Increased N2 Fixation Research Articles

Phototrophic N2 fixation suppressed by activated sulfate reduction in anoxic rice soil slurries.

Interactions between sulfate reduction (SR) and phototrophic nitrogenase activities were investigated in rice soil slurries mixed with rice straw. Activation of SR by adding exogenous sulfate suppressed acetylene-reducing activity (ARA) of the slurries, which was associated with phototrophic purple bacteria (PB) enumerated to 108-109 MPN g-1 dry weight (dw) soil. Adding 5 mm sodium molybdate, an inhibitor of SR, markedly increased ARA. However, in the slurries receiving both molybdate and exogenous sulfate, the effects declined simultaneously with partial recovery of SR. These results indicate outcompetition of sulfate-reducing bacteria (SRB) with PB in rice soil, when sulfate concentrations are high enough to support SR. The increasing effects of molybdate on ARA continued during the incubation in the sulfate-depleted condition, probably because of absence of SR and toxicity of molybdate to methanogenesis. Accordingly, stopping activities of the competitive microorganisms may be efficient to increase N2 fixation in rice soil.

Supply of O2 regulates demand for O2 and uptake of malate by N2-fixing bacteroids from soybean nodules.

Bacteroids, prepared anaerobically from soybean root nodules by fractional centrifugation or by sucrose or Percoll density-gradient methods, were retained within a stirred, flow-through reaction chamber and used to determine rates of respiration and N2 fixation at various rates of O2 supply. Liquid reaction solutions containing malate, oxyleghaemoglobin, dissolved N2 and various levels of dissolved O2 were passed through the reaction chamber at measured rates of flow. The relative oxygenation of leghaemoglobin in the chamber was determined automatically by spectrophotometry of the effluent solution, and the concentrations of free, dissolved O2 ([O2(free)]) and the rates of O2 consumption were calculated. N2 fixation was measured by analysis of fractions of effluent. The principal finding was that stepwise increases in the flow rate (increasing the supply of O2 and malate) induced an increase in O2 demand (respiration) resulting in a decrease in [O2(free)] and increased N2 fixation. In some experiments, samples of bacteroids were withdrawn from the flow chamber during steady states and the rates of malate uptake were measured in standard, microaerobic assays. Progressive taking of samples from the flow chamber whilst maintaining constant flow rates (increasing the supply of O2 and malate per bacteroid) also resulted in increased O2 demand and declines in [O2(free)]. With increased bacteroid respiration, transport of malate into bacteroids (linear with time between 1 and 5 min after starting each assay) increased proportionally. This suggests that the rate of malate transport is tightly coupled with bacteroid respiration. Thus, bacteroid respiration, coupled with malate uptake, must be regulated by the rate of O2 supply, rather than by the [O2(free)] prevailing in the stirred chamber as found or assumed in previous work. These features are discussed in relation to N2 fixation by anaerobically isolated bacteroids.

Trends in rhizobial inoculant production and use

Rhizobia inoculants have contributed to increase N2 fixation and yield in legumes crops. However, most of the inoculants produced world-wide are of poor or suboptimal quality. We discuss here why some of them are poor products and how to improve their quality and efficacy. Reported data on the inoculation rate effect can be used to design good inoculants. Technologies are now available to produce inoculants with a shelf-life of more than 1 year. Available quality control methods can help to improve the quality of inoculants although they do not take into account the physiological satus of the rhizobia. Unfortunately quality control is not commonly used except in major inoculant companies and the quality of inoculants sold on the market is low. The need for an increase in quality standards is discussed especially for the number of rhizobia delivered per seed and for the presence of contaminants. Some new technologies which able to increase efficacy and reliability of inoculation are discussed.

Symbiotic Performance of Herbaceous Legumes in Tropical Cover Cropping Systems

Increasing use of herbaceous legumes such as mucuna (Mucuna pruriens var. utilis [Wright] Bruck) and lablab (Lablab purpureus [L.] Sweet) in the derived savannas of West Africa can be attributed to their potential to fix atmospheric nitrogen (N2). The effects of management practices on N2 fixation in mucuna and lablab were examined using 15N isotope dilution technique. Dry matter yield of both legumes at 12 weeks was two to five times more in in situ mulch (IM) than live mulch (LM) systems. Land Equivalent Ratios, however, showed 8 to 30% more efficient utilization of resources required for biomass production under LM than IM systems. Live mulching reduced nodule numbers in the legumes by one third compared to values in the IM systems. Similarly, nodule mass was reduced by 34 to 58% under LM compared to the IM systems. The proportion of fixed N2 in the legumes was 18% higher in LM than IM systems. Except for inoculated mucuna, the amounts of N fixed by both legumes were greater in IM than LM systems. Rhizobia inoculation of the legumes did not significantly increase N2 fixation compared to uninoculated plots. Application of N fertilizer reduced N2 fixed in the legumes by 36 to 51% compared to inoculated or uninoculated systems. The implications of cover cropping, N fertilization, and rhizobia inoculation on N contributions of legumes into tropical low-input systems were discussed.

Variability in yield and yield component responses to genistein pre-incubated Bradyrhizobium japonicum by soybean [Glycine max (L.) Merr] cultivars

The sub-tropical legume, soybean [Glycine max (L.) Merr.], has lower grain yields at low temperatures, mainly due to reduced nitrogen fixation. The isoflavone genistein has been identified as one of the major compounds in soybean seed and root extracts responsible for inducing the expression of the B. japonicum nod genes. A 2-year field study was conducted in 1997 and 1998 with 11 soybean cultivars recommended for Quebec, and representing a range of yield potentials and maturity groups. The objective of this study was to assess the variability among soybean cultivar maturity groups in terms of response to genistein application under Canadian short season and cool-spring conditions. The experiments were organized in a randomized complete block design with three replications. The two genistein treatments included B. japonicum inoculant pre-incubated with 20 μm genistein and B. japonicum inoculant only. The inoculants were applied into the furrow at the time of planting. The results of this study showed that genistein pre-incubated B. japonicum increased soybean grain yield and protein content over two years. In 1998, pod number per plant−1 and seed number plant−1 were also clearly increased. When 20 μm genistein was applied in 1998, cultivars in the late maturity group had 28 and 70% more shoot and total protein content, respectively, than the early maturity groups with or without genistein, or the late maturity without genistein, in 1998. There was no interaction between genistein application and soybean cultivar in this study, indicating that both early and late maturing cultivars responded similarly to genistein pretreated inocula. Pre-incubation of B. japonicum with genistein can increase N2 fixation potential in short season areas. Key words: Soybean, cultivars, genistein, yield, and yield components

Soybean Cultivar Differences in Ureides and the Relationship to Drought Tolerant Nitrogen Fixation and Manganese Nutrition

Water deficit in soybean [Glycine max (L.) Merr.] results in the accumulation of the products of N2 fixation (ureides) in shoots, and this may lead to feedback inhibition of N2 fixation. Manganese is required for ureide degradation in leaves, and it was hypothesized that increased leaf Mn+2 would alleviate ureide accumulation during drought, lessen feedback inhibition, and prolong N2 fixation. In a growth chamber experiment, ureides supplied through roots decreased N2 fixation in well‐watered plants, and a soybean cultivar with demonstrated tolerance of N2 fixation to water deficit (Jackson) had a lesser concentration of shoot ureides following exogenous ureide application than a cultivar sensitive to water deficit (KS4895). In a greenhouse experiment, N2 fixation in Jackson under moderate water deficit was not different from the control in the absence or presence of soil‐applied Mn+2, whereas N2 fixation in KS4895 was 30% of the control in the absence of soil‐applied Mn+2 and 111% of the control in the presence of soil‐applied Mn+2 Increased N2 fixation in KS4895 with soil‐applied Mn+2 under water deficit was associated with decreased shoot ureide concentration. In field experiments, Jackson consistently had lesser shoot ureide concentrations than did KS4895, and enzymatic degradation of ureides was greater for Jackson on one date than for KS4895. We concluded that ureides inhibit N2 fixation, that genetic variation in the ability to degrade ureides may be important in drought tolerance, and that increased leaf Mn+2 concentration promotes ureide breakdown and prolongs N2 fixation under water deficit.

Nitrogen nutrition in nodulated field plants of the shrub tea legume Aspalathus linearis assessed using 15N natural abundance

Provision of N, P, and Ca to field plants of A. linearis markedly (P<0.05) increased growth and N nutrition in a very acidic nutrient-poor soil. Application of P and Ca promoted a significant increase in %N derived from fixation and amounts of N fixed compared to those receiving no nutrients. N2 fixation measured under field conditions ranged from 3.8 g N plant-1 in unfertilized control to 7.1 g N plant-1 in fertilized plants. Overall, about 85% increase in N2 fixation was observed with P supply. The high N2-fixing activity in P-treated plants was confirmed by their lower (more negative) ∂15N values. Age of plants also influenced growth and symbiotic activity as the ∂15N values, %N derived from fixation, and N fixed were lower in 1- and 2-year-old plants compared to 3-year-old. The contribution of symbiotic fixation in unfertilized A. linearis to the N economy of the ecosystem ranged from 105 kg N ha-1 in 1-year-old plants to 128 kg N ha-1 in 3-year-old plants, clearly indicating the remarkable adaptation of this symbiosis to the very nutrient-poor, low pH conditions of Cedarberg soils.

Water deficit stress effects on N2 fixation in cowpea inoculated with different Bradyrhizobium strains

The objectives of this experiment were to select strains of Bradyrhizobium sp. resistant to water stress, envisaging an increase in N2 fixation in cowpea (Vigna unguiculata [L.] Walp.), and to verify the plant's adaptive physiological responses to water stress. The experiment was carried out in greenhouse conditions using random complete blocks subdivided into plots adjusted to soil water potential levels of −6.0, −75.0, and −85.0 kPa, and subplots containing strains of Bradyrhizobium sp. (SEMIA 6145, 6086, 6002 and NFB 700), with four blocks. The soil was a Yellow Latosol with pH 6.3. The crop used was cowpea cv. IPA 204. Stress was applied continuously beginning 15 d after planting, by the control of water potential through a porous cup. Various parameters were evaluated every 7 days, until final harvest at 45 d. There was significant interaction between Bradyrhizobium strains and water stress. At the more negative ψm, plants inoculated with the SEMIA 6145 had higher LHb concentration, ureide-N, ψw and root dry matter, forming associations of greater symbiotic efficiency, while plants inoculated with SEMIA 6086 were not resistant to stress. LHb concentration apparently was not inhibited at ψw −1.0 MPa in cowpea. The Bradyrhizobium strains may have affected the metabolism of N assimilation and/or transport. Key words: Bradyrhizobium, N2 fixation, porous cup, Vigna unguiculata, water stress, Yellow Latosol

Biocontrol of soil‐borne plant pathogens attacking cucumber (Cucumis sativus) by Rhizobacteria in a semiarid environment

Three new strains of growth‐promoting N2‐fixing rhizobacteria, namely, Azospirillum brasilense SBR, Azotobacter chroococcum ZCR, and Klebsiella pneumoneae KPR, engineered for increased N2 fixation, and a commercial product H ALEX® were tested for growth promotion and as remediation agents against the soil‐borne plant pathogens Fusarium oxysporum f. sp. lycopersici, Rhizoctonia solani and Pythium sp., which attack cucumbers in the semiarid environment of Egypt. In vitro, the bacteria reduced the dry weight of mycelium of Fusarium by 90–96%, that of Rhizoctonia by 72–94%, Pythium by 71–95%, and Sclerotinia by 100%. Bioassaying the effect of HALEX on the pathogens showed a 56% decrease in the incidence of damping‐off and a 66% decrease in flowering at the age of 8 weeks. It showed an increase in emergence by 4%, photosynthetic area by 22%, shoot and root dry weights by 30% and 80%, respectively, and shoot N content by 89%. The term Sigmoid nutrition is introduced to describe the continuous and close provision of growth promoters and nutrients to the plant all along the Sigmoid growth curve.

Stimulation of Symbiotic N2 Fixation in Trifolium repens L. under Elevated Atmospheric pCO2 in a Grassland Ecosystem.

Symbiotic N2 fixation is one of the main processes that introduces N into terrestrial ecosystems. As such, it may be crucial for the sequestration of the extra C available in a world of continuously increasing atmospheric CO2 partial pressure (pCO2). The effect of elevated pCO2 (60 Pa) on symbiotic N2 fixation (15N-isotope dilution method) was investigated using Free-Air-CO2-Enrichment technology over a period of 3 years. Trifolium repens was cultivated either alone or together with Lolium perenne (a nonfixing reference crop) in mixed swards. Two different N fertilization levels and defoliation frequencies were applied. The total N yield increased consistently and the percentage of plant N derived from symbiotic N2 fixation increased significantly in T. repens under elevated pCO2. All additionally assimilated N was derived from symbiotic N2 fixation, not from the soil. In the mixtures exposed to elevated pCO2, an increased amount of symbiotically fixed N (+7.8, 8.2, and 6.2 g m-2 a-1 in 1993, 1994, and 1995, respectively) was introduced into the system. Increased N2 fixation is a competitive advantage for T. repens in mixed swards with pasture grasses and may be a crucial factor in maintaining the C:N ratio in the ecosystem as a whole.

Comparison between parental and variant soybeanBradyrhizobium strains with regard to the production of lipo-chitin nodulation signals, early stages of root infection, nodule occupancy, and N2 fixation rates

Soybean is the most important leguminous crop in Brazil and the nitrogen required for plant growth is supplied byBradyrhizobium bacteria through the symbiotic relation established by the inoculation process. Since 1992, two new strains, CPAC 7 and CPAC 15, which have been shown to increase yields in several field experiments, have been recommended in Brazilian commercial inoculants. CPAC 15 is a natural variant of theB. elkanii SEMIA 566 strain, and was isolated after several years of adaptation to a Brazilian Cerrado soil, while CPAC 7 is a variant ofB. japonicum strain CB 1809, selected under laboratory conditions for higher nodulation and yield. The comparison between parental and variant strains, under greenhouse conditions, showed that both CPAC 15 and CPAC 7 increased N2 fixation rates in relation to the parental strains. The better performance of CPAC 15 was related to an increase in nodule efficiency (mg N2 fixed mg-1 nodule) while with CPAC 7 the higher N2 fixation rates were due to increased nodulation. Both CPAC 15 and CPAC 7 increased nodule occupancy, when co-inoculated at a ratio of 1:1 withB. elkanii 29w, in relation to their parental strains. Variant strains also differed from parental in their ability to increase numbers of root hairs (Hai phenotype) either when inoculated onto plants, or when supernatants of bacteria exposed to seed exudates were used as inoculants. This results lead to the hypothesis that a modification in some of the “common” nodulation genes had occurred. However, the increase in Hai phenotype with CPAC 7 was dependent on the soybean cultivar, indicating a possible alteration in some genotypic specific nodulation gene. Apparently, there were no differences in Nod metabolites produced by strains CPAC 15 and SEMIA 566, but a more detailed chemical analysis would be required to rule out subtle differences. On the contrary, significant differences were found between CPAC 7 and the parental strain CP 1809, in the profile of Nod metabolites. Consequently, it may be possible that diffusable molecules, responsible for Hai phenotype, would be related to nodulation ability, competiviveness, and N2 fixation, resulting in the higher yields that have been associated with CPAC 7 and CPAC 15. For the CPAC 7 strain, the increase in Hai phenotype could be atributed to the differences found in the Nod molecules. Consequently, a high degree of physiological and genetic variability can result from the adaptation of rhizobial strains to the soil. Also, this variability can be found under laboratory conditions, when searching single colonies with specific properties. ei]Section editor: R O D Dixon

Inhibition of N2 fixation by white clover (Trifolium repens L.) at low concentrations of NO inf3 sup? in flowing solution culture

The impact of sustained low external concentrations of NO 3 − (0, 10, 100 and 1000 mmol m−3) on plant growth and the relative acquisition of N through N2 fixation and NO 3 − uptake by established, nodulated white clover (Trifolium repens L. cv. Blanca) was studied over 28 days in flowing solution culture. Nitrogen fixation was measured by N difference and 15N dilution methods. Plants supplied with NO 3 − achieved higher relative growth rates (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3702!\[\bar x\]=0.091 d−1) compared with ‘control’ plants dependent on N2 fixation (0.073 d−1). Nitrate plants showed progressive increases in shoot: root d.w. ratios from 4 to 6.5–7.6 between days 0–28, compared with 5.1 on day 28 for control plants. Increases in both nodule d.w. and numbers per plant were inhibited after day seven at all concentrations of NO 3 − . The severity of inhibition of N2 fixation increased with increasing NO 3 − concentration and with time. The total amounts of N2 fixed per plant between days 0–7 after supplying 10, 100 and 1000 mmol m−3 NO 3 − , respectively, were 37–39, 28–30 and 0–13%, of the total N acquired. Between days 7–28 the proportional contributions of N2 fixation to total N acquisition declined to 3, 0.5 and 0%, respectively, in these treatments. The corresponding mean specific rates of N2 fixation between days 0–7 were, respectively, 5.4, 3.2, and 2.0 mmol N d−1 g−1 nodule d.w., compared with 7.9 mmol N d−1 g−1 nodule d.w. for zero NO 3 − plants. There was no evidence of a transitory increase in N2 fixation following the addition of NO 3 − , even at the lowest supply concentration.

Sowing time and tillage practice affect chickpea yield and nitrogen fixation. 2. Nitrogen accumulation, nitrogen fixation and soil nitrogen balance

Following long-term studies at Warra, on the western Darling Downs, chckpea (Cicer anetinum) was selected as a useful grain legume cash crop with potential for improvement of its nitrogen (N) fixing ability through management. This 2-year study examined the effect of sowing time and tillage practice on dry matter yield, grain yield (Horn et al. 1996), N accumulation, N2 fixation, and the subsequent soil N balance. Generally, greater N accumulation resulted from sowing in late autumn-early winter (89-117 kg N/ha) than sowing in late winter (76-90 kg N/ha). The amount of N2 fixed was low in both years (15-32 kg N/ha), and was not significantly affected by sowing time or tillage. The potential for N2 fixation was reduced in both years due to high initial soil nitrate levels and low total biomass of chickpea because of low rainfall. Nitrogen accumulation by grain was higher under zero tillage (ZT) than conventional tillage (CT) for all sowing times, and this affected the level of grain N export. The consequence of low N2 fixation and high N export in chickpea grain was a net loss of total soil N, (2-48 kg N/ha under CT and 22-59 kg N/ha under ZT). Management practices to ensure larger biomass production and lower soil nitrate-N levels may result in increased N2 fixation by chickpea and thus a positive soil N balance.

Dynamics of nodulation, nitrogen fixation, nitrogen use and biomass yield over time in pot-grown Leucaena leucocephala (Lam.) de Wit

The dynamics of nodulation, N2-fixation and N use in Leucaena leucocephala cv. K28 over time was investigated in a screenhouse at 4, 8, 12 and 16 months after planting (MAP) using the 15N-labelling method. Leucaena had a consistently increasing pattern of nodulation, dry biomass and nitrogen yield. A sharp rise in nodulation was observed between 12 and 16 MAP, whereas for biomass, N accumulation and N2-fixation, and N2-fixation, an upward surge occurred between 4 and 12 months. Nodulation, N accumulation, N2-fixation and biomass yield all peaked at 16 MAP. Along with the steady increase in N2-fixation throughout the 16-month growth period, the % N derived from the atmosphere rose from 17.9% to 61.5%, 70.1% and 74%, equivalent to 191, 1623, 2395 and 3385 mg N2 fixed plant-1 at 4, 8, 12 and 16 MAP, respectively. Nitrogen assimilation from soil and fertilizer decreased inversely to the increase in symbiotic nitrogen fixation with time.

Ecophysiology of stromatolitic microbial mats, Stocking Island, exuma cays, Bahamas

Intertidal stromatolites, covered by cyanobacterial mats, were recently discovered at Stocking Island, Exuma Cays, Bahamas. Ecophysiological responses (CO2 fixation, N2 fixation, and photoacclimation) of these cyanobacterial mats to experimental manipulations were examined to identify potential environmental variables controlling community structure and function. The mats exhibit horizontal zonation that shifts from soft to crusty to hard in a seaward direction. Cluster analysis of chemotaxonomic photopigments (chlorophylls and carotenoids) revealed that visually distinct mat types are composed of distinct phototrophic assemblages. Under reduced irradiance, diatoms within the mats photoacclimated by increasing accessory photopigments (diadinoxanthin, fucoxanthin, and chlorophyll c 1 c 2) and cyanobacteria reduced the photoprotective carotenoid echinenone. In a 4-day nutrient addition bioassay experiment, nitrate, phosphate, dissolved organic carbon, and trace metal enrichments did not enhance CO2 fixation, but phosphate enrichments tripled N2 fixation rates. The addition of DCMU increased N2 fixation rates relative to nonamended light and dark rates, indicating light (photosystem I) enhanced nitrogenase activity. Soft mats appear to represent the early stages of colonization and stabilization of mat communities. Active growth following stabilization results in the formation of partially-lithified crusty mats, which eventually become highly-lithified and form hard mats. Collectively, our results suggest that Stocking Island stromatolitic mats have low growth rates and consequently exhibit slow responses to increased nutrient availability and changes in ambient irradiance. In general, intertidal stromatolitic mats at Stocking Island appear to exhibit low rates of CO2 and N2 fixation relative to nonlithifying temperate cyanobacteral mats. Although production is low, respiration is likewise low, leading to the suggestion that high production to respiration ratios (P:R) may be necessary for lithification of intertidal stromatolitic mats.

Nitrogen fixation by trees in relation to soil nitrogen economy

The N2-fixing potential (NFP) (i.e. the amount of fixed N2 in a constraint-free environment) of N2-fixing trees (NFTs) varies with the genotype. The NFP can be higher than 30-50 g N2 fixed tree−1 year−1 in the most active species, be they leguminous trees such asAlbizia lebbeck, Gliricidia sepium andLeucaena leucocephala, or actinorhizal trees such asCasuarina equisetifolia. The actual amount of nitrogen fixed (ANF) (i.e. the amount of N2 fixed in the field) is lower than the NFP or even nil because of various constraints, especially drought, nutrient deficiencies, excess of available N and pathogenic nematodes. As tree litters are mineralized, the amount of available N in the soil increases with time, this process leading to the cessation of N2 fixation in aging plantations. When the mineralization rate is slowed down or inhibited, N2 fixation can continue. NFTs improve the N status of soils, but the transfer of fixed N to associated plants is not always ensured. Three main approaches are appropriate to increase N2 fixation: clonal selection of trees combined with vegetative propagation, inoculation with effective rhizobium orFrankia strains, and proper fertilization (especially P). In the absence of major environmental constraints, a positive response to inoculation is expected only when specific (non-promiscuous) NFTs are grown in sites where the density of compatible rhizobia is low or nil. The potentialities of NFTs are far from being fully exploited. Further investigations are proposed and the economics of NFT management is briefly discussed.

Ontogenic variation in nitrogen fixation and accumulation of nitrogen in mungbean, blackgram, cowpea, and groundnut

Ontogenic variations in N2 fixation and accumulation of N by the mungbean (Vigna radiata L. Wilczek), blackgram (Vigna mungo L. Hepper), cowpea (Vigna unguiculata L. Walp.), and groundnut (Arachis hypogaea L.) were studied by a 15N-dilution technique. Pots filled with 7 kg of red yellow podzolic soil were used. Samples were taken 20, 40, 60, and 80 days after emergence which approximately corresponded to preflowering, flowering, early/mid-pod filling and late pod filling stages, respectively. During early growth (up to 40 days after emergence), the carryover of seed N accounted for a considerable fraction of the total plant N in the legumes, the highest being in the groundnut. With a correction for carryover, the groundnut derived over 45% of its N content from the atmosphere 20 days after emergence whereas the corresponding figures were 33% for the blackgram and about 28% for the cowpea and mungbean. Between flowering and early pod fill, there was a rapid increase in N2 fixation in all legumes except in groundnut which showed highest fixation from 60 to 80 days after emergence. In the mungbean, N2 fixation and uptake of soil N were insignificant 60 days after emergence while in other legumes these processes continued beyond this time. All legumes derived about 90% of their N from atmosphere by 80 days after emergence. However, due to considerable interspecific differences in total N yield the final amount of N2 fixed showed an appreciable variation among legumes. It was highest in the groundnut (443 mg N plant-1) followed by the cowpea (385), blackgram (273), and mungbean (145), respectively. The groundnut maintained nodules until the late pod filling stage while in other legumes, nodules senesced progressively following the mid-pod filling stage. During pod filling there was a net mobilization of N from vegetative tissues to developing pods in the mungbean, which amounted to about 20% of N in seeds. This mobilization was not evident in other legumes.

Field Response of Soybean to Increased Dinitrogen Fixation

A number of studies have shown an association between soybean [Glycine max (L.) Merr.] seed yield and plant N accumulation, suggesting that yield could be increased by nodulation by superior strains of Bradyrhizobium japonicum. Attempts to evaluate the response of field‐grown soybean to bradyrhizobia‐mediated increases in N accumulation have been hindered by an inability of introduced strains to compete successfully with indigenous strains. Therefore, to determine the effect of increased N2 fixation on soybean growth and seed yield, an experiment was conducted using soybean seedlings that were nodulated under greenhouse conditions and then transplanted to the field. Two inoculation treatments, USDA 122 and a mixture of bradyrhizobia indigenous to the field site (control), were applied to the seed and seedlings were developed for 21 d in the greenhouse. Prenodulated seedlings were then transplanted to 1.8 m2 field microplots on a Matapeake silt loam (fine‐loamy, mixed, mesic Typic Hapludult) with a most probable number of indigenous bradyrhizobial population of 6.1 × l08 cells kg−1 soil. Relative to the control, nodulation by USDA 122 increased both N2 fixed and total shoot N by a 2‐yr average of 29%. Plants nodulated by USDA 122 also assimilated more soil N. The increase in N2 fixation and total N accumulation occurred prior to, but not during, seedfill. The USDA 122 treatment increased seed yield by an average of 31% (90 g m−2). The yield increase was associated with increases in pod number, seedfill rate, remobilized dry matter, remobilized N, and vegetative biomass.

Soybean Stover Nitrogen Affected by Dinitrogen Fixation and Cultivar

AbstractGrain legumes often are credited with leaving residual N for crops that follow in the rotation. This study was conducted to determine if increasing N2 fixation in soybean [Glycine max (L.) Merr.] would result in higher amounts of stover N at harvest. Eleven early‐maturing cultivars (Maturity group 00 and 0) in 1988 and 16 in 1989 were grown with five inoculant strain treatments and alone as uninoculated controls. There were slight cultivar‐by‐inoculant interactions but main effects were much larger. The order of N2 fixation with inoculant treatments, measured by the difference method, was 532C or three‐strain mixture &gt; USDA 110 &gt; CB 1809 &gt; HH303. Dinitrogen fixation ranged from 60 to 125 kg ha−1 in 1988 and from 31 to 84 in 1989. Higher N2 fixation caused related significant (P ≤ 0.05) increases in stover dry matter and stover N yields in both years, despite concomitant increases in the apparent harvest index (AHI) and apparent NHI. Stover N concentration was not affected by increased N2 fixation in either year. In 1988, cultivars differed only slightly in stover N content. In 1989, amounts of stover N in different cultivars depended on how completely N was translocated to seed. Over the 2 yr, amounts of N in harvestable stover of the early‐maturing cultivars ranged from 9 to 24 kg ha−1, averaged across inoculant treatments. Although stover N increased with N2 fixation in both years, the amount of stover N available for return to the soil was quite low.

Sowing date and varietal effects on the N2 fixation of field pea and implications for improvement of soil nitrogen

Dry matter, biomass N, N2 fixation (determined by the 15N isotope dilution method), grain yield and grain N, were determined for five pea cultivars grown with three sowing times at six sites in south-eastern Australia. Earlier sowing (late April to early May) increased N2 fixation (by as much as 96 kg N/ha compared to sowing in late June to early July). Early sowing improved the probability of peas contributing to soil total N. The potential increment in soil N from the pea stubbles left after harvesting grain, averaged over the varieties, was as high as 98 kg N/ha with early sowing, and as low as -38 kg N/ha with late sowing. The benefits from earlier sowing were due to larger dry matter production with a higher N concentration and a greater proportion of plant N from N2 fixation. Varietal variation in fixed N and potential for augmenting soil total N, was generally smaller than the variation in these parameters due to different sowing dates.