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Showing papers on "Nitrogen fixation published in 2000"


Journal ArticleDOI
TL;DR: Biological nitrogen (N 2 ) fixation is key to sustainable agricultural systems in tropical soils, which are frequently deficient in N, but high temperature, drought and soil acidity constrain legume root-nodule formation and function in the tropics.

610 citations


Journal ArticleDOI
01 Jan 2000-Ecology
TL;DR: In this article, the authors used two independent methods to determine the dynamics of soil carbon and nitrogen following abandonment of agricultural fields on a Minnesota sand plain, and the two methods yielded similar results.
Abstract: We used two independent methods to determine the dynamics of soil carbon and nitrogen following abandonment of agricultural fields on a Minnesota sand plain. First, we used a chronosequence of 19 fields abandoned from 1927 to 1982 to infer soil carbon and nitrogen dynamics. Second, we directly observed dynamics of carbon and nitrogen over a 12-yr period in 1900 permanent plots in these fields. These observed dynamics were used in a differential equation model to predict soil carbon and nitrogen dynamics. The two methods yielded similar results. Resampling the 1900 plots showed that the rates of accumulation of nitrogen and carbon over 12 yr depended on ambient carbon and nitrogen levels in the soil, with rates of accumulation declining at higher carbon and nitrogen levels. A dynamic model fitted to the observed rates of change predicted logistic dynamics for nitrogen and carbon accumulation. On average, agricultural practices resulted in a 75% loss of soil nitrogen and an 89% loss of soil carbon at the time of abandonment. Recovery to 95% of the preagricultural levels is predicted to require 180 yr for nitrogen and 230 yr for carbon. This model accurately predicted the soil carbon, nitrogen, and carbon : nitrogen ratio patterns observed in the chronosequence of old fields, suggesting that the chronose- quence may be indicative of actual changes in soil carbon and nitrogen. Our results suggest that the rate of carbon accumulation was controlled by the rate of nitrogen accumulation, which in turn depended on atmospheric nitrogen deposition and symbiotic nitrogen fixation by legumes. Our data support the hypothesis that these aban- doned fields initially retain essentially all nitrogen and have a closed nitrogen cycle. Multiple regression suggests that vegetation composition had a significant influence on the rates of accumulation of both nitrogen and carbon; legumes increased these rates, and C 3 grasses and forbs decreased them. C4 grasses increased the C:N ratio of the soil organic matter and thereby increased the rate of carbon accumulation, but not nitrogen accumulation.

552 citations


Journal ArticleDOI
TL;DR: The results indicate that certain strains of rhizobia can promote rice growth and yield, most likely through mechanisms that involve changes in growth physiology or root morphology rather than BNF.
Abstract: Growth-promoting diazotrophs can enhance the growth and development of associated crops by transferring fixed N or by improving nutrient uptake through modulation of hormone-linked phenomena in inoculated plants. Six rhizobial diazotrophs isolated from a wide range of legume hosts were investigated to determine their growth-promoting activities in lowland rice (Oryza sativa L.) during 1997. Seeds and seedlings of rice Pankaj were inoculated with different rhizobia and grown in potted soil supplemented with varied amounts of mineral N. Inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium sp. IRBG74, and Bradyrhizobium sp. IRBG271 increased rice grain and straw yields by 8 to 22 and 4 to 19%, respectively, at different N rates. Nitrogen, P, and K uptake were increased by 10 to 28% due to rhizobial inoculation. Nitrogen-15-based studies indicated that the increased N uptake was not due to biological N 2 fixation (BNF). Inoculation also increased Fe uptake in rice by 15 to 64%. Indole-3-acetic acid (IAA) accumulated in the external root environment of rice plants when grown gnotobiotically with rhizobia. The results indicate that certain strains of rhizobia can promote rice growth and yield, most likely through mechanisms that involve changes in growth physiology or root morphology rather than BNF.

429 citations


Journal ArticleDOI
TL;DR: Only an integrated and multidisciplinary approach can determine if (a) N2-fixing symbioses with Gramineae really exist, and (b) if they are effective in agronomic terms.

417 citations


Journal ArticleDOI
TL;DR: In this paper, a survey of long-term trends in N2 fixation by selected grain legumes is included, and some possible explanations for the observed stagnation in efforts to increase N 2 fixation under field conditions are presented.

390 citations






Journal ArticleDOI
TL;DR: Results from research programs to define genetic variation in N 2 fixation of soybean and to enhance N 2fixing through breeding are presented, with emphasis on symbiotic tolerance to nitrate, mutagenesis-induced supernodulation and promiscuous and selective nodulation.

156 citations


Journal ArticleDOI
TL;DR: There is a general consensus that plant genotype is a key factor to higher contributions of BNF, especially for BNF inputs and ecological studies under field conditions.
Abstract: Biological nitrogen fixation (BNF) in the Gramineae family has been well documented, but a complete understanding of this issue is needed to turn the research into a practical approach. The literature has a long and diverse list of diazotrophic bacteria found colonizing several plant tissues, such as roots, stems, leaves, and trash as well as the rhizosphere. However, only a limited amount of research has focussed on existing associations of N2-fixing microorganisms with grasses or cereal, especially for BNF inputs and ecological studies under field conditions. The recent discovery of the endophytic diazotroph bacteria such as Acetobacter diazotrophicus, Herbaspirillum spp. and Azoarcus spp. colonizing the interior of sugarcane, rice, Kallar grass (Leptochloa fusca (L.) Kunth), respectively, and other species of grasses as well as cereals has led to a considerable interest in exploring these novel associations. There is a general consensus that plant genotype is a key factor to higher contributions of BNF...

Journal ArticleDOI
TL;DR: The paradigms of nitrogenase function, transcriptional control and post-translational regulation, as well as the variations on these schemes, described in various nitrogen-fixing bacteria are described.
Abstract: Biological nitrogen fixation, a process found only in some prokaryotes, is catalyzed by the nitrogenase enzyme complex. Bacteria containing nitrogenase occupy an indispensable ecological niche, supplying fixed nitrogen to the global nitrogen cycle. Due to this inceptive role in the nitrogen cycle, diazotrophs are present in virtually all ecosystems, with representatives in environments as varied as aerobic soils (e.g., Azotobacter species), the ocean surface layer (Trichodesmium) and specialized nodules in legume roots (Rhizobium). In any ecosystem, diazotrophs must respond to varied environmental conditions to regulate the tremendously taxing nitrogen fixation process. All characterized diazotrophs regulate nitrogenase at the transcriptional level. A smaller set also possesses a fast-acting post-translational regulation system. Although there is little apparent variation in the sequences and structures of nitrogenases, there appear to be almost as many nitrogenase-regulating schemes as there are nitrogen-fixing species. Herein are described the paradigms of nitrogenase function, transcriptional control and post-translational regulation, as well as the variations on these schemes, described in various nitrogen-fixing bacteria. Regulation is described on a molecular basis, focusing on the functional and structural characteristics of the proteins responsible for control of nitrogen fixation.

Journal ArticleDOI
TL;DR: Nitrogen-fixing root nodules are found in 10 plant families containing trees and shrubs associated with rhizobia or Frankia, and future research to detail nodulation status, nitrogen fixation rates, physiological adaptations and genetic diversity in both legumes and actinorhizal trees will provide fundamental knowledge to further conservation and utilisation of these plants.

Journal ArticleDOI
TL;DR: The methanogenic Archaea bring a broadened perspective to the field of nitrogen fixation and suggest that nitrogen fixation may have originated in a common ancestor of the Bacteria and the Archaea.
Abstract: The methanogenic Archaea bring a broadened perspective to the field of nitrogen fixation. Biochemical and genetic studies show that nitrogen fixation in Archaea is evolutionarily related to nitrogen fixation in Bacteria and operates by the same fundamental mechanism. At least six nif genes present in Bacteria (nif H, D, K, E, N and X) are also found in the diazotrophic methanogens. Most nitrogenases in methanogens are probably of the molybdenum type. However, differences exist in gene organization and regulation. All six known nif genes of methanogens, plus two homologues of the bacterial nitrogen sensor-regulator glnB, occur in a single operon in Methanococcus maripaludis. nif gene transcription in methanogens is regulated by what appears to be a classical prokaryotic repression mechanism. At least one aspect of regulation, post-transcriptional ammonia switch-off, involves novel members of the glnB family. Phylogenetic analysis suggests that nitrogen fixation may have originated in a common ancestor of the Bacteria and the Archaea.

Journal ArticleDOI
TL;DR: It is demonstrated that, in fertile soil and under temperate climatic conditions, symbiotic N2 fixation per se is responsible for the considerably greater amount of above-ground biomass and the higher N yield under elevated atmospheric pCO2, which supports the assumption that symbioticN2 fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO2-rich world.
Abstract: Summary Although legumes showed a clearly superior yield response to elevated atmospheric pCO2 compared to nonlegumes in a variety of field experiments, the extent to which this is due to symbiotic N2 fixation per se has yet to be determined. Thus, effectively and ineffectively nodulating lucerne (Medicago sativa L.) plants with a very similar genetic background were grown in competition with each other on fertile soil in the Swiss FACE experiment in order to monitor their CO2 response. Under elevated atmospheric pCO2, effectively nodulating lucerne, thus capable of symbiotically fixing N2, strongly increased the harvestable biomass and the N yield, independent of N fertilization. In contrast, the harvestable biomass and N yield of ineffectively nodulating plants were affected negatively by elevated atmospheric pCO2 when N fertilization was low. Large amounts of N fertilizer enabled the plants to respond more favourably to elevated atmospheric pCO2, although not as strongly as effectively nodulating plants. The CO2-induced increase in N yield of the effectively nodulating plants was attributed solely to an increase in symbiotic N2 fixation of 50–175%, depending on the N fertilization treatment. N yield derived from the uptake of mineral N from the soil was, however, not affected by elevated pCO2. This result demonstrates that, in fertile soil and under temperate climatic conditions, symbiotic N2 fixation per se is responsible for the considerably greater amount of above-ground biomass and the higher N yield under elevated atmospheric pCO2. This supports the assumption that symbiotic N2 fixation plays a key role in maintaining the C/N balance in terrestrial ecosystems in a CO2-rich world.

Journal ArticleDOI
TL;DR: The study shows that in the early stage of a cyanobacterial bloom new nitrogen is provided for the pelagic foodweb through release of recently fixed nitrogen by the diazotrophs, whereas in the late stage the input of new nitrogen occurs mainly through lysing of decaying filamentous cyanobacteria cells.


Journal ArticleDOI
TL;DR: Although the associative Azospirillum bacteria (MS) slightly reduced effects relative to single mycorrhizal inoculation (M), the multilevel treatments, with both of the diazotrophs (MRS), showed a further enhancement (a synergistic effect) for almost all of the tested parameters and substrates.


Journal ArticleDOI
TL;DR: In this article, the effect of nitrogen and phosphorus on nodulation and N2 fixation in a common bean (Phaseolus vulgaris) was investigated, considering the effects of different amounts of P and N. The effect of N on the uptake of P by plants was estimated by analysing rhizospheric pH and P concentration in xylem sap and in plant shoots.
Abstract: Availability of nitrogen (N) and phosphorus (P) might significantly affect N2 fixation in legumes. The interaction of N and P was studied in common bean (Phaseolus vulgaris), considering their effects on nodulation and N2 fixation, nitrate reductase activity, and the composition of N compounds in xylem sap. The effect of N on the uptake of P by plants was estimated by analysing rhizospheric pH and P concentration in xylem sap and in plant shoots. Inoculated bean plants were grown in pots containing perlite/vermiculite in two experiments with different amounts of P and N. In a third experiment, bean plants were grown on two soil types or on river sand supplied with different concentrations of N. At harvest, shoot growth, number of nodules and mass, and nitrogenase activity were determined. Xylem sap was collected for the determination of ureides, amino acids, nitrate and phosphate concentration. At low nitrate concentration (1 mM), increasing amounts of P promoted both nodule formation and N2 fixation, measured as ureide content in the xylem sap. However, at high nitrate concentration (10 mM), nodulation and N2 fixation did not improve with increased P supply. Glutamine and aspartate were the main organic N compounds transported in the xylem sap of plants grown in low nitrate, whereas asparagine was the dominant N compound in xylem sap from plants grown in high nitrate. Nitrate reductase activity in roots was higher than in shoots of plants grown with low P and high N. In both soils and in the sand experiment, increased application of N decreased nodule mass and number, nitrogenase activity and xylem ureides but increased the concentration of asparagine in xylem sap. Increasing P nutrition improved symbiotic N2 fixation in bean only at low N concentrations. It did not alleviate the inhibitory effect of high nitrate concentration on N2 fixation. A decrease in plant P uptake was observed, as indicated by a lower concentration of P in the xylem sap and shoots, correlating with the amount of N supplied. Simultaneously with the specific inhibition of N2 fixation, high nitrate concentrations might decrease P availability, thus inhibiting even further the symbiotic association because of the high P requirement for nodulation and N2 fixation.

Journal ArticleDOI
TL;DR: The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited, and increased input of phosphorus would profoundly change their structure and functions.
Abstract: Cyanobacterial mats are important components of oligotrophic wetland ecosystems in the limestone-based regions of the Caribbean. Our goals were to: (1) Estimate the biomass and primary production of cyanobacterial mats, quantify the extent of nitrogen fixation and measure the activity of alkaline phosphatase (APA) in representative marshes of northern Belize; (2) Record changes in these variables following nutrient additions. The mat biomass ranged from 200 to 700 g m−2 AFDM, with the epipelon contributing up to 87% of the total. Tissue nitrogen was similar in all marshes (1.1–1.5%), while tissue phosphorus was extremely low (0.0055–0.0129%) and well correlated with the N:P ratio in water. Nitrogen fixation expressed as nitrogenase activity was high in some marshes (17.5 nmol C2H4 cm−2 h−1) and low (< 5 nmol C2H4 cm−2 h−1) in others depending mainly on the proportion of heterocyst-forming cyanobacteria (Nostocales, Stigonematales) in the mat. Alkaline phosphatase activity was positively correlated with the N:P ratio of the mat. Experimental addition of phosphorus resulted in significant increase in primary production and nitrogen fixation while it suppressed the APA activity. The presented data clearly showed that oligotrophic marshes of northern Belize are strongly P limited. Increased input of phosphorus would profoundly change their structure and functions.

Journal ArticleDOI
TL;DR: In this article, a short and long-term growth chamber experiment was conducted on Sphagnum recurvum P. var. mucronatum (Russ) grown at 360 (ambient) and 700 mu L L-1 (elevated) atmospheric [CO2] in combination with different nitrogen deposition rates (6, 15, 23 g N m m(-2) y(-1)), in a short-and longterm grow chamber experiment.
Abstract: Sphagnum bogs play an important role when considering the impacts of global change on global carbon and nitrogen cycles. Sphagnum recurvum P. Beauv. var. mucronatum (Russ.) was grown at 360 (ambient) and 700 mu L L-1 (elevated) atmospheric [CO2] in combination with different nitrogen deposition rates (6, 15, 23 g N m(-2) y(-1)), in a short- and long-term growth chamber experiment. After 6 months, elevated atmospheric [CO2] in combination with the lowest nitrogen deposition rate, increased plant dry mass by 17%. In combination with a high nitrogen deposition rate, biomass production was not significantly stimulated. At the start of the experiment, photosynthesis was stimulated by elevated atmospheric [CO2], but it was downregulated to control levels after three days of exposure. Elevated [CO2] substantially reduced dark respiration, which resulted in a continuous increase in soluble sugar content in capitula. Differences in growth response among different nitrogen and CO2 treatments could not be related to measured carbon exchange rates, which was mainly due to interference of microbial respiration. Doubling atmospheric [CO2] reduced total nitrogen content in capitula but not in stems at all nitrogen deposition rates. Reduction in total nitrogen content coincided with a decrease in amino acids, but soluble protein levels remained unaffected. Thus, elevated [CO2] induced a substantial shift in the partitioning of nitrogen compounds in capitula. Soluble sugar concentration was negatively correlated with total nitrogen content, which implies that the reduction in amino acid content in capitula, exposed to elevated [CO2], might be caused by the accumulation of soluble sugars. Growth was not stimulated by increased nitrogen deposition. High nitrogen deposition, resulting in a capitulum nitrogen content in excess of 15 mg g(-1) dw, was detrimental to photosynthesis, reduced water content and induced necrosis. We propose a capitulum nitrogen content of 15 mg g(-1) dw as a possible bioindicator for the detection of nitrogen pollution stress in oligo-mesotrophic peat bog ecosystems. At the lowest nitrogen deposition level, nitrogen recovery was higher than 100%, which indicates substantial dry deposition and/or gaseous nitrogen fixation by bacteria, associated with Sphagnum. Increasing nitrogen deposition rates decreased nitrogen recovery percentages, which indicates reduced efficiency of nitrogen fixation.

Journal ArticleDOI
TL;DR: The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial.
Abstract: We measured nitrogenase activity (acetylene reduction) of asymbiotic, heterotrophic, nitrogen-fixing bacteria on leaf litter from the tree Metrosideros polymorpha collected from six sites on the Hawaiian archipelago. At all sites M. polymorpha was the dominant tree, and its litter was the most abundant on the forest floor. The sites spanned a soil chronosequence of 300 to 4.1 million y. We estimated potential nitrogen fixation associated with this leaf litter to be highest at the youngest site (1.25 kg ha-1 y-1), declining to between 0.05 and 0.22 kg ha-1 y-1 at the oldest four sites on the chronosequence. To investigate how the availability of weathered elements influences N fixation rates at different stages of soil development, we sampled M. polymorpha leaf litter from complete, factorial fertilization experiments located at the 300-y, 20,000-y and 4.1 million–y sites. At the youngest and oldest sites, nitrogenase activity on leaf litter increased significantly in the plots fertilized with phosphorus and “total” (all nutrients except N and P); no significant increases in nitrogenase activity were measured in leaf litter from treatments at the middle-aged site. The results suggest that the highest rates of N fixation are sustained during the “building” or early phase of ecosystem development when N is accumulating and inputs of geologically cycled (lithophilic) nutrients from weathering are substantial.

Journal ArticleDOI
TL;DR: The analysis of nodule occupancy in 12 cultivars grown either under sterile conditions and receiving a double inoculum and N-free nutrient solution, or in pots containing soil with an established population of bradyrhizobia, showed the preference of cultivars for specific strains.
Abstract: In order to identify soybean cultivars with higher biological N2 fixation capacities, North American and Brazilian soybean [Glycine max (L.) Merrill] cultivars, belonging to maturity groups VI–VIII, were evaluated for nodulation parameters and N2 fixation rates. The symbiotic performance of 152 cultivars was evaluated in pots containing 4 kg soil with an established population of the three Bradyrhizobium elkanii strains [29w (SEMIA 5019):SEMIA 566 : SEMIA 587, 22%:36%:34%] which are established in most Brazilian soils cultivated with soybean. Differences were verified among cultivars, with some accumulating up to twice as much nodule dry weight and N in tissues as others. The variability among cultivars was also confirmed when six of them were used in a field experiment, resulting in differences in nodulation, yield and total N accumulated in grains. The analysis of nodule occupancy in 12 cultivars grown either under sterile conditions and receiving a double inoculum and N-free nutrient solution, or in pots containing soil with an established population of bradyrhizobia, showed the preference of cultivars for specific strains.

Journal ArticleDOI
TL;DR: Soybean recombinant inbred lines derived from a cross between two cultivars of different genetic origins, each with a high seed protein content, were analyzed for their dinitrogen fixation and nitrate assimilation abilities and for their seed traits.

Journal ArticleDOI
TL;DR: Symbiotic N2 fixation is well adapted to the climatic and acidic soil conditions in the Alps and contributes, up to the altitudinal limit, a significant amount of N to the N nutrition of legumes.
Abstract: Symbiotic N2 fixation may be an important source of N for legumes in alpine ecosystems, though, this has hardly been investigated. Symbiotic N2 fixation in nine legume species in permanent grassland over an altitudinal gradient (from 900 up to 2600 m a.s.l.) was investigated in the Swiss Alps on strictly siliceous soils. To assess symbiotic N2 fixation, an enriched 15N isotope dilution method was established for low N input, permanent grasslands and was evaluated with the 15N natural abundance method. The non-N2-fixing reference species used in both methods differed significantly in their 15N atom%-excess. However, when several reference species were combined, the enriched 15N isotope dilution method was reliable and led to the conclusion that up to their altitudinal limit, legumes may acquire from 59% to more than 90% of their N through symbiotic N2 fixation depending on the species. These findings were confirmed by the 15N natural abundance method. Even at the legumes’ altitudinal limit all plants investigated showed apparently active nodules. Moreover, a clear host-microsymbiont specificity between plant and rhizobia was evident at high altitudes. This suggests that symbiotic N2 fixation is well adapted to the climatic and acidic soil conditions in the Alps and contributes, up to the altitudinal limit, a significant amount of N to the N nutrition of legumes.

Journal ArticleDOI
TL;DR: Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application, and recovery of 15N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers.
Abstract: This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N2-fixing cyanobacteria abundance and N2 fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha-1. No further increase was observed with 140 kg N ha-1. Soil N was the main source of N for rice, only 8–14% of the total N incorporated by plants derived from 15N fertilizer. Recovery of applied 15N-ammonium sulphate by the soil–plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3–1% of applied N was lost by denitrification. Recovery of 15N from labeled cyanobacteria by the soil–plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application.

Journal ArticleDOI
TL;DR: If inorganic nitrogen is depleted after the spring bloom and if inorganic phosphorus still remains in significant concentrations, then a continuous decrease in phosphorus may be coupled to the net nitrogen fixation rate by cyanobacteria.

Journal ArticleDOI
TL;DR: The current dissatisfaction with low productivity of annual medic pastures has highlighted the need to seek alternative legumes to provide efficient N2 fixation in low rainfall, alkaline soil environments of southern Australia, and management options for rhizobial populations to improve legume diversity and productivity are discussed.
Abstract: The current dissatisfaction with low productivity of annual medic (Medicago spp.) pastures has highlighted the need to seek alternative legumes to provide efficient N2 fixation in low rainfall, alkaline soil environments of southern Australia. Clover species adapted to these environments will have limited N2 fixation if effective rhizobia are not present in sufficient quantities. A survey of 61 sites was conducted across South Australia to determine the size, distribution and effectiveness of Rhizobium leguminosarum bv. trifolii (clover rhizobia) populations resident in these low rainfall, alkaline soil environments. Clover rhizobia were detected at 56 of the sites, with a median density of 230–920 rhizobia/g soil. Most rhizobial populations were poor in their capacity to fix nitrogen. Rhizobial populations from fields provided 11–89% and 10–85% of the shoot biomass of commercial reference strains when inoculated onto host legumes T. purpureum (purple clover) and T. resupinatum (persian clover), respectively. Rhizobial population size was correlated negatively to pH and the percentage of CaCO3 in the soil, and was significantly increased in the rhizospheres of naturalised clover, found at 17 sites. Management options for rhizobial populations to improve legume diversity and productivity are discussed in terms of rhizobial population dynamics and likely soil constraints to successful rhizobial colonisation.

Journal ArticleDOI
TL;DR: The result showed that P has a direct effect on the N2-fixation parameters, rather than an indirect effect via increased plant growth, which indicates the importance to studying the effects of more than one nutrient at a time.
Abstract: Red clover (Trifolium pratense L.) is one of the most important plants in forage production, especially in northern areas. Fertilisation practices are focused on high yield and forage quality but effects of nutrients on nodulation and N2 fixation are poorly understood. The aim of this work was to study how nitrogen (N) and phosphorus (P) separately as well as in combination affected nodulation. Red clover plants were grown in pots with gravel in a greenhouse for 11 weeks. To resemble field conditions the root temperature was kept lower than the shoot temperature. Plants were given five different combinations of N and P concentrations during growth. The result showed that at high N concentrations P had a counteracting effect on the N inhibition. The N2-fixation parameters, nodule number, nodule dry matter and specific nitrogenase activity, were six times higher in plants grown with high N and high P than in plants with high N and low P. When the N2-fixation parameters and the dry matter of roots and shoots...