Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method

Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N₂ fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the ¹⁵N dilution method. The two Sesbania species as N₂-fixing reference plants and four aquatic weed species as non-N₂-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with ¹⁵N at 0.26 atom % ¹⁵N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % ¹⁵N excess. The results showed that ¹⁵N enrichment of soil NH ₄ ⁺ -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in ¹⁵N enrichment, in both N₂-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in ¹⁵N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N₂-fixing and non-fixing species was estimated by the technique of varying the ¹⁵N level. In the second crop, this ratio in non-fixing species was higher than that in N₂-fixing species. Comparable estimates of % Ndfa were obtained by using ¹⁵N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using ¹⁵N enrichment of non-fixing species and those by using soil NH ₄ ⁺ -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N₂ due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the ¹⁵N dilution method are discussed with reference to these results.     

Estimating N2 fixation by field-grown lupins (Lupinus angustifolius L.) using soil and plant 15N enrichment

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.     

N2 fixation in two Sesbania species and its transfer to rice (Oryza sativa L.) as revealed by 15N technology

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.     

A yield-independent, 15N-isotope dilution method to estimate legume symbiotic dependence without a non-N2-fixing reference plant

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.     

Evaluation of N2 fixation by applying 15N labeled plant material and ammonium sulfate

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.     

Sesbania rostrata as a green manure for lowland rice: Growth,N2 fixation,Azorhizobium sp. inoculation,and effects on succeeding crop yields and nitrogen balance

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.     

N2 fixation and growth of legumes as affected by sulphur fertilization

The influence of three sulphur application rates in combination with two nitrogen application rates on N₂ fixation and growth of different legumes was investigated. N was applied as N-labelled ¹⁵NH₄ ¹⁵NO₃. The ¹⁵N isotope dilution technique was used to estimate N₂ fixation. At both N increments dry matter yield was highest with high S supply. Independently of the N supply, the high S application rate resulted in a significantly higher N accumulation, which was mainly caused by a higher N₂ fixation rate. With the grain legumes the weight of nodules was increased by the high S application rate. The higher number of nodules per pot with optimum S supply was the result of a better root growth. Rates of acetylene reduction correlated significantly with S supply.     

Symbiotic efficiency of Sesbania rostrata and S. cannabina as affected by agronomic practices

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.     

Comparison of 15N natural abundance and difference methods for estimating N2 fixation of soybeans grown in a tropical soil

Abstract

A study was carried out to compare the difference or N-yield method with the ¹⁵N natural abundance method for the estimation of the fractional contribution of biological N₂ fixation in the different plant parts of nodulating and non-nodulating isolines of soybeans. The results indicated that the δ¹⁵N values of most plant parts of soybeans were significantly lower (p<0.05) in the nodulating than in the non-nodulating isoline. However, in the case of the root+nodule component, the δ¹⁵N value was higher in the nodulating than in the non-nodulating isoline possibly due to isotopic discrimination of ¹⁵N over ¹⁴N which may have occurred in the nodules. Inoculation of soybeans with the Bradyrhizobium japonicum strain CB 1809 increased significantly (p<0.05) the δ¹⁵N value of the root+nodule component implying that the effectiveness of the soybean-rhizobium symbiosis had increased by inoculation.

Percentage of plant N derived from atmospheric N₂ fixation (%Ndfa) estimated by the ¹⁵N natural abundance method was highly correlated (r=0.762, p<0.01) with that by the difference or N-yield method and the differences between the two methods were not statistically significant. The agreement between the two methods was closer at maturity than at the early reproductive stage.

The %Ndfa obtained by the difference method ranged from 48.4 to 92.6% whereas the %Ndfa obtained by the ¹⁵N natural abundance method ranged from 43.2 to 92.4% in the different plant parts. Based on the ¹⁵N natural abundance method, approximately 15% of the N in pod, shoot, grain, and shell was derived from the soil but in the case of stover, this fraction was about 55%.     

Use of 15N isotope dilution for quantification of N2 fixation associated with roots of kallar grass (Leptochloa fusca (L.))

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.     

Yield,nodulation, and N2 fixation by cowpea cultivars at different phosphorus levels

A greenhouse experiment was conducted to investigate the effect of a P application (0 vs. 50 mg P kg^-1) on yield, nodulation, and N₂ fixation by three cowpea cultivars (Soronko, Amantin, and IT81D-1137) using the ¹⁵N isotope-dilution method. When P was not applied the inoculated cowpea genotypes showed significant differences (Soronko>Amantin> IT81D-1137) in N accumulation, in contrast to the uninoculated cowpea cultivars, which accumulated similar amounts of N. The differences in shoot N in inoculated plants were thus caused by differences in N₂ fixation. The average values of N fixed (for both P levels) were 74% in Soronko, 59% in Amantin, and 42% in IT81D-1137, corresponding to 80, 51, and 24 mg N plant^-1, respectively. Inoculation increased the total shoot-N accumulation in cv. Soronko by 270% without P and by 204% with P, cv. Amantin by 152 and 104%, and cv. IT81D-1137 by 74 and 58%, respectively. With P, the % N derived from atmosphere (%Ndfa) was 42% for IT81D-1137, 62% for Amantin, and 76% for Soronko. The high value for Soronko indicates that in a soil of medium fertility, certain cowpea cultivars are capable of satisfying their total N requirement through N₂ fixation. The P effect on N₂ fixation was mainly in the total amount of N fixed rather than on the percentage derived from the atmosphere.     

Extractability of 15N-labeled corn-shoot tissue in a sandy and a clay soil by 0.01 MCaCl2 method in laboratory incubation experiments

Management of N fertilization depends not only on the mineral N measured at the beginning of the growing season but also on the status of the low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the ¹⁵N applied in labeled cornshoot tissue would be recovered in 0.01 M CaCl₂-extractable ¹⁵N fractions and wheter a decrease in the CaCl₂-extractable ¹⁵N fraction quantitatively followed the trend in net mineralization of the ¹⁵N applied in corn-shoot tissue during an incubation period. The effects of adding ¹⁵N-labeled young corn-shoot tissue to a sandy soil and a clay soil were investigated for 46 days in an aerobic incubation experiment at 25°C. The application of 80 mg N kg^-1 soil in the form of labeled corn-shoot tissue (24.62 mg ¹⁵N kg^-1 soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in comparison with the untreated soils during the incubation. The labeled organic-N fraction was significantly higher in the sandy soil than in the clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in ¹⁵NO _inf3 ^sup- during the incubation. Ammonification of applied plant N resulted in a significant increase in the 1 M HCl-extractable non-exchangeable ¹⁵NH _inf4 ^sup+ fraction in the clay soik, owing to the vermiculite content. The ¹⁵N recovery was analyzed by the 0.01 M CaCl₂ extraction method; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incubation, recovery increased to 47.2 and 43.8% in the sandy and clay soils, respectively. Net mineralization of the ¹⁵N applied in corn-shoot tissue determined after the 46-day incubation was 6.60 mg ¹⁵N kg^-1 soil (=34.9% of the applied organic ¹⁵N) and 4.37 mg ¹⁵N kg^-1 soil (=23.1% of the applied organic ¹⁵N) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl₂ over the whole incubation period was 3.14 and 2.33 mg ¹⁵N kg^-1 soil in the sandy and clay soil, respectively. These results indicate that net mineralization of ¹⁵N was not consistent with the decrease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl₂ to extract or desorb all of the applied organic ¹⁵N that was mineralized during the incubation period.     

Field measurements of nitrogen fixation in leguminous trees used in agroforestry systems: Influence of 15N-labeling approaches and reference trees

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.     

Field evaluation of N2 fixation and N partitioning in climbing bean (Phaseolus vulgaris L.) using 15N

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.     

Nitrogen accumulation in three legumes and two cereals with emphasis on estimation of N2 fixation in the legumes by the natural 15N-abundance technique

Summary N accumulation and natural ¹⁵N abundance in three legumes (groundnuts, cowpeas, and soybeans) and in two cereals (sorghum and maize) were investigated over two seasons in Alfisols with and without N fertilization. Using the N uptake and natural ¹⁵N abundance of non-nodulating plants as the indication of N derived from soil and fertilizer, the per cent N derived from atmospheric N₂ was calculated for nodulated plants. In the first experiment, the groundnut genotype contained 85% atmosphere-derived N, but the percentage decreased with N application. Estimates of atmosphere-derived N by the N-difference and ¹⁵N-abundance techniques gave identical results. The percentages of atmosphere-derived N estimated by the two methods at different stages of groundnut growth were also similar. In the second experiment, atmosphere-derived N was estimated in plants grown with 0–200 kg ha^-1 applied N. The estimated atmosphere-derived N ranged from 42% to 61% for groundnuts from 33% to 77% for cowpeas, and from 24% to 48% for soybeans, depending on the amount of N applied. Inoculation with a Bradyrhizobium strain increased the percentage of atmospherederived N in soybean plants grown without any fertilizer N. The natural ¹⁵N abundance of sorghum and maize was very close to that of the non-nodulating groundnut, suggesting that these cereals can be used as reference plants in the estimation of atmosphere-derived N by the natural ¹⁵N-abundance method.ICRISAT Journal Article No. 876     

15N标记的土壤估测某些豆科植物N2固定的温室实验

Pot experiments were carried out to estimate N2 fixation by vetch,milk vetch,sickle alfalfa and broadbean in pure stand using a ^15N-labelled soil.Winter wheat was used as the non-fixing control.The 15N-labelled soil used was prepared by growing corn-wheat-corn successively on a nearly organic-matter-free Xiashu loess supplemented with adequate amounts of (15NH4)2SO4,P,K and micronutrients,then incorporating these 15N-labelled plant materials into the soil after each havest,and allowing the plant materials to be decomposed aerobically for 410d after incorporation of the plant material of the thire crop.The 15N enrichment of wheat plant-N varied slightly with organs,with a maximum difference of 9.8%,Based on 15N enrichment of soil N inferred from the mean value of the 15N enrichment in different organs of wheat 79%-91% of total N in the tops and 67%-74% of total N in the roots of legumes studied were derived from atmosphere .Estimate by isotope dilution method was in good agreement with that by the conventional difference method provided values obtained by the latter were corrected for seed N,and also with that from the measurement of N accumulated in the tops of the legumes.     

Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

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     

N supply by groundnuts to maize in a maize plus groundnut intercropping system,as affected by the genotype

Summary Identification of legume genotypes with high N-supplying ability is important in improving and sustaining the productivity of low-input cropping systems. Hence, ¹⁵N-aided studies were made to ascertain the relative N-supplying ability of some cultivars of groundnut, a widely grown tropical legume. The study was conducted outdoors in 1991 at Kamburupitiya, Sri Lanka, in tanks filled with 64 kg soil which had been tagged by incorporating ¹⁵N-labelled plant material. Maize cv. Badra was grown as a monocrop and as an intercrop with five genotypes of groundnut, X-14, MI-1, Red Spanish, ICGV 87127, and a non-nodulating line. All the nodulating genotypes derived over 90% of their N from the atmosphere. Significant genotypic differences in N₂ fixation were observed. X-14 fixed the highest amount (1.95 g plant^-1), while Red Spanish the lowest (0.88 g plant^-1). Intercropping of maize with nodulating groundnut significantly decreased the ¹⁵N atom excess of maize, depending on the genotype. However, this decrease did not appear to be related to the amount of N₂ fixed, based on aboveground material. The per cent N derived by maize from the intercropped groundnuts varied from 17% (X-14) to 39% (Red Spanish), indicating a marked genotypic variability in N-suppling ability. X-14, which fixed the largest amounts of N₂, grew most vigorously compared to other genotypes, causing a growth depression in the maize. The genotype that fixes the most N₂ may therefore not necessarily have the greatest N-supplying ability. The transfer of N from the legume and the consequent improvement of N nutrition in the associated cereal in low-fertility situations is therefore expected to be high when the growth of the legume is intermediate and does not suppress the growth of the cereal.     

The fate of markerAzospirillum lipoferum inoculated into rice and its effect on growth,yield and N2 fixation of plants studied by acetylene reduction,15N2 feeding and15N dilution techniques

Summary A spontaneous mutant ofAzospirillum lipoferum, resistant to streptomycin and rifampicin, was inoculated into the soil immediately before and 10 days after transplanting of rice (Oryza sativa L.). Two rice varieties with high and low nitrogen-fixing supporting traits, Hua-chou-chi-mo-mor (Hua) and OS4, were used for the plant bacterial interaction study. The effect of inoculation on growth and grain and dry matter yields was evaluated in relation to nitrogen fixation, by in situ acetylene reduction assay,¹⁵N₂ feeding and¹⁵N dilution techniques. A survey of the population of marker bacteria at maximum tillering, booting and heading revealed poor effectivety. The population of nativeAzospirillum followed no definite pattern. Acetylene-reducing activity (ARA) did not differ due to inoculation at two early stages but decreased in the inoculated plants at heading. In contrast, inoculation increased tiller number, plant height of Hua and early reproductive growth of both varieties. Grain yield of both varieties significantly increased along with the dry matter. Total N also increased in inoculated plants, which was less compared with dry matter increase.¹⁵N₂ feeding of OS4 at heading showed more¹⁵N₂ incorporation in the control than in the inoculated plants. The ARA,¹⁵N and N balance studies did not provide clear evidence that the promotion of growth and nitrogen uptake was due to higher N₂ fixation.     

Influence of mycorrhiza vs. soluble phosphate on growth,nodulation, and N2 fixation (15N) in alfalfa under different levels of water potential

Summary The legume Medicago sativa (+Rhizobium melilott) was grown under controlled conditions to study the interactions between soluble P in soil (four levels), or a mycorrhizal inoculum, and the degree of water potential (four levels) in relation to plant development and N₂ fixation. ¹⁵N-labelled ammonium sulphate was added to each pot for a qualitative estimate of N₂ fixation, in order to rank the effects of the different treatments.Dry-matter yield, nutrient content and nodulation increased with the amount of plant-available P in the soil, and decreased as the water stress increased, for each P-level. The mycorrhizal effect on dry matter, N yield, and on nodulation was little affected by the water potential. Since P uptake was affected by the water content in mycorrhizal plants, additional mechanisms, other than those mediated by P, must be involved in the mycorrhizal activity.There was a positive correlation between N yield and nodulation for the different P levels and the mycorrhizal treatment at all water levels. A high correlation between plant unlabelled N content and atom% ¹⁵N excess was also found for all levels of P. In mycorrhizal plants, however, the correlation between unlabelled N yield and ¹⁵N was lower. This suggests that mycorrhiza supply plants with other N sources in addition to those derived from the improvement on N₂ fixation.