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1.
Interstrain competitiveness is a key factor affecting the performance of rhizobium inoculant. In the present study five native strains of Bradyrhizobium japonicum, namely SSF 4, SSF 5, SSF 6, SSF 7 and SSF 8, were assessed for their competitiveness in nodulating soybean using serological methods. The strains were inoculated individually or with the type strain USDA 110 at a 1:1 ratio. Nodule occupancy determined by immunofluorescence and dot immunoblot assay revealed that under in vitro conditions SSF 8 is more competitive than USDA 110 whereas the others were less competitive. The competitive ability of these strains was also estimated in pot culture in the field. In red soil both SSF 8 and USDA 110 were equally competitive whereas in black soil SSF 8 competed better than USDA 110 and produced more nodules. In a black soil field trial using a randomized block design, USDA 110 or SSF 8, when inoculated alone, occupied the majority of the nodules and enhanced nodule dry weight and shoot biomass. SSF 8 was more competitive when the strains were co-inoculated. Received: 1 November 1996  相似文献   

2.
 Field experiments were conducted to investigate the performance of three soybean cultivars with five foreign bradyrhizobia strains in different regions. The experiments at the two sites were designed with soybean (Glycine max L.) cultivars as the main factor and bradyrhizobia strains (USDA 136, TAL 122, USDA 6, TAL 377 and TAL 102) as the sub-factor. The experiments were arranged in randomised complete block design with four replications. Results show that nodule number, nodule dry weight and shoot dry weight, total N and seed yield were significantly increased when soybean cultivars were inoculated with foreign bradyrhizobia in two locations in the south east of Nigeria. At 63 days after planting the percentage increase in nodule number and dry weight after inoculation of soybean cultivars with bradyrhizobia strains ranged from 71 to 486% and from 0 to 200%, respectively. The percentage increase in shoot dry matter, %N and total N after bradyrhizobia inoculation ranged between 2–130%, 18–62% and 35–191%, respectively at Awka, and at the Igbariam site the percentage increase in shoot dry weight, %N and total N ranged between 3–76%, 0–43% and 19–125%, respectively. Seed yields after bradyrhizobia inoculation of soybean cultivar TGX 1485–1D at Igbariam ranged between 1.20 and 2.18 t ha–1 against the uninoculated plants, which had seed yields of 1.05 t ha–1. The poorest yield response after inoculation with bradyrhizobia strains was observed in soybean cultivar M-351, with a seed yield ranging from 0.60 to 0.98 t ha–1. The fact that foreign bradyrhizobia strains were more effective than the indigenous strains for all the parameters studied suggests that there is a need to use bradyrhizobia inoculants for increased soybean production in Nigeria. The variations in the strain performance with the different soybean cultivars at the two sites, emphasises the need for careful Bradyrhizobium spp. strain selection. The fact that inoculation response was cultivar- and site-specific suggests that strategies for improving inoculation response in soybean cultivars should also consider the soil environment where the soybean is to be produced. Received: 25 May 1999  相似文献   

3.
The competitiveness of a mesquite Rhizobium (AZ-M1) and its ability to survive in desert soils was compared to a selected commercial strain (31A5). In a greenhouse study, the native isolate out-competed strain 31A5 in nodule occupancy, when applied as a mixed inoculant to seed germinated and grown in sand culture, and irrigated with N-free nutrients. A high incidence of nodule double occupancy was found when double strain inoculants were used. The survival rate of the two strains was tested in three desert soils in a controlled laboratory study. The desert strain AZ-M1 grew and survived in all the soils for 1 month. The commercial strain 31A5, did not grow, and the population decreased in 14 days from 108 cells g?1 dry soil to below 104 cells g?1. Both strains survived to a lesser extent in a saline-sodic soil. A significant morphological change from a rod to a coccus was observed 2 days after strain 31A5 had been introduced into the desert soils.  相似文献   

4.
We examined whether strains of Bradyrhizobium japonicum selected for growth on acid media in vitro would also survive and grow better in acid soils. Four agar screening media for acid-tolerant rhizobia, which differed in the number of acid soil stresses imposed (pH, low calcium (Ca) and phosphorus (P), high aluminum (Al) and manganese (Mn)), were assessed for their effects on the survival of 14 Indonesian strains and two commercial strains of B. japonicum. Survival of B. japonicum in the agar media was compared with that in two acid soils. A repeat stab inoculation method which provided a declining range of inoculum cell number to 103 cells per stab was used to assess the daily growth of the strains on the screening media at 5 pH levels (3.8, 4.2, 4.5, 5.0, and 6.8). The growth and survival of the 16 strains were then measured at days 1, 8, 18, and 28 after inoculation in two acid soils (pH 4.24 and 4.35) sterilized using γ-irradiation at 5.0 Mrad. Selectivity of the agar media improved as more acid stress factors were incorporated in the media. Those strains of Bradyrhizobium identified as acid, Al and Mn-tolerant in acidic agar media, also had better survival in the low pH soils. There was no relationship between acid or alkali production on agar media and acid tolerance on agar or in soil. There was no apparent relationship between symbiotic performance and acid tolerance, and one acid-tolerant strain was as effective as the commercial inoculant strain CB1809. The most acid-tolerant strain was also the most ineffective.  相似文献   

5.
Summary We assessed the effectiveness of three locally made lignite, subbituminous coal and cowmanure-based cowpea Bradyrhizobium inoculants in comparison with a peat-based imported Bradyrhizobium incoculant in a two-field plot investigation. The local inoculants were prepared by incorporating three rhizobia strains (Ife CR9, Ife CR15 and Bradyrhizobium japonicum) into each of the above carrier materials and were used to inoculate three cowpea seed varieties: TVU 1190, IT 82E-60 and Ife brown. With lignite-based He CR9 inoculated into TVU 1190 seeds, total N content of the plants was 178.6 mg/plant compared with only 64.3 mg/plant for the uninoculated nitrate-free control plants. With Nigerian lignite, sub-bituminous coal and cow manure as carriers for cowpea rhizobia, the cowpea yield of the inoculated plants increased by 72%, 54% and 10%, respectively, compared with uninoculated plants, while the peat-based inoculant gave a 25% increase in cowpea yield. With lignite-based Ife CR9 inoculated into Ife brown seeds, total N content of the plants was 149.1 mg/plant, but with inoculation by lignite-based B. japonicum, total N content of the treated Ife brown plant was 132.4 mg/plant. Thus, the native Ife CR9 strain seems to be slightly better adapted to tropical conditions than the imported B. japonicum.  相似文献   

6.
Volcanic ash soil, which is widely distributed in Japan, contains a large amount of well-structured soil aggregates. By using these aggregates as carrier materials, we prepared (brady)rhizobial inoculants for red kidney bean (Phaseolus vulgaris) and soybean (Glycine max). Autoclaved soil aggregates were inoculated with Rhizobium tropici CIATS99R or Bradyrhizobium japonicum USDA110R, incubated for 15 or 21 d at 30°C, slowly air-dried at 20°C to prepare the aggregate-based inoculants, and stored at various temperatures. The populations of CIATS99R and USDA110R in the aggregate-based inoculants were maintained during several months of storage at 20°C. When the aggregate-based inoculants were mixed with soil, CIATS99R and USDA110R cells showed a remarkably improved survival in soils compared with those mixed with soil without carrier material. The effect of the aggregate-based inoculants on the growth of red kidney bean and soybean was examined in pot experiments. By placing a small amount of the inoculant just beneath the seeds at the time of sowing, plant growth was significantly enhanced compared with the use of traditional peat-based inoculant. In addition, nodule formation on the upper part of soybean roots and nodule occupancy by the inoculated strain were remarkably enhanced by the aggregate-based inoculant. It is suggested that soil aggregates might be suitable carrier materials for preparing cheap and effective (brady)rhizobial inoculants.  相似文献   

7.
 Most soils sown with field beans (Phaseolus vulgaris L.) contain indigenous rhizobia which might interfere with the establishment of inoculated strains. As a consequence, the benefits of bean inoculation are usually questioned, and the use of N fertilizer is gradually becoming a common practice. The present study had the objective of evaluating the effectiveness of inoculation and N fertilization in field soil with (site 1) and without (site 2) a previous bean-cropping history. At site 1, which had a rhizobial population of 7×102 cells g–1 soil, inoculation had no effect on nodulation or yield, whereas at site 2 (<10 cells g–1 soil) inoculation increased nodulation, nodule occupancy by the inoculated strain and grain yield. N fertilizer decreased nodulation at both sites, but increased grain yield at site 1 but not at site 2, indicating that the response to inoculation and N fertilization depends on the cropping history. When bean was cultivated for the first time, indigenous populations of rhizobia were low and high yields were accomplished solely with seed inoculation, with no further response to N fertilizer. In contrast, previous cultivation of bean increases soil rhizobia, preventing nodule formation by inoculated strains, and N fertilizer may be necessary for maximum yields. A significant interaction effect between N fertilizer and inoculation was detected for serogroup distribution only at site 2, with N fertilizer decreasing nodule occupancy by the inoculated strain and increasing the occurrence of indigenous strains. Consequently, although no benefits were obtained by the combination of inoculation and N fertilizer, this practice may be feasible with the selection of appropriate N-tolerant strains from the indigenous rhizobial population. Received: 26 May 1999  相似文献   

8.
High yield culture medium is fundamental for production of inoculants for plant growth-promoting bacteria. Based on substitution of glucose in tryptone–yeast extract–glucose medium by Na-gluconate or glycerol, two new culture media were developed for mass cultivation of the commonly used plant growth-promoting bacterium Azospirillum sp. After 18 h of incubation, these modifications increased populations of different strains of Azospirillum (to ∼1011 cells ml−1 [single cell count] and ∼5 × 109 CFU ml−1 [plate count method]), significantly reduced generation time, and were also suitable for production of common synthetic inoculants.  相似文献   

9.
In acid soil, low pH, reduced availability of nutrients, and toxicity of Al and Mn limit plant growth and the survival and effectiveness of rhizobia. The symbiosis between legumes and rhizobia is particularly sensitive to acid soil stress. A pot experiment evaluated whether Bradyrhizobium japonicum strain growth on acidic agar media would predict ability to colonize the rhizosphere and form effective nodules in acidic soils. Three Indonesian strains of B. japonicum with similar effectiveness at neutral pH in sand culture but with different tolerance of acid soil stress factors in agar media, and an acid-tolerant commercial strain (CB1809) of comparable effectiveness, were tested in three acid soils using the Al tolerant soybean (Glycine max cv PI 416937). At 7 days after inoculation all strains had achieved large rhizosphere populations, but by day 14 the rhizosphere population of the acid-sensitive strain had decreased, while the more acid-tolerant strains increased. The acid-tolerant strains had significantly greater nodulation and symbiotic effectiveness than plants inoculated with the acid-sensitive strain. Laboratory prescreening of B. japonicum for acid, Al and Mn tolerance in acid media successfully identified strains which were symbiotically competent in low pH soils.  相似文献   

10.
Bradyrhizobium strains were isolated from nodules obtained from field-grown soybean plants sampled in 12 soybean production locations in Argentina. These fields had been annually cropped with soybean and did not show decreases in yields even though they had been neither N-fertilized nor inoculated for at least the last 5 years. We hypothesized that the isolated strains maintained high competitiveness and efficiency in fixing adequate N2 levels. A set of strains that showed the highest nodular occupancy in each sampling location were assayed for symbiotic performance under greenhouse and field conditions and comparatively evaluated with Bradyrhizobium japonicum E109, the strain officially recommended for inoculant formulation in Argentina. An inoculant pool, formed by four strains obtained from nodules collected from Cañada Rica, developed higher nodular biomass than B. japonicum E 109 in assays carried out in greenhouses under well irrigated conditions. Additionally, neither nodule production nor specific nitrogenase activity decreased with respect to B. japonicum E 109 when plants were drought stressed during 7 days from sowing. The mean yields obtained under field conditions and plotted against the principal component one (CP1) obtained with an additive main effect and multiplicative interaction (AMMI) model showed that the inoculant pool from Cañada Rica had higher contribution to yield than strain E 109, although with lower environmental stability. The inoculant pool from Cañada Rica could be considered an improved inoculant and be used for preliminary assays, to formulate inoculants in Argentina.  相似文献   

11.
The impact of plant growth-promoting rhizobacteria (PGPR) inoculants on the growth, yield and interactions of spring wheat with arbuscular mycorrhizal fungi (AMF) was assessed in field studies. The pseudomonad inoculants P. cepacia R55, R85, P. aeruginosa R80, P. fluorescens R92 and P. putida R104, which enhance growth and yield of winter wheat, were applied at a rate of ca. 107–108 cfu seed-1 and plots established on pea stubble or summer fallow at two different sites in Saskatchewan. Plant shoot and root biomass, yield and AMF colonization were determined at four intervals. Plant growth responses were variable and dependent on the inoculant strain, harvest date and growth parameter evaluated. Significant increases or decreases were measured at different intervals but these were usually transient and final seed yield was not significantly affected. Harvest index was consistently increased by all pseudomonad inoculants; responses to strain R55 and R104 were significant. Root biomass to 60 cm depth was not significantly affected by inoculants except strain R104, which significantly reduced root dry weight. However, root distribution, root length and AMF colonization of roots within the soil profile to 60 cm were significantly altered by inoculants. Most of these responses were reductions in the assessed parameter and occurred at depths below 15 cm; however, strains R85 and R92 significantly increased root dry weight in the 0- to 15-cm zone. These results indicate that some PGPR inoculants may adversely affect mutualistic associations between plants and indigenous soil microorganisms, and suggest a possible reason as to why spring wheat growth was not consistently enhanced by these pseudomonad PGPR.  相似文献   

12.
Abstract

Cobb and Coker 488, late‐season (maturity group VIII) cultivars of soybean [Glycine max(L.) Merr], were grovn under irrigated and non‐irrigated conditions on a Norfolk loamy sand in a two‐year field experiment. Each cultivar was inoculated withBradyrhizobium japonicumstrains [USDA 3I1b110; Brazil 587; NifTAL 184 and 102 (NifTAL cultures of Brazil 587 and USDA 110, respectively); and North Carolina 1001, 1004, 1005, 1010, and 1029). Drought conditions were present both years, and irrigation significantly increased the overall yield (2.49 vs 1.92 Mg ha‐1). Coker 488 was significantly higher in seed yield than Cobb (2.55 vs 2.02 Mg ha‐1). Strain ofB.japonicumalso affected seed yields. NC1010‐inoculated soybean was significantly higher in seed yield rank than all other soybean at the P<0.01 level, when compared by single degree of freedom contrast (sdfc). The yield ranking of soybean inoculated with NC1001 was significantly lower than soybean inoculated with all other strains, when compared by sdfc (P<0.10). Other strains differed in responses which ranged from good to poor inoculants under specific water management conditions. For instance, under nonirrigated conditions, soybean inoculated with strains ofB.japonicumfrom North Carolina was significantly higher in seed yield than those inoculated with the cultures of USDA 110, B587, or the control, when compared by sdfc (P>0.03, 0.05, 0.06, respectively). Since soybean inoculated with either strain of USDA 110 was generally high in yield rank under irrigated conditions, their response to irrigation was large relative to soybean inoculated with the NC strain (P<0.04). Neither seed nitrogen nor xylem water potential was highly correlated to seed yield. Since seed yield and N content were not highly correlated, the amount of N accumulated in soybean dry mass and that removed in seed were not highly correlated. Thus, the amount of N returned to the soil would be affected by management combinations of late‐season determinate soybean cultivar,B.japonicumstrain, and irrigation  相似文献   

13.
 Thirty-five Azospirillum strains (13 strains from plant roots and 22 strains from soils) were isolated from Ishigaki island, Japan, which has a subtropical climate. These strains were different from each other according to polymerase-chain-reaction band patterns obtained by using a random primer (OPT-08). Two Azospirillum strains (AZ43 and AZ92-2) were also examined for use in further experiments. Inoculation of lowland rice with these strains enhanced early growth of rice to various degrees. Inoculation of strains VIII.P1-2, AZ92-2, V.S2-2, and V.P5 in sterilized soil yielded higher shoot dry weights than the application of 90 μg N g–1 soil without inoculation. Only inoculation with strains AZ92-2 and VIII.P1-2 caused higher N uptake than the application of 90 μg N g–1 soil. Three strains were selected for the next experiment based on the results of their effect on the early growth of rice. An investigation was conducted to determine the ability of two indigenous Azospirillum strains (V.S2-2 and VIII.P1-2) and one stock strain (AZ92-2) to promote growth and nutrient-uptake of lowland rice in unsterilized soil under several levels of N application (0, 80, 160, and 240 mg N pot–1). Inoculation with these strains without N application increased shoot dry weight by 12–15% compared to the uninoculated treatment. Inoculation with Azospirillum V.S2-2 together with the application of 160 mg N pot–1 resulted in a shoot dry weight as high as that obtained in the treatment with 240 mg N pot–1 without inoculation. Thus, in this former case, the amount of N applied could be reduced by 80 mg pot–1 due to the effect of the microbial inoculum without a significant change in the high, targeted, yield.  相似文献   

14.
Due to their ecologic and economic importance, bradyrhizobia have been extensively studied in recent years. Since 1992, Bradyrhizobium elkanii SEMIA 587 and SEMIA 5019 and Bradyrhizobium japonicum SEMIA 5079 and SEMIA 5080 have been widely used in most Brazilian soybean fields. The objective of this work was to estimate the genetic variability of bradyrhizobial isolates recovered from soils under rhizobial inoculation and different soil managements. Only 25% of the isolates demonstrated high similarities to the original strains, and a strong correlation was obtained between the bradyrhizobial genetic variability and soil management. A high level of genetic diversity was observed both within isolates (H = 5.46) as well as among the different soil practices. Soil under no-tillage presented a higher bradyrhizobia diversity compared with bradyrhizobia isolated from soil under conventional tillage. Serological characterization also indicated that B. elkanii strains SEMIA 587 and SEMIA 5019 were more competitive and presented a higher nodular occupancy capacity than strains belonging to B. japonicum species in Southern Brazilian soils.  相似文献   

15.
The survival of an antibiotic-resistant mutant of a commercial inoculant Bradyrhizobium japonicum strain, A1017ks, was studied in a volcanic ash soil (Andosol) in comparison with a non-volcanic ash soil (Fluvisol) over a period of 84 days. In a non-sterile soil system, the population decline in the Andosol (15% or 1.2 log units) was larger than in the Fluvisol (6% or 0.54 log units). In both soils, however, the inoculant bradyrhizobium survived at fairly high population levels after the period of incubation [106 and 107 colony-forming units (CFU) g-1 dry soil in the Andosol and Fluvisol, respectively]. In sterile control soil, viable bradyrhizobium cells could not be detected after 1 week of incubation in the Andosol, whereas in the Fluvisol population of introduced bradyrhizobium was maintained throughout the period of incubation. Overall changes in the population of indigenous bacteria and fungi were also monitored. However, no clear pattern of interaction between the inoculant Bradyrhizobium japonicum and the indigenous microbes could be identified. The antibiotic-resistant mutant maintained its resistance in the Fluvisol throughout the 3-month period of incubation, making it a useful model for conducting ecological studies in the soil.  相似文献   

16.
ABSTRACT

Common bean (Phaseolus vulgaris L.) is relatively poor in dinitrogen (N2) fixation, so selecting compatible host cultivar and Rhizobium strain combinations may offer an improvement. The effectiveness of six rhizobial strains was evaluated using five bean cultivars of bean (three pinto and two black bean) in a growth-room experiment. We subsequently selected the three best strains to assess whether multi-strain inoculation had advantages over single-strain inoculation in growth-room and field experiments. In the first-growth-room experiment, Rhizobium strains UMR 1899, RCR 3618, and USDA 2676 were selected for high nodulation, plant dry weight, shoot nitrogen (N), and N2 fixation. In a second growth-room experiment, the individual strains and a mixture of the three strains generally did not differ in the parameters evaluated. Total shoot N accumulated ranged from 172.9 to 162.8 mg plant?1, of which 32.1% to 33.6% (equivalent to 54.0 to 59.2 mg plant? 1) was fixed. In field experiments, plant biomass and seed N2 fixed did not differ among the inoculants at any site. These results suggest that the three strains were equally effective and that the multi-strain inoculant offered no consistent advantage over the single-strain inoculants.  相似文献   

17.
 The effect of cucumber roots on survival patterns of the biocontrol soil inoculant Pseudomonas fluorescens CHA0-Rif was assessed for 22 days in two non-sterile soils, using a combination of total immunofluorescence cell counts, Kogure's direct viable counts and colony counts on plates containing rifampicin. In Eschikon soil (high fertility status for cucumber), CHA0-Rif persisted as culturable cells in bulk soil and in the rhizosphere, but colony counts were lower than viable counts and total cell counts inside root tissues. The occurrence of viable but non-culturable (VBNC) cells inside root tissues (5 log cells g–1 root) was unlikely to have resulted from the hydrogen peroxide treatment used to disinfect the root surface, as hydrogen peroxide caused the death of CHA0-Rif cells in vitro. In Siglistorf soil (low fertility status for cucumber), the inoculant was found mostly as non-culturable cells. Colony counts and viable counts of CHA0-Rif were similar, both in bulk soil and inside root tissues, whereas in the rhizosphere viable counts exceeded colony counts at the last two samplings (giving about 7 log VBNC cells g–1). In conclusion, soil type had a significant influence on the occurrence of VBNC cells of CHA0-Rif, although these cells were found in root-associated habitats (i.e. rhizosphere and root tissues) and not in bulk soil. Received: 12 November 1999  相似文献   

18.
Low effectiveness of native strains remains a limitation to soybean productivity in sub-Saharan Africa; while in other countries commercial inoculants are produced that provide effective strains that stimulate N fixation and growth. An experiment was set up to evaluate the response of a dual purpose promiscuous soybean variety (TGx1740-2F) and a non-promiscuous variety (Nyala) to commercial rhizobium inoculants in soils from central and coastal Kenya. Highest nodulation was observed in some of the treatments with commercial inoculants applied with nodule weights of 4.5 and 1.0 g plant−1 for TGx1740-2F and Nyala, respectively. Average biomass yields of TGx1740-2F (16 g plant−1) were twice as large as of Nyala (7.5 g plant−1) at the podding stage. Nitrogen fixation was higher in TGx1740-2F than in Nyala, and positively affected by a number of commercial inoculants with more than 50% N derived from the atmosphere. Nodule occupancy was 100% on both soybean varieties, indicating that the commercial strains were extremely infective in both of the tested soils. These results showed that commercial strains can be used to inoculate promiscuous soybean and enhance N fixation and yield.  相似文献   

19.
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.  相似文献   

20.
We investigated Cd, Zn, and Cd + Zn toxicity to soil microbial biomass and activity, and indigenous Rhizobium leguminosarum biovar trifolii, in two near neutral pH clay loam soils, under long-term arable and grassland management, in a 6-month laboratory incubation, with a view to determining the causative metal. Both soils were amended with Cd- or Zn-enriched sewage sludge, to produce soils with total Cd concentrations at four times (12 mg Cd g−1 soil), and total Zn concentrations (300 mg Zn kg−1 soil) at the EU upper permitted limit. The additive effects of Cd plus Zn at these soil concentrations were also investigated. There were no significant differences in microbial biomass C (B C), biomass ninhydrin N (B N), ATP, or microbial respiration between the different treatments. Microbial metabolic quotient (defined as qCO2 = units of CO2–C evolved unit−1 biomass C unit−1 time) also did not differ significantly between treatments. However, the microbial maintenance energy (in this study defined as qCO2-to-μ ratio value, where μ is the growth rate) indicated that more energy was required for microbial synthesis in metal-rich sludge-treated soils (especially Zn) than in control sludge-treated soils. Indigenous R. leguminosarum bv. trifolii numbers were not significantly different between untreated and sludge-treated grassland soils after 24 weeks regardless of metal or metal concentrations. However, rhizobial numbers in the arable soils treated with metal-contaminated sludges decreased significantly (P < 0.05) compared to the untreated control and uncontaminated sludge-treated soils after 24 weeks. The order of decreasing toxicity to rhizobia in the arable soils was Zn > Cd > Cd + Zn.  相似文献   

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