Phosphorus Deficiency Delays the Onset of Nodule Function in Soybean

Effects of phosphorus (P) deficiency on nodulation were examined in soybean grown in nutrient solution for 7 weeks. Increasing P supply increased shoot growth of nitrogen (N₂)-fixing plants from week 5 and that of nitrate-fed plant from week 4 after treatment. Nitrogen (N₂)-fixing plants had a greater P requirement for maximum growth at week 5. Increasing P supply from 1 to 16 μ M increased N concentration in N₂-fixing plants at week 4 but did not affect it from week 5. By contrast, P deficiency increased N concentration in nitrate-fed plants. Increasing P supply improved nodule formation from week 3. Nodule mass was affected more by P supply than nodule number, which, in turn, was affected more than plant growth. However, P supply did not decrease nodule specific N₂ fixation from week 5. The results suggest that P deficiency impaired symbiotic N₂ fixation through delaying onset of nodule function and decreasing nodule development.     

Effects of CO2 concentration in rhizosphere on nodulation and N2 fixation of soybean and cowpea

Soybean (Glycine max L. Merr.) cvs. Akisengoku and Peking, and cowpea (Vigna unguiculata Walp.) cv. Kegonnotaki were inoculated with Bradyrhizobium japonicum AlO17, Shinorhizobium fredii USDAI93, and B. sp. Vigna MAFF03-03063, respectively and were cultured hydroponically with supply of CO₂-free air, 3dm³ m^-3 CO₂ air, or 25 dm³ m^-3 CO₂ air to study the effects of the CO₂ concentration in the rhizosphere on plant growth, nodulation, and nitrogen fixation. Increase of the CO₂ concentration in the rhizosphere led to the increase of the plant dry weight in the symbiosis between Peking and USDAI93, and that between Kegonnotaki and MAFF03-03063. On the other hand, dry matter accumulation in the symbiosis between Akisengoku and AI017 decreased under the supply of 25 dm³ m^-3 CO₂ air aimed at increasing the CO₂ concentration in the rhizosphere beyond the optimum CO₂ concentration for growth. Nodule mass and nodule number per plant were highest in Akisengoku, followed by Kegonnotaki and lowest in Peking. Also the increase of the CO₂ concentration in the rhizosphere led to the increase of the nodule mass and number in Kegonnotaki, while no changes were observed in Akisengoku and Peking. Biological nitrogen fixation (BNF) was highest in Akisengoku, followed by Kegonnotaki, and lowest or near zero in Peking. BNF in Akisengoku and Kegonnotaki showed a similar tendency to that of dry matter accumulation. BNF of Peking was especially low under the supply of CO₂-free air, and it increased with the increase of the CO₂ concentration in the rhizosphere. For the symbiosis of Bradyrhizobium strains with soybean and cowpea, the most suitable CO₂ concentration for N₂ fixation and plant growth was estimated to be about 10 dm³ m^-3, while for the symbiosis of S. fredii with soybean, the value was estimated to be above 30 dm³ m^-3.     

Time-course of dinitrogen fixation of promiscuous soybean cultivars measured by the isotope dilution method

 Soybean cultivars capable of nodulating with indigenous Bradyrhizobium spp. have been developed by the International Institute of Tropical Agriculture (IITA) and national programs in Africa in order to avoid artificial inoculation by resource-poor farmers in Africa. The current selection procedure for enhanced N₂ fixation is based on an assessment of nodule formation which does not directly quantify the proportions of crop N derived from the atmosphere. We have monitored N accumulation patterns and N₂ fixation in nine promiscuous soybean cultivars with different maturity periods, using the ¹⁵N dilution technique. Nodule development generally peaked at the early podfill stage for all cultivars except Tgx 1519-1D and Tgx 1447-2D in which it continued to increase. The proportion of crop N derived from fixation (%NDFA) ranged between 51% and 67%, 77% and 84%, and 66% and 73% at full bloom, early podfill, and physiological maturity stages, respectively. Total N accumulation increased in all soybean genotypes with increasing plant age. Significant correlations (P<0.001) were established between nodule weight and %NDFA, even though this did not explain the relationship between nodule development and N₂ fixation in cultivars such as Tgx 1519-1D. Promiscuous soybean cultivars retained between 10% and 19% of total N accumulated at the final harvest, in belowground biomass. Our results indicated that these soybean cultivars can derive substantial proportions of plant N from N₂ fixation in soils where compatible indigenous bradyrhizobia populations are adequate and effective. Also, we have substantiated the claims that qualitative nodulation parameters currently used to select varieties with a high N₂ fixation capacity need to be validated with other measurements of N₂ fixation. Received: 5 November 1998     

Nodulation and N2 fixation of faba bean (Vicia faba L.) after soil amendment with crop residues

The effect of prior soil amendment with different N sources at 50 mg N (kg soil)^—1 on nodulation and N₂ fixation of faba bean (Vicia faba L. cv. Troy) using wheat (Triticum aestivum L. cv. Star) as reference crop was assessed in a pot experiment. Four treatments viz legume manure (LEGM) as clover shoots, cereal manure (CEREM) as barley straw, N fertilizer (FERT‐N) as Ca(NO₃)₂, and no‐manure control (NOMAN) were investigated consecutively at 45, 70, and 90 days after sowing (DAS). Faba bean nodulated profusely, with an increase on average from 629 nodules per pot at 45 DAS to nearly 2.3‐ and 3.3‐fold at 70 and 90 DAS, respectively. Low nodule numbers and nodule dry matter occurred under FERT‐N and CEREM, whereas high values were found for NOMAN and LEGM. Soil amendment affected percent N₂ fixation in relation to N source and plant age. Highest percent N₂ fixation (≥ 90 %) was found under the lowest N‐supplying amendments, no‐manure, and cereal manure, respectively. FERT‐N depressed N₂ fixation particularly at 45 DAS when N₂ fixation was reduced to as low as 23 %. The rise in N₂ fixation thereafter suggests that faba bean adjusted after depletion of mineral N in the soil. N₂ fixation was also decreased after cereal straw application, even though N concentration in faba bean plants was high. The results indicate that plant residues, both with high and low N concentration, applied to soil to raise its fertility may interfere with N₂ fixation of faba bean.     

Dinitrogen fixation and soil n uptake by soybean as affected by phosphorus availability

The effects of phosphorus supply (0, 30, and 90 mg P kg^‐1) on growth, N₂ fixation, and soil N uptake by soybean (Glycine max (L.) Merr.) were studied in a pot experiment using the ¹⁵N isotope technique. Phosphorus supply increased the top dry matter production at flowering and the dry matter production of seeds, straw, pod shells, and roots at late pod filling of inoculated soybeans. Phosphorus supply reduced the N concentration of plant tops at flowering, but increased the amount of N accumulated at both flowering and late pod filling. In inoculated soybeans total N accumulation paralleled the dry matter production. The P concentration in above‐ground plant parts of nodulated soybeans was not affected by P application. At flowering only 18 to 34% of total N was derived from N₂ fixation, whereas as much as 74% was derived from N₂ fixation at late pod filling. Only the addition of 90 mg P kg^‐1 soil significantly increased the amount of N₂ fixed at the late pod filling stage. Phosphorus supply did not influence the uptake of fertilizer or soil N in soybeans, even if the root mass was increased up to 60% by the P supply.     

Phosphorus‐deficiency effects on response of symbiotic N2 fixation and carbohydrate status in soybean to atmospheric CO2 enrichment

The impact of phosphorus (P) deficiency on response of symbiotic N₂ fixation and carbohydrate accumulation in soybean (Glycine max [L.] Merr.) to atmospheric CO₂ enrichment was examined. Plants inoculated with Bradyrhizobium japonicum MN 110 were grown in growth chambers with controlled atmospheres of 400 and 800 μL CO₂ L^‐1 and supplied either 1.0 mM‐P (P‐sufficient) or 0.05 mM‐P (P‐deficient) nitrogen (N)‐free nutrient solution. When plants were supplied with sufficient P, CO₂ enrichment significantly increased whole plant dry mass (83%), nodule mass (67%), total nitrogenase activity (58%), and N (35%) and P (47%) accumulation at 35 days after transplanting (DAT). Under sufficient P supply, CO₂ enrichment significantly increased starch concentrations in nodules compared to the normal atmospheric CO₂ treatment. Under normal CO₂ levels (400 μL L^‐1) nonstructural carbohydrate concentration (starch plus soluble sugar) was significantly higher in leaves of P‐deficient plants than in leaves of P‐sufficient plants in which nonstructural carbohydrate concentration exhibited a strong diurnal pattern. Under deficient P supply whole plant dry mass, symbiotic N₂‐fixation parameters, and N and P accumulation were not enhanced by atmospheric CO₂ enrichment. Phosphorus deficiency decreased nonstructural carbohydrate accumulation in nodules at the end of a 10‐day period in which functional activity was developing by 86% relative to P‐sufficient controls. While P deficiency elicited significant increases in the nonstructural carbohydrate concentration in leaves, it caused significant decreases in the nonstructural carbohydrate concentration in nodules over the diurnal cycle from 30 to 31 DAT. Collectively, these results indicate that the lack of a symbiotic N₂‐fixation response to atmospheric CO₂ enrichment by P‐deficient plants may be related to the decreased carbohydrate status of nodules.     

Effect of soil physical properties on soybean nodulation and N2 fixation at the early growth stage in heavy soil field in Hachirougata Polder,Japan

We studied the effect of the soil physical properties on soybean nodulation and N₂ fixation in the heavy soil of an upland field (UF) and an upland field converted from a paddy field (UCPF) in the Hachirougata polder, Japan. Seeds of the soybean cultivar Ryuho were sown in each field with or without inoculation of Bradyrhizobium japonicum A1017. The soybean plants were sampled at 35 (V3) and 65 (Rl) d after sowing (DAS), and then nodulation and the percentage of N derived from N₂ fixation in the xylem sap were determined. The soil physical properties were different between UF and UCPF, especially the air permeability and soil water regime. Nodule growth was restricted in UCPF irrespective of rhizobial inoculation, though rhizobial infection was not inhibited by the unfavorable soil physical conditions. Soybean plant growth was closely related to the nodule mass and N₂ fixation activity, and the inoculation of a superior rhizobium strain was effective only at 35 DAS. These results indicate that soybean nodulation and N₂ fixation was considerably affected by the physical properties of heavy soil, and that it is important to maintain the N₂ fixation activity and inoculate the soybean plants with a superior rhizobium strain at a later growth stage in order to increase soybean production in heavy soil fields.     

Low levels of ferredoxin, ATP and leghemoglobin contribute to limited N2 fixation of peas (Pisum sativum L.) and alfalfa (Medicago sativa L.) under S deficiency conditions

Sulphur (S) has become a major limiting factor for plant production in industrial as well as in remote industrial rural areas. Limitation of S can reduce legume N₂ fixation by affecting nodule development and function. In pot experiments with pea (Pisum sativum L.) and alfalfa (Medicago sativa L.), we investigated the influence of S on growth, ferredoxin, ATP and leghemoglobin concentrations. Addition of 200 mg S pot⁻¹ increased yield of shoots, roots and nodules of both plant species significantly. However, the influence of S on nodule yield formation was most pronounced. Pea and alfalfa roots were found to have higher S concentrations than shoots and being up to 2.9 times the S concentration in the shoots of peas under S-sufficient conditions. Sulphur addition also increased N₂ fixation significantly. The ferredoxin concentration in bacteroids of root nodules of pea was increased significantly by S only 10 weeks after planting and in bacteroids of root nodules of alfalfa 10 and 17 weeks after planting, while on per pot base the amounts of ferredoxin were higher throughout the experimental period of time. The ATP concentration of bacteroids of root nodules of both plant species as well as of mitochondria of root nodules of pea were significantly higher with optimum S supply. The effects of S deficiency on N₂ fixation are likely to be caused by the shortage of ferredoxin and ATP. The amount of leghemoglobin was reduced in comparison to nodules of the S-sufficient plants.     

Effects of hairy vetch foliage application on nodulation and nitrogen fixation in soybean cultivated in three soil types

We investigated the effects of applying hairy vetch foliage on nodulation and atmospheric nitrogen (N₂) fixation in soybean cultivated in three soil types in pot experiments. Soybean plants were grown in Gley Lowland soil (GLS), Non-allophanic Andosol (NAS), and Sand-dune Regosol (SDR) with hairy vetch foliage application in a greenhouse for 45 days. In GLS, the nodule number was not influenced by the application, however, nodule dry weight and N₂ fixation activity tended to increase. In NAS and SDR, nodule formation was depressed by foliage application. Soybean plant growth was promoted in GLS and SDR but not in NAS. These promotive effects of hairy vetch foliage application on soybean plant growth in GLS were considered to be mainly caused by the increase in N₂ fixation activity of the nodules, whereas it was considered to be mainly caused by the increase in nitrogen uptake activity of the roots in SDR. The varying effects of hairy vetch foliage application on soybean nodulation may be due to soil chemical properties such as pH and cation exchange capacity, which are related to soil texture. Therefore, we conclude that it is important to use hairy vetch for soybean cultivation based on the different effects of hairy vetch on soybean plant growth in different soil types.     

Effects of co-inoculation of Bradyrhizobium elkanii BLY3-8 and Streptomyces griseoflavus P4 on Rj4 soybean varieties

Co-inoculation of selected nitrogen-fixing bacteria with plant growth-promoting bacteria is the promising way for the improvement of soybean production through enhancing plant growth, nodulation, and N₂ fixation. Therefore, this experiment was conducted to study the effects of co-inoculation of Bradyrhizobium elkanii BLY3-8 with Streptomyces griseoflavus P4 on plant growth, nodulation, N₂ fixation, N uptake, and seed yield of Rj₄ soybean varieties. Two experiments with completely randomized design and three replicates were done in this study. N₂-fixation ability of soybean was evaluated by acetylene reduction activity (ARA) and relative ureide method. In the first experiment, synergetic effect in N₂ fixation and nodulation was occurred in co-inoculation treatment (BLY3-8 + P4) in Yezin-3 and Fukuyutaka. Based on these results, co-inoculation effect of BLY3-8 and P4 was assessed on Yezin-3 and Fukuyutaka varieties at three different growth stages, using Futsukaichi soil under natural environmental conditions. This study shows that co-inoculation of BLY3-8 and P4 significantly increased N₂ fixation at V6 stage; plant growth, nodulation, N₂ fixation, and N uptake at R3.5 stage; and shoot growth, N uptake, and seed yield at R8 stage, in Rj₄ soybean varieties compared with the control. Significant difference in plant growth, nodulation, N₂ fixation, N uptake, and yield between co-inoculation and control, not between single inoculation and control, suggests that there is a synergetic effect due to co-inoculation of BLY3-8 and P4.     

Nodule phosphorus requirement and O2 uptake in common bean genotypes under phosphorus deficiency

Abstract

The effect of P deficiency on nodulation, nodule P content, nodule O₂ permeability and N fixation rates in Phaseolus vulgaris–rhizobia symbiosis was studied under glasshouse conditions. Four recombinant inbred lines (L34, L83, L115 and L147) and one variety cultivated in Morocco (Concesa) were inoculated with Rhizobium tropici CIAT 899 in hydroaeroponic culture. Two P levels i.e. 75 (deficient level) and 250 µmol plant^?1 week^?1 P (sufficient level) were applied and the trial was assessed 42 days after transplanting that coincide with plant flowering stage. Under P-deficiency, decrease of plant growth (18%) and nodule biomass (19%) was detected and significantly pronounced in the sensitive line L147 compared with the remaining genotypes. Additionally, under P-deficiency, the efficiency in use of rhizobial symbiosis, estimated by the slope of the regression model of shoot biomass as a function of nodule biomass, was significantly increased in the four lines. This constraint did not significantly influence nodule P content in Concesa, but it was 24 and 41% lower in the tolerant and in the sensitive lines, respectively. Nodule P content was positively correlated to nodule biomass, r=0.75, and shoot N, r=0.92. These genotypic variations were associated with variability in nodule O₂ permeability that was significantly affected by the P level-bean genotype interaction. Under P-deficiency, nodule O₂ permeability was significantly reduced in the tested genotypes and accompanied with a decrease in shoot N content, especially in the sensitive lines (35%). Moreover, the ratios plant N fixed: nodule P content and plant N fixed:nodule dry weight were affected under P-deficiency in four lines with an exception observed in Concesa. Depending on the observed data we concluded that N₂ fixation efficiency could be influenced by nodulation and level of nodule P requirement which depend on both bean genotypes and P level.     

Assimilation of ammonia by the azolla‐anabaena symbiosis

¹⁵N‐labeled ammonia was rapidly assimilated by Azolla caroliniana and incorporated into plant material even though sustained growth of the fern‐algae symbiosis cannot be maintained with ammonia as nitrogen source. During ammonia uptake, the nitrogenous pool of the fern rapidly increases and contains large amounts of free ammonia and glutamine. N₂ fixation activity of the algal symbiont declines during assimilation of ammonia, but it is restored to a high level upon transfer of plants to nitrogen‐free media, as the pool ammonia content decreases. During growth of the fern on N₂, the algal symbiont supplies ammonia in a manner permitting sustained growth of the plant. Exogenous ammonia, therefore, appears to interrupt regulation of inorganic nitrogen metabolism of the plant‐algal symbiosis.     

Alleviating salt stress on soybean (Glycine max (L.) Merr.) – Bradyrhizobium japonicum symbiosis,using signal molecule genistein

For the onset of symbiosis process between soybean (Glycine max (L.) Merr.) and Bradyrhizobium japonicum, signals should be exchanged. Salinity has inhibitory effects on the symbiosis between the two partners. Hence, a greenhouse experiment was planned to: (1) determine the stressful effects of salinity on soybean and B. japonicum symbiosis, hypothesizing that they can inhibit the signal exchange process between the two partners, and (2) determine if the addition of genistein (a nod gene inducer) to B. japonicum (strain 532C) inocula could overcome the stressful effects of salinity on the Bradyrhizobium – soybean symbiosis. Three levels of salinity (control, 36 and 61 mmolar or 3.6 and 6.1 mmhos/cm) and three levels of genistein (0, 5 and 20 μM) were combined in a factorial fashion in four replicates. Soybean plants were harvested at three different times including 20, 40 and 60 days after inoculation (DAI). Genistein enhanced soybean nodulation and growth, and such effects became greater with time under high salinity levels. For example, at 60 DAI the enhancing effects of genistein on the symbiosis process in soybean was more pronounced at the highest level of salinity. The significant interaction effect between genistein 5 μM and salinity 61 mmolar may reveal the direct role of genistein 5 μM in overcoming the stressful effects of salinity on the symbiosis between B. japonicum and soybean, and hence, plant growth. This novel finding may be very useful to increase soybean yields in salty croplands.     

Combined effects of plant growth‐promoting rhizobacteria and genistein on nitrogen fixation in soybean at suboptimal root zone temperatures

Application of plant growth‐promoting rhizobacteria (PGPR) or the plant to bacteria signal molecule genistein has been shown to increase nodulation and nitrogen (N) fixation by soybean [Glycine max (L.) Merr.] over a range of root zone temperatures (RZTs) and, specifically, off‐sets at least some of the ill‐effects of low RZTs. Two sets of controlled‐environment experiments, one on a growth bench and the other in a greenhouse, were conducted to examine the combined ability of both PGPR and genistein to reduce the negative effects of low RZT on soybean nodulation and N fixation. Each of two the PGPR strains, Serratia proteamaculans 1–102 and Serratia liquefaciens 2–68 were co‐inoculated with Bradyrhizobium japonicum USDA 110 preincubated with 17.5 (somewhat inhibitory), and 15°C (very inhibitory). At RZTs of 25 and 17.5°C PGPR strains and genistein in combination increased the number of nodules and the amount of Nn fixed. The most stimulatory effect was observed at 17.5°C for the combination: S. proteamaculans 1–102 plus B. japonicum USDA 110 pre‐incubated in 15 μM genistein under greenhouse conditions. For most treatment combinations the stimulatory effects of PGPR and genistein were additive at RZTs of 17.5 and 25°C. Surprisingly, the combination of these two factors resulted in antagonism at the very inhibitory RZT of 15°C. The results suggest that the negative effects of certain low RZTs could be more effectively off‐set by combined treatments of PGPR plus geneistin pre‐incubation of rhizobial cultures than by their individual treatment.     

Iron dependent ureides and allantionase synthesis in pigeon pea

The effect of iron (Fe) on ureide metabolism was examined in 45‐day‐old pigeon pea (Cajanus cajan L.) (ureide plant) and alfalfa (Medicago sativa) (amide plant). Plants were either inoculated with Rhizobium or fertilized with ammonium nitrate (NH₄NO₃). The ureides, allantoin and allantoate, and allanotinase activity were increased in pigeon pea with Fe supplementation. Specific effect of Fe on ureide metabolism of pigeon pea was indicated by the lack of same effect in alfalfa under similar conditions. Nitrogenase activity was elevated with increasing concentrations of Fe in pigeon pea (ureide) as well as alfalfa (amide) symbiosis. Nitrogen (N₂) fixation, ureides, and allantoinase activity were reduced at 10 ppm and above concentration of Fe.     

Inhibitory effects of acidic minesoil on the sericea lespedeza/Bradyrhizobium symbiotic relationship 1

Effects of acidic minesoil on sericea lespedeza [Lespedeza juncea (L.F.) var. sericea (Mig.)] and its nitrogen (N₂)‐fixing symbiotic relationship with Bradyrhizobium spp. were examined. Sericea lespedeza was grown in pots with N fertilization, without N fertilization, or with commercial Bradyrhizobium as a seed inoculant. Minesoil (pH 5.2) was fertilized with calcium (Ca), phosphorus (P), molybdenum (Mo), and potassium (K), and the pH level was adjusted to 4.8 or 4.5 with aluminum or iron sulfate [Al₂(SO₄)₃; Fe₂(SO₄)₃]. Minesoil was also limed to pH 6.1. Shoot dry weights, shoot N concentrations, nodule dry weights, and nodule numbers were significantly lower (P < 0.05) when inoculated plants were grown in soil at pH 4.5 and 4.8 compared to limed soil. Thus, the N₂ fixation process was adversely affected below pH 5.0. Nitrogen‐fertilized plants grew well in acidified soil, and there were no significant differences in shoot dry weights of such plants among the soil acidification treatments including limed soil. Thus, the N₂‐fixing symbiosis appeared to be more sensitive to acidified soil than the plant host. The effects of Al toxicity versus other factors could not be determined because Al₂(SO₄)₃‐ and Fe₂(SO₄)₃‐amended soils contained similar levels of toxic Al at the highest pH (4.8) that prevented N₂ fixation.

Time periods required for cells of Bradyrhizobium strains to multiply by a factor of 10⁴ were significantly longer (P ≤ 0.05) in extracts of Al₂(SO₄)₃‐amended soil (pH 4.8 and 4.5) than in extracts of calcium carbonate [CaCO₃]‐amended soil (pH 6.1). These increases suggested that reduced multiplication of Bradyrhizobium in acidified minesoils may have been at least partially responsible for the large decreases in nodulation and N₂ fixation observed in these soils. It was also reasoned that the inability of existing bacteria to infect and nodulate plant roots may also have been a factor, based on the high inoculation rates used and the abilities of Bradyrhizobium cells to survive and multiply (albeit at a reduced rate) in extracts of acidified soil. Sericea lespedeza is known to tolerate soils of pH 4.5. However, results of this study suggested sericea lespedeza may not fix appreciable N₂ in acidic soil below pH 5 when inoculated with commercial Bradyrhizobium, even after the establishment of lespedeza plants tolerant of such conditions.     

Overcoming the Stressful Effects of Salinity and Acidity on Soybean Nodulation and Yields Using Signal Molecule Genistein Under Field Conditions

Soil stresses such as salinity and acidity may adversely affect nitrogen (N)₂-fixation. The hypothesis of this study is that soil salinity and acidity inhibit soybean [Glycine max (L.) Merr.] nodulation and N₂-fixation due to, at least in part, disruption of the signal exchange process. The objectives were: 1) to determine the effects of stressful soil salinity and acidity on the signal exchange processes between soybean and Bradyrhizobium japonicum, and 2) to determine whether or not the addition of signal molecule genistein to B. japonicum can overcome at least part of the inhibition of nodulation, caused by stressful soil salinity and acidity. Salt (sodium chloride) and sulfur (S) were applied. Genistein (0, 5, and 20μ M) was tested. Genistein addition could partially overcome the salt and acidity stresses by increasing soybean yields up to 21% and 23%, respectively. These novel findings may be very useful for planting soybean under salinity and acidity stresses.     

Effect of organic amendments on nodulation and nitrogen fixation by cowpea

Cornstover, garbage compost, and cowdung were added to an Alfisol (Iwo series) at the rates of 0, 23, 69, and 115 Mg ha^‐1 and incubated for 4 weeks in the greenhouse prior to sowing cowpea [Vigna unguiculata (L.) var. Ife Brown]. Two successive plantings were made per pot. Harvesting of the plants was at onset of flowering, 42 days after planting. Nitrogen (N) fixed was assessed using the N₂ difference method. Soil reaction (pH), organic carbon (C), and tissue N contents were increased by all the organic amendments. At both harvests, dry matter yields were significantly enhanced by garbage compost and cowdung. While garbage compost and cowdung increased the amounts of N₂ fixed, the efficiency of N₂ fixation was higher in the cornstover treatments. The cornstover additions also significantly increased nodule size and number. These results suggest that organic amendments would benefit N₂ fixation especially in soils low in indigenous organic matter. The beneficial effects were in the order: cowdung > garbage compost > cornstover. To enhance N₂ fixation in cowpea, a rate of 23 Mg ha^‐1 of these organic amendments is suggested.     

Drought and nitrogen source effects on nitrogen nutrition,seed growth,and yield in soybean

Drought in soybean [Glycine max (L.) Merr.] decreases yield‐related processes and N₂ fixation is more sensitive to drought than are many other of these processes. Therefore, application of nitrogen (N) fertilizer may increase drought tolerance over those plants primarily dependent on N₂ fixation. In a field experiment, NH₄NO₃ applications (+N) to drought‐stressed soybean resulted in biomass and N accumulation rates similar to those rates for an irrigated treatment without N fertilizer (‐N). In contrast, biomass and N accumulation rates were decreased for the ‐N treatment. N fertilization increased seed growth rate and decreased seed fill duration for irrigated and drought treatments. In the drought treatment, N application increased seed number per unit area, which resulted in higher yields. In a greenhouse experiment, fertilization with either KN0₃ or NH₄C1 increased biomass and N accumulation rates during drought over those of plants dependent solely on N₂ fixation. It was concluded that application of N fertilizer to soybean increases drought tolerance because of the extreme sensitivity of N₂ fixation to drought.     

Comparative growth and symbiotic performance of four Acacia mangium provenances from Papua New Guinea in response to the supply of phosphorus at various concentrations

Plant growth performance, the P content in root and nodule tissues, and nodulation and N₂-fixing ability were studied in four provenances of Acacia mangium from Papua New Guinea following different levels of P fertilizer application. A. mangium did not seem to need high levels of P for growth and N₂ fixation. The response by this leguminous tree to the P supply varied significantly according to provenance and to P concentrations in the culture solution. The provenances of A. mangium were classified into three types according to their P response: (1) Growth performance, nodulation, and N₂ fixation of plants were stimulated as concentrations of P increased (provenance PH 482); (2) the maximal effect of P on plant growth was found only at P concentrations higher than 500 M (provenance PH 484); and (3) the plant response to P fertilization was low, even with nutrient solutions containing P concentratins higher than 500 M (provenances PH 483 and PH 485). Provenance PH 483 was distinguished by its low nodulating ability. However, this provenance grew well, probably because of its high N₂ fixation efficiency as expressed by specific acetylene reduction activity and its high P content in nodule tissues. Therefore, in certain cases, these two parameters may be useful criteria in selecting leguminous plants for field use. Statistical analyses of the study results showed that the effect of the factor P supply on N₂ fixation efficiency and nodule development was only significant at P concentrations lower than 250 M whereas the effect of the factor plant provenance was significant regardless of the P concentration used. This observation emphasizes the value of provenance screening in the identification of plants for use in a wide range of soil types.