Nitrogen Fixation Efficiency Research Articles

What determines symbiotic nitrogen fixation efficiency in rhizobium: recent insights into Rhizobium leguminosarum.

Symbiotic nitrogen fixation (SNF) by rhizobium, a Gram-negative soil bacterium, is an essential component in the nitrogen cycle and is a sustainable green way to maintain soil fertility without chemical energy consumption. SNF, which results from the processes of nodulation, rhizobial infection, bacteroid differentiation and nitrogen-fixing reaction, requires the expression of various genes from both symbionts with adaptation to the changing environment. To achieve successful nitrogen fixation, rhizobia and their hosts cooperate closely for precise regulation of symbiotic genes, metabolic processes and internal environment homeostasis. Many researches have progressed to reveal the ample information about regulatory aspects of SNF during recent decades, but the major bottlenecks regarding improvement of nitrogen-fixing efficiency has proven to be complex. In this mini-review, we summarize recent advances that have contributed to understanding the rhizobial regulatory aspects that determine SNF efficiency, focusing on the coordinated regulatory mechanism of symbiotic genes, oxygen, carbon metabolism, amino acid metabolism, combined nitrogen, non-coding RNAs and internal environment homeostasis. Unraveling regulatory determinants of SNF in the nitrogen-fixing protagonist rhizobium is expected to promote an improvement of nitrogen-fixing efficiency in crop production.

Magnetic field stabilized atmospheric pressure plasma nitrogen fixation: Effect of electric field and gas temperature

In this paper, we investigate the influence of plasma characteristics on nitrogen fixation efficiency and explore the optimization of discharge parameters by utilizing a magnetic field stabilized atmospheric pressure plasma. The gas temperature and electric field of the plasma are maintained at a constant level and can be independently adjusted by controlling the discharge current, gas flow rate, and external magnetic field. The spatial distribution of the gas temperature of the plasma is measured by laser-induced Rayleigh scattering. The results show that reducing the electric field and gas temperature leads to an increase in NOx production. The optimal parameters for nitrogen fixation are identified as a discharge current of 55 mA, a gas flow rate of 6 l·min−1, and an O2 fraction of 40%. These settings result in the lowest recorded energy cost of 2.29 MJ·mol−1 and a NOx concentration of approximately 15 925 ppm. The stable characteristics of the magnetically stabilized atmospheric pressure plasma make it suitable for further investigations into the effect of plasma characteristics on nitrogen fixation.

Boron-doped graphene quantum dot/bismuth molybdate composite photocatalysts for efficient photocatalytic nitrogen fixation reactions

Bismuth molybdate (BMO) is a promising visible-driven photocatalyst and constructing heterojunctions in BMO-based materials is an effective way to enhance photocatalytic performance. In this study, boron-doped graphene quantum dots (BGQDs) were synthesized by one-step pyrolysis and carbonization, followed by the preparation of bismuth molybdate/boron-doped graphene quantum dots (BGQDs/BMO) heterojunction photocatalysts using in-situ growth method. The introduction of BGQDs significantly improved the photocatalytic nitrogen fixation activity under the irradiation of visible light and without scavengers. The highest NH3 yield was achieved with BGQDs/BMO-10, which was 3.48 times higher than pure phase BMO. This improvement was due to the formation of Z-scheme heterojunctions between BGQDs and BMO with the synergistic mechanism of interfacial charge transport and the generation of more protons. This study provides useful guidance for enhancing the visible-light nitrogen fixation performance of BMO materials.

Boron doped mesoporous g-C3N4 nanosheets: Boosting photocatalytic nitrogen fixation performance under visible light

A simple two-step method was used to prepare g-C3N4 nanosheets doped with B atoms. The multi-layer porous morphology prolongs the transmission pathway of visible light inside, thereby improving the ability to capture visible light. The doped photosensitive B atoms can decrease the original bandgap of g-C3N4 and increase the material's sensitivity to visible light. According to the findings, B/g-C3N4 performs superbly in photocatalytic nitrogen fixation, with the best performance being 2 wt% B/g-C3N4 (81.56 mol/L-1.h−1), which has a nitrogen fixation efficiency 2.44 times higher than g-C3N4 (33.52 mol/L-1.h−1). Additionally, 2 wt% B/g-C3N4 exhibits strong photocatalytic stability due to the doping of B atoms.

Anchoring heterometallic cluster on P-doped carbon nitride for efficient photocatalytic nitrogen fixation in water and air ambient

Light-driven nitrogen fixation to produce ammonia is a green and economical technology of nitrogen reduction but is still quite challenging, especially in an air atmosphere without any sacrificial reagents. Herein, we demonstrate efficient photocatalytic nitrogen fixation using water and air directly by loading lanthanide–transition metal (4f–3d) cluster NdCo3 on two-dimensional P-doped graphitic carbon nitrides (PCN) material surface. Benefiting from the increase in the number of nitrogen vacancies (NVs) and highly matched band gap structure and excellent hole trapping ability of clusters, the NdCo3/PCN photocatalyst exhibits efficient nitrogen reduction activity with 371 (in air) and 825 µmol h−1 g−1 (in pure nitrogen) without any sacrificial reagents. The introduction of potassium sulfate inhibits hydrogen production and promotes nitrogen reduction activation. This work suggests that anchoring precisely structured clusters on 2D materials may enhance photocatalytic nitrogen reduction under normal temperature and pressure.

Rhizobial nitrogen fixation efficiency shapes endosphere bacterial communities and Medicago truncatula host growth

BackgroundDespite the knowledge that the soil–plant–microbiome nexus is shaped by interactions amongst its members, very little is known about how individual symbioses regulate this shaping. Even less is known about how the agriculturally important symbiosis of nitrogen-fixing rhizobia with legumes is impacted according to soil type, yet this knowledge is crucial if we are to harness or improve it. We asked how the plant, soil and microbiome are modulated by symbiosis between the model legume Medicago truncatula and different strains of Sinorhizobium meliloti or Sinorhizobium medicae whose nitrogen-fixing efficiency varies, in three distinct soil types that differ in nutrient fertility, to examine the role of the soil environment upon the plant–microbe interaction during nodulation.ResultsThe outcome of symbiosis results in installment of a potentially beneficial microbiome that leads to increased nutrient uptake that is not simply proportional to soil nutrient abundance. A number of soil edaphic factors including Zn and Mo, and not just the classical N/P/K nutrients, group with microbial community changes, and alterations in the microbiome can be seen across different soil fertility types. Root endosphere emerged as the plant microhabitat more affected by this rhizobial efficiency-driven community reshaping, manifested by the accumulation of members of the phylum Actinobacteria. The plant in turn plays an active role in regulating its root community, including sanctioning low nitrogen efficiency rhizobial strains, leading to nodule senescence in particular plant–soil–rhizobia strain combinations.ConclusionsThe microbiome–soil–rhizobial dynamic strongly influences plant nutrient uptake and growth, with the endosphere and rhizosphere shaped differentially according to plant–rhizobial interactions with strains that vary in nitrogen-fixing efficiency levels. These results open up the possibility to select inoculation partners best suited for plant, soil type and microbial community.Dv-bX6Q_Z8DTbrQMTFWdQvVideo

Facile synthesis of Fe single-atom porous photocatalysts via direct metal atomization achieving efficient photocatalytic nitrogen fixation

Photocatalytic nitrogen fixation has been explored as a feasible pathway for ammonia synthesis. However, the convenient and efficient preparation of photocatalysts for nitrogen fixation remains a challenge. Meanwhile, the reaction pathway and mechanism of photocatalytic nitrogen fixation are unclear. Herein, single-atom Fe-porous g-C3N4 (FPx) samples were manufactured using a one-step anneal technique via bubble template and direct metal atomization. Characterization results indicate that FPx has a porous structure and single-atom Fe. The porous structure exposed more active centers. Simultaneously, single-atom Fe changes the adsorption mode of N2 from physical to chemical and turns the photocatalytic nitrogen fixation from the associative distal pathway to the associative alternating pathway. Consequently, without any sacrificial agent or cocatalysts, FPx presents a prominent increase in photocatalytic activity, reaching 62.42 µmol h−1 g−1, over fivefold larger than that of bulk g-C3N4. This work provides new insights into photocatalytic nitrogen fixation and achieves efficient N2 photoreduction by constructing single-atom photocatalysts.

In-situ construction of novel sulfur-vacancy-rich Bi/Bi2S3/SnS2 Z-scheme heterostructure photocatalysts for efficient Cr(VI) reduction and nitrogen fixation

Highly competent and economical photocatalysts are one of the most charming targets in environmental restoration and clean production. Herein, a novel sulfur-vacancy-rich Bi/Bi2S3/SnS2 Z-scheme heterostructure was constructed in situ and applied for the photoreduction Cr(VI) and nitrogen fixation. The fabricated Bi/Bi2S3/SnS2–2 exhibits the optimum photoreduction Cr(VI) performance with the efficiency of 94.5% within 15 min visible light irradiation. The remarkably enhanced catalytic efficiency derived from the synergistic effect of the construction of intimate contacted interface, abundant sulfur vacancy and surface plasmon resonance (SPR) effect of metal Bi. Meanwhile, the excellent photocatalytic nitrogen fixation property (96.4 μmol g–1 h–1) was achieved by Bi/Bi2S3/SnS2–2 under full solar illumination because sulfur vacancy could provide sufficient catalytic sites to accelerate the adsorption and nitrogen activation. The Z-scheme heterostructure was proposed to expound the photocatalytic mechanism. This work offers a new perspective on hierarchical heterostructure with plentiful vacancies for environmental remediation and energy development.

Biochar amendment reduces biological nitrogen fixation and nitrogen use efficiency in cadmium-contaminated paddy fields

Cadmium (Cd) contamination poses a considerable threat to human health through grain enrichment and limits biological nitrogen fixation (BNF) in paddy fields. Biochar has shown great potential for agricultural soil remediation because it inactivates Cd, but uncertainties remain as to how biochar amendments affect BNF and grain N use efficiency in paddies. To elucidate these issues, we investigated the effects of biochar amendment on the structure and function of diazotrophic bacterial communities in different rice growth stages in Cd-contaminated paddy fields, and evaluated the contribution of BNF to grain N use efficiency under biochar amendment. The results showed that biochar amendment significantly increased the abundance of diazotrophic bacteria in the tillering and jointing stages. Furthermore, the community structure of soil diazotrophic bacteria markedly changed with biochar amendment, with a significant reduction in the abundances of Euryarchaeota, Desulfobacterales (Proteobacteria), and Sphingomonadales (Bacteroidetes) in the tillering stage. Changes in the soil carbon/nitrogen (C/N) ratio was the main factor driving diazotrophic microbial community characteristics caused by the release of available C from biochar at the tillering stage, rather than the Cd. Moreover, biochar amendment increased the efficiency of BNF (especially for autotrophic N2 fixation) in the vegetative phase of rice growth. Notably, biochar amendment significantly decreased BNF efficiency during the filling stage and reduced grain N use efficiency. The limited available nutrients in biochar and the toxicity of polycyclic aromatics and phenols in biochar-derived dissolved organic matter were responsible for the varied impacts of biochar on BNF in different rice growth stages. For the first time, we report that biochar amendment in paddy soils reduces Cd toxicity but also inhibits BNF and thereby decreases N use efficiency. Therefore, before applying biochar to inactivate Cd in paddy fields, there should be a trade-off between agricultural production and ecological safety to achieve sustainable agriculture.

Heme catabolism mediated by heme oxygenase in uninfected interstitial cells enables efficient symbiotic nitrogen fixation in Lotus japonicus nodules.

Legume nodules produce large quantities of heme required for the synthesis of leghemoglobin (Lb) and other hemoproteins. Despite the crucial function of Lb in nitrogen fixation and the toxicity of free heme, the mechanisms of heme homeostasis remain elusive. Biochemical, cellular, and genetic approaches were used to study the role of heme oxygenases (HOs) in heme degradation in the model legume Lotus japonicus. Heme and biliverdin were quantified and localized, HOs were characterized, and knockout LORE1 and CRISPR/Cas9 mutants for LjHO1 were generated and phenotyped. We show that LjHO1, but not the LjHO2 isoform, is responsible for heme catabolism in nodules and identify biliverdin as the invivo product of the enzyme in senescing green nodules. Spatiotemporal expression analysis revealed that LjHO1 expression and biliverdin production are restricted to the plastids of uninfected interstitial cells. The nodules of ho1 mutants showed decreased nitrogen fixation, and the development of brown, rather than green, nodules during senescence. Increased superoxide production was observed in ho1 nodules, underscoring the importance of LjHO1 in antioxidant defense. We conclude that LjHO1 plays an essential role in degradation of Lb heme, uncovering a novel function of nodule plastids and uninfected interstitial cells in nitrogen fixation.

Enhanced artificial nitrogen fixation efficiency induced by construction of ternary TiO2/MIL-88A(Fe)/g-C3N4 Z-scheme heterojunction

Herein, a ternary TiO2/MIL-88A(Fe)/g-C3N4 heterojunction is successfully constructed through a facile hydrothermal strategy for enhancing solar energy harvesting and efficiency of catalytic nitrogen reduction induced by enlarged light absorption range, increasing interfacial charge transfer ability and desirable stability. Under the simulated sunlight irradiation, the N2 fixation experiment shows that the yield of NH3 reaches 1084.31 μmol/(g·h) over the TiO2/MIL-88A(Fe)/g-C3N4 photocatalyst, and the yield is significantly enhanced, which is 33.68 and 13.94 times that is higher than the pure TiO2 and g-C3N4, respectively. In a mean time, the excellent performance of the photocatalytic N2 fixation over the ternary TiO2/MIL-88A(Fe)/g-C3N4 is verified based on density function theory calculation and the decisive step over the composite is investigated by calculating Gibbs free energies of nitrogen reduction paths. The performance enhancement mechanism of TiO2/MIL-88A(Fe)/g-C3N4 is speculated, which indicates that the hybridized three-component system presents a desirable Z-scheme band alignment, resulting in the improvement of separation and transfer efficiency of photoinduced charge carriers. The article shows a new and high-efficiency TiO2/MIL-88A(Fe)/g-C3N4 photocatalysis for excellent nitrogen reduction ability.

Transcriptomic and Metabolomic Analyses Reveal the Roles of Flavonoids and Auxin on Peanut Nodulation.

Rhizobia form symbiotic relationships with legumes, fixing atmospheric nitrogen into a plant-accessible form within their root nodules. Nitrogen fixation is vital for sustainable soil improvements in agriculture. Peanut (Arachis hypogaea) is a leguminous crop whose nodulation mechanism requires further elucidation. In this study, comprehensive transcriptomic and metabolomic analyses were conducted to assess the differences between a non-nodulating peanut variety and a nodulating peanut variety. Total RNA was extracted from peanut roots, then first-strand and second-strand cDNA were synthesized and purified. After sequencing adaptors were added to the fragments, the cDNA libraries were sequenced. Our transcriptomic analysis identified 3362 differentially expressed genes (DEGs) between the two varieties. Gene ontology and Kyoto Encyclopedia of Genes and Genomes analyses revealed that the DEGs were mainly involved in metabolic pathways, hormone signal transduction, secondary metabolic biosynthesis, phenylpropanoid biosynthesis, or ABC transport. Further analyses indicated that the biosynthesis of flavonoids, such as isoflavones, flavonols, and flavonoids, was important for peanut nodulation. A lack of flavonoid transport into the rhizosphere (soil) could prevent rhizobial chemotaxis and the activation of their nodulation genes. The downregulation of AUXIN-RESPONSE FACTOR (ARF) genes and lower auxin content could reduce rhizobia's invasion of peanut roots, ultimately reducing nodule formation. Auxin is the major hormone that influences the cell-cycle initiation and progression required for nodule initiation and accumulates during different stages of nodule development. These findings lay the foundation for subsequent research into the nitrogen-fixation efficiency of peanut nodules.

Evaluation of Legume-Rhizobial Symbiotic Interactions Beyond Nitrogen Fixation That Help the Host Survival and Diversification in Hostile Environments.

Plants often experience unfavorable conditions during their life cycle that impact their growth and sometimes their survival. A temporary phase of such stress, which can result from heavy metals, drought, salinity, or extremes of temperature or pH, can cause mild to enormous damage to the plant depending on its duration and intensity. Besides environmental stress, plants are the target of many microbial pathogens, causing diseases of varying severity. In plants that harbor mutualistic bacteria, stress can affect the symbiotic interaction and its outcome. To achieve the full potential of a symbiotic relationship between the host and rhizobia, it is important that the host plant maintains good growth characteristics and stay healthy under challenging environmental conditions. The host plant cannot provide good accommodation for the symbiont if it is infested with diseases and prone to other predators. Because the bacterium relies on metabolites for survival and multiplication, it is in its best interests to keep the host plant as stress-free as possible and to keep the supply stable. Although plants have developed many mitigation strategies to cope with stress, the symbiotic bacterium has developed the capability to augment the plant's defense mechanisms against environmental stress. They also provide the host with protection against certain diseases. The protective features of rhizobial-host interaction along with nitrogen fixation appear to have played a significant role in legume diversification. When considering a legume-rhizobial symbiosis, extra benefits to the host are sometimes overlooked in favor of the symbionts' nitrogen fixation efficiency. This review examines all of those additional considerations of a symbiotic interaction that enable the host to withstand a wide range of stresses, enabling plant survival under hostile regimes. In addition, the review focuses on the rhizosphere microbiome, which has emerged as a strong pillar of evolutionary reserve to equip the symbiotic interaction in the interests of both the rhizobia and host. The evaluation would draw the researchers' attention to the symbiotic relationship as being advantageous to the host plant as a whole and the role it plays in the plant's adaptation to unfavorable environmental conditions.

Highly efficient visible-light photocatalytic nitrogen fixation via single-atom iron catalyst site and synergistic effect of Zr-cluster in zirconium-based porphyrinic metal-organic frameworks (PCN-222)

Zirconium-based porphyrinic metal-organic frameworks (MOFs), including PCN-222 (nonmetal), and PCN-222 (Fe) were employed as the photocatalyst for photocatalytic nitrogen (N2) fixation to NH3 under visible light irradiation. The as-synthesized MOFs of PCN-222 (nonmetal) and PCN-222 (Fe) exhibited a notable increase in photocatalytic N2 fixation compared to the pure (Metallo) porphyrin ligands of H2TCPP and FeTCPP, respectively. The comparative studies illustrated that using the Zr-cluster as a stable node and the active redox site is a successful strategy to increase the stability and photocatalytic activity of the MOF-based photocatalysts. On the other, the single-atom Fe site as an active center of absorption and activation of nitrogen molecules improved the N2 fixation activity. Remarkably, among them all, PCN-222 (Fe) demonstrated the highest photocatalytic N2 fixation activity with a rate of 1502.51 μmol g-1h−1 which could be related to the simultaneous presence of Zr-cluster and single-atom Fe site which promote the performance of the photocatalyst. So, the photocatalytic activity of PCN-222 (Fe) is 1.9 and 2.5 times higher than the activity of PCN-222 (nonmetal) and pure FeTCPP, respectively. High photocatalytic activity, sacrificial agent-free conditions, excellent efficiency in short response time, reusability, and mild reaction conditions are the attractive characteristics of the as-synthesized photocatalysts that make them suitable candidates for photocatalytic NH3 production.

Unique Tubular BiOBr/g-C3 N4 Heterojunction with Efficient Separation of Charge Carriers for Photocatalytic Nitrogen Fixation.

The industrial ammonia synthesis process consumes a lot of energy and causes serious environmental pollution. As a sustainable approach for ammonia synthesis, photocatalytic nitrogen reduction employing water as the reducing agent has a lot of potential. A simple surfactant-assisted solvothermal method is used to synthesize g-C3 N4 nanotubes with flower-like spherical BiOBr grown inside and outside (BiOBr/g-C3 N4 , BC). The hollow tubular structure realizes the full use of visible light by the multi-scattering effect of light. Large surface areas and more active sites for N2 adsorption and activation are present in the distinctive spatially dispersed hierarchical structures. Particularly, the quick separation and transfer of electrons and holes are facilitated by the sandwich tubular heterojunctions and tight contact interface of BiOBr and g-C3 N4 . The maximal NH3 generation rate of the BiOBr/g-C3 N4 composite catalysts can reach 255.04 μmol⋅ g-1 ⋅ h-1 , and it is 13.9 and 5.8 times that of pure BiOBr and g-C3 N4 . This work provides a novel method for designing and constructing unique heterojunctions for efficient photocatalytic nitrogen fixation.

Reduction in the Use of Some Herbicides Favors Nitrogen Fixation Efficiency in Phaseolus vulgaris and Medicago sativa.

In recent decades, the quality of agricultural soils has been seriously affected by the excessive application of pesticides, with herbicides being one of the most abundant. Continuous use of herbicides alters the soil microbial community and beneficial interactions between plants and bacteria such as legume-rhizobia spp. symbiosis, causing a decrease in the biological nitrogen fixation, which is essential for soil fertility. Therefore, the aim of this work was to study the effect of two commonly used herbicides (pendimethalin and clethodim) on the legume-rhizobia spp. symbiosis to improve the effectiveness of this process. Phaseolus vulgaris plants grown in pots with a mixture of soil:perlite (3:1 v/v), showed a 44% inhibition of nitrogen fixation rate with pendimethalin. However, clethodim, specifically used against monocots, did not induce significant differences. Additionally, we analyzed the effect of herbicides on root exudate composition, detecting alterations that might be interfering with the symbiosis establishment. In order to assess the effect of the herbicides at the early nodulation steps, nodulation kinetics in Medicago sativa plants inoculated with Sinorhizobium meliloti were performed. Clethodim caused a 30% reduction in nodulation while pendimethalin totally inhibited nodulation, producing a reduction in bacterial growth and motility as well. In conclusion, pendimethalin and clethodim application reduced the capacity of Phaseolus vulgaris and Medicago sativa to fix nitrogen by inhibiting root growth and modifying root exudate composition as well as bacterial fitness. Thus, a reduction in the use of these herbicides in these crops should be addressed to favor a state of natural fertilization of the soil through greater efficiency of leguminous crops.

Electrocatalytic Disproportionation of Nitric Oxide Toward Efficient Nitrogen Fixation (Adv. Energy Mater. 14/2023)

Nitrogen Fixation In article number 2204231, Angjian Wu, Hao Bin Wu, Jianhua Yan, and co-workers achieve electrocatalytic disproportionation of nitric oxide in a bipolar membrane based electrolysis system with decoupled acid–base electrolytes, which aims for efficient nitrogen fixation via coupling the cathodic NO reduction reaction with the anodic NO electrooxidation simultaneously.

Selected indigenous drought tolerant rhizobium strains as promising biostimulants for common bean in Northern Spain.

Drought is the most detrimental abiotic stress in agriculture, limiting crop growth and yield and, currently, its risk is increasing due to climate change. Thereby, ensuring food security will be one of the greatest challenges of the agriculture in the nearest future, accordingly it is essential to look for sustainable strategies to cope the negative impact of drought on crops. Inoculation of pulses with biostimulants such as rhizobium strains with high nitrogen fixation efficiency and drought-tolerance, has emerged as a promising and sustainable production strategy. However, some commercial inoculums are not effective under field conditions due to its lower effectiveness against indigenous rhizobium strains in the establishment of the symbiosis. Thus, in the present study, we evaluated the ability to improve drought tolerance in common bean plants of different indigenous rhizobia strains isolated from nearby crop fields in the Basque Country either affected by drought or salinity. The plants in this trial were grown in a climatic chamber under controlled conditions and exposed to values of 30% relative soil water content at the time of harvest, which is considered a severe drought. From the nine bacteria strains evaluated, three were found to be highly efficient under drought (namely 353, A12 and A13). These strains sustained high infectiveness (nodulation capacity) and effectiveness (shoot biomass production) under drought, even surpassing the plants inoculated with the CIAT899 reference strain, as well as the chemically N-fertilized plants. The tolerance mechanisms developed by plants inoculated with 353, A12 and A13 strains were a better adjustment of the cell wall elasticity that prevents mechanical damages in the plasma membrane, a higher WUE and an avoidance of the phenological delay caused by drought, developing a greater number of flowers. These results provide the basis for the development of efficient common bean inoculants able to increase the yield of this crop under drought conditions in the Northern Spain and, thus, to be used as biostimulants. In addition, the use of these efficient nitrogen fixation bacteria strains is a sustainable alternative to chemical fertilization, reducing cost and minimizing its negative impact on environment.

Changes in Soil Rhizobia Diversity and Their Effects on the Symbiotic Efficiency of Soybean Intercropped with Maize

It has been established that maize/soybean intercropping can improve nitrogen use efficiency. However, few studies have addressed how maize/soybean intercropping affects nitrogen-fixing bacterial diversity and N fixation efficiency of intercropped soybean. In this study, nitrogen-fixing bacterial communities, N fixation efficiency, and their relationships with soil properties under three nitrogen fertilization application rates (N0 0 kg/ha, N1 40 kg/ha, N2 80 kg/ha) were explored through field experiments. Nitrogen fixation and nitrogen-fixing bacteria diversity were assessed using 15N natural abundance, Illumina high-throughput sequencing, and nifH (nitrogen fixation) gene copies quantification in the rhizosphere soil of intercropped soybean. The results showed that nitrogen application rates significantly decreased the nitrogen-fixing bacteria diversity, nitrogen fixation efficiency, and nifH gene copies in the rhizosphere soil. Nitrogen fixation efficiency, nodule number, and dry weight of intercropped soybean were highest in the N0 treatment, and nitrogen fixation was the highest in the N1 treatment. The nitrogen-fixing efficiency in N0, N1, and N2 treatments increased by 69%, 59%, and 42% and the nodule number of soybean was 10%, 22%, and 21%, respectively, compared with monocultures. The soybean nitrogen-fixing bacteria diversity in intercropping under N0 and N1 treatments significantly increased compared with monocultures. There was a significant positive correlation between soil nifH gene copies and N fixation efficiency and a negative correlation with soil available nitrogen. Bradyrhizobium abundance in soybean rhizosphere soil decreased significantly with the increase in nitrogen application rates and was significantly correlated with soil AN (available nitrogen) and pH content in the soybean rhizosphere. These results help us to understand the mechanisms by which nitrogen use efficiency was improved, and nitrogen fertilizer could be reduced in legume/Gramineae intercropping, which is important to improve the sustainability of agricultural production.

Mapping of QTLs Associated with Biological Nitrogen Fixation Traits in Peanuts (Arachis hypogaea L.) Using an Interspecific Population Derived from the Cross between the Cultivated Species and Its Wild Ancestors.

Peanuts (Arachis hypogaea L.) are an allotetraploid grain legume mainly cultivated by poor farmers in Africa, in degraded soil and with low input systems. Further understanding nodulation genetic mechanisms could be a relevant option to facilitate the improvement of yield and lift up soil without synthetic fertilizers. We used a subset of 83 chromosome segment substitution lines (CSSLs) derived from the cross between a wild synthetic tetraploid AiAd (Arachis ipaensis × Arachis duranensis)4× and the cultivated variety Fleur11, and evaluated them for traits related to BNF under shade-house conditions. Three treatments were tested: without nitrogen; with nitrogen; and without nitrogen, but with added0 Bradyrhizobium vignae strain ISRA400. The leaf chlorophyll content and total biomass were used as surrogate traits for BNF. We found significant variations for both traits specially linked to BNF, and four QTLs (quantitative trait loci) were consistently mapped. At all QTLs, the wild alleles decreased the value of the trait, indicating a negative effect on BNF. A detailed characterization of the lines carrying those QTLs in controlled conditions showed that the QTLs affected the nitrogen fixation efficiency, nodule colonization, and development. Our results provide new insights into peanut nodulation mechanisms and could be used to target BNF traits in peanut breeding programs.