Legume Biological Nitrogen Fixation Research Articles

Determination of abundance and symbiotic effectiveness of native rhizobia nodulating soybean and other legumes in Rwanda.

Rhizobia diversity in the rhizosphere is one of the key promoters of biological nitrogen fixation between host legumes and microsymbionts, although related complex interaction may depend on various factors. This research was intended to assess the abundance of indigenous rhizobia isolates under various soil conditions, as well as their effectiveness to nodulate legumes such as soybeans. Factors such as soil properties and legume species influence the volume and symbiotic effectiveness of native rhizobia to nodulate crop legumes. To investigate the abundance of rhizobia isolates, legume crops were uprooted to obtain nodules for most probable number (MPN) determination of rhizobia isolates, and soybean (Glycine max.) was used to verify the presence of suitable and efficient rhizobia strains for nitrogen fixation. Soil samples were obtained from the holes out of which nodules were collected, and the laboratory analysis included pH, Mg, K, available P, organic C, Ca, and N to establish the correlation between the soil status and number of rhizobia isolates' cells. Significant variations (p-value <.05) were observed in the cell counts of Rhizobia isolates from Glycine max, Phaseolus vulgaris, Pisum sativum, and Vigna unguiculata, particularly when compared to Arachis hypogaea isolates under acidic conditions. Notably, Pisum sativum and Vigna unguiculata showed consistent performance across all pH conditions. The number of rhizobia isolates was found to be significantly linked to total N and P deficiencies (p < .05). It was also established that total N was dependent on the number of rhizobia cells and that there is a strong correlation between organic carbon and N content. This study highlights the crucial role of understanding and optimizing conditions for rhizobia nodulation in diverse soil environments, emphasizing its potential impact on enhancing biological nitrogen fixation in legumes.

15 N labeling technology reveals enhancement of nitrogen uptake and transfer by root interaction in cotton/soybean intercropping.

Biological nitrogen fixation in legumes and their transfer of nitrogen to non-legumes in legume/non-legume intercropping systems are considered to be important for the improvement of productivity. However, research on interspecific interaction and root nitrogen transfer in cotton/soybean intercropping systems has rarely been undertaken. In this study, the roots of cotton and soybean were separated with either complete root barriers (CB), using plastic film, or semi-root barriers (SB), using nylon net. No root barrier (NB) was used as the control. The results showed that cotton produced more above-ground dry matter (DM) than soybean. The above-ground DM and nitrogen uptake of cotton was greatest with the NB treatment. The above-ground DM and nitrogen uptake of soybean was greatest with the CB treatment. At the harvest stage, the nitrogen transfer rate from soybean to cotton was 22.47% with the SB treatment and 40.41% with the NB treatment. Interspecific root interaction increased the nitrogen transfer amount, especially for the cotton roots in the 0-15 cm soil layer and for the soybean roots in the 0-30 cm soil layer. The root distribution of soybean was the key factor affecting nitrogen transfer amount, and nitrogen transfer amount was the key factor affecting nitrogen uptake of cotton in the cotton/soybean intercropping system. These results indicated that nitrogen transfer from soybean to cotton through root interaction improved cotton above-ground DM and nitrogen uptake. © 2023 Society of Chemical Industry.

A high-resolution transcriptomic atlas depicting nitrogen fixation and nodule development in soybean.

Although root nodules are essential for biological nitrogen fixation in legumes, the cell types and molecular regulatory mechanisms contributing to nodule development and nitrogen fixation in determinate nodule legumes, such as soybean (Glycine max), remain incompletely understood. Here, we generated a single-nucleus resolution transcriptomic atlas of soybean roots and nodules at 14 days post inoculation (dpi) and annotated 17 major cell types, including six that are specific to nodules. We identified the specific cell types responsible for each step in the ureides synthesis pathway, which enables spatial compartmentalization of biochemical reactions during soybean nitrogen fixation. By utilizing RNA velocity analysis, we reconstructed the differentiation dynamics of soybean nodules, which differs from those of indeterminate nodules in Medicago truncatula. Moreover, we identified several putative regulators of soybean nodulation and two of these genes, GmbHLH93 and GmSCL1, were as-yet uncharacterized in soybean. Overexpression of each gene in soybean hairy root systems validated their respective roles in nodulation. Notably, enrichment for cytokinin-related genes in soybean nodules led to identification of the cytokinin receptor, GmCRE1, as a prominent component of the nodulation pathway. GmCRE1 knockout in soybean resulted in a striking nodule phenotype with decreased nitrogen fixation zone and depletion of leghemoglobins, accompanied by downregulation of nodule-specific gene expression, as well as almost complete abrogation of biological nitrogen fixation. In summary, this study provides a comprehensive perspective of the cellular landscape during soybean nodulation, shedding light on the underlying metabolic and developmental mechanisms of soybean nodule formation.

An agronomic study of legacy effects from annual legume pastures in acid soils

AbstractThe use of acid soil‐tolerant annual legume pasture species and liming to raise soil pH can have legacy effects that improve subsequent wheat crop growth in acid soils. We established a two‐year experiment in a semi‐arid environment with and without historical lime application to investigate how acid soils (±lime) influence the legacy effects of annual legume pastures. We measured legume and cereal growth and yield, soil‐borne disease incidence, rhizosheath organic acid type and amount, legume biological nitrogen fixation (BNF) and soil moisture in Year 1, and soil microbial biomass carbon (MBC) and wheat yield and quality in Year 2. Minor differences in soil moisture and soil‐borne diseases occurred between cereals and legumes, and the poor BNF contribution of all pasture legumes improved in lime treatments. Pasture legumes produced more organic acids than cereals, which increased in limed treatments. Soil MBC in Year 2 was greater following legumes than cereals. Year 2 wheat grain yield increased following pasture legumes, more so in limed treatments. The contribution of organic acid exudation by annual legume pastures to microbial community legacy effects warrants further scrutiny in semi‐arid environments, particularly when acid soils limit typical rotational benefits.

The soybean β‐expansin gene GmINS1 contributes to nodule development in response to phosphate starvation

Legume biological nitrogen fixation (BNF) is the most important N source in agricultural ecosystems. Nodule organogenesis from the primordia to the development of mature nodules with the ability to fix N2 largely determines BNF capacity. However, nodule growth is often limited by low phosphorus (P) availability, while the mechanisms underlying nodule development responses to P deficiency remain largely unknown. In this study, we found that nodule enlargement is severely inhibited by P deficiency, as reflected by the smaller individual nodule size from a soybean core collection in the field. Wide-ranging natural diversity in nodule size was further identified in soybeans reared in low P soils, with the FC-1 genotype outperforming FC-2 in assessments of nodulation under low P conditions. Among β-expansin members, GmINS1 expression is most abundantly enhanced by P deficiency in FC-1 nodules, and its transcript level is further displayed to be tightly associated with nodule enlargement. Four single nucleotide polymorphisms discovered in the GmINS1 promoter distinguished the FC-1 and FC-2 genotypes and accounted for the differential expression levels of GmINS1 responses to P deficiency. GmINS1 overexpression led to increases in nodule size, infected cell abundance, and N2 fixation capacity and subsequently promoted increases in N and P content, soybean biomass and yield. Our findings provide a candidate gene for optimizing BNF capacity responses to low P stress in soybean molecular breeding programs. This article is protected by copyright. All rights reserved.

Will Phosphate Bio-Solubilization Stimulate Biological Nitrogen Fixation in Grain Legumes?

Biological nitrogen fixation (BNF) refers to a bacterially mediated process by which atmospheric N2is reduced, either symbiotically or non-symbiotically, into ammonia (NH3) in the presence of the enzyme complex nitrogenase. In N2-fixing grain legumes, BNF is often hampered under low phosphorus (P) availability. The P status of legumes, particularly nodules, as well as P availability in the rhizosphere, play a vital role in regulating BNF. Aside from increasing P availability via fertilization, other plant traits (i.e., extensive rooting system and their spatial distribution, hyper-nodulation, root exudates, rhizosphere acidification, and heterogeneity) contribute to greater P uptake and hence more effective BNF. The positive interaction between P availability and BNF can be exploited through beneficial soil P solubilizing microorganisms (PSM). These microorganisms can increase plant-available P by modifying either rhizosphere soil processes or promoting plant traits, which lead to increased P uptake by the production of plant growth-promoting substances, both of which could indirectly influence the efficiency of BNF in legumes. In this review, we report on the importance of microbial P bio-solubilization as a pathway for improving BNF in grain legumes via PSM and P solubilizing bacteria (PSB). Because BNF in legumes is a P-requiring agro-ecological process, the ability of soil PSB to synergize with the rhizobial strains is likely a key belowground process worth investigating for advanced research aiming to improve rhizosphere biological functions necessary for sustainable legume-based cropping systems.

Mineral Fertilizer Demand for Optimum Biological Nitrogen Fixation and Yield Potentials of Legumes in Northern Ethiopia

Farmers in Northern Ethiopia integrate legumes in their cropping systems to improve soil fertility. However, biological nitrogen fixation (BNF) potentials of different legumes and their mineral nitrogen (N) and phosphorus (P) demands for optimum BNF and yields are less studied. This study aimed to generate the necessary knowledge to enable development of informed nutrient management recommendations, guide governmental public policy and assist farmer decision making. The experiment was conducted at farmers’ fields with four N levels, three P levels, and three replications. Nodule number and dry biomass per plant were assessed. Nitrogen difference method was used to estimate the amount of fixed N by assuming legume BNF was responsible for differences in plant N and soil mineral N measured between legume treatments and wheat. The result revealed that the highest grain yields of faba bean (2531 kg ha−1), field pea (2493 kg ha−1) and dekeko (1694 kg ha−1) were recorded with the combined application of 20 kg N ha−1 and 20 kg P ha−1. Faba bean, field pea and dekeko also fixed 97, 38 and 49 kg N ha−1, respectively, with the combined application of 20 kg N ha−1 and 20 kg P ha−1; however, lentil fixed 20 kg ha−1 with the combined application of 10 kg N ha−1 and 10 kg P ha−1. The average BNF of legumes in the average of all N and P interaction rates were 67, 23, 32 and 16 kg N ha−1 for faba bean, field pea, dekeko and lentil, respectively. Moreover, faba bean, field pea, dekeko and lentil accumulated a surplus soil N of 37, 21, 26 and 13 kg ha−1, respectively, over the wheat plot. The application of 20 kg N ha−1 and 20 kg P ha−1 levels alone and combined significantly (p &lt; 0.05) increased the nodulation, BNF and yield of legumes; however, 46 kg N ha-1 significantly decreased BNF. This indicated that the combination of 20 kg N ha−1 and 20 kg P ha−1 levels is what mineral fertilizer demands to optimize the BNF and yield of legumes. The results of this study can lead to the development of policy and farmer guidelines, as intensification of the use of legumes supplied with starter N and P fertilizers in Northern Ethiopian cropping systems has the multiple benefits of enhancing inputs of fixed N, improving the soil N status for following crops, and becoming a sustainable option for sustainable soil fertility management practice.

Rhizobium leguminosarum strain combination effects on nodulation and biological nitrogen fixation with Vicia villosa

Biological nitrogen fixation (BNF) resulting from symbiosis between legumes and rhizobia helps improve soil N fertility. Inoculated soils with specific strains of rhizobia may increase potential BNF in legumes, but the efficacy of these rhizobia in promoting BNF may be limited by competition from resident rhizobia already present in soils. We evaluated the ability of four strains to nodulate and increase BNF in Vicia villosa (hairy vetch) as individual and combined inoculants in a laboratory experiment. Plants were inoculated with a single rhizobia strain or equal ratios of 2–4 strains and grown under controlled conditions. After 46 d, entire plant shoots were harvested and analyzed for biomass, N content, nodule number, and nodule mass. Nodule occupancy was assessed using DNA fingerprinting of characteristic rhizobia genes. Negative-N and positive-N control plants averaged 6.3 and 83.2 mg total shoot N, respectively. Average total shoot N of inoculated vetch treatments was between that of uninoculated control treatments. Nodule number, total nodule mass, and BNF efficiency (total shoot N mass per total nodule mass) did not significantly differ between individual strains. Neither BNF efficiency nor nodule number were indicative of competitive ability for nodule occupancy. The rhizobia did not display any consistent synergistic or antagonistic effect on BNF when combined in inoculants and nodule occupancy did not favor any specific strain. Because vetch inoculated with local Rlv strains produced similar amounts of N as uninoculated plants fertilized with N, our work suggests that rhizobia sourced from local soils may produce sufficient BNF with vetch.

A convenient, soil-free method for the production of root nodules in soybean to study the effects of exogenous additives.

Legumes develop root nodules that harbor endosymbiotic bacteria, rhizobia. These rhizobia convert nitrogen to ammonia by biological nitrogen fixation. A thorough understanding of the biological nitrogen fixation in legumes and its regulation is key to develop sustainable agriculture. It is well known that plant hormones affect nodule formation; however, most studies are limited to model legumes due to their suitability for in vitro, plate‐based assays. Specifically, it is almost impossible to measure the effects of exogenous hormones or other additives during nodule development in crop legumes such as soybean as they have huge root system in soil. To circumvent this issue, the present research develops suitable media and growth conditions for efficient nodule development under in vitro, soil‐free conditions in an important legume crop, soybean. Moreover, we also evaluate the effects of all major phytohormones on soybean nodule development under identical growing conditions. Phytohormones such as abscisic acid (ABA) and jasmonic acid (JA) had an overall inhibitory effect and those such as gibberellic acid (GA) or brassinosteroids (BRs) had an overall positive effect on nodule formation. This versatile, inexpensive, scalable, and simple protocol provides several advantages over previously established methods. It is extremely time‐ and resource‐efficient, does not require special training or equipment, and produces highly reproducible results. The approach is expandable to other large legumes as well as for other exogenous additives.

QUANTITATIVE ANALYSIS OF BIOLOGICAL NITROGEN FIXATION IN VARIOUS MODELS OF LEGUMES AND THE FACTORS INFLUENCING THE PROCESS: A REVIEW

Biological nitrogen fixation refers to the biological reduction of dinitrogen gas to ammonia under normal temperature and pressure. Leguminous crops can combine this abundantly present gas of the atmosphere, which has proven to be beneficial for both the crops themselves and any other succeeding crops growing in the same soil, thereby downsizing the utility of nitrogenous fertilizers. Several techniques are available for the direct quantitative measurement of legume biological nitrogen fixation in the field and controlled environments. However, these are time-consuming and therefore, costly and generate data relevant only to the time and place of measurement. As an alternative, legume biological nitrogen fixation is calculable by either empirical models or dynamic mechanistic simulation models. Comparatively, simulation by a dynamic model is calculable for quantifying legume biological nitrogen fixation, because of its capability to simulate the response of N fixation to a good vary of environmental variables and legume growth standing presently. In this review, an attempt was made to discuss and compare the strategies used to estimate the potential N fixation rate, and therefore the response functions to simulate legume biological nitrogen fixation in nine widely cited models over the last thirty years. Further, assessing their relative strength in simulating legume biological nitrogen fixation with varying biotic and abiotic factors and determines the discrepancies between experimental findings and simulation was carried out. In conclusion, this review clarifies, the current progress in legume biological nitrogen fixation quantification in simulation models, and guides their development, combining elementary experimental and modelling work.

Leveraging biological nitrogen fixation and soybean yield by planting duration under varying onset of rainy seasons in Uganda

Rainfall distribution and onset of rainy seasons, both of which are key determinants of the decision to plant most annual crops in sub-Saharan Africa (SSA), are increasingly difficult to predict due to climate change. Consequently, planting schedules affect both crops and vital natural systems such as biological nitrogen fixation in legumes. The objective of this study was to establish the optimum planting duration for soybean (Glycine max L.), from the onset of the rainy season, in the context of nodulation and grain yield, and diversity of rhizobia in rhizosphere soil and nodules. A field experiment was set up with treatments, Bradyrhizobia inoculated and non- inoculated soybean variety Maksoy3N planted at four dates; 0, 10, 20 and 30 days from the onset of rain season. Diversity of rhizobia in the rhizosphere soil and nodules were fingerprinted using Single Strand Conformation Polymorphism (SSCP) method. Inoculation significantly (P&lt;0.05) increased the number but not effectiveness of nodules and grain yield; which was attributed to presence of native strains in the soil. Planting after 10 days following the onset of a rainy season significantly (P&lt;0.05) reduced grain yields and total yield loss was realised when seasonal cumulative rainfall was about 500 mm. Planting after 20 days from onset of rainy season significantly (P&lt; 0.05) reduced the total number of nodules. A total of 8 and 12 clusters of bacteria from the soil and nodules at 5% dissimilarity, respectively, were recorded. The Bradyrhizobia species consisted of B. japonicum, B. japonicum (USDA 6), B. elkanii, B. liaoningense, B. canariense, and B. yuanmingense.Key words: Climate change, inoculation, nodulation, rhizobia

Biological Nitrogen Fixation by Local and Improved Genotypes of Cowpea in Burkina Faso (West Africa): Total Nitrogen Accumulated can be used for Quick Estimation

Biological nitrogen fixation (BNF) by legumes is an indicator of their potential contribution to recycling nitrogen in cropping systems. Many techniques exist for the quantitative measurement of legume BNF. The isotopic dilution (ID) methods are the most accurate but are too expensive, time-consuming and require technical expertise. There is a gap between the simple but less accurate Total Nitrogen Difference (TND) method and the Isotopic Dilution (ID) methods. By measuring the BNF of 11 cowpea (Vigna unguiculata) genotypes, this study aimed to develop a simple model as an improved tool for the quick estimation of BNF. Total N accumulated by traditional genotypes from Burkina Faso varied from 23 to 41 kg ha−1. Approximately 40 to 65% of this was nitrogen derived from the atmosphere (Ndfa) when the TND method was used (Ndfa-TND), while the ID method indicated that 29 to 37% of N accumulated was derived from the atmosphere (Ndfa-ID). The TND method overestimated the BNF of high N-yielding genotypes but underestimated the BNF of low N-yielding genotypes (N-accumulated below 31 kg N ha−1). The relationship between N-accumulated and Ndfa-ID was described by a polynomial regression: Yi = 0.0127 Xi2 - 0.5354 Xi + 17.44, where Yi and Xi represent Ndfa-ID and N-accumulated, respectively (P&lt;0.05, R2 =0.92). The model was validated and could be used for quick estimation of BNF directly from the N accumulated.

In silico structural homology modeling of nif A protein of rhizobial strains in selective legume plants.

Symbiosis is a complex genetic regulatory biological evolution which is highly specific pertaining to plant species and microbial strains. Biological nitrogen fixation in legumes is a functional combination of nodulation by nod genes and regulation by nif, fix genes. Three rhizobial strains (Rhizobium leguminosarum, Bradyrhizobium japonicum, and Mesorhizobium ciceri) that we considered for in silico analysis of nif A are proved to be the best isolates with respect to N2 fixing for ground nut, chick pea and soya bean (in vitro) out of 47 forest soil samples. An attempt has been made to understand the structural characteristics and variations of nif genes that may reveal the factors influencing the nitrogen fixation. The primary, secondary and tertiary structure of nif A protein was analyzed by using multiple bioinformatics tools such as chou-Fasman, GOR, ExPasy ProtParam tools, Prosa -web. Literature shows that the homology modeling of nif A protein have not been explored yet which insisted the immediate development for better understanding of nif A structure and its influence on biological nitrogen fixation. In the present predicted 3D structure, the nif A protein was analyzed by three different software tools (Phyre2, Swiss model, Modeller) and validated accordingly which can be considered as an acceptable model. However further in silico studies are suggested to determine the specific factors responsible for nitrogen fixing in the present three rhizobial strains.

Enhanced biological nitrogen fixation and competitive advantage of legumes in mixed pastures diminish with biochar aging

Soil amendment with biochar can increase biological nitrogen fixation (BNF) and improve growing conditions for legumes, while such effects may alter when biochar properties change with time (biochar aging). We examined BNF and competitiveness of legumes in a mixed pasture (dominated by clover, Trifolium repens) after 26 months of biochar amendment (field aged biochar) and in a pot study with grass (Lolium perenne) and clover mixture using fresh and chemically aged biochars. In both studies, the fraction of N derived from symbiotic fixation (%Ndfa) was not affected by aged biochar treatments, while it reduced from 73% in the control to 68% with fresh biochar addition in the pot study. However, this reduction in %Ndfa was compensated with greater biomass production resulting in the largest amount of N fixed with fresh biochar application. The amount of N fixed was reduced by field aged biochar, although it was not affected by chemically aged biochars. Moreover, in the field study grasses became more competitive than legumes with biochar, while such an advantage by grasses was not observed in the pot study. Aging of biochar can reduce BNF and competitiveness of legumes in mixed pastures with significant implications for pasture management.

Functional traits in cover crop mixtures: Biological nitrogen fixation and multifunctionality

Abstract Cover crop mixtures with complementary plant functional traits including biological nitrogen fixation (BNF) may supply nitrogen (N) to farm fields while simultaneously providing other ecosystem functions such as N retention and weed suppression (i.e., multifunctionality). Understanding variation in these relationships across farms can help advance trait‐based research in agroecology and ecological approaches to nutrient management. This on‐farm experiment explored the contributions of two‐ and three‐species cover crop mixtures, which combined legumes, brassicas and cool season grasses, to ecosystem functions across a gradient of soil fertility levels driven by farm management history. I evaluated the predictions that functional trait diversity of the cover crops would explain variation in multifunctionality, and that legume biomass and BNF within mixtures would be inversely correlated with indicators of soil N availability from organic matter across the farm gradient. Ecosystem functions varied widely across farms. As expected, functional diversity was a significant predictor of multifunctionality, although the relationship was weak. Cover crop mixtures had significantly greater multifunctionality than a cereal rye monoculture, though not at the highest observed levels of each function, indicating trade‐offs among functions. Linear regression models showed that legume biomass and BNF were negatively correlated with soil properties indicative of N availability from soil organic matter, whereas non‐legume and weed biomass were positively correlated with other measures of soil fertility. Synthesis and applications. Cover crop mixtures can increase functional diversity within crop rotations. Designing mixtures with complementary plant traits may be particularly effective for increasing multifunctionality and agroecosystem sustainability. On‐farm research to understand variation in biological nitrogen fixation, which is both a plant trait and a key ecosystem function, across heterogeneous soil conditions, can inform management of soil fertility based on ecological principles.

Vegetative Composition, Arbuscular Mycorrhizal Fungi Root Colonization, and Biological Nitrogen Fixation Distinguish Organic and Conventional Perennial Forage Systems

Core Ideas Conventional and organic forage fields did not differ in total forage yield. Legume content was more than two times greater under organic management. Total forage biological nitrogen fixation was greater under organic management. Arbuscular mycorrhizal fungi root colonization was greater under organic management. Arbuscular mycorrhizal fungi root colonization was inversely related to soil test P concentrations. Low concentrations of Olsen soil test phosphorus (STP) on organic dairy farms in Canada have been reported, however, the resulting impact on crop productivity is unclear. This study evaluated mixed forage (alfalfa [Medicago sativa L.]/timothy [Phleum pratense L.]) productivity, nutrient uptake, and related soil biological and biochemical properties on organic and conventional dairy farms in Ontario, Canada. Despite a lower STP concentration, mean organic forage yield (8.0 Mg DM ha−1) did not significantly differ from conventional (9.5 Mg DM ha−1). The proportion of N derived from biological nitrogen fixation (BNF) in alfalfa did not differ by management system, however the total forage BNF‐N was greater in the organic fields due to a twofold increase in legume proportions. Alfalfa tissue P concentrations, above the suggested critical value of 2.0 g kg−1 in all systems, were positively correlated with STP (r = 0.82, P = &lt; 0.0001) and significantly greater in conventional (3.3 g kg−1) than organic (2.4 g kg−1). Mean arbuscular mycorrhizal fungi (AMF) root colonization of alfalfa was greater in organic systems (37 vs. 23%) and was negatively correlated with STP (P = 0.0006). Management systems did not differ in soil alkaline and acid phosphatase activity, or phytase‐labile P. It was concluded that legume‐based forages can produce acceptable yields at low STP concentrations (&lt;10 mg kg−1) and supports the importance of legumes, legume BNF, and biological processes linked to soil P cycling in maintaining productivity in organic dairy farming systems.

INTEGRATING SCIENTIFIC AND LOCAL SOILS KNOWLEDGE TO EXAMINE OPTIONS BY CONTEXT INTERACTIONS FOR PHOSPHORUS ADDITION TO LEGUMES IN AN ANDEAN AGROECOSYSTEM

SUMMARYThis research sought to link Andean soil knowledge and farmer categorization of soil fertility to soil science characterization of soils, and use these to understand the impacts of phosphorus (P) fertilization of legumes using rock phosphate and soluble P fertilizer in 17 smallholder-managed sites with varying soil properties. We found that farmer high/low categorization of soils corresponded to soil P fertility and distance from farmer dwellings. Measures of soil P fertility also were inversely related to mycorrhizal colonization of vetch roots and directly related to the potential for P release by legume residues (C:P ratio). However, particular soil properties (texture and calcium phosphate pools) were better in explaining the response of legume biological nitrogen fixation to P addition, with maximal impacts in low-clay soils and soils with low calcium phosphates, as assessed with a dilute HCl extraction. In these conducive contexts, legume BNF increased 67 and 150% for RP and TSP, respectively (p &amp;lt; 0.05), with similar increases in biomass P stocks that showed promise for soil regeneration. In low-P fields, added P increased potential P release from legume residues via lower C:P ratios (p &amp;lt; 0.05). Percent cover of soil increased from 35% in the control to 45 and 55% with RP and TSP (p &amp;lt; 0.05), which shows potential for reduced erosion risk under P fertilization of legumes. We discuss results with respect to the integration of local farmer knowledge systems with exogenous scientific knowledge.

Soil Fertility Map for Food Legumes Production Areas in China.

Given the limited resources of fossil energy, and the environmental risks of excess fertilizer on crops, it is time to reappraise the potential role of food legume biological nitrogen fixation (BNF) as sources of nitrogen for cropping systems in China. 150 soil samples across 17 provinces and 2 municipalities of China were collected and analyzed. A distribution map of the soil fertilities and their patterns of distribution was constructed. The pH results indicated that soils were neutral to slightly alkaline overall. The soil organic matter (SOM) and the available nitrogen (AN) content were relatively low, while the available phosphorus (AP) and available potassium (AK) contents were from moderate to high. Production areas of food legumes (faba bean, pea, adzuki bean, mung bean and common bean) were clearly separated into 4 soil fertility type clusters. In addition, regions with SOM, AN, AP and AK deficiency, high acidity and high alkalinity were listed as target areas for further soil improvement. The potential was considered for biological nitrogen fixation to substitute for the application of mineral nitrogen fertiliser.

The NIN Transcription Factor Coordinates Diverse Nodulation Programs in Different Tissues of the Medicago truncatula Root.

Biological nitrogen fixation in legumes occurs in nodules that are initiated in the root cortex following Nod factor recognition at the root surface, and this requires coordination of diverse developmental programs in these different tissues. We show that while early Nod factor signaling associated with calcium oscillations is limited to the root surface, the resultant activation of Nodule Inception (NIN) in the root epidermis is sufficient to promote cytokinin signaling and nodule organogenesis in the inner root cortex. NIN or a product of its action must be associated with the transmission of a signal between the root surface and the cortical cells where nodule organogenesis is initiated. NIN appears to have distinct functions in the root epidermis and the root cortex. In the epidermis, NIN restricts the extent of Early Nodulin 11 (ENOD11) expression and does so through competitive inhibition of ERF Required for Nodulation (ERN1). In contrast, NIN is sufficient to promote the expression of the cytokinin receptor Cytokinin Response 1 (CRE1), which is restricted to the root cortex. Our work in Medicago truncatula highlights the complexity of NIN action and places NIN as a central player in the coordination of the symbiotic developmental programs occurring in differing tissues of the root that combined are necessary for a nitrogen-fixing symbiosis.

Effect of nitrogen fertilization on yield, N content, and nitrogen fixation of alfalfa and smooth bromegrass grown alone or in mixture in greenhouse pots

Planting grass and legume mixtures on improved grasslands has the potential advantage of realizing both higher yields and lower environmental pollution by optimizing the balance between applied N fertilizer and the natural process of legume biological nitrogen fixation. However, the optimal level of N fertilization for grass-legume mixtures, to obtain the highest yield, quality, and contribution of N2 fixation, varies with species. A greenhouse pot experiment was conducted to study the temporal dynamics of N2 fixation of alfalfa (Medicago sativa L.) grown alone and in mixture with smooth bromegrass (Bromus inermis Leyss.) in response to the addition of fertilizer N. Three levels of N (0, 75, and 150 kg ha−1) were examined using 15N-labeled urea to evaluate N2 fixation via the 15N isotope dilution method. Treatments were designated N0 (0.001 g per pot), N75 (1.07 g per pot) and N150 (2.14 g per pot). Alfalfa grown alone did not benefit from the addition of fertilizer N; dry matter was not significantly increased. In contrast, dry weight and N content of smooth bromegrass grown alone was increased significantly by N application. When grown as a mixture, smooth bromegrass biomass was increased significantly by N application, resulted in a decrease in alfalfa biomass. In addition, individual alfalfa plant dry weight (shoots+roots) was significantly lower in the mixture than when grown alone at all N levels. Smooth bromegrass shoot and root dry weight were significantly higher when grown with alfalfa than when grown alone, regardless of N application level. When grown alone, alfalfa's N2 fixation was reduced with N fertilization (R2=0.9376, P=0.0057). When grown in a mixture with smooth bromegrass, with 75 kg ha−1 of N fertilizer, the percentage of atmospheric N2 fixation contribution to total N in alfalfa (%Ndfa) had a maximum of 84.07 and 83.05% in the 2nd and 3rd harvests, respectively. Total 3-harvest %Ndfa was higher when alfalfa was grown in a mixture than when grown alone (shoots: |t|=3.39, P=0.0096; root: |t|=3.57, P=0.0073). We believe this was due to smooth bromegrass being better able to absorb available soil N (due to its fibrous root system), resulting in lower soil N availability and allowing alfalfa to develop an effective N2 fixing symbiosis prior to the 1st harvest. Once soil N levels were depleted, alfalfa was able to fix N2, resulting in the majority of its tissue N being derived from biological nitrogen fixation (BNF) in the 2nd and 3rd harvests. When grown in a mixture, with added N, alfalfa established an effective symbiosis earlier than when grown alone; in monoculture BNF did not contribute a significant portion of plant N in the N75 and N150 treatments, whereas in the mixture, BNF contributed 17.90 and 16.28% for these treatments respectively. Alfalfa has a higher BNF efficiency when grown in a mixture, initiating BNF earlier, and having higher N2 fixation due to less inhibition by soil-available N. For the greatest N-use-efficiency and sustainable production, grass-legume mixtures are recommended for improving grasslands, using a moderate amount of N fertilizer (75 kg N ha−1) to provide optimum benefits.