Significance of<Emphasis Type="Italic">Herbaspirillum seropedicae</Emphasis> inoculation and/or straw amendment on growth and dinitrogen fixation of wheat using<Superscript>15</Superscript>N-dilution method

The effect of Herbaspirillum seropedicae inoculation and/or maize straw (0, 5 and 10 Mg/hm2) amendment on the growth and N2 fixation of wheat was determined in pot experiments using 15N-dilution method. Inoculation resulted in accumulation of fixed nitrogen, and % N from atmosphere being 24.6 and 26.5% in wheat shoot and grain, respectively. Straw amendment reduced % Natm to 16.1 and 20.2% at high straw level (10 Mg/hm2). Rational nitrogen fertilization (180 kg N/hm2) completely inhibited N2 fixation by H. seropedicae inoculation. Bacterial inoculation increased dry shoot and grain yield up to 23 and 31%, respectively. The highest levels of shoot and grain dry mass (46.5 and 42.4%) were obtained by N-fertilization in both inoculated and uninoculated plants. Total shoot and grain N-yield increased irrespective of organic matter amendment by inoculation up to 9 and 25%, respectively. N-fertilized plants recorded a maximum increase in N-yield (57 and 51%). H. seropedicae was reisolated from inoculated wheat histosphere after harvesting (90 d from sowing). Neither organic matter nor mineral nitrogen applications had any marked effect on bacterial total counts colonizing wheat histosphere. Moreover, no symptoms of mottled stripe disease were observed on leaves and stems of inoculated plants.     

Inoculation Response of Legumes in Relation to the Number and Effectiveness of Indigenous Rhizobium Populations

The response of legumes to inoculation with rhizobia can be affected by many factors. Little work has been undertaken to examine how indigenous populations or rhizobia affect this response. We conducted a series of inoculation trials in four Hawaiian soils with six legume species (Glycine max, Vigna unguiculata, Phaseolus lunatus, Leucaena leucocephala, Arachis hypogaea, and Phaseolus vulgaris) and characterized the native rhizobial populations for each species in terms of the number and effectiveness of the population for a particular host. Inoculated plants had, on average, 76% of the nodules formed by the inoculum strain, which effectively eliminated competition from native strains as a variable between soils. Rhizobia populations ranged from less than 6 × 10⁰/g of soil to 1 × 10⁴/g of soil. The concentration of nitrogen in shoots of inoculated plants was not higher than that in uninoculated controls when the most probable number MPN counts of rhizobia were at or above 2 × 10¹/g of soil unless the native population was completely ineffective. Tests of random isolates from nodules of uninoculated plants revealed that within most soil populations there was a wide range of effectiveness for N₂ fixation. All populations had isolates that were ineffective in fixing N₂. The inoculum strains generally did not fix more N₂ than the average isolate from the soil population in single-isolate tests. Even when the inoculum strain proved to be a better symbiont than the soil rhizobia, there was no response to inoculation. Enhanced N₂ fixation after inoculation was related to increased nodule dry weights. Although inoculation generally increased nodule number when there were less than 1 × 10² rhizobia per g of soil, there was no corresponding increase in nodule dry weight when native populations were effective. Most species compensated for reduced nodulation in soils with few rhizobia by increasing the size of nodules and therefore maintaining a nodule dry weight similar to that of inoculated plants with more nodules. Even when competition by native soil strains was overcome with a selected inoculum strain, it was not always possible to enhance N₂ fixation when soil populations were above a threshold number and had some effective strains.     

Nodulation and growth ofLeucaena leucocephala (Lam.) de Wit as affected by inoculation and N fertilizer

Leonard jar, pot and field experiments examined the effects of inoculation and the influence of nitrogen fertilizer on nodulation, nitrogen fixation and growth ofLeucaena leucocephala (Lam.) de Wit at IITA, Ibadan, Nigeria. Leucaena responded to both inoculation and/or nitrogen application. Shoot growth and total N and P of inoculated plants were comparable to those of the highest N treatment, and the values were about 55% greater than those of uninoculated ones. Field data indicated that toal N yields of inoculated leucaena were increased by 50% with 40 or 80 kg ha^–1 of N fertilizer. However, N fertilizer depressed N fixation by 56% as was expected from nodule mass data. N-fixation was delayed for about 8 weeks in the plots without N. Application of small amounts of N starter (20 ppm) proved to be beneficial to satisfy the plant need during the early stage of leucaena growth. The rhizobial strains IRc 1045 and IRc 1050 were effective, competitive and survived well in the field one year after their establishment.     

Growth of an invasive legume is symbiont limited in newly occupied habitats

Mutualisms may play an important role in the establishment and invasion success of introduced species, but their influence is little studied. To test whether a lack of root nodule symbionts may limit the performance of invasive legumes, seedlings of Cytisus scoparius were introduced to an old-field habitat and then either inoculated with Bradyrhizobium strains from existing C. scoparius populations, or left uninoculated. In two separate years, inoculation more than doubled average plant biomass. For uninoculated transplants, nodule formation was positively correlated with proximity to plants of the native legume Desmodium canadense , but not related to distance from a second legume species, Apios americana. Polymerase chain reaction assays and DNA sequencing confirmed that bacteria isolated from uninoculated C. scoparius plants were indistinguishable from Bradyrhizobium strains in root nodules of D. canadense . By contrast, bacterial strains associated with A. americana were never found in C. scoparius nodules. Transplants in seven other habitats across a 160 km region also showed a highly significant, fivefold biomass increase in response to inoculation. Thus, colonizing legumes can suffer from a scarcity of nodule symbionts. However, certain indigenous legumes may create favourable microhabitats for invasion, by increasing symbiont availability in their vicinity.     

Evaluation of symbiotic properties and nitrogen contribution of mucuna to maize grown in the derived savanna of West Africa

The severity and increase of the Imperata cylindrica constraint as a weed, the decline of the traditional fallow systems as a means of soil fertility management and the lack of inorganic fertilizer appear to have created opportunities for adoption of mucuna (Mucuna pruriens) technology by smallholder farmers in some areas in the derived savanna of West Africa. What is not known, however, is the extent to which the establishment and N contribution of mucuna in these areas depend on symbiotic properties such as effective nodulation and mycorrhizal infection. Short term surveys carried out in 34 farmer's arable fields located in four different sites in the derived savanna, southern Benin, West Africa, together with results of greenhouse and field experiments showed that mycorrhizal infection rate of mucuma ranged from 2 to 31% and correlated positively with nodulation and shoot dry matter production. Nodulation occurred in 79% of the fields with numbers of nodules ranging from 0 to 135 plant^–1. Mucuna responded both to inoculation and N fertilizer in degraded soils but growth response depended on the rhizobia strains and mucuna varieties. Mucuna accumulated in 12 weeks about 313 kg N ha^–1 as either a sole crop or 166 kg N ha^–1 when mixed/intercropped with maize, respectively. Across all cropping systems it derived an average of 70% of its N from atmospheric N₂ (estimates made by the ¹⁵N isotope dilution method), representing 167 kg N ha^–1 per 12 weeks in the field. Mucuna interplanted with maize obtained a greater proportion of its nitrogen (74%) from fixation than did mucuna grown alone (66%) suggesting that competition for soil N influences the proportion of nitrogen fixed by mucuna. The total amount of N₂ fixed per hectare was, however, reduced significantly by intercropping mucuna with maize. A preceding mucuna crop provided a maize yield equivalent to 120 kg N kg ha^–1 of inorganic N fertilizer.     

Growth and nodulation of tropical food legumes in dilute solution culture

Twenty-two tropical food legumes were grown in dilute nutrient solution with or without rhizobium inoculation and supplied with either low or adequate amounts of inorganic N. Growth of legumes supplied with adequate inorganic N was generally satisfactory. However, solution phosphorus (P) concentration (15μM) was excessive for black gram, while the initial solution manganese concentration (1.8μM) was excessive for green gram. Growth responses to inoculation with rhizobium at low inorganic N supply were obtained in only 9 of the 22 legumes studied, and shoot dry matter yields were ≤ 51% of those obtained with adequate N supply. Poor growth by inoculated plants with a low N supply was attributed to failure of the inoculated strain of Bradyrhizobium to infect roots (lima bean and Mexican yam bean), to low nodule numbers (green gram, black gram and navy bean), or to excessive uptake of P (black gram, adzuki bean, pigeonpea, winged bean and cowpea cv. Vita 4) and/or manganese (green gram and black gram). High solution temperatures may have limited N fixation by some of the legumes, particularly chickpea.     

Influence of the Size of Indigenous Rhizobial Populations on Establishment and Symbiotic Performance of Introduced Rhizobia on Field-Grown Legumes

Indigenous rhizobia in soil present a competition barrier to the establishment of inoculant strains, possibly leading to inoculation failure. In this study, we used the natural diversity of rhizobial species and numbers in our fields to define, in quantitative terms, the relationship between indigenous rhizobial populations and inoculation response. Eight standardized inoculation trials were conducted at five well-characterized field sites on the island of Maui, Hawaii. Soil rhizobial populations ranged from 0 to over 3.5 × 10⁴ g of soil^-1 for the different legumes used. At each site, no less than four but as many as seven legume species were planted from among the following: soybean (Glycine max), lima bean (Phaseolus lunatus), cowpea (Vigna unguiculata), bush bean (Phaseolus vulgaris), peanut (Arachis hypogaea), Leucaena leucocephala, tinga pea (Lathyrus tingeatus), alfalfa (Medicago sativa), and clover (Trifolium repens). Each legume was (i) inoculated with an equal mixture of three effective strains of homologous rhizobia, (ii) fertilized at high rates with urea, or (iii) left uninoculated. For soybeans, a nonnodulating isoline was used in all trials as the rhizobia-negative control. Inoculation increased economic yield for 22 of the 29 (76%) legume species-site combinations. While the yield increase was greater than 100 kg ha^-1 in all cases, in only 11 (38%) of the species-site combinations was the increase statistically significant (P ≤ 0.05). On average, inoculation increased yield by 62%. Soybean (G. max) responded to inoculation most frequently, while cowpea (V. unguiculata) failed to respond in all trials. Inoculation responses in the other legumes were site dependent. The response to inoculation and the competitive success of inoculant rhizobia were inversely related to numbers of indigenous rhizobia. As few as 50 rhizobia g of soil^-1 eliminated inoculation response. When fewer than 10 indigenous rhizobia g of soil^-1 were present, economic yield was significantly increased 85% of the time. Yield was significantly increased in only 6% of the observations when numbers of indigenous rhizobia were greater than 10 cells g of soil^-1. A significant response to N application, significant increases in nodule parameters, and greater than 50% nodule occupancy by inoculant rhizobia did not necessarily coincide with significant inoculation responses. No less than a doubling of nodule mass and 66% nodule occupancy by inoculant rhizobia were required to significantly increase the yield of inoculated crops over that of uninoculated crops. However, lack of an inoculation response was common even when inoculum strains occupied the majority of nodules. In these trials, the symbiotic yield of crops was, on average, only 88% of the maximum yield potential, as defined by the fertilizer N treatment. The difference between the yield of N-fertilized crops and that of N₂-fixing crops indicates a potential for improving inoculation technology, the N₂ fixation capacity of rhizobial strains, and the efficiency of symbiosis. In this study, we show that the probability of enhancing yield with existing inoculation technology decreases dramatically with increasing numbers of indigenous rhizobia.     

Low concentrations of nitrate and ammonium stimulate nodulation and N2 fixation while inhibiting specific nodulation (nodule DW g−1 root dry weight) and specific N2 fixation (N2 fixed g−1 root dry weight) in soybean

Nitrate N is a major inhibitor of the soybean/Bradyrhizobium symbiosis in legumes and although this inhibition has been studied for many years, as yet no consensus has been reached on the specific and quantitative interactions between nitrate and ammonium supply and N₂ fixation. The effect of nitrate and ammonium supply on plant growth, nodulation and N₂ fixation capacity during the full growth cycle was investigated in both greenhouse and growth chamber experiments with three soybean genotypes. The results show that a high concentration of mineral N (10 mM), either as nitrate or ammonium or ammonium nitrate significantly suppressed nodule number, nodule dry weight and total N₂ fixed per plant of nodulated soybeans. However, lower mineral N concentrations, either 1 mM or 3.75 mM significantly enhanced nodule number, nodule dry weight and total N₂ fixed per plant, while specific nodulation (nodule dry weight g^–1 root DW, SNOD) and specific N₂ fixation (total N₂ fixed g^–1 root DW, SNF) were significantly reduced, particularly at the early vegetative growth stage V4, compared to the treatment with N₂ fixation as the only N source, in both growth chamber and greenhouse experiments. Therefore, we suggest that SNOD or SNF might be better indicators to express the suppressing effect of mineral N addition on nodule performance and N₂ fixed. Our studies also showed that ammonium alone was the more efficient N source than either ammonium nitrate or nitrate for soybean, as it resulted in higher biomass accumulation, nodule dry weight, total N accumulation and total N₂ fixed by 23, 20, 18 and 44%, respectively, compared to NO₃ ^– as the N source.     

Nitrogen dynamics in grain crop and legume pasture systems under elevated atmospheric carbon dioxide concentration: A meta‐analysis

Understanding nitrogen (N) removal and replenishment is crucial to crop sustainability under rising atmospheric carbon dioxide concentration ([CO₂]). While a significant portion of N is removed in grains, the soil N taken from agroecosystems can be replenished by fertilizer application and N₂ fixation by legumes. The effects of elevated [CO₂] on N dynamics in grain crop and legume pasture systems were evaluated using meta‐analytic techniques (366 observations from 127 studies). The information analysed for non‐legume crops included grain N removal, residue C : N ratio, fertilizer N recovery and nitrous oxide (N₂O) emission. In addition to these parameters, nodule number and mass, nitrogenase activity, the percentage and amount of N fixed from the atmosphere were also assessed in legumes. Elevated [CO₂] increased grain N removal of C₃ non‐legumes (11%), legumes (36%) and C₄ crops (14%). The C : N ratio of residues from C₃ non‐legumes and legumes increased under elevated [CO₂] by 16% and 8%, respectively, but the increase for C₄ crops (9%) was not statistically significant. Under elevated [CO₂], there was a 38% increase in the amount of N fixed from the atmosphere by legumes, which was accompanied by greater whole plant nodule number (33%), nodule mass (39%), nitrogenase activity (37%) and %N derived from the atmosphere (10%; non‐significant). Elevated [CO₂] increased the plant uptake of fertilizer N by 17%, and N₂O emission by 27%. These results suggest that N demand and removal in grain cropping systems will increase under future CO₂‐enriched environments, and that current N management practices (fertilizer application and legume incorporation) will need to be revised.     

Amount of nitrogen fixed by forage,pasture and grain legumes in Cyprus,estimated by the A-value and a modified difference method

The increasing need for protein at low cost has created a need to evaluate the biological nitrogen fixing potential of legumes in Cyprus. In field studies which were conducted over the growing years of 1982–3 and 1983–4, legumes which are traditionally grown in the country were evaluated for dry matter and nitrogen yield and biological nitrogen fixation (BNF). The legumes studied were medic (Medicago truncatula Gearth), ochrus vetch (Lathyrus ochrus L.), bitter vetch (Vicia ervilia L.) and faba bean (Vicia faba L. var major) in the first year and in addition chickpea (Cicer arietinum L.), woollypod vetch (Vicia dasycarpa Ten.) and tickbean (Vicia faba L. var minor) in the second year. Using the A-value method with barley and oats as reference crops, nitrogen (N) fixed by the various legumes in the first year was 30–50% and from 55–67% of total N yield for the two reference crops, respectively. In the second year the estimates of N fixed ranged from 70 to 80% with similar results obtained for the two reference crops barley and ryegrass. However, in the second year chickpea, which had limited nodulation, fixed only 40% of its N yield. Estimates of nitrogen from the atmosphere (Ndfa) obtained by the difference method (DM) were 10 to 14% lower than those from the A-value method. These results were obtained after correcting for the amount of N derived from the applied fertilizer. The two methods were highly correlated (r=0.98) for estimates of amount of BNF. The rates of N₂ fixation of uninoculated legumes which are nodulated by the indigenous populations of Rhizobium in Cyprus are comparable to those of legumes inoculated with selected strains of Rhizobium in other countries. An exception was the amount of N fixed by chickpea. The appearance of the first nodules at late stages of growth may be the reason for the low BNF of this crop.     

Response of cowpea (Vigna unguiculata) to inoculation with VA-mycorrhizal fungi and to rock phosphate fertilization in some unsterilized Nigerian soils

Summary The effects on cowpea of inoculation with vesicular-arbuscular (VA) mycorrhizal fungi and rock phosphate (RP) fertilization were studied in pots using Alagba and Araromi series soils and in the field on Alagba, Apomu and Egbeda series soils. Inoculation of the plants with VA-mycorrhizal fungi caused very rapid infection of the roots. A higher per cent mycorrhizal infection was maintained during subsequent plant growth in the field. RP application reduced the degree of infection without affecting plant growth in the field and in pot experiments. Nodulation, nitrogen fixation and utilization of RP were increased by inoculation with mycorrhizal fungi in the pot experiments but not in the field experiments. In the pot experiments, inoculated plants supplied with RP flowered earlier, and took up more phosphorus than either inoculated plants without RP or uninoculated plants. The largest response to inoculation in terms of shoot dry matter, nodule yield and nitrogen content of shoots was obtained in Alagba soil under both pot and field conditions.IITA Journal Series Paper No. 136.     

Nitrogen fixation and growth of soybean as influenced by varying the methods of inoculation with Bradyrhizobium japonicum

The effects of inoculating soil with a water suspension of Bradyrhizobium japonicum (i) at seeding, (ii) 7, or (iii) 14 days after planting (DAP), (iv) seed slurry inoculation and (v) seed slurry supplemented with postemergence inoculation of a water suspension of Bradyrhizobium at 7 or (vi) 14 DAP, on nodulation, N₂ fixation and yield of soybean (Glycine max. [L.] Merrill) were compared in the greenhouse. The ¹⁵N isotope dilution technique was used to quantify N₂ fixed at flowering, early pod filling and physiological maturity stages (36, 52 and 70 DAP, respectively). On average, the water suspension inoculation formed the greatest number of nodules, and seed plus postemergence inoculation formed slightly more nodules than the seed-only inoculated plants (27, 19 and 12 nodules/plant respectively at physiological maturity). Seed slurry inoculation followed by postemergence inoculation at 14 DAP gave the highest nodule weight, with the plants fixing significantly more (P<0.05) N₂ (125 mg N plant⁻¹ or 56% N) than any other treatment (mean, 75 mg plant⁻¹ or 35% N). However, the higher N₂ fixation was not translated into higher N or dry matter yields. Estimates of N₂ fixed by the ostemergence Bradyrhizobium inoculations as well as plant yield were not significantly different from those of the seed slurry inoculation. Thus, delaying inoculation (e.g., by two weeks as in this study) did not reduce the symbiotic ability of soybean plants.     

The effect of Erwinia carotovora subsp. atroseptica (blackleg) on potato plants. II. Compensatory growth

In field experiments in 1981 and 1982, uninoculated seed tubers (cv. Désirée) and those inoculated with Erwinia carotovora subsp. atroseptica at the rose (apical) or heel (stolon attachment) ends were planted at normal (35 cm) or double spacing; in additional plots, inoculated and uninoculated tubers were planted alternately. Inoculation, especially at the rose end, decreased plant height and sometimes resulted in blackleg symptoms. Individual plant yields were recorded at the end of the season. In plots of uniform seed type at normal spacing, inoculation decreased total yield compared with uninoculated by 12–13% (heel-end inoculation) or 26–40% (rose-end inoculation). At double spacing, yields increased compared with normal spacing by 44–58% (uninoculated or heel-end inoculation) or 30–39% (rose-end inoculation). When rose-end-inoculated and uninoculated seed tubers were planted alternately, inoculated plants yielded less and uninoculated plants more than in plots planted throughout with the same seed treatment. The abilities of inoculated and uninoculated plants to compensate for weak or missing neighbours were combined using equations to predict the yields of crops with different proportions of diseased or missing plants.     

土壤氮磷添加下豆科草本植物生物固氮与磷获取策略的权衡机制

豆科草本植物固氮是陆地生态系统重要的自然氮输入方式, 影响着草地生产的经济性和可持续性。为探讨氮磷交互作用影响豆科草本植物生物固氮率的潜在生理生态机制, 该研究选取8种豆科草本植物分别种植在对照、氮肥添加、磷肥添加和氮磷耦合添加处理的土壤中, 进行野外盆栽实验。测定了初花期植物生物量和营养含量、根部碳水化合物含量、根际pH、根际柠檬酸含量、根际有效磷含量、植物根瘤生物量、磷含量及其生物固氮率。主要结果: 依赖于豆科物种, 氮添加显著促进了豆科草本植物根际磷的活化, 降低了根生物量分配以及根系非结构性碳水化合物含量。在两种磷添加处理下, 氮添加导致8种豆科草本植物根瘤生物量平均下降27%-36%, 生物固氮率平均下降20%-33%。磷添加降低了根际的磷活化, 但促进了豆科草本植物根系发育和非结构性碳水化合物的积累。在施氮和不施氮条件下, 磷添加分别使8种豆科草本植物的生物固氮率提高了45%-69%和0-47%。氮添加降低豆科草本植物生物固氮率, 其原因是氮添加提高了植物磷需求, 为活化更多磷, 豆科草本植物降低根系生物量和根系非结构性碳水化合物的含量, 导致根瘤发育受到限制。在氮添加的同时进行磷添加, 能够改善土壤氮磷平衡, 促进根系生长和非结构性碳水化合物积累, 缓解了增氮对生物固氮的抑制作用。     

Response of field-grownCasuarina equisetifolia to inoculation withFrankia strain ORS 021001 entrapped in alginate beads

A large scale field experiment (ca 1 ha) was carried out in Senegal, to evaluate the response ofCasuarina equisetifolia to inoculation withFrankia strain ORS 021001 entrapped in alginate beads. Biomasses (expressed as dry weight or total nitrogen) of assimilatory branchlets, wood and roots, and nodules were measured in uninoculated and inoculated trees, randomly sampled 1,2 and 3 years after transplantation in the field. When biomasses were expressed as dry weight, increases due to inoculation were similar at the three sampling dates, 45, 36 and 40%, respectively. When biomasses were expressed as total nitrogen, the response to inoculation with time was much higher in the 2nd year than in the 1st and 3rd year. N₂ fixation, estimated using the difference method reached 2.48, 12.25 and 13.44 g N₂ fixed annually per tree. Correspondingly, nodule dry weights, expressed in g per tree, were 2.5, 12.18 and 22.75 at the end of the 1 st, 2nd and 3rd year, respectively. In spite of the positive response of field-grownCasuarina equisetifolia to inoculation, the decrease of N₂ fixation observed in the third year was probably due to unfavorable climatic conditions coupled with insect attacks at the beginning of the third year.     

Cutting regime affects the amount and allocation of symbiotically fixed N in green manure leys

Cutting strategy effect on N₂ fixation and distribution of fixed N above and below ground in red clover (Trifolium pratense L.) and mixed red clover/perennial ryegrass (Lolium perenne L.) green manure leys was quantified in field experiments including in situ mezotrons and microplots. Symbiotically fixed N in clover, transfer of fixed N to grass in the mixed stands and the fate of ¹⁵N contained in mulch were estimated by isotope dilution. Below ground clover-derived N was estimated by leaf labelling. Total N₂ fixation was estimated by correcting fixed N in plant shoots with plant-derived N below ground and recycled N from mulch. The total N₂ fixation was larger in harvested and mulched stands (average 45 g?m^?2) than in the intact stands (32 g?m^?2). Of the fixed N, 53% (intact), 46% (harvested) and 60% (mulched) was found below ground. The average recycling of N in mulch was 21% and contributed 13.7% (pure clover) and 2.2% (mixed) of the clover N in the regrowth. Recycling of N did not decrease N₂ fixation in the mulched compared with harvested stands. The results indicate that cutting regime should be considered when estimating total amounts of N fixed by green manure leys.     

Global inputs of biological nitrogen fixation in agricultural systems

Biological dinitrogen (N₂) fixation is a natural process of significant importance in world agriculture. The demand for accurate determinations of global inputs of biologically-fixed nitrogen (N) is strong and will continue to be fuelled by the need to understand and effectively manage the global N cycle. In this paper we review and update long-standing and more recent estimates of biological N₂ fixation for the different agricultural systems, including the extensive, uncultivated tropical savannas used for grazing. Our methodology was to combine data on the areas and yields of legumes and cereals from the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N₂ fixation. As the FAO lists grain legumes only, and not forage, fodder and green manure legumes, other literature was accessed to obtain approximate estimates in these cases. Below-ground plant N was factored into the estimations. The most important N₂-fixing agents in agricultural systems are the symbiotic associations between crop and forage/fodder legumes and rhizobia. Annual inputs of fixed N are calculated to be 2.95 Tg for the pulses and 18.5 Tg for the oilseed legumes. Soybean (Glycine max) is the dominant crop legume, representing 50% of the global crop legume area and 68% of global production. We calculate soybean to fix 16.4 Tg N annually, representing 77% of the N fixed by the crop legumes. Annual N₂ fixation by soybean in the U.S., Brazil and Argentina is calculated at 5.7, 4.6 and 3.4 Tg, respectively. Accurately estimating global N₂ fixation for the symbioses of the forage and fodder legumes is challenging because statistics on the areas and productivity of these legumes are almost impossible to obtain. The uncertainty increases as we move to the other agricultural-production systems—rice (Oryza sativa), sugar cane (Saccharum spp.), cereal and oilseed (non-legume) crop lands and extensive, grazed savannas. Nonetheless, the estimates of annual N₂ fixation inputs are 12–25 Tg (pasture and fodder legumes), 5 Tg (rice), 0.5 Tg (sugar cane), <4 Tg (non-legume crop lands) and <14 Tg (extensive savannas). Aggregating these individual estimates provides an overall estimate of 50–70 Tg N fixed biologically in agricultural systems. The uncertainty of this range would be reduced with the publication of more accurate statistics on areas and productivity of forage and fodder legumes and the publication of many more estimates of N₂ fixation, particularly in the cereal, oilseed and non-legume crop lands and extensive tropical savannas used for grazing.     

The abilities of three Frankia isolates to nodulate and fix nitrogen with four species of Casuarina

Three isolates of Frankia from nodules of Casuarina sens, strict. (JCT287. JCT295 and 20607) were compared in their abilities to nodulate and fix N, when associated with four species of Casuarina ( C. cunninghamiana Miq., C. equisetifolia Forst., C obesa Miq. and C. glauca Sieb. ex Spreng) growing in a N-deficient soil.
All three Frankia isolates nodulated each of the four species of Casuarina . At 27 weeks after inoculation, growth (dry weight) of inoculated plants was 3.6 to 5.0 times greater than that of uninoculated plants. There were no significant differences in plant dry weight, the N concentration of shoots or roots, or the amount of N, fixed per plant among the Frankia isolates for each of the species of Casuarina studied. The infectivity and effectiveness in N, fixation of Frankia strain JCT287 with C. cunninghamiana was similar when two different defined media were used for culture of the inoculum.     

In situ lateral transfer of symbiosis islands results in rapid evolution of diverse competitive strains of mesorhizobia suboptimal in symbiotic nitrogen fixation on the pasture legume Biserrula pelecinus L

The multi-billion dollar asset attributed to symbiotic nitrogen fixation is often threatened by the nodulation of legumes by rhizobia that are ineffective or poorly effective in N(2) fixation. This study investigated the development of rhizobial diversity for the pasture legume Biserrula pelecinus L., 6 years after its introduction, and inoculation with Mesorhizobium ciceri bv. biserrulae strain WSM1271, to Western Australia. Molecular fingerprinting of 88 nodule isolates indicated seven were distinctive. Two of these were ineffective while five were poorly effective in N(2) fixation on B. pelecinus. Three novel isolates had wider host ranges for nodulation than WSM1271, and four had distinct carbon utilization patterns. Novel isolates were identified as Mesorhizobium sp. using 16S rRNA, dnaK and GSII phylogenies. In a second study, a large number of nodules were collected from commercially grown B. pelecinus from a broader geographical area. These plants were originally inoculated with M. c bv. biserrulae WSM1497 5-6 years prior to isolation of strains for this study. Nearly 50% of isolates from these nodules had distinct molecular fingerprints. At two sites diverse strains dominated nodule occupancy indicating recently evolved strains are highly competitive. All isolates tested were less effective and six were ineffective in N(2) fixation. Twelve randomly selected diverse isolates clustered together, based on dnaK sequences, within Mesorhizobium and distantly to M. c bv. biserrulae. All 12 had identical sequences for the symbiosis island insertion region with WSM1497. This study shows the rapid evolution of competitive, yet suboptimal strains for N(2) fixation on B. pelecinus following the lateral transfer of a symbiosis island from inoculants to other soil bacteria.