Ontogenic variation in nitrogen fixation and accumulation of nitrogen in mungbean,blackgram, cowpea,and groundnut

Ontogenic variations in N₂ fixation and accumulation of N by the mungbean (Vigna radiata L. Wilczek), blackgram (Vigna mungo L. Hepper), cowpea (Vigna unguiculata L. Walp.), and groundnut (Arachis hypogaea L.) were studied by a ¹⁵N-dilution technique. Pots filled with 7 kg of red yellow podzolic soil were used. Samples were taken 20, 40, 60, and 80 days after emergence which approximately corresponded to preflowering, flowering, early/mid-pod filling and late pod filling stages, respectively. During early growth (up to 40 days after emergence), the carryover of seed N accounted for a considerable fraction of the total plant N in the legumes, the highest being in the groundnut. With a correction for carryover, the groundnut derived over 45% of its N content from the atmosphere 20 days after emergence whereas the corresponding figures were 33% for the blackgram and about 28% for the cowpea and mungbean. Between flowering and early pod fill, there was a rapid increase in N₂ fixation in all legumes except in groundnut which showed highest fixation from 60 to 80 days after emergence. In the mungbean, N₂ fixation and uptake of soil N were insignificant 60 days after emergence while in other legumes these processes continued beyond this time. All legumes derived about 90% of their N from atmosphere by 80 days after emergence. However, due to considerable interspecific differences in total N yield the final amount of N₂ fixed showed an appreciable variation among legumes. It was highest in the groundnut (443 mg N plant^-1) followed by the cowpea (385), blackgram (273), and mungbean (145), respectively. The groundnut maintained nodules until the late pod filling stage while in other legumes, nodules senesced progressively following the mid-pod filling stage. During pod filling there was a net mobilization of N from vegetative tissues to developing pods in the mungbean, which amounted to about 20% of N in seeds. This mobilization was not evident in other legumes.     

A comparison of nitrogen fixation in genotypes of groundnut (Arachis hypogaea L.) using 15N-isotope dilution

Summary Nitrogen fixation in seven groundnut genotypes was measured by ¹⁵N-isotope dilution using a non-nodulating cultivar of groundnut as the nonfixing reference plant. Nitrogen fixation varied between 100 kg N ha^–1 in genotype J-11 and 153 kg N ha^–1 in Robut 33-1. The amount of plant-available soil N was small, so that 86%–92% of plant nitrogen was derived from N₂-fixation. Thus differences in N₂-fixation between genotypes closely reflected differences in their total N accumulation.ICRISAT Journal Article no. 600     

Nitrogen and phosphorus use in maize sole cropping and maize/cowpea mixed cropping systems on an Alfisol in the northern Guinea Savanna of Ghana

Summary The use of N and P by mixed and by sole cropping (crop rotation) of maize and cowpeas were compared in a field experiment on an Alfisol at the Nyankpala Agricultural Experiment Station in the northern Guinea Savanna of Ghana, using two levels of N (0 and 80 kg N ha^-1 year^-1 as urea) and P application (0 and 60 kg P ha^-1 year^-1 as Volta phosphate rock). Maize grain yields were significantly reduced in the mixed cropping system. This yield difference became smaller with the application of N and P fertilizer. The N and P concentrations in maize ear leaves at silking indicated that a deficiency in N and P contributed to the maize yield depression in mixed cropping. Competition for soil and fertilizer N between maize and cowpeas was suggested by: (1) A similarity in total N uptake between the two cropping systems; (2) efficient use of soil nitrate by the cowpeas; and (3) low N₂ fixation by the cowpeas, calculated with the aid of an extended-difference method. In general, N₂ fixation was low, with the highest values in the sole cropping (53 kg ha^-1) and a substantial reduction in the mixed cropping system. The application of N fertilizer further reduced N₂ fixation. This was substantiated by nodule counts. The lower N₂ fixation in the mixed cropping system was only partly explained by the lower density of cowpeas in this system. In addition, dry spells during the cropping season and shading by the maize component could have reduced the nodulation efficiency. No N transfer from the legume/rhizobium to the non-legume crop was observed. Impaired P nutrition in the mixed compared with the sole-cropped maize might have been due to less P mobility in the soil. This was indicated by lower soil moisture contents in the topsoil under mixed cropping, especially during the dry year of 1986. The results show that mixed cropping of maize and cowpeas did not lead to improved use of soil and fertilizer N and P or to an enhanced N₂ fixation. On the contrary, an annual rotation of maize and cowpeas was clearly superior.     

Time-course of nitrogen fixation in two bambara groundnut (Vigna subterranea L. Verdc.) cultivars

TheA-value method, involving the application of a higher¹⁵N rate to a reference non-N₂-fixing plant, was used to assess the magnitude of N₂ fixation in two bambara groundnut cultivars at four growth stages [vegetative, 0–47 days after planting (DAP); early pod-filling, 47–99 DAP; mid-pod-filling, 99–120 DAP; physiological maturity, 120–148 DAP). The cultivars were Ex-Ada, a bunchy type, and CS-88-11, a slightly spreading type. They were grown on a loamy sand. Uninoculated Ex-Ada and CS-88-11 were used as reference plants to measure the N₂ fixed in the inoculated bambara groundnuts. In this greenhouse study, soil was the major source of N in bambara groundnuts during vegetative growth, and during this period it accounted for over 80% of the N accumulaed in the plants. However, N₂ fixation became the major source of plant N during reproductive growth. There were significant differences between the two cultivars in the ability to fix N₂, and at physiological maturity, almost 75% of the N in CS-88-11 was derived from the atmosphere compared to 55% in Ex-Ada. Also, the total N fixed in CS-88-11 at physiological maturity was almost double that in Ex-Ada. Our data indicate that the higher N₂ fixation in CS-88-11 was due to two factors, a higher intensity of N₂ fixation and a longer active period of N₂ fixation. The results also suggest that bambara groundnut genotypes could be selected for higher N₂ fixation in farining systems.     

Biological nitrogen fixation in Azospirillum strain-maize genotype associations as evaluated by the 15N isotope dilution technique

Few studies of the inoculation of cereal crops with N₂-fixing bacteria have included more than one or two plant genotypes. In a recent study performed in Argentina using 12 different maize genotypes, it was found in 2 consecutive field experiments that several of them responded consistently, either negatively or positively, to inoculation with a mixture of strains of Azospirillum spp. The present study in post was performed to investigate the effect of inoculation of individual strains (and a mixture) of Azospirillum spp., and their nitrate reductase negative (NR-) mutants, on the growth of four of these maize genotypes. Two of these genotypes were grown in ¹⁵N-labelled soil with the aim of quantifying any contributions of biological N₂ fixation. Two genotypes (Morgan 318 and Dekalb 4D-70) produced similar increases in grain yield when they were inoculated with a mixture of Azospirillum spp. strains or fertilized with the equivalent of 100 kg N ha^-1. The other genotypes (Dekalb 2F-11 and CMS 22) showed little response to inoculation or N fertilization. The Morgan 318 and Dekalb 4D-70 genotypes showed a large increase in total N accumulation, suggesting that the response was due to increased N acquisition, but not due to bacterial nitrate reductase as the NR- mutants generally caused plant responses similar to those of the parent strains. Despite problems with the stabilization of the ¹⁵N enrichment in the soil, the ¹⁵N isotope dilution results indicated that there were very significant biological nitrogen fixation (BNF) contributions to the Dekalb 4D-70 and CMS 22 maize genotypes.Dedicated to Professor J.C.G. Ottow on the occasion of his 60th birthday     

Nitrogen fixation and beneficial effects of some grain legumes and green-manure crops on rice

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.     

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 studies on the transfer of legume-fixed nitrogen to associated cereals in intercropping systems

Summary An attempt has been made to estimate quantitatively the amount of N fixed by legume and transferred to the cereal in association in intercropping systems of wheat (Triticum aestivum L.) — gram (Cicer arietinum L.) and maize (Zea mays L.) —cowpea (Vigna unguiculate L.) by labelling soil and fertilizer nitrogen with ¹⁵N. The intercropped legumes have been found to fix significantly higher amounts of N as compared with legumes in sole cropping if the intercropped cereal-legume received the same dose of fertilizer N as the sole cereal crop. But when half of the dose of the fertilizer N applied to sole cereal crop was received by intercropped plants, the amount of N fixed by legumes in association with cereals was significantly less than that fixed by sole legumes. Under field conditions 28% of the total N uptake by maize (21.2 kg N ha^–1) was of atmospheric origin and was obtained by transfer of fixed N by cowpea grown in association with maize. Under greenhouse conditions gram and summer and monsoon season cowpea have been found to contribute 14%–20%, 16% and 32% of the total N uptake by associated wheat and summer and monsoon maize crops, respectively. Inoculation of cowpea seeds with Rhizobium increased both the amount of N fixed by cowpea and transferred to maize in intercropping system.     

Carbon flow in the plant-soil-microbe continuum at different growth stages of maize grown in a Mollisol

Understanding the photosynthetic carbon (C) dynamics in the plant–soil–microbe continuum is critical to the C sequestration in soils. However, such information is limited in maize (Zea mays L.) in Mollisols. Pot-grown maize was labelled with ¹³CO₂ at the 10-leaf, 15-leaf, heading, milk and dent stages to investigate the photosynthetic C flow in a maize–soil system and its contribution to soil organic carbon (SOC) in Mollisols. The majority of fixed ¹³C was recovered in shoots, ranging from 44.7% to 78.6%. The allocation of ¹³C fixed at different growth stages to belowground (roots and soil) gradually decreased over the growing period, indicating that the strength of root C sink is stronger at the early stages. However, the proportion of ¹³C in dissolved organic C and microbial biomass C to that in SOC significantly increased as the growth stages advanced. Over the entire growth period, the contribution of root-derived C to SOC was estimated to be 5461 mg C plant^?1 growth period^?1, of which approximately 79% was synthesized during the vegetative stages. Therefore, the input of photosynthetic C by maize plants into SOC mainly occurred during the younger stages of the plant, favouring the storage of SOC in Mollisols.     

GREENHOUSE GAS EMISSIONS FROM AN ALKALINE SALINE SOIL CULTIVATED WITH MAIZE (ZEA MAYS L.) AND AMENDED WITH ANAEROBICALLY DIGESTED COW MANURE: A GREENHOUSE EXPERIMENT

Sludge derived from cow manure anaerobically digested to produce biogas (methane; CH₄) was applied to maize (Zea mays L.) cultivated in a nutrient-low, alkaline, saline soil with electrolytic conductivity 9.4 dS m^?1 and pH 9.3. Carbon dioxide (CO₂) emission increased 3.1 times when sludge was applied to soil, 1.6 times when cultivated with maize and 3.5 times in sludge-amended maize cultivated soil compared to the unamended uncultivated soil (1.51 mg C kg^?1 soil day^?1). Nitrous oxide (N₂O) emission from unamended soil was -0.0004 μg nitrogen (N) kg^?1 soil day^?1 and similar from soil cultivated with maize (0.27 μg N kg^?1 soil day^?1). Application of sludge increased the N₂O emission to 4.59 μg N kg^?1 soil day^?1, but cultivating this soil reduced it to 2.42 μg N kg^?1 soil day^?1. It was found that application of anaerobic digested cow manure stimulated maize development in an alkaline saline soil and increased emissions of CO₂ and N₂O.     

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%.     

施氮和豌豆/玉米间作对土壤无机氮时空分布的影响

为探明甘肃河西走廊绿洲灌区豌豆/玉米间作体系土壤无机氮时空分布现状和过量施用氮肥对环境的影响,2011年在田间试验条件下,采用土钻法采集土壤剖面样品,采用Ca Cl2溶液浸提、流动分析仪测定土壤无机氮含量的方法,研究了不同氮水平[0 kg(N)·hm?2、75 kg(N)·hm?2、150 kg(N)·hm?2、300 kg(N)·hm?2、450 kg(N)·hm?2]下豌豆/玉米间作体系土壤无机氮时空分布规律。结果表明:作物整个生育期内,灌漠土无机氮以硝态氮为主,其含量是铵态氮的7.55倍。在玉米整个生育期内,与不施氮相比,75 kg(N)·hm?2、150 kg(N)·hm?2、300 kg(N)·hm?2和450 kg(N)·hm?2处理的土壤硝态氮含量分别增加29.7%、67.5%、88.2%和134.3%。与豌豆收获期相比,在玉米收获时土壤硝态氮含量平均降低44.2%。间作豌豆和间作玉米分别比对应的单作在0~120 cm土层硝态氮含量降低6.1%和5.1%。豌豆/玉米间作体系土壤无机氮累积量在不同施氮量和不同生育时期都是表层(0~20 cm)最高。豌豆收获后,0~60 cm土层土壤无机氮累积量间作豌豆和间作玉米分别比相应单作降低4.9%和1.9%,60~120 cm土层降低10.8%和9.2%;玉米收获后0~60 cm土层平均降低28.2%和9.4%,60~120 cm土层平均降低23.5%和12.5%。土壤无机氮残留量间作豌豆比单作豌豆在0~60 cm土层降低4.9%,60~120 cm降低10.9%。因此,施用氮肥显著增加了土壤无机氮含量和累积量,且主要影响土壤硝态氮。过量的氮肥投入会因作物不能及时全部吸收而被大水漫灌和降雨等途径淋洗到土壤深层,造成氮肥损失和农田环境污染。间作能显著降低土壤无机氮浓度和累积量,特别在作物生长后期对土壤无机氮累积的降低作用更加明显。     

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.     

华北平原玉米季优化管理的密集型农业系统下一氧化二氮和甲烷通量研究

Addressing concerns about mitigating greenhouse gas （GHG） emissions while maintaining high grain yield requires improved management practices that achieve sustainable intensification of cereal production systems. In the North China Plain, a field experiment was conducted to measure nitrous oxide （N2O） and methane （CH4） fluxes during the maize （Zea mays L.） season under various agricultural management regimes including conventional treatment （CONT） with high N fertilizer application at a rate of 300 kg N ha-1 and overuse of groundwater by flood irrigation, optimal fertilization 1 treatment （OPTIT）, optimal fertilization 2 treatment （OPT2T）, and controlled-release urea treatment （CRUT） with reduced N fertilizer application and irrigation, and a control （CK） with no N fertilizer. In contrast to CONT, balanced N fertilization treatments （OPT1T, OPT2T, and CRUT） and CK demonstrated a significant drop in cumulative N20 emission （1.70 v.s. 0.43-1.07 kg N ha-l）, indicating that balanced N fertilization substantially reduced N20 emission. The vMues of the N20 emission factor were 0.42%, 0.29%, 0.32%, and 0.27% for CONT, OPTIT, OPT2T, and CRUT, respectively. Global warming potentials, which were predominantly determined by N20 emission, were estimated to be 188 kg CO2-eq ha-1 for CK and 419-765 kg CO2-eq ha-1 for the N fertilization treatments. Global warming potential intensity calculated by considering maize yield was significantly lower for OPT1T, OPT2T, CRUT, and CK than for CONT. Therefore, OPTIT, OPT2T, and CRUT were recommended as promising management practices for sustaining maize yield and reducing GHG emissions in the North China Plain.     

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.     

Diversity of diazotroph populations in the rhizosphere of maize (Zea mays L.) growing on different French soils

Summary We studied the dominant diazotrophs associated with maize roots and rhizosphere soil originating from three different locations in France. An aseptically grown maize plantlet, the spermosphere model, was used to isolate N₂-fixing (acetylene-reducing) bacteria. Bacillus circulans was the dominant N₂-fixing bacterium in the rhizosphere of maize-growing soils from Ramonville and Trogny, but was not found in maize-growing sandy soil from Pissos. In the latter soil, Enterobacter cloacae, Klebsiella terrigena, and Pseudomonas sp. were the most abundant diazotrophs. Azospirillum sp., which has been frequently reported as an important diazotroph accociated with the maize rhizosphere, was not isolated from any of these soils. The strains were compared for their acetylene-reducing activity in the spermosphere model. The Bacillus circulans strains, which were more frequently isolated, also exhibited significantly greater acetylene-reducing activity (3100 nmol ethylene day^-1 plant^-1) than the Enterobacteriaceae strains (180 nmol ethylene day^-1 plant^-1). This work indicates for the first time that Bacillus circulans is an important maizerhizosphere-associated bacterium and a potential plant growth-promoting rhizobacterium.     

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.     

Effect of successive cuttings on uptake and partitioning of 15N among plant parts of Leucaena leucocephala

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.     

Evaluation of 15N-isotope dilution for measurement of nitrogen fixation in chickpea (Cicer arietinum L.)

Summary N accumulation, nodulation, and acetylene reduction activity were measured at frequent intervals during the growth of two chickpea genotypes, and N₂ fixation was estimated by an isotope-dilution method, using safflower as a non-N₂-fixing reference. Safflower was more efficient at N uptake than both the chickpea genotypes for at least the first 50 days and thus could not be used as an accurate reference control. We recommend that further work should employ non-nodulatiog genotypes of chickpea as reference plants and use slow-release forms of 15N fertilizer. Direct genotype comparison by isotope dilution estimated that genotype K 850 fixed 16–18 kg ha^–1 more N than G 130, and this difference was supported by the greater nodule mass and acetylene reduction activity in the K 850 cultivar. Inoculation with an ineffective chickpea Rhizobium sp. led to 69% nodulation on cultivar K 850 but only 33% on G 130. While nodule weight, N uptake, and acetylene reduction activity decreased with inoculation in K 850, the isotope dilutions were similar for both inoculation treatments. The lack of a significant effect on N₂ fixation was ascribed to the partial success of inoculant establishment.Published as Journal Article No. JA 692 of the International Crops Research Institute for the Semi Arid Tropics, Patancheru, A.P. 502324, India     

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.