钙磷对酸铝土壤中苜蓿根瘤菌迁移定殖和群体感应的影响

采用分室(内、外室)培养法,以紫花苜蓿及其耐酸根瘤菌91522为材料,在酸性土壤(pH4.45)中额外补充Al3+至中度铝毒水平,从培养装置根箱外接种,探讨了补充Ca2+、P后,耐酸苜蓿根瘤菌在酸性土壤上的存活、迁移以及群体感应的变化动态。结果表明,补充5 mmol kg-1Ca2+处理的土壤根瘤菌的数量在97 d种植期内均显著高于对照(Ca0P0,即无Ca2+无P处理),Ca2+5 mmol kg-1+P 0μmol kg-1处理近根区(距苜蓿植株根1～2 cm)土壤最大根瘤菌数量为同期对照的6.15倍;在此基础上补充P后根瘤菌的数量进一步增加,"Ca2+5 mmol kg-1+P 4μmol kg-1"处理近根区最大根瘤菌数量为同期对照处理的9.40倍。因此推断Ca2+和P的上述作用存在交互效应。施加Ca2+、P能够显著提高土壤中根瘤菌群体感应物质N-酰基高丝氨酸内酯衍生物(N-acyl-homoserine actones,AHLs)的含量,且Ca2+5 mmol kg-1效果好于Ca2+10mmol kg-1处理。根瘤菌数量在远根区(距苜蓿植株根6～8 cm)与近根区变化规律一致,即接种1周后根瘤菌数量由远根区向近根区逐渐增加,接种后30 d内达到最高值,之后数量下降并趋于稳定。但近根区根瘤菌数量和AHLs含量均高于远根区,说明宿主根际微环境也能够影响根瘤菌数量和群体感应。初步认为,酸性有铝土壤上补充Ca2+5 mmol kg-1和P 30μmol kg-1对耐酸根瘤菌的存活、迁移和群体感应有良好的改善效果。该处理最终显著地增强了对酸性土壤极为敏感的苜蓿植株的耐酸铝胁迫能力,较对照显著(p0.05)改善了酸铝胁迫下苜蓿的植株根鲜重(为对照的4.67倍)、地上部鲜重(3.10倍)、含氮量(2.47倍)和根瘤数(14.74倍)等农艺性状。     

Size, symbiotic effectiveness and genetic diversity of field pea rhizobia (Rhizobium leguminosarum bv. viciae) populations in South Australian soils

Field pea (Pisum sativum L.) is widely grown in South Australia (SA), often without inoculation with commercial rhizobia. To establish if symbiotic factors are limiting the growth of field pea we examined the size, symbiotic effectiveness and diversity of populations of field pea rhizobia (Rhizobium leguminosarum bv. viciae) that have become naturalised in South Australian soils and nodulate many pea crops. Most probable number plant infection tests on 33 soils showed that R. l. bv. viciae populations ranged from undetectable (six soils) to 32×10³ rhizobia g⁻¹ of dry soil. Twenty-four of the 33 soils contained more than 100 rhizobia g⁻¹ soil. Three of the six soils in which no R. l. bv. viciae were detected had not grown a host legume (field pea, faba bean, vetch or lentil). For soils that had grown a host legume, there was no correlation between the size of R. l. bv. viciae populations and either the time since a host legume had been grown or any measured soil factor (pH, inorganic N and organic C). In glasshouse experiments, inoculation of the field pea cultivar Parafield with the commercial Rhizobium strain SU303 resulted in a highly effective symbiosis. The SU303 treatment produced as much shoot dry weight as the mineral N treatment and more than 2.9 times the shoot dry weight of the uninoculated treatment. Twenty-two of the 33 naturalised populations of rhizobia (applied to pea plants as soil suspensions) produced prompt and abundant nodulation. These symbioses were generally effective at N₂ fixation, with shoot dry weight ranging from 98% (soil 21) down to 61% (soil 30) of the SU303 treatment, the least effective population of rhizobia still producing nearly double the growth of the uninoculated treatment. Low shoot dry weights resulting from most of the remaining soil treatments were associated with delayed or erratic nodulation caused by low numbers of rhizobia. Random amplified polymorphic DNA (RAPD) polymerase chain reaction (PCR) fingerprinting of 70 rhizobial isolates recovered from five of the 33 soils (14 isolates from each soil) showed that naturalised populations were composed of multiple (5-9) strain types. There was little evidence of strain dominance, with a single strain type occupying more than 30% of trap host nodules in only two of the five populations. Cluster analysis of RAPD PCR banding patterns showed that strain types in naturalised populations were not closely related to the current commercial inoculant strain for field pea (SU303, ≥75% dissimilarity), six previous field pea inoculant strains (≥55% dissimilarity) or a former commercial inoculant strain for faba bean (WSM1274, ≥66% dissimilarity). Two of the most closely related strain types (≤15% dissimilarity) were found at widely separate locations in SA and may have potential as commercial inoculant strains. Given the size and diversity of the naturalised pea rhizobia populations in SA soils and their relative effectiveness, it is unlikely that inoculation with a commercial strain of rhizobia will improve N₂ fixation in field pea crops, unless the number of rhizobia in the soil is very low or absent (e.g. where a legume host has not been previously grown and for three soils from western Eyre Peninsula). The general effectiveness of the pea rhizobia populations also indicates that reduced N₂ fixation is unlikely to be the major cause of the declining field pea yields observed in recent times.     

Cropping history affects nodulation and symbiotic efficiency of distinct hairy vetch (Vicia villosa Roth.) genotypes with resident soil rhizobia

Compatible rhizobia strains are essential for nodulation and biological nitrogen fixation (BNF) of hairy vetch (Vicia villosa Roth, HV). We evaluated how past HV cultivation affected nodulation and BNF across host genotypes. Five groups of similar HV genotypes were inoculated with soil dilutions from six paired fields, three with 10-year HV cultivation history (HV+) and three with no history (HV?), and used to determine efficiency of rhizobia nodulation and BNF. Nodulation was equated to nodule number and mass, BNF to plant N and Rhizobium leguminosarum biovar viceae (Rlv) soil cell counts using qPCR to generate an amplicon of targeted Rlv nodD genes. Both HV cultivation history and genotype affected BNF parameters. Plants inoculated with HV+ soil dilutions averaged 60 and 70 % greater nodule number and mass, respectively. Such plants also had greater biomass and tissue N than those inoculated with HV? soil. Plant biomass and tissue N were strongly correlated to nodule mass (r ²?=?0.80 and 0.50, respectively), while correlations to nodule number were low (r ²?=?0.50 and 0.31, respectively). Although hairy vetch rhizobia occur naturally in soils, past cultivation of HV was shown in this study to enhance nodulation gene-carrying Rlv population size and/or efficiency of rhizobia capable of nodulation and N fixation.     

Sampling effects on the assessment of genetic diversity of rhizobia associated with soybean and common bean

Biological nitrogen fixation plays a key role in agriculture sustainability, and assessment of rhizobial diversity contributes to worldwide knowledge of biodiversity of soil microorganisms, to the usefulness of rhizobial collections and to the establishment of long-term strategies aimed at increasing contributions of legume-fixed N to agriculture. Although in recent decades the use of molecular techniques has contributed greatly to enhancing knowledge of rhizobial diversity, concerns remain over simple issues such as the effects of sampling on estimates of diversity. In this study, rhizobia were isolated from nodules of plants grown under field conditions, in pots containing soil, or in Leonard jars receiving a 10⁻² or a 10⁻⁴ serially-diluted soil inoculum, using one exotic (soybean, Glycine max) and one indigenous (common bean, Phaseolus vulgaris) legume species. The experiments were performed using an oxisol with a high population (10⁵ cells g⁻¹ soil) of both soybean rhizobia, composed of naturalized strains introduced in inoculants and of indigenous common-bean rhizobia. BOX-PCR was used to evaluate strain diversity, while RFLP-PCR of the ITS (internally transcribed spacer) region with five restriction enzymes aimed at discriminating rhizobial species. In both analyses the genetic diversity of common-bean rhizobia was greater than that of soybean. For the common bean, diversity was greatly enhanced at the 10⁻⁴ dilution, while for the soybean dilution decreased diversity. Qualitative differences were also observed, as the DNA profiles differed for each treatment in both host plants. Differences obtained can be attributed to dissimilarity in the history of the introduction of both the host plant and the rhizobia (exotic vs. indigenous), to host-plant specificity, rhizobial competitiveness, and population structure, including ease with which some types are released from microcolonies in soil. Therefore, sampling method should be considered both in the interpretation and comparison of the results obtained in different studies, and in the setting of the goals of any study, e.g. selection of competitive strains, or collection of a larger spectrum of rhizobia. Furthermore, effects of sampling should be investigated for each symbiosis.     

Regulation of parasitism by host density: The Bdellovibrio-Rhizobium interrelationship

Rhizobium strains of the cowpea group did not lose viability readily when added to soil, but Bdellovibrio acting on these rhizobia were found in 32 of 90 soils examined. Bdellovibrio did not initiate replication in liquid media at low host densities, but it did multiply once the Rhizohium numbers increased through growth to about 10⁸ ml^?1. From about 10⁴ to 6 × 10⁵ ml^?1Rhizohium cells survived attack by the parasites in liquid media. In nutrient-free buffer, no significant increase in vibrio abundance was evident if the rhizobial frequency was low. whereas Rhizobium populations containing 6 × 10⁸ cells ml^?1 were lysed rapidly. Bdellovibrio did not multiply when introduced into sterile soil with small numbers of the host, but it replicated when the rhizobia were abundant because of the latter's use of soil organic matter for growth or because of the deliberate addition of 10⁸Rhizohium g^?1. Nevertheless, the host persisted in such vibrio-rich soil samples. The abundance of indigenous bdellovibrios increased appreciably in nonsterile soil if the rhizobia were introduced in large but not small numbers. It is suggested that a major reason for the lack of elimination of the host population in soil by its parasites is the need for a critical host cell frequency, large Rhizobium numbers being required for Btiellovibrio to initiate replication and low numbers of surviving hosts no longer being able to support the parasite.     

Population densities of clover rhizobia in Texas pastures and response to liming

Summary Clovers are widely used forage legumes on acidic soils in Texas and need inoculation with appropriate rhizobia when first introduced. Acidic soils are not conducive to survival of clover rhizobia. A survey of pastures was undertaken to determine the number of rhizobia present. The effect of liming acidic soils on the survival of clover rhizobia was also evaluated in the laboratory. The number of clover rhizobia was more than 100 cells g^-1 soil in 70% of the pastures surveyed but populations within pastures varied by more than two orders of magnitude. The number of years of clover production beyond 1 year did not affect the rhizobial population density. The soil pH of twelve samples was below 5.0 and six samples had populations of rhizobial lower than 100 g^-1 soil. Eleven out of sixteen samples from fields that had grown clover and had pH values above 6.0 had populations exceeding 1000 g^-1 soil and only three samples had populations lower than 100 g^-1 soil. Incubating indigenous or inoculated rhizobia in well-mixed soils having pH values of 5.1 or below resulted in populations declining to below 10 g^-1 soil in 6 weeks. Mixing of soils with pH values of up to 5.4 induced reduction of rhizobial numbers, possibly by destroying microsites. Liming of soils to increase pH values above 5.5 improved survival of native or inoculated rhizobia in most cases.     

Nodulation of tree legumes and the ecology of their native rhizobial populations in tropical soils

A legume introduced into a new area will only form nodules and fix nitrogen if compatible rhizobia are present in the soil. Using 25 (60 in the case of Sesbania sesban) soils sampled from tropical areas of Africa, Asia and Latin America, we examined the nodulation of four agroforestry tree species (Calliandra calothyrsus, Gliricidia sepium, Leucaena leucocephala and S. sesban), their symbiotic interactions with the native rhizobial populations, and some of the ecological indicators of rhizobial population dynamics. Rhizobial population sizes estimated by the legume species ranged from undetectable numbers to 3.16×10⁴ cells per g of soil depending on the trap host species. Although C. calothyrsus had the highest nodulation rate in the soils used, inoculation tests showed L. leucocephala to be the most promiscuous species while G. sepium had the most effective symbiosis. S. sesban was the most specific for both nodulation and symbiotic effectiveness. Symbiotic effectiveness did not bear any close relationship with specific soil parameters, but rhizobial numbers were highly correlated with soil acidity, particle size and exchangeable bases. Soil acidity was also the main factor that was highly correlated with genetic diversity among the rhizobial populations.     

Displacement of native rhizobia nodulating chickpea (Cicer arietinum L.) by an inoculant strain through soil solarization

Summary Soil solarization greatly reduced the native chickpea Rhizobium population. With inoculation, it was possible to increase the population of the Rhizobium in solarized plots. In the 1st year, 47% nodulation was obtained with chickpea inoculant strain IC 59 when introduced with a cereal crop 2 weeks after the soil solarization and having a native Rhizobium count of <10 g^-1 soil, and only 13% when introduced 16 weeks after solarization at the time the chickpeas were sown, with 2.0×10² native rhizobia g^-1 soil. In the non-solarized plots inoculated with 5.6×10³ native rhizobia g^-1 soil, only 6% nodulation was obtained with the inoculant. In the succeeding year, non-inoculated chickpea was grown on the same plots without any solarization or Rhizobium inoculation. The treatment that showed good establishment of the inoculant strain in year 1 formed 68% inoculant nodules. Other treatments indicated a further reduction in inoculant success, from 1%–13% to 1%–9%. Soil solarization thus allowed an inoculant strain to successfully displace the high native population in the field and can serve as a research tool to compare strains in the field, irrespective of competitive ability. In year 1, Rhizobium inoculation of chickpea gave increased nodulation and increased plant growth 20 and 51 days after sowing, and increased dry matter, grain yield, and grain protein yield at maturity. These beneficial effects of inoculation on plant growth and yield were not measured in the 2nd year.Submitted as Journal Article No. JA 945 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Andhra Pradesh 502 324, India     

Growth of perennial forage legumes in acidic soils of the Appalachian Hill‐Lands after liming

A major constraint to the renovation of forage legume‐based pastures on acidic soils of the Appalachian hill‐lands is thought to be the absence of effective rhizobia. A growth chamber experiment was done with aluminum (Al) toxic, low pH (≥ 4.2) soils from four series (Berks, Lily, Tate, and Westmoreland) that were planted with alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), white clover (Trifolium repens L.), or birdsfoot trefoil (Lotus corniculatus L.). These soils, without lime addition, were previously shown not to contain effective, naturalized populations of rhizobia for these plant species. However, a non‐toxic, pH 6.8, Watauga soil was shown to have such rhizobia but only for alfalfa. In the present study, these five soils were reexamined after liming to pH ≥ 5.5 for effective, naturalized populations of rhizobia and the efficacy of soil inoculation with commercially available rhizobia. In addition to effective, naturalized R. meliloti for alfalfa in the Watauga soil, similar populations of R. trifolii for red clover, and R. lotus for birdsfoot trefoil, were now found. Such rhizobia were also found for alfalfa in the Lily soil and for red clover in the Lily and Tate soil. Thus, liming allowed the expression of effectiveness of natural rhizobia that otherwise would not have been detected in soil pot experiments without lime. Inoculation of the toxic soils after lime addition with commercial rhizobia was effective in about half of the soil‐plant combinations that did not contain populations of effective, naturalized rhizobia. Asymbiotic shoot growth of all the plant species was significantly (P ≤ 0.05) correlated with soil pH over a range of 5.5–6.6. These results indicate that, in the absence of effective, naturalized populations of rhizobia, improvement of rhizobial inocula could increase forage production by ～34% for some species on some of the toxic soils, even after the pH of the soils is increased to ≥ 5.5.     

Co-selection of compatible rhizobia and vesicular-arbuscular mycorrhizal fungi for cowpea in sterilized and non-sterilized soils

Summary We selected two isolates of Rhizobium for cowpea (Vigna unguiculata) with sterilized soil tests and two different isolates by non-sterilized soil testing. The four rhizobia were then paired individually with either Glomus pallidum, Glomus aggregatum, or Sclerocystis microcarpa in separate, sterilized, or non-sterilized soil experiments. The purpose of the experiments was to determine the effect of soil sterilization on the selection of effective cowpea rhizobia, and to see whether these rhizobia differed in their effects on cowpea growth when paired with various vesicular-arbuscular mycorrhizal (VAM) fungi. Our experiments showed that the rhizobia selected in sterilized soil tests produced few growth responses in the cowpea compared to the other introduced rhizobia, irrespective of pairing with VAM fungi in sterilized or non-sterilized soil. In contrast, the two rhizobia initially selected by non-sterilized soil testing significantly improved cowpea growth in non-sterilized soil, especially when paired with G. pallidum. Our results suggest that it is important to select for effective rhizobia in non-sterilized soil, and that pairing these rhizobia with specific, coselected VAM fungi can significantly improve the legume growth response.     

A microcosm study on the influence of pH and the host-plant on the soil persistence of two alfalfa-nodulating rhizobia with different saprophytic and symbiotic characteristics

The acid tolerance of Sinorhizobium meliloti in culture media and in soils is considered a useful criteria to select for strains with improved survival in agricultural acidic soils. Using a glass tube system with gamma-irradiated soil at different pH values, we analysed the survival of two different alfalfa-nodulating rhizobia: S. meliloti (pH_limit for growth 5.6–6.0) and the acid-tolerant Rhizobium sp. LPU83, closely related to the strain Rhizobium sp. Or191 (pH_limit for growth below 5.0). Although the acid-tolerant rhizobia showed a slightly better survival during the first months in acid soil (pH=5.6), none of the strains could be detected 2 months after inoculation (bacterial counts were below 10³ colony-forming units (cfu)/30 g of soil). The inclusion of two alfalfa plants/glass tube with soil, however, supported the persistence of both types of rhizobia at pH 5.6 for over 2 months with counts higher than 9×10⁶ cfu/30 g of soil. Remarkably, in the presence of alfalfa the cell densities reached by S. meliloti were higher than those reached by strain LPU83, which started to decline 1 week after inoculation. Although more acid-sensitive in the culture medium than the Or191-like rhizobia, in the presence of the host plant the S. meliloti strains showed to be better adapted to the free-living condition, irrespective of the pH of the soil.     

Competitive ability of selected Cyclopia Vent. rhizobia under glasshouse and field conditions

This study tested the competitive ability of three locally isolated Cyclopia rhizobia and strain PPRICI3, the strain currently recommended for the cultivation of Cyclopia, a tea-producing legume. Under sterile glasshouse conditions, the three locally isolated strains were equally competitive with strain PPRICI3. In field soils, the inoculant strains were largely outcompeted by native rhizobia present in the soil, although nodule occupancy was higher in nodules growing close to the root crown (the original inoculation area). In glasshouse experiments using field soil, the test strains again performed poorly, gaining less than 6% nodule occupancy in the one soil type. The presence of Cyclopia-compatible rhizobia in field soils, together with the poor competitive ability of inoculant strains, resulted in inoculation having no effect on Cyclopia yield, nodule number or nodule mass. The native rhizobial population did not only effectively nodulate uninoculated control plants, they also out-competed introduced strains for nodule occupancy in inoculated plants. Nonetheless, the Cyclopia produced high crop yields, possibly due to an adequate supply of soil N.     

Chickpea rhizobium populations: Survey of influence of season,soil depth and cropping pattern

Chickpea Rhizobium populations in soil samples from research stations and farmers' fields in different geographic regions of India ranged from <10 to > 10⁴ rhizobia g⁻¹ soil. Fields on research stations with a known history of chickpea cropping had more rhizobia (calc. 10³ to 10⁵ rhizobia g^1&#x0304; soil) than the majority of farmers' fields (calc. < 10 to 10³ rhizobia g⁻¹ soil). In the absence of chickpea in the cropping pattern, soils generally had < 10² rhizobia g^1&#x0304; and crops in such fields nodulated poorly. However, poor nodulation was also observed when populations of rhizobia were high, indicating that other factors were also important for nodulation. There was no obvious consistent correlation of Rhizobium population with pH, electrical conductivity and nitrate-nitrogen status of the soil.Rhizobium populations declined with soil depth and were highest (about 10⁴ rhizobia g⁻¹ soil) in the top 30 cm of the profile and lowest, but still present (calc. 10³–10³ rhizobia g'1 soil), at 90–120 cm—a depth where no nodules are found. Populations fluctuated most in the top 5 cm, being reduced during periods of high soil temperature in summer and recovering after rains. Rhizobium populations were at a maximum after chickpea but survived well under pigeonpea, groundnut and maize. When rice followed an inoculated chickpea crop, there was about a 100-fold decrease in the Rhizobium population.     

Soybean [<Emphasis Type="Italic">Glycine max</Emphasis> (L.) Merrill] rhizobial diversity in Brazilian oxisols under various soil,cropping, and inoculation managements

In this study, soybean nodules were collected from 12 sites in the State of Mato Grosso, in the Brazilian Cerrados, where both exotic soybean [Glycine max (L.) Merrill] and bradyrhizobial strains have been introduced from 1 to 18 years before. All soils were originally devoid of rhizobia capable of effectively nodulating soybean and varied in terms of chemical and physical properties, inoculation procedures, and cropping systems. Rhizobial genetic diversity was assessed on 240 isolates by rep-PCR fingerprinting with BOX primer, and indices of diversity (abundance-based coverage estimator and traditional and modified Shannon indices) were applied to the profiles obtained. The genetic diversity was much greater than expected, as after the introduction of a maximum of four strains, up to 13 profiles were identified, some sharing many similar bands with the inoculant strains, but others quite distinct from the putative parental genotypes. The increase in the number of rep-PCR profiles could be attributed to genetic variability due to the stressful tropical environmental conditions, but also might indicate that indigenous rhizobia become capable of nodulating the host legume. After the third year of cropping with the host legume, inoculation did not affect rhizobial diversity. A high content of clay decreased diversity in comparison with that seen in a sandy soil, probably due to reduced aeration. Diversity was higher under the no-tillage system when compared to the conventional tillage management, highlighting the importance of maintaining crop residues in tropical soils. Understanding the ecology of exotic rhizobia after being introduced into new cropping areas represents a first step towards the establishment of better strategies of inoculation, which in turn may result in sustainability and higher plant yields.     

Rhizobia in tropical legumes—IX. pot and field trials with inoculants for Psophocarpus tetragonolobus (L.) DC.

Antigenically identifiable inoculants for Psophocarpus tetragonolobus were evaluated in three non-sterile soils contained in pots (sandy-clay, Renggam series; a loamy-sand, Sungei Buloh series; silty-clay, Munchong series). Most-probable-numbers of indigenous rhizobia ranged from 4 (Renggam series) to 13 (Munchong series) g^?1. Only two (RRIM 56 and 968) of the eight rhizobia tested formed > 50% of the nodules in all soils. Recovery of two strains (RRIM 968 and UMKL 12) was significantly poorer from the Munchong series soil which had the most indigenous rhizobia and the highest silt plus clay content. In a field trial using a Sungei Buloh series soil containing 700 rhizobia g^?1 capable of nodulating P. tetragonolobus, none of the applied strains formed > 18% of the nodules; two formed no nodules. There were no significant increases in plant yield in response to inoculation in the field trial and in two soils in the pot trials. In Sungei Buloh series soil, RRIM 56 formed 90% of the nodules when the indigenous rhizobia were 5 cells g^?, and 14% when the population was 700 g^?1. This raises the question of the need to inoculate seed sown into soils with high indigenous rhizobial populations, but there was some indication of increasing representation of inoculant strains in nodules with time.     

Competitiveness and persistence of introduced rhizobia on oversown clover: Influence of strain,inoculation rate and lime pelleting

Five strains of R. trifolii were evaluated, at two inoculation levels in the presence or absence of lime pelleting, for their ability to compete and persist in a tussock grassland soil containing a naturalized population of rhizobia. Effects on the growth of the oversown white clover (Trifolium repens) were also investigated.Strains showed marked differences in their ability to form nodules on the host in competition with the naturalized population of rhizobia and also differed in their persistence over 15 months. The most competitive and persistent strain was PDDCC 2163 followed by 2153, 2666 and 2668 and the least competitive was 4144. Lime pelleting or increasing the rate of inoculation increased the competitive ability of strains. Strains that were highly competitive increased clover dry matter and N uptake.     

Biochar particle size and Rhizobia strains effect on the uptake and efficiency of nitrogen in lentils

Abstract

Biochar has attained significant attention as a beneficial soil amendment amongst growers and researchers. However, the impact of particle size of biochar is yet to be investigated. Here in the present study, we studied three particle sizes (<2?mm, 2–5?mm, and >5?mm) of biochar and two rhizobia strains (Rhizobium leguminoserum (RL) and Rhizogold (RG)) for their effect on the uptake and efficiency of nitrogen (N) in lentils. The two years experiment followed a randomized complete block design with split plot arrangement replicated three times. The data revealed that grain N, straw N, N uptake, N recovery efficiency (NRE), and N agronomic efficiency (NAE) were maximum with biochar smallest size (<2?mm). However, the N physiological efficiency, number of branches and plant height decreased with reduced particle size. Furthermore, the smallest particle size showed more number of pod plant^?1. Biofertilizer strain (RL and RG) significantly increased the straw N but not the grain N. Both strains showed increased NRE and NAE, however, the RL demonstrated 7% more grain N than the RG. Both strains (RL and RG) demonstrated 16% and 20% increase in number of branches plant^?1, 62% and 48% in plant height and 2% and 5% in root length, respectively. The RL strain improved the number of branches plant^?1 at the lowest (<2?mm) and medium size (2–5?mm) particles size but both RL and RG strains demonstrated increased plant height under the maximum particle size. These results indicated that a mere increase in surface area with decreasing biochar particle size may not serve for enhancing biofertilizer strains performance since reducing particle size may immobilize the starter N applied. However, reducing particle size effect on N cycling into soil plant system was favorable.     

Native rhizobium strains are lacking in some agricultural soils in NE South Africa

ABSTRACT

Rhizobia is a group of gram-negative soil-borne bacteria with several beneficial strains for biological nitrogen fixation in legume crops. Rhizobium strains are found native in the soil but where they are absent, commercial strains are inoculated on crops. We assessed the availability of native rhizobia in chickpea fields at two sites, with contrasting soil types, in NE South Africa. Serial dilutions were used to identify bacteria from soil samples and chickpea nodules sampled before sowing and at flowering, respectively. Our results indicated the absence of rhizobia strains at both sites. Burkholderia cenocepacia, Klebsiella variicola, Bacillus subtilis and Ochrobactrum spp, which are not important in agriculture but are often reported in clinical environments, were identified. Therefore, inoculating chickpea with compatible rhizobia strains may be necessary in some soils in this region.     

Influence of Pseudomonas syringae R25 and P. putida R105 on the growth and N2 fixation (acetylene reduction activity) of pea (Pisum sativum L.) and field bean (Phaseolus vulgaris L.)

The effects of inoculating field peas (Pisum sativum L.) with Rhizobium leguminosarum and field beans (Phaseolus vulgaris L.) with R. phaseoli, alone or in combination with Pseudomonas syringae R25 and/or P. putida R105, were assessed under gnotobiotic conditions in growth pouches and in potted soil in a growth chamber. Inoculation of peas with P. syringae R25 or P. putida R105 alone had no effect on plant growth in pouches. In soil, however, the isolate R25 inhibited nitrogenase activity (as assessed by acetylene reduction assay) of nodules formed by indigenous rhizobia; strain R105 stimulated pea seedling emergence and nodulation. P. syringae R25 inhibited the growth of beans in either plant-growth system. P. putida R105, however, had no effects on beans in pouches, but reduced plant root biomass and nodulation by indigenous rhizobia in soil. Coinoculation of pea seeds with R. leguminosarum and either of the pseudomonads significantly (P<0.01) increased shoot, root, and total plant weight in growth pouches, but had no effect in soil. Co-inoculation of field beans with R. phaseoli and P. putida R105 had no effects on plant biomass in growth pouches or in soil, but the number of nodules and the acetylene reduction activity was significantly (P<0.01) increased in the soil. In contrast, co-inoculation of beans with rhizobia and P. syringae R25 had severe, deleterious effects on seedling mergence, plant biomass, and nodulation in soil and growth pouches. Isolate R25 was responsible for the deleterious effects observed. Although plant growth-promoting rhizobacteria may interact synergistically with root-nodulating rhizobia, the PGPR selected for one crop should be assessed for potential hazardous effects on other crops before being used as inoculants.     

Estimation of number of root-nodule bacteria by a nodulation-dilution frequency method

Rhizobia spend most of their life in the soil outside the host plants. Soil conditions, therefore, may exert influence on the survival and variation of rhizobia so as to contribute to the nitrogen fixation of the legumes. The knowledge on the ecology of Rhizobium in soils may expected to give basis for the success of seed inoculation and establishment of effective strains in the field. In addition, it will render service for the study of the ecological soil microbiology, for example, ecology of pathogens of soil-borne plant diseases.