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The performance of the soybean (Glycine max) inoculated with Bradyrhizobium is influenced by cropping history. However, re-inoculation with elite strains of Bradyrhizobium generally provides increases in soybean grain yield. The Amazonian savanna has an agricultural frontier area where soybeans have recently been introduced. In this study, we evaluated soybean nodulation, N accumulation, N2 fixation, and grain yield responses to inoculation with the commercial Bradyrhizobium strains SEMIA 587 and SEMIA 5019 of B. elkanii, SEMIA 5079 of B. japonicum, and SEMIA 5080 of B. diazoefficiens. Four field experiments were performed in areas with or without previous cropping with inoculated soybean that contained preexisting bradyrhizobia populations of ≥ 103 or

Intercropping is commonly used in low-input systems but could also be a strategy for higher input systems. Three ratios of substitutive oat–pea intercrops were tested on a fertile soil in eastern A...

An acid Alfisol (Typic Paleustalfs) was amended in triplicates at the rate of 5 g kg−1 soil (12 t/ha) with organic amendment (cow dung (CD), corn cob (CC) and 1:1 CD:CC (CDCC), and 20 kg N ha−1 as ...

Biological nitrogen fixation (BNF) is an important nitrogen source for both N2-fixers and their neighboring plants in natural and managed ecosystems. Biological N fixation can vary considerably depending on soil conditions, yet there is a lack of knowledge on the impact of varying soils on the contribution of N from N2-fixers in mixed swards. In this study, the amount and proportion of BNF from red clover were assessed using three grassland soils. Three soil samples, Hallsworth (HH), Crediton (CN), and Halstow (HW) series, were collected from three grassland sites in Devon, UK. A pot experiment with 15N natural abundance was conducted to estimate BNF from red clover, and the proportion of N transferred from red clover to the non-N2 fixing grass in a grass-clover system. The results showed that BNF in red clover sourced from atmosphere in the HH soil was 2.92 mg N plant−1, which was significantly lower than that of the CN (6.18 mg N plant−1) and HW (8.01 mg N plant−1) soils. Nitrogen in grass sourced from BNF via belowground was 0.46 mg N plant−1 in the HH soil, which was significantly greater than that in CN and HW soils. However, proportionally there were no significant differences in the percentage N content of both red clover and grass sourced from BNF via belowground among soils, at 65%, 67%, 65% and 35%, 27%, 31% in HH, CN, and HW, respectively. Our observations indicate that the amount of BNF by red clover varies among grassland soils, as does the amount of N sourced from BNF that is transferred to neighboring plants, which is linked to biomass production. Proportionally there was no difference among soils in N sourced from BNF in both the red clover plants and transferred to neighboring plants.

The accurate estimate of N2 fixation by legume trees using the 15N natural abundant method ((15)NNAM) requires the presence of a reference plant, whose δ(15)N reflects that of the soil derived-N. Our study was conducted in four cocoa (Theobroma cacao L.) plantations located in two cocoa growing regions of Ghana. Gliricidia trees (Gliricidia sepium Jacq. Kunth ex Walp.), a legume which fixes N2 were intercropped in the investigated plantations. The cocoa trees were planted in rows with a density of 3×3 m, while Gliricidia trees were dispersed at variable distances from cocoa ( 10 m). Our objective was to find out to which extent the presence of Gliricidia in an intercropping system influences δ15N of cocoa (no ability to fix N2) growing at different distances from Gliricidia. We used the (15)NNAM which is based on the difference in δ15N values between the soil-mineral N (assessed by the reference plant cocoa) and atmospheric N2, to assess N2-fixation. At the end of both the rainy and dry season, we sampled young, but fully expanded leaves of cocoa trees which were at least 12 m away (referred as ‘cocoa far’) or at 5 m or closer (referred as ‘cocoa close’) to the nearest Gliricidia tree. Gliricidia leaves were sampled as well. Our data show that, except in one site, the δ(15)N of ‘cocoa far’ was always higher (p

Sulfur (S) and Nitrogen (N) metabolisms in plants are interacted and it is known that S deficiency decrease N absorption and metabolism. In leguminous plants S deficiency also decreases N2 fixation by rhizobia in the nodules. Deep placement of a controlled-release N fertilizer is a good method to provide nitrogen to soybean without inhibiting N2 fixation; thus, it was hypothesized that this method is able to provide nitrogen effectively to sulfur-deficient soybean plants. In this study effects of deep placement of coated urea on S-N physicological interaction, growth and productivity in soybean plants were examined using pot experiments. Soybean plants were grown with sulfate concentrations of 30, 100, or 1000 μM, with or without deep placement of coated urea. Shoot weights at the developing stage were not affected by S deficiency. SPAD values of leaves during the flowering stage decreased with S deficiency and increased with the deep placement of coated urea. S deficiency decreased seed weight per plant at the harvesting stage, but this decrease was attenuated by the deep placement of coated urea. N and S content in shoots at the developing stage increased with the deep placement of coated urea, whereas in seeds, only the N content increased. N2 fixation activity based on the relative ureide-N content in xylem sap indicated that the deep placement of coated urea did not inhibit N2 fixation activity at the early flowering stage. Without deep placement of coated urea, the relative ureide-N content decreased under S deficiency at the seed filling stage. These results suggest that the deep placement of coated urea is an efficient method to supply N to support soybean yield under S deficiency. Abbreviations: Deep+: with deep placement of coated urea; Deep–: without deep placement of coated urea

We investigated Bambara groundnut, groundnut, mung bean, cowpea, and black gram for use as biofertilizers in cropping systems. The 15N natural abundance technique was used to measure N2 fixation in this study. The percent of N derived from fixation by mung bean (Vigna radiata L. Wilczek), Bambara groundnut (Vigna subterranea L. Verdc.), cowpea (Vigna unguiculata L. Walp.), black gram (Vigna mungo L.), and groundnut (Arachis hypogaea L) was 98, 83, 79, 66, and 45% respectively. Nitrogen contribution from these legumes was 83, 67, 39, 36, and 32 kg.ha−1 respectively for Bambara groundnut, groundnut, mung bean, black gram, and cowpea. Maize grain yield without N fertilizer was 2,449, 2,291, 2,204, 2,046, and 1,671 kg.ha−1, respectively, for maize following groundnut, Bambara groundnut, cowpea, mung bean, black gram, and maize. Grain yield increase of maize after legumes without N fertilizer was 47, 46, 37, 32, and 22%, respectively, for groundnut, Bambara groundnut, cowpea, mung bean, and black gram. Supplying 0 to 60 kg N ha−1 to maize plants increased shoot DM from 3,264 to 4,279 kg.ha−1, grain yield from 2,184 to 3,586 kg.ha−1, and whole-plant DM from 5,448 to 7,865 kg.ha−1, which represented a 31, 64, and 44% increase with N fertilizer supply from 0 to 60 kg N ha−1. Symbiotic N benefit of preceding legumes to maize without N fertilizer was 20–40 kg N. ha−1 in fertilizer equivalents. The preceding legumes increased maize grain concentrations of P, Ca, S, Fe, Mn, and Zn in zero-N plots relative to maize after maize. There was a 225, 222, 154, 149, and 108% increase in marginal returns of maize after groundnut, Bambara groundnut, cowpea, mung bean, and black gram, respectively, without N fertilizer.

Hexaploid Caucasian clover was grown in soil sampled at three New Zealand South Island high country sites to which specific rhizobium inoculum had been added with sowing of the legume in 1975, 1992...

Legumes tend to be nodulated by competitive rhizobia that do not maximize nitrogen (N 2 ) fixation, resulting in suboptimal yields. Rhizobial nodulation competitiveness and effectiveness at N 2 fixation are independent traits, making their measurement extremely time-consuming with low experimental throughput. To transform the experimental assessment of rhizobial competitiveness and effectiveness, we have used synthetic biology to develop reporter plasmids that allow simultaneous high-throughput measurement of N 2 fixation in individual nodules using green fluorescent protein (GFP) and barcode strain identification (Plasmid ID) through next generation sequencing (NGS). In a proof-of-concept experiment using this technology in an agricultural soil, we simultaneously monitored 84 different Rhizobium leguminosarum strains, identifying a supercompetitive and highly effective rhizobial symbiont for peas. We also observed a remarkable frequency of nodule coinfection by rhizobia, with mixed occupancy identified in ∼20% of nodules, containing up to six different strains. Critically, this process can be adapted to multiple Rhizobium -legume symbioses, soil types, and environmental conditions to permit easy identification of optimal rhizobial inoculants for field testing to maximize agricultural yield.

In Mediterranean areas, sustainable production of rainfed pastures is associated with plant species which withstand low soil fertility and variable annual drought, phosphorus (P) being the ...

Nutrient-poor soils coupled with micronutrient deficiency among many rural resource-poor communities remain a challenge in sub-Saharan Africa. Nutrient-poor soils can be managed through various soil amendment or fertilisation strategies. Micronutrients can be supplied through plants. The study was aimed at determining the symbiotic nitrogen fixation of cowpea as well as the contribution of inter-cropping under varying levels of nitrogen, phosphorus, and potassium (NPK) fertilisation. In addition, the amount of micronutrients supplied by cowpea and amaranth were determined. The experiment was laid out in a 2 &times, 4 factorial treatment structure in a completely randomised design, with inter-cropping (cowpea and amaranth) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels) as treatment factors with four replications. Symbiotic N2 fixation of cowpea decreased from 341&ndash, 448 kgN.ha&minus, 1 to 77&ndash, 91 kgN.ha&minus, 1 for the first year and 557&ndash, 227 kgN.ha&minus, 1 to 92&minus, 164 kgN.ha&minus, 1 for the second year with fertilisation. The iron and zinc nutritional yield increased (61&ndash, 210 g.ha&minus, 1 for first year and 304&ndash, 867 g.ha&minus, 1, for second year), proportional to fertiliser application to both crops. The research shows the benefits of leguminous crops in soil nutrient fertility and inorganic fertilisation with inter-cropping in managing micronutrient deficiency to meet the nutritional needs of rural communities.

Symbiotic N2 fixation research thus far has been primarily focused on selection of bacteria. However, little progress in impacting crop yields has resulted from this approach. Bacteria introduced in field soils rarely compete well with indigenous bacteria, including mutated lines selected for high nitrogen fixation capacity. Consequently, introduction of 'elite' bacteria in fields commonly does not result in crop yield increase. This review highlights that the primary regulation of N2 fixation is a result of response of integrated physiological activity at the plant level. Nitrogen feedback from the host plant plays an important role in regulating the N2 fixation rate. Rapid sequestration of fixed nitrogen by the plant is especially important for high N2 fixation activity. In addition, water cycling in the plant between the shoot and nodules plays a key role in sustaining high N2 fixation activity. Therefore, attention in selecting the host-plant genotype is suggested to be the next step to increasing N2 fixation activity of grain legumes.

Background and Aims. An excessive and prolonged use of fertilizers undermines soils’ quality and, consequently, that of the crops they support, thus reducing the content of organic matter and generating environmental damages and problems to human health. Therefore, the use of biofertilizers such as cyanobacteria becomes a promising alternative. However, it is not always possible to generalize these fertilizers’ applicability, because microorganisms may be impacted by the physical and chemical variations of their environment. We will describe the action mechanisms or the characteristics of cyanobacteria involved in plant growth promotion for different crops in situ through a systematic review of scientific literature. Methods. A comprehensive search for original articles in two different databases, ScienceDirect and Scopus, was performed. We included in our search documents published from 2009 to 2018. After the screening process and the addition of gray literature publications, we obtained 23 articles for theoretical analysis. Results. The studies were distributed mainly in Asia and part of Africa, without any important temporal variation. They also showed a tendency to describe the use of cyanobacteria genera such as Anabaena sp., Nostoc sp., and Calothrix sp., besides mechanisms as N2 fixation, phosphate solubilization, phytohormone production, bioactive compounds excretion, and symbiotic associations, mainly on rice, wheat and corn crops. Conclusions. Cyanobacteria fertilizers used in situ are a widespread strategy, mainly in cereal crops. Their use is predominant in countries where cereal crops make an important contribution to their national economy. The great variety of mechanisms and characteristics of cyanobacteria used to promote plant growth in the field demonstrate the dramatic influence that physical, chemical, and biological variables have in their development.

Groundnut production constitutes an integral part of the livelihoods of the people in the Guinea savanna of West Africa. This region accounts for over 70% of the total groundnut production in Ghana, 90% in Nigeria, and 100% in Mali and Burkina Faso. However, harsh environmental conditions often result in drastic yield reductions. In this study, we identified groundnut genotypes with superior symbiotic efficiency, greater pod yield, and plant water-use efficiency from 21 advanced groundnut breeding lines from ICRISAT after testing them at three locations in the Guinea savanna of Ghana over two consecutive years. Average N contribution by the groundnut genotypes ranged from 48 to 108 kg N ha−1, and mean pod yield from 580 to 2,100 kg ha−1. Genotype 17 (ICGV-IS 08837) produced about 2.5-fold more pods than genotype 1 (Chinese), which was the most widely cultivated variety by farmers. Of the 21 genotypes studied, genotype 16 (ICGV 99247) recorded the highest shoot δ13C value and was superior in water-use efficiency, which was consistent with stability estimates and mean performance. We also measured the effects of G × E on pod yield, N2 fixation, shoot δ13C, and mega-environments for testing groundnut in the Guinea savanna, and these were all significant, although the effect was minimal on shoot δ13C values. Of the locations studied, Nyankpala and Damongo were more discriminating, and each constituted a mega-environment for conducting future groundnut trials in the Guinea savanna. Genotype 3 (ICG 6222) emerged as the best cultivar for the Damongo mega-environment, while genotype 17 was the best genotype for the Nyankpala mega-environment. The genotypes exhibiting the highest sensitivity of N2 fixation in the environment included genotype 3 (ICG 6222), genotype 4 (ICGV 00068), and genotype 10 (ICGV 03315) (bi > 1.3), while Pi estimates ranked genotypes 3, 10, and 17 as the best groundnut cultivars in terms of symbiotic N contribution. Based on the results of this study, genotype 17 (ICGV-IS 08837), genotype 3 (ICG 6222), genotype 10 (ICGV 03315), and genotype 4 (ICGV 00068), which were the most outstanding in terms of the overall pod yield, shoot biomass production, and amount of N-fixed, were the most suitable candidates to recommend for use in developing new varieties for the Guinea savanna of Ghana. Genotype 17 (ICGV-IS 08837) has already been released as a commercial variety for the Guinea savanna of Ghana since October 2018.

Testing rhizobial inoculation of common bean (Phaseolus vulgaris L.) in hydroponics enables accurate quantification of biological N2 fixation (BNF) and provides information about the potential of reducing inorganic N fertilizer use. In view of this background, common bean grown on pumice was inoculated with Rhizobium tropici CIAT899 (Rt) and supplied with either full-N (total nitrogen 11.2 mmol L-1 ), 1/3 of full-N or N-free nutrient solution (NS). BNF was quantified at the early pod-filling stage using the 15 N natural abundance method.; Results: Full-N supply to Rt-inoculated plants resulted in markedly smaller nodules than less- or zero-N supply, and no BNF. Rt inoculation of full-N-treated plants did not increase biomass and pod yield compared with non-inoculation. Restriction (1/3 of full-N) or omission of inorganic N resulted in successful nodulation and BNF (54.3 and 49.2 kg N ha-1 , corresponding to 58 and 100% of total plant N content respectively) but suppressed dry shoot biomass from 191.7 (full-N, +Rt) to 107.4 and 43.2 g per plant respectively. Nutrient cation uptake was reduced when inorganic N supply was less or omitted.; Conclusion: Rt inoculation of hydroponic bean provides no advantage when full-N NS is supplied, while 1/3 of full-N or N-free NS suppresses plant biomass and yield, partly because the restricted NO3- supply impairs cation uptake. © 2017 Society of Chemical Industry.; © 2017 Society of Chemical Industry.

Legumes represent an important N source in organic vegetable rotations. Since the amount of N2 fixed as well as N mineralisation from legume biomass are highly variable, N availability does often not match the requirements of following crops. The production of grain legume seeds followed by their temporary storage and application as N fertilizer may represent a flexible and controllable alternative to traditional legume green manures. In two field experiments, narrow-leaved lupine was investigated as N source for subsequent white cabbage and beetroot and was compared to two grass-clover treatments, in which cut herbage was either removed from the sward or remained as mulch. Symbiotic N2 fixation was highest for cut grass-clover, averaging about 350 kg ha−1. Mulching reduced N2 fixation by on average 57%. Lupine N2 fixation was largely comparable to that of mulched grass-clover swards. Net N mineralisation from grass-clover residues within the year of their incorporation was positively related to t...

Intercropping has been considered to be an effective approach for producing large quantities of grain per unit of land. Maize (Zea mays L.)/pea (Pisum sativum L.) strip intercropping may serve as a model for effectively boosting a system’s productivity. However, how intercropped pea may compete for soil N sources with intercropped maize under various levels of N availability is unknown. Here, we determined the level of interspecific competition during the pea/maize cogrowth period, N2 fixation of pea, complementary growth effect on maize, and yield responses of the two component crops. The field experiment was conducted at Wuwei Experimental Station in northwestern China from 2012 to 2014. Different N management practices were implemented in the pea/maize systems. Intercropped pea was the dominant plant, as shown by the highly positive competitive ratio (averaging 1.35) and its aggressivity (averaging 0.31) values compared with intercropped maize. Ameliorating N application in the maize/pea strip intercropping intensified the interspecific competition, improved the N2 fixation of intercropped pea and increased the complementary growth of intercropped maize. On average, the N management system with 45 kg N ha−1 applied as the first topdressing plus 135 kg N ha−1 as the third topdressing increased the competitive ratio and aggressivity by 8% and 32%, respectively; improved N2 fixation of the pea by 39%; enhanced the complementary growth of maize by 10%; and boosted the grain yield by 13% (maize) and 6% (pea) compared to the N management system with 135 kg N ha−1 as the first and 45 kg N ha−1 as the third topdressing. Significant positive correlations were found among the interspecific competition, N2 fixation, and grain yield, clearly showing that improved coordination of interspecific competition can boost system productivity in maize/pea strip intercropping.

This research examined the effect of pod elimination on growth and N2 fixation ability in three annual Medicago species during drought stress and subsequent recovery. This study was carried out as a factorial experiment based on a completely randomized design with four replications. The treatments included annual Medicago species, presence or removal of the pod, and water stress at four levels: 100% (as a control), 80%, 60%, and 40% field capacity (FC) during the period of stress. The results showed that annual Medicago species can be considered as semi-drought tolerant species and their recovery ability from mild and moderate drought stress is relatively good. M. polymorpha performed better and showed higher recovery ability in comparison with other tested species. The results of this study showed that elimination of pods from annual Medicago species before sowing enhanced seedling performance under drought stress. However, the effect of pod elimination showed higher significance on growth parameters than nitrogen fixation. The information from this research would be useful for better understanding the physiological basis of changes in drought resistance, as well as targeted breeding programs, to improve the performance of annual Medicago species under conditions of limited or scarce water supply.

Vachellia sieberiana is a Fynbos legume that is highly adapted to nutrient-poor savannah ecosystems and can withstand competition from invasive shrubs like Chromolaena odorata by utilizing both atmospheric and soil nitrogen sources. The shift in the use of N resources is driven by soil-borne symbionts to the plant in the Family Rhizobiaceae, which enhance below-ground allocation to nodules and make them more efficient at fixing biological N. In this way, V. sieberiana seedlings subjected to competition have C growth costs when integrated over the whole plant growth cycle compared with V. sieberiana seedlings growing with no competition, and allocate more biomass to the below-ground structures that let them survive in impoverished soils., Vachellia sieberiana fixes atmospheric nitrogen (N) and distributes it back into ecosystems. We hypothesize that biological nitrogen fixation in this plant species is limited by competition from the invasive shrub, Chromolaena odorata. Competition would therefore result in the legume plant switching its limited nitrogen (N) sources in phosphorus-poor soils in savannah ecosystems when resources have to be shared. This study investigated the different patterns of N use and growth costs by a native and an introduced leguminous shrubby species. We propose that the two species sharing the same environment might result in competition. The competitive effect would induce in the indigenous legume to better utilize atmospheric-derived N modifying plant growth kinetics and plant mineral concentrations. Seedlings of V. sieberiana were cultivated in natural soil inoculum with low levels of phosphorus (mg L−1 ± SE) of 3.67 ± 0.88. The experiments were divided into two treatments where (i) seedlings of V. sieberiana were subjected to competition by cultivating them together with seedlings of C. odorata, and (ii) seedlings of V. sieberiana were cultivated independently. Although V. sieberiana was subjected to competition, the N2-fixing bacteria that occupied the nodules was Mesorhizobium species, similar to plants not subjected to competition. Total plant biomass was similar between treatments although V. sieberiana plants subjected to competition accumulated more below-ground biomass and showed higher carbon construction costs than plants growing individually. Total plant phosphorus and nitrogen decreased in seedlings of V. sieberiana under competition, whereas no differences were observed in percent N derived from the atmosphere (%NDFA) between treatments. The specific nitrogen utilization rate (SNUR) was higher in V. sieberiana plants subjected to competition while specific nitrogen absorption rate (SNAR) showed the opposite response. Vachellia sieberiana is highly adapted to nutrient-poor savannah ecosystems and can withstand competition from invasive shrubs by utilizing both atmospheric and soil nitrogen sources.

Naturally occurring cyanobacterial growth on soil surfaces with udic moisture regimes has received far less study than biological soil crusts (BSCs) of xeric or aridic biomes. Because they are ephemeral and recurrent on udic soils, we refer to such cyanobacterial biofilms as soil surface consortia (SSCs) to distinguish them from classical BSCs. We assessed the ability of SSCs to fix N2 as well as take up NO3−-N in fertilized soils by testing a cyanobacterial enrichment from a local agricultural field. The metagenome of this consortium, designated DG1, consisted of Cylindrospermum sp. (90%) and genomes of six non-photosynthetic bacteria. We evaluated N2 fixation by DG1 in the presence of inorganic N by measuring biomass uptake of 15N2 during 7-days incubations in a controlled-atmosphere chamber in media containing 0, 62, 124, or 247 mg L−1NO3−-N. After 7 days, mean 15N atom % excess in DG1 biomass was 0.0143, 0.0029, 0.0037, and 0.0038 at the four NO3−-N concentrations, respectively. Mean 15N atom % excess in dead cell controls was not significantly larger than zero. Mean N2 fixation rates of 101.3, 18.9, 25.6, and 26.6 μg N g−1 dry biomass d−1, respectively, indicated that DG1 continued to fix N2 in the presence of NO3−-N, but at rates 4- to 5-fold lower than in N-free medium. We also assessed the potential for the SSC to retain soil NO3−-N by applying simulated rainfall to soil microcosms inoculated with three levels of DG1 grown for 1, 3, and 7 days at varied NO3−-N concentrations. Overall, inoculation resulted in 50–70% more soil N retained after rainfall (p < 0.001) compared to non-inoculated microcosms. The effect of establishment time was significant (p = 0.043). Since water infiltration rates through microcosms were not significantly affected, we inferred that SSC biomass absorbed and/or immobilized NO3−-N. These results show how SSCs can modulate soil N, either by fixing more N2 under N-limited conditions or by immobilizing inorganic N when concentrations are higher. Thus, naturally occurring or intentionally inoculated SSCs represent potential renewable sources of biologically fixed N and means for soil stabilization and N retention in diverse agricultural systems.

Farid, M. and Navabi, A. 2015. N2 fixation ability of different dry bean genotypes. Can. J. Plant Sci. 95: 1243–1257. Common bean (Phaseolous vulgaris L.) is generally known as a weak N2 fixer compared with other legumes. The availability of genetic variation for N2 fixation potential of common bean supports the idea that symbiotic nitrogen fixation (SNF) in common bean can be improved through breeding. To assess the potential N2 fixation of selected common bean genotypes, 12 bean varieties including three Andean and nine Middle American were screened for SNF ability and related traits in controlled environments and field trials in Ontario, Canada. A non-nodulating mutant, R99, was used as the reference plant to estimate nitrogen derived from the atmosphere (Ndfa) through the natural 15N abundance method. Significant variation was found among the bean genotypes for Ndfa and its related traits. Environmental and genotype by environmental effects significantly influenced Ndfa and its related traits. The three Andean bean genotypes were superior to Middle American genotypes for nodulation ability, while the Middle American genotypes were generally stronger nitrogen fixers in optimum soil moisture conditions. In general, nitrogen fixation was found to be significantly associated with seed yield and carbon isotope discrimination, an indicator of water use efficiency.

Legumes have a significant role in effective management of fertilizers and improving soil health in sustainable agriculture. Because of the high phosphorus (P) requirements of N2-fixing nodule, P deficiency represents an important constraint for legume crop production, especially in tropical marginal countries. P deficiency is an important constraint for legume crop production, especially in poor soils present in many tropical degraded areas. Unlike nitrogen, mineral P sources are nonrenewable, and high-grade rock phosphates are expected to be depleted in the near future. Accordingly, developing legume cultivars with effective N2 fixation under P-limited conditions could have a profound significance for improving agricultural sustainability. Legumes have evolved strategies at both morphological and physiological levels to adapt to P deficiency. Molecular mechanisms underlying the adaptive strategies to P deficiency have been elucidated in legumes. These include maintenance of the P-homeostasis in nodules as a main adaptive strategy for rhizobia-legume symbiosis under P deficiency. The stabilization of P levels in the symbiotic tissues can be achieved through several mechanisms, including elevated P allocation to nodules, formation of a strong P sink in nodules, direct P acquisition via nodule surface and P remobilization from organic-P containing substances. The detailed biochemical, physiological and molecular understanding will be essential to the advancement of genetic and molecular approaches for enhancement of legume adaptation to P deficiency. In this review, we evaluate recent progress made to gain further and deeper insights into the physiological, biochemical and molecular reprogramming that legumes use to maintain P-homeostasis in nodules during P scarcity.

Water deficit is one of the major constraints for soybean production in Vietnam. The soybean breeding research efforts conducted at the Agriculture Genetics Institute (AGI) of Vietnam resulted in the development of promising soybean genotypes, suitable for the drought-stressed areas in Vietnam and other countries. Such a variety, namely, DT2008, was recommended by AGI and widely used throughout the country. The aim of this work was to assess the growth of shoots, roots, and nodules of DT2008 versus Williams 82 (W82) in response to drought and subsequent rehydration in symbiotic association as a means to provide genetic resources for genomic research. Better shoot, root, and nodule growth and development were observed in the cultivar DT2008 under sufficient, water deficit, and recovery conditions. Our results represent a good foundation for further comparison of DT2008 and W82 at molecular levels using high throughput omic technologies, which will provide huge amounts of data, enabling us to understand the genetic network involved in regulation of soybean responses to water deficit and increasing the chances of developing drought-tolerant cultivars.

The potential effects of carbonaceous nanomaterials (CNMs) on agricultural plants are of concern. However, little research has been performed using plants cultivated to maturity in soils contaminated with various CNMs at different concentrations. Here, we grew soybean for 39 days to seed production in soil amended with 0.1, 100, or 1000 mg kg-1 of either multiwalled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), or carbon black (CB) and studied plant growth, nodulation, and dinitrogen (N2) fixation potential. Plants in all CNM treatments flowered earlier (producing 60% to 372% more flowers when reproduction started) than the unamended controls. The low MWCNT-treated plants were shorter (by 15%) with slower leaf cover expansion (by 26%) and less final leaf area (by 24%) than the controls. Nodulation and N2 fixation potential appeared negatively impacted by CNMs, with stronger effects at lower CNM concentrations. All CNM treatments reduced the whole-plant N2 fixation potential, with the highest reductions (by over 91%) in the low and medium CB and the low MWCNT treatments. CB and GNPs appeared to accumulate inside nodules as observed by transmission electron microscopy. CNM dispersal in aqueous soil extracts was studied to explain the inverse dose-response relationships, showing that CNMs at higher concentrations were more agglomerated (over 90% CNMs settled as agglomerates >3 μm after 12 h) and therefore proportionally less bioavailable. Overall, our findings suggest that lower concentrations of CNMs in soils could be more impactful to leguminous N2 fixation, owing to greater CNM dispersal and therefore increased bioavailability at lower concentrations.

Although often overlooked in forest research, the canopy can play an important role in forest nutrient cycling. Since the canopy is spatially isolated from the forest floor, nutrient cycling in the two areas may differ as terrestrial nutrients accumulate. We measured rates of free-living N2 fixation along an elevation gradient (1,000, 2,000 and 3,000 m) of tropical montane canopy soils, compared these to rates measured in the top 5 cm of forest floor soils (excluding fresh litter), and assessed the effects of elevated nutrient inputs to the forest floor. N2 fixation was measured using the acetylene reduction assay. Measurements occurred in the field, in the wet and dry seasons, using intact cores of soil. The forest floor had been fertilized biannually with moderate amounts of nitrogen (N) and phosphorus (P) for 4 years; treatments included control, N, P and N + P. N2 fixation rates exhibited little variation with elevation but were higher in the dry season than the wet season. Fixation was inhibited in forest floor N plots compared to control and P plots, and stimulated in canopy P plots compared to control. At 2,000 m, the canopy contributed 12 % of measured canopy and forest floor N2 fixation (1.2 kg N ha−1 year−1). Results suggest that N2 fixation is an active process in canopy soils, which is variable across seasons and sensitive to changes in terrestrial nutrient availability. Long-term terrestrial accumulation of N and/or P has the potential to significantly change the dynamics of soil N cycling in these canopies.

Objetivou-se neste trabalho avaliar o efeito de diferentes formulações à base de glyphosate sobre a colonização micorrízica e nodulação no cultivar de soja RR TMG 125. Os experimentos foram desenvolvidos em condições de campo, em Latossolo Vermelho-Amarelo, durante os anos agrícolas 2010/2011 e 2011/2012. Os tratamentos constaram da aplicação da dose de 720 g e.a. ha-1 das seguintes formulações de glyphosate: Trop®, Roundup Original®, Roundup Ultra®, Roundup WG®, Roundup Transorb R® e Zapp Qi®, além de duas testemunhas (capinada e não capinada). Utilizou-se o delineamento em blocos casualizados com quatro repetições. A matéria seca da parte aérea, nodulação e produtividade de soja não foram influenciadas pelas diferentes formulações de glyphosate. Para a colonização micorrízica, observou-se efeito positivo da formulação Roundup Original®, em relação à testemunha capinada; no entanto, esse efeito não foi contínuo nos dois anos de experimento. A variável número de nódulos foi mais afetada pelas formulações de glyphosate testadas. The aim of this study was to evaluate the effect of different glyphosate-based formulations on mycorrhizal colonization and nodulation in soybean cultivar RR TMG 125. The experiments were conducted under field conditions in Red-Yellow Latosol, during 2010/2011 and 2011/2012. The formulations Trop®, Roundup Original®, Roundup Ultra®, Roundup WG®, Roundup Transorb R® and ZappQi® were used at a glyphosate dose of 720 g a.e. ha-1. The experiment was arranged in a randomized block design with four replications and two controls (hoed control and non-hoed control). The variables shoot, nodule dry matter, and soybean yield were not influenced by the different formulations of glyphosate. For mycorrhizal colonization, it was verified that a positive effect of the herbicide Roundup Original® regarding the hoed control was not continuous in the two years of the experiment. The variable number of nodules was the most affected by the glyphosate formulations tested.

Paradoxically, symbiotic dinitrogen (N2 ) fixers are abundant in nitrogen (N)-rich, phosphorus (P)-poor lowland tropical rain forests. One hypothesis to explain this pattern states that N2 fixers have an advantage in acquiring soil P by producing more N-rich enzymes (phosphatases) that mineralise organic P than non-N2 fixers. We assessed soil and root phosphatase activity between fixers and non-fixers in two lowland tropical rain forest sites, but also addressed the hypothesis that arbuscular mycorrhizal (AM) colonisation (another P acquisition strategy) is greater on fixers than non-fixers. Root phosphatase activity and AM colonisation were higher for fixers than non-fixers, and strong correlations between AM colonisation and N2 fixation at both sites suggest that the N-P interactions mediated by fixers may generally apply across tropical forests. We suggest that phosphatase enzymes and AM fungi enhance the capacity of N2 fixers to acquire soil P, thus contributing to their high abundance in tropical forests.

The contribution of N(2) fixation to overall soybean N uptake has most commonly been quantified by N isotope-based methods, which rely on isotopic differences in plant N between legumes and non-fixing reference plants. The choice of non-fixing reference plants is critical for the accuracy of isotope-based methods, and mismatched reference plants remain a potential source of error. Accurate estimates of soybean N(2) fixation also require information on N isotopic fractionation within soybean. On the basis of a previous observation of a close correlation between an expression of N fractionation within soybean and the proportion of plant N derived from atmosphere (%Ndfa) determined by (15) N natural abundance, this field study aimed at assessing the relationship between various expressions describing intraplant (15) N or N partitioning and %Ndfa during soybean development. Starting from a late vegetative stage until beginning senescence, the N content and N isotopic composition of shoots, roots and nodules of nodulated and non-nodulated soybeans was determined at eight different developmental stages. Regression analysis showed that %Ndfa most closely correlated with the difference in the N isotopic composition of shoot N minus that of root including nodule N, and that this relationship was similar to that obtained in a previous multi-site field study. We therefore consider this expression to hold promise as a means of quantifying %Ndfa independent of a reference plant, which would avoid some of the external sources of error introduced by the use of reference plants in determining %Ndfa.

Soybean research has found that nodule traits, especially nodule biomass, are associated with N2 fixation ability. Two genotypes, differing in nodule number per plant and individual nodule weight, KS4895 and Jackson, were mated to create 17 F3- and 80 F5-derived RILs. The population was mapped with 664 informative markers with an average distance of less than 20 cM between adjacent markers. Nodule traits were evaluated in 3-year field trials. Broad-sense heritability for nodule number (no. plant−1), individual nodule dry weight (mg nodule−1), individual nodule size (mm nodule−1), and total nodule dry weight (g plant−1) was 0.41, 0.42, 0.45, and 0.27, respectively. Nodule number was negatively correlated with individual nodule weight and size. Nodule number, individual nodule weight, and size are major components which likely contributed to increased total nodule weight per plant. Composite interval mapping (CIM) identified eight QTLs for nodule number with R2 values ranging from 0.14 to 0.20. Multiple interval mapping (MIM) identified two QTLs for nodule number, one of which was located close to the QTL identified with CIM. Six QTLs for individual nodule weight were detected with CIM, and one QTL was identified with MIM. For nodule size, CIM identified seven QTLs with R2 values ranging from 0.14 to 0.27. Five QTLs for total nodule weight were detected with CIM, one of which was located close to a QTL identified with MIM. These results document the first QTL information on nodule traits in soybean from field experiments utilizing a dense, complete linkage map.

Nitrogen (N) fixation was estimated for three Acacia senegal (L.) (A. senegal) Willd. varieties (A. senegal var. senegal, kerensis, and leiorhachis) growing naturally in different sites in the dryland areas of Kenya. The quantities of N2 fixed were estimated by the 15N natural abundance method, using leaves as the sampling material. Balanites aegyptiaca (B. aegyptiaca) was selected as the reference species growing in the same area. Soil samples were also collected under A. senegal trees for nodule assessment. Leaf 15N natural abundance values (δ15N) were significantly different between A. senegal and B. aegyptiaca. These values averaged 6.35, 4.67, and 3.03% for A. senegal var. kerensis, leiorhachis, and senegal, respectively, and were lower than those of the adjacent reference species. There were also significant differences in the amount of N2 fixed (%Ndfa) among the varieties. A. senegal var. senegal showed the highest levels of N2 fixation with a mean of 36% while A. senegal var. kerensis and leiorhac...

Transfer of fixed N from legumes to non-legume reference plants may alter the 15N signature of the reference plant as compared to the soil N available to the legume. This study investigates how N transfer influences the result of 15N-based N2 fixation measurements. We labelled either legumes or non-legumes with 15N and performed detailed analyses of 15N enrichment in mixed plant communities in the field. The results were used in a conceptual model comparing how different N transfer scenarios influenced the 15N signatures of legumes and reference plants, and how the resulting N2 fixation estimate was influenced by using reference plants in pure stand or in mixture with the legume. Based on isotopic signatures, N transfer was detected in all directions: from legume to legume, from legume to non-legume, from non-legume to legume, from non-legume to non-legume. In the scenario of multidirectional N transfer, N2 fixation was overestimated by using a reference plant in pure stand. Fixed N transferred to neighbouring reference plants modifies the 15N signature of the soil N available both to the reference plant and the N2-fixing legume. This provides strong support for using reference plants growing in mixture with the legumes for reliable quantifications of N2 fixation.

This paper reviews the main results from a set of experiments carried out in a semiarid Mediterranean environment during the past 25 years on faba bean (Vicia faba L.), a crop traditionally grown in southern Italy and Sicily under rainfed conditions. These experiments focused on the residual effects of faba bean on subsequent crop(s) and assessment of the nitrogen (N) balance during the crop cycle, paying attention to both the environmental release of N (losses via volatilisation and denitrification) and estimates of N2 fixation as influenced by tillage system, intercropping, and presence/absence of mycorrhizal inoculum. Faba bean relied on N2 fixation more than other grain legumes typically grown in the Mediterranean region (e.g., chickpea). Contributing reasons were the higher plant N demand of faba bean and its lower capacity to use soil mineral N. This implies higher N benefits for subsequent crop(s) as well as higher risk of N losses from the plant–soil– atmosphere system via leaching, denitrification, and volatilisation. Results from these experiments contribute to better defining the role of faba bean in Mediterranean agro-ecosystems and to identifying technical solutions that maximise the potential benefits of faba bean as a fertility-building crop.

Leguminous plants grown in sewage sludge-amended soils can acquire nitrogen by assimilation of nitrate and ammonium from the soil solution or from atmospheric-dinitrogen (N2) fixation through association with N2-fixing bacteria. We proposed that operation of both metabolic processes could contribute to alleviate the impact of drought in sludge-treated plants. A greenhouse experiment was conducted to evaluate the involvement of nodule metabolism in the use efficiency of water and N in sludge-treated plants. Treatments comprised (1) plants inoculated with rhizobia and amended with sewage sludge; (2) plants inoculated with rhizobia without any amendment; and (3) noninoculated plants supplied with ammonium nitrate, each under well-watered and drought conditions. Under drought, sludge-treated plants had increased plant growth and higher photosynthetic and water-use efficiencies than untreated plants. Drought stimulated nitrate reductase and GS/GOGAT activities but did not affect the activities of phosphoenolpyruvate carboxylase and malate dehydrogenase or the leghemoglobin concentration. The results suggest that under drought conditions, both N2 fixation and nitrate assimilation in nodules of sludge-treated plants contributed to improve plant N supply and to increase the drought tolerance of alfalfa. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., Authors thank NILSA (Navarra de Infraestructuras Locales, S.A.) for providing sewage sludge and sludge analysis, A. Urdiáin and M. Oyárzun for technical assistance during measurements. M. L. Fiasconaro was the recipient of a grant from Asociación de Amigos de la Universidad de Navarra.

Nitrogen (N) transfer is well documented in legume-cereal intercropping but this is less often reported for legume-Brassica intercrops even though Brassica crops require higher levels of N fertilizers. The present study was carried out to quantify N transfer from legumes (Lupinus albus L., Trifolium incarnatum L. or Vicia sativa L.) to rapeseed (Brassica napus L.) using the split-root (15)N-labelling method. After three months we observed that legumes did not alter the growth of rapeseed. Vetch showed the lowest growth and demonstrated low (15)N shoot to root translocation and no significant N transfer to rapeseed. In contrast, significant (15)N enrichment was found in lupine and clover and (15)N was transferred to the associated rapeseed plants (around 6 and 4 mg N plant(-1), respectively), which contributed 2 to 3% of the rapeseed total N. Additionally, the data revealed that N2 fixation dominated the N nutrition in lupine despite the high N level provided in the donor compartment, suggesting a greater niche segregation between companion plants. Based on the results of this study we suggest that intercropping can be a relevant contributor to rapeseed N nutrition. Among the three legumes tested, clover and lupine seemed to be the best intercropping candidates.

Engineering cereal crops that are self-supported by nitrogen fixation has been a dream since the 1970s when nitrogenase was transferred from Klebsiella pneumoniae to Escherichia coli. A renewed interest in this area has generated several new approaches with the common aim of transferring nitrogen fixation to cereal crops. Advances in synthetic biology have afforded the tools to rationally engineer microorganisms with traits of interest. Nitrogenase biosynthesis has been a recent target for the application of new synthetic engineering tools. Early successes in this area suggest that the transfer of nitrogenase and other supporting traits to microorganisms that already closely associate with cereal crops is a logical approach to deliver nitrogen to cereal crops.

Biological Nitrogen Fixation (BNF) process benefits the agriculture sector especially for reducing cost of nitrogenfertilizer. In the process, the diazotrophs convert N2 into ammonia (NH3) which is useable by plants. The BNF process iscatalysed by nitrogenase enzyme that involved protons and electrons together with evolution of H2 therefore, theassessment of N2 fixation is also available via H2 production and electron allocation analysis. Thus, the aims of thisexperiment were to estimate the nitrogenase enzyme activities and observe the influence of diazothrophs on growth oflegume (soybean) and non legume (rice) plants. Host plants were inoculated with respective inocula; Bradyrhizobiumjaponicum (strain 532C) for soybean while Azospirillum brasilense (Sp7) and locally isolated diazotroph (isolate 5) forrice. At harvest, the plants were observed for plant growth parameters, H2 evolution, N2 fixation and electron allocationcoefficient (EAC) values. The experiment recorded N2 fixation activities of inoculated soybean plants at 141.2 μmol N2 h-1g-1 dry weight nodule, and the evolution of H2 at 144.4 μmol H2 h-1 g-1 dry weight nodule. The electron allocationcoefficient (EAC) of soybean was recorded at 0.982. For inoculated rice plants, none of the observations was successfully recorded. However, results for chlorophyll contents and plant dry weight of both plants inoculated with respective inocula were similar to the control treatments supplied with full nitrogen fertilization (+N). The experiment clearly showed that inoculation of diazotrophic bacteria could enhance growth of the host plants similar to plants treated with nitrogenous fertilizer due to efficient N2 fixation process

Effects of endophytic actinomycetes and two Bradyrhizobium japonicum (Kirchner 1896) Jordan 1982 strains on growth, nodulation, nitrogen (N) fixation and seed weight of different soybean (Glycine max L. Merr.) varieties from Myanmar (Hinthada), Thailand (SJ5) and Cambodia (DT84) were studied in pot experiments in Thailand. The six treatments consisted of an uninoculated control (T1), a single inoculation with endophytic actinomycetes (Streptomyces sp. strain, P4) (T2), a single inoculation with Bradyrhizobium japonicum strains, USDA110 (T3) or THA7 (T4) and a dual inoculation of P4 with USDA110 (T5) or THA7 (T6). Sterile soil was used for cultivation of all treatments under open field conditions, using tap water for irrigation. N2 fixation activity of soybean was evaluated by the relative ureide method using xylem solute from the root bleeding sap at the early pod-fill stage. It was found that the single inoculation of P4 did not show significant effects on nodulation, N2 fixation, shoot dry weight and se...

Background & aims Understanding the mechanism of how phosphorus (P) regulates the response of legumes to elevated CO2 (eCO2) is important for developing P management strategies to cope with increasing atmospheric CO2 concentration. This study aimed to explore this mechanism by investigating interactive effects of CO2 and P supply on root morphology, nodulation and soil P fractions in the rhizosphere.