N. The distribution of 15N in different fractions of the soil-N indicted isotopic dilution due to bacterial fixation of atmospheric N2.  相似文献   

15.
Studies were conducted on paddy soils to ascertain N2 fixation, growth, and N supplying ability of some green-manure crops and grain legumes. In a 60-day pot trial, sunhemp (Crotalaria juncia) produced a significantly higher dry matter content and N yield than Sesbania sesban, S. rostrata, cowpeas (Vigna unguiculata), and blackgram (V. mungo), deriving 91% of its N content from the atmosphere. Dry matter production and N yield by the legumes were significantly correlated with the quantity of N2 fixed. In a lowland field study involving sunhemp, blackgram, cowpeas, and mungbean, the former produced the highest stover yield and the stover N content, accumulating 160–250 kg N ha-1 in 60 days, and showed great promise as a biofertilizer for rice. The grain legumes showed good adaptability to rice-based cropping systems and produced a seed yield of 1125–2080 kg ha-1, depending on the location, species, and cultivar. Significant inter- and intraspecific differences in the stover N content were evident among the grain legumes, with blackgram having the highest N (104–155 kg N ha-1). In a trial on sequential cropping, the groundnut (Arachis hypogaea) showed a significantly higher N2 fixation and residual N effect on the succeeding rice crop than cowpeas, blackgram, mungbeans (V. radiata), and pigeonpeas (Cajanus cajan). The growth and N yield of the rice crop were positively correlated with the quantity of N2 fixed by the preceding legume crop.  相似文献   

16.
Summary Root and stem nodulation, nitrogen fixation (acetylene-reducing activity), growth and N accumulation bySesbania rostrata as affected by season and inoculation were studied in a pot experiment. The effects ofS. rostrata as a green manure on succeeding wet-season and dry-season rice yields and total N balance were also studied.S. rostrata grown during the wet season showed better growth, nodulation, and greater acetylene-reducing activity than that grown during the dry season. Inoculation withAzorhizobium caulinodans ORS 571 StrSpc® (resistant to streptomycin and spectinomycin) on the stem alone or on both root and stem significantly increased N2 fixation by the plants. Soil and seed inoculation yielded active root nodules under flooded conditions. Plants that were not inoculated on the stem did not develop stem nodules. The nitrogenase activity of the root nodules was greater than that of the stem nodules in about 50-day-oldS. rostrata. S. rostrata incorporation, irrespective of inoculation, significantly increased the grain yield and N uptake of the succeeding wet season and dry season rice crops. The inoculated treatments produced a significantly greater N gain (873 mg N pot–1) than the noinoculation (712 mg N pot–1) treatment. About 80% of the N gained was transferred to the succeeding rice crops and about 20% remained in the soil. The soil N in the flooded fallow-rice treatment significantly declined (–140 mg N pot–1) but significantly increased in bothS. rostrata-rice treatments (159 and 151 mg N pot–1 in uninoculated and inoculated treatments respectively). The N-balance data gave extrapolated values of N2 fixed per hectare at about 303 kg N ha–1 per two crops forS. rostrata (uninoculated)-rice and 383 forS. rostrata (inoculated)-rice.  相似文献   

17.
Pot experiments were conducted with two soils, from Rottenhaus and Seibersdorf in Austria, to ascertain whether the rate of fertilizer N application and the test crop would influence the amount of N available in the soil as assessed by the A-value method. 15N-labelled fertilizer was applied at rates of 10, 25, 40, 60, and 100 mg N kg-1 soil, corresponding approximately to 20, 50, 80, 120 and 200 kg N ha-1 respectively, and two crop species, barley (Hordeum vulgareL.) and non-nodulating soybean (Glycine max L.) were used to determine the soil A N value under the various fertilizer regimes. The results showed that the Rottenhaus soil had a higher A N value than the Seibersdorf soil, suggesting that the former was more fertile than the latter. The A N values of both soils were significantly affected by the level of N application. When grown in the same soil, the two test crops showed significantly different fertilizer use efficiency and per cent N derived from fertilizer when the rate of N application exceeded 20 kg ha-1. Thus, the A N value as determined by the two test crops differed significantly for the same soil when the rate of N application was greater than 20 kg/ha. The difference was greater when the soil fertility level was high. The dependence of the A N value on the level of N application and the species of crop seriously compromises the suitability of this method for determining plant-associated N2 fixation. Hence, considerable caution is required when using this method to estimate plant-associated N2 fixation.  相似文献   

18.
The variation in P uptake and use efficiency and N accumulation by Gliricidia sepium (N2-fixing tree), Senna siamea and S. spectabilis (leguminous non-N2-fixing trees) were examined in the field at Fashola (savanna zone), southwestern Nigeria, using four P rates, 0, 20, 40 and 80 kg P ha-1. Growth of G. sepium and S. spectabilis responded to P application at 24 weeks after planting (WAP) and average yield increases of 58% and 145% were observed by the application of 40 kg P ha-1 for the two species, respectively. Such a P response was not found in S. siamea at 24 WAP and for any of the species at 48 WAP. G. sepium accumulated more P (on average 162%) than S. siamea and S. spectabilis at 24 WAP and had greater root length and a higher percentage of mycorrhizal infection. However, at 48 WAP S. siamea had 2.5 times more P than G. sepium. Differences in the physiological P use efficiency (PPUE) between G. sepium and the non-N2-fixing trees were significant at the 0 P level, being higher for S. siamea (average, 0.61 g shoot mg-1 P) than for G. sepium (0.27 g shoot mg-1 P). G. sepium had a consistently lower atom % 15N than S. spectabilis, while that of S. siamea for most of the time did not differ from that of G. sepium. The reference plant affected N2 fixation extimates, with negative values and a higher variability (CV 60%) associated with S. siamea than with S. spectabilis (CV<20%). Consequently, S. spectabilis was selected as a better reference plant for measuring N2 fixation in G. sepium. G. sepium fixed on average 35% and 54% of its N at 24 and 48 WAP, respectively. Except at the lowest P rate, percentage and amount of N fixed were not generally enhanced by P application.  相似文献   

19.
Effect of different 15N labeled sources on the estimation of N2 fixation was investigated. The combination of 15N labeled ammonium sulfate, 15N labeled plant material, and 15N labeled ammonium sulfate with unlabeled plant material, was examined in pot experiments. Two cultivars of soybean (Glycine max) and one of mungbean (Vigna radiata) were used. No significant difference was observed among the treatments for the estimation of N2 fixation. This was due to the homogeneity and stability of the 15N abundance in soil which resulted in a similar N uptake from the soil by the N2 fixing and reference crops. The plant yield, total N uptake and amount of N2 fixed were higher in the Yellow Soil than in the Andosol. The amount of N2 fixed was strongly influenced by the plant growth and consequently it affected the plant yield. The slow decomposition of plant material in the Andosol resulted in a low yield in both the N2 fixing and reference crops. Thus, the artificial decrease of the available N content in soil, by application of plant material, did not stimulate N, fixation but suppressed plant growth and N2 fixation.  相似文献   

20.
Estimating symbiotic di‐nitrogen (N2) fixation is challenging, especially when working with woody N2 fixers in field trials. Fortunately, isotope methods based on 15N natural abundance or on 15N artificial enrichment (dilution method) make it possible to estimate the proportion of nitrogen derived from the atmosphere (Ndfa) in N2‐fixing species. These methods have been extensively used in the field for herbaceous species, much less for tree species such as alder and acacia, and rarely for black locust (Robinia pseudoacacia). The objectives of this study were to characterize the fixation potential of black locust in a plantation by using the two 15N isotope methods in the field, and to document values of isotope fractionation occurring during N2 fixation (the B value). B values were estimated both by growing trees on an N‐free medium in controlled conditions (Blab) and by making Ndfa calculated with the natural abundance method converge with Ndfa calculated with the 15N dilution method in the field (Bfield). The two methods gave consistent estimates of the B value. B values ranging between –1.4 and –3.2‰ were found, varying with the age of the plant material. Up to 76% of the N in the black locust trees came from the atmosphere, representing more than 45 kg N ha?1 over five years and confirming that black locust may be well adapted to N‐poor soils.  相似文献   

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