The Experts below are selected from a list of 18540 Experts worldwide ranked by ideXlab platform
Ford R Denison - One of the best experts on this subject based on the ideXlab platform.
-
failure to fix nitrogen by non reproductive symbiotic Rhizobia triggers host sanctions that reduce fitness of their reproductive clonemates
Proceedings of The Royal Society B: Biological Sciences, 2011Co-Authors: Ryoko Oono, Carolyn G Anderson, Ford R DenisonAbstract:The legume–Rhizobia symbiosis is a classical mutualism where fixed carbon and nitrogen are exchanged between the species. Nonetheless, the plant carbon that fuels nitrogen (N2) fixation could be diverted to Rhizobial reproduction by ‘cheaters’—Rhizobial strains that fix less N2 but potentially gain the benefit of fixation by other Rhizobia. Host sanctions can decrease the relative fitness of less-beneficial reproductive bacteroids and prevent cheaters from breaking down the mutualism. However, in certain legume species, only undifferentiated Rhizobia reproduce, while only terminally differentiated Rhizobial bacteroids fix nitrogen. Sanctions were, therefore, tested in two legume species that host non-reproductive bacteroids. We demonstrate that even legume species that host non-reproductive bacteroids, specifically pea and alfalfa, can severely sanction undifferentiated Rhizobia when bacteroids within the same nodule fail to fix N2. Hence, host sanctions by a diverse set of legumes play a role in maintaining N2 fixation.
-
controlling the reproductive fate of Rhizobia how universal are legume sanctions
New Phytologist, 2009Co-Authors: Ryoko Oono, Ford R Denison, Toby E KiersAbstract:When a single host plant is infected by more than one strain of Rhizobia, they face a tragedy of the commons. Although these Rhizobia benefit collectively from nitrogen fixation, which increases host-plant photosynthesis, each strain might nonetheless increase its own reproduction, relative to competing strains, by diverting resources away from nitrogen fixation. Host sanctions can limit the evolutionary success of such Rhizobial cheaters (strains that would otherwise benefit by fixing less nitrogen). Host sanctions have been shown in soybean (Glycine max) nodules, where the next generation of symbiotic Rhizobia is descended from bacteroids (the differentiated cells that can fix nitrogen). Evidence for sanctions is less clear in legume species that induce Rhizobial dimorphism inside their nodules. There, bacteroids are swollen and cannot reproduce regardless of how much nitrogen they fix, but sanctions could reduce reproduction of their undifferentiated clonemates within the same nodule. This Rhizobial dimorphism can affect Rhizobial evolution, including cheating options, in ways that may affect future generations of legumes. Both the importance of sanctions to hosts and possible physiological mechanisms for sanctions may depend on whether bacteroids are potentially reproductive.
-
legume sanctions and the evolution of symbiotic cooperation by Rhizobia
The American Naturalist, 2000Co-Authors: Ford R DenisonAbstract:abstract: The legume‐rhizobium symbiosis is an ideal model for studying the factors that limit the evolution of microbial mutualists into parasites. Legumes are unable to consistently recognize parasitic Rhizobia that, once established inside plant cells, use plant resources for their own reproduction rather than for N2 fixation. Evolution of parasitism in Rhizobia, driven partly by competition among multiple Rhizobial strains infecting the same plant, may be countered by postinfection legume sanctions. Both the biochemical options for Rhizobial cheating and the evolutionary effect of legume sanctions depend on differences in Rhizobial life history associated with nodule type. In legumes with determinate nodule growth, Rhizobia typically retain the ability to reproduce after differentiating into N2‐fixing bacteroids. Sanctions against individual bacteroids (e.g., acid hydrolases) would therefore select for cooperative Rhizobia. In nodules with indeterminate growth, bacteroids generally lose the ability to...
Wen Xin Chen - One of the best experts on this subject based on the ideXlab platform.
-
competition between Rhizobia under different environmental conditions affects the nodulation of a legume
Systematic and Applied Microbiology, 2017Co-Authors: Hui Yan, Wen Feng Chen, En Tao Wang, Qing Guo Cui, Wen Xin ChenAbstract:Mutualistic symbiosis and nitrogen fixation of legume Rhizobia play a key role in ecological environments. Although many different Rhizobial species can form nodules with a specific legume, there is often a dominant microsymbiont, which has the highest nodule occupancy rates, and they are often known as the "most favorable Rhizobia". Shifts in the most favorable Rhizobia for a legume in different geographical regions or soil types are not well understood. Therefore, in order to explore the shift model, an experiment was designed using successive inoculations of Rhizobia on one legume. The plants were grown in either sterile vermiculite or a sandy soil. Results showed that, depending on the environment, a legume could select its preferential Rhizobial partner in order to establish symbiosis. For perennial legumes, nodulation is a continuous and sequential process. In this study, when the most favorable Rhizobial strain was available to infect the plant first, it was dominant in the nodules, regardless of the existence of other Rhizobial strains in the rhizosphere. Other Rhizobial strains had an opportunity to establish symbiosis with the plant when the most favorable Rhizobial strain was not present in the rhizosphere. Nodule occupancy rates of the most favorable Rhizobial strain depended on the competitiveness of other Rhizobial strains in the rhizosphere and the environmental adaptability of the favorable Rhizobial strain (in this case, to mild vermiculite or hostile sandy soil). To produce high nodulation and efficient nitrogen fixation, the most favorable Rhizobial strain should be selected and inoculated into the rhizosphere of legume plants under optimum environmental conditions.
-
comparative genomics of Rhizobia nodulating soybean suggests extensive recruitment of lineage specific genes in adaptations
Proceedings of the National Academy of Sciences of the United States of America, 2012Co-Authors: Chang Fu Tian, Yan Ming Zhang, Yunzeng Zhang, Yuan Jie Zhou, Shuang Wang, Jun Wang, Luz B Gilbert, Wen Xin ChenAbstract:The rhizobium–legume symbiosis has been widely studied as the model of mutualistic evolution and the essential component of sustainable agriculture. Extensive genetic and recent genomic studies have led to the hypothesis that many distinct strategies, regardless of Rhizobial phylogeny, contributed to the varied rhizobium–legume symbiosis. We sequenced 26 genomes of Sinorhizobium and Bradyrhizobium nodulating soybean to test this hypothesis. The Bradyrhizobium core genome is disproportionally enriched in lipid and secondary metabolism, whereas several gene clusters known to be involved in osmoprotection and adaptation to alkaline pH are specific to the Sinorhizobium core genome. These features are consistent with biogeographic patterns of these bacteria. Surprisingly, no genes are specifically shared by these soybean microsymbionts compared with other legume microsymbionts. On the other hand, phyletic patterns of 561 known symbiosis genes of Rhizobia reflected the species phylogeny of these soybean microsymbionts and other Rhizobia. Similar analyses with 887 known functional genes or the whole pan genome of Rhizobia revealed that only the phyletic distribution of functional genes was consistent with the species tree of Rhizobia. Further evolutionary genetics revealed that recombination dominated the evolution of core genome. Taken together, our results suggested that faithfully vertical genes were rare compared with those with history of recombination including lateral gene transfer, although Rhizobial adaptations to symbiotic interactions and other environmental conditions extensively recruited lineage-specific shell genes under direct or indirect control through the speciation process.
-
diversity and biogeography of Rhizobia isolated from root nodules of glycine max grown in hebei province china
Microbial Ecology, 2011Co-Authors: En Tao Wang, Xin Hua Sui, Chang Fu Tian, Wen Feng Chen, Yan Ming Zhang, Yunzeng Zhang, Wen Xin ChenAbstract:A total of 215 Rhizobial strains were isolated and analyzed with 16S rRNA gene, 16S–23S intergenic spacer, housekeeping genes atpD, recA, and glnII, and symbiotic genes nifH and nodC to understand the genetic diversity of soybean Rhizobia in Hebei province, China. All the strains except one were symbiotic bacteria classified into nine genospecies in the genera of Bradyrhizobium and Sinorhizobium. Surveys on the distribution of these Rhizobia in different regions showed that Bradyrhizobium japonicum and Bradyrhizobium elkanii strains were found only in neutral to slightly alkaline soils whereas Bradyrhizobium yuanmingense, Bradyrhizobium liaoningense-related strains and strains of five Sinorhizobium genospecies were found in alkaline–saline soils. Correspondence and canonical correspondence analyses on the relationship of Rhizobial distribution and their soil characteristics reveal that high soil pH, electrical conductivity, and potassium content favor distribution of the B. yuanmingense and the five Sinorhizobium species but inhibit B. japonicum and B. elkanii. High contents of available phosphorus and organic matters benefit Sinorhizobium fredii and B. liaoningense-related strains and inhibit the others groups mentioned above. The symbiotic gene (nifH and nodC) lineages among B. elkanii, B. japonicum, B. yuanmingense, and Sinorhizobium spp. were observed in the strains, signifying that vertical gene transfer was the main mechanism to maintain these genes in the soybean Rhizobia. However, lateral transfer of symbiotic genes commonly in Sinorhizobium spp. and rarely in Bradyrhizobium spp. was also detected. These results showed the genetic diversity, the biogeography, and the soil determinant factors of soybean Rhizobia in Hebei province of China.
-
characterization of Rhizobia isolated from albizia spp in comparison with microsymbionts of acacia spp and leucaena leucocephala grown in china
Systematic and Applied Microbiology, 2006Co-Authors: Feng-qin Wang, Yongfa Zhang, En Tao Wang, Wen Xin ChenAbstract:This is the first systematic study of Rhizobia associated with Albizia trees. The analyses of PCR-RFLP and sequencing of 16S rRNA genes, SDS–PAGE of whole-cell proteins and clustering of phenotypic characters grouped the 31 Rhizobial strains isolated from Albizia into eight putative species within the genera Bradyrhizobium, Mesorhizobium and Rhizobium. Among these eight Rhizobial species, five were unique to Albizia and the remaining three were shared with Acacia and Leucaena, two legume trees coexisting with Albizia in China. These results indicated that Albizia species nodulate with a wide range of Rhizobial species and had preference of microsymbionts different from Acacia and Leucaena. The definition of four novel groups, Mesorhizobium sp., Rhizobium sp. I, Rhizobium sp. II and “R. giardinii”, indicates that further studies with enlarged Rhizobial population are necessary to better understand the diversity and to clarify the taxonomic relationships of Albizia-associated Rhizobia.
En Tao Wang - One of the best experts on this subject based on the ideXlab platform.
-
competition between Rhizobia under different environmental conditions affects the nodulation of a legume
Systematic and Applied Microbiology, 2017Co-Authors: Hui Yan, Wen Feng Chen, En Tao Wang, Qing Guo Cui, Wen Xin ChenAbstract:Mutualistic symbiosis and nitrogen fixation of legume Rhizobia play a key role in ecological environments. Although many different Rhizobial species can form nodules with a specific legume, there is often a dominant microsymbiont, which has the highest nodule occupancy rates, and they are often known as the "most favorable Rhizobia". Shifts in the most favorable Rhizobia for a legume in different geographical regions or soil types are not well understood. Therefore, in order to explore the shift model, an experiment was designed using successive inoculations of Rhizobia on one legume. The plants were grown in either sterile vermiculite or a sandy soil. Results showed that, depending on the environment, a legume could select its preferential Rhizobial partner in order to establish symbiosis. For perennial legumes, nodulation is a continuous and sequential process. In this study, when the most favorable Rhizobial strain was available to infect the plant first, it was dominant in the nodules, regardless of the existence of other Rhizobial strains in the rhizosphere. Other Rhizobial strains had an opportunity to establish symbiosis with the plant when the most favorable Rhizobial strain was not present in the rhizosphere. Nodule occupancy rates of the most favorable Rhizobial strain depended on the competitiveness of other Rhizobial strains in the rhizosphere and the environmental adaptability of the favorable Rhizobial strain (in this case, to mild vermiculite or hostile sandy soil). To produce high nodulation and efficient nitrogen fixation, the most favorable Rhizobial strain should be selected and inoculated into the rhizosphere of legume plants under optimum environmental conditions.
-
genetic diversity and community structure of Rhizobia nodulating sesbania cannabina in saline alkaline soils
Systematic and Applied Microbiology, 2016Co-Authors: Yajing Liu, En Tao Wang, Chenggang Ren, Wei Liu, Nan Jiang, Xiaoli Zhang, Zhi Hong XieAbstract:Sesbania cannabina is a plant that grows naturally along the seashores in Rudong County, China (RDC) and it has been introduced into the Yellow River Delta (YRD) as a pioneer plant to improve the saline–alkaline soils. In order to investigate the diversity of S. cannabina Rhizobia in these soils, a total of 198 Rhizobial isolates were characterized and phylogenetic trees were constructed based on data from multilocus sequence analysis (MLSA) of the housekeeping genes recA, atpD and glnII, as well as 16S rRNA. Symbiotic features were also studied by establishing the phylogeny of the symbiotic genes nodA and nifH, and by performing nodulation assays. The isolates had highly conserved symbiotic genes and were classified into nine genospecies belonging to the genera Ensifer, Agrobacterium, Neorhizobium and Rhizobium. A unique community structure was detected in the Rhizobia associated with S. cannabina in the saline–alkaline soils that was characterized by five novel genospecies and four defined species. In addition, Ensifer sp. I was the predominant Rhizobia in YRD, whereas Ensifer meliloti and Neorhizobium huautlense were the dominant species in RDC. Therefore, the study demonstrated for the first time that this plant strongly selected the symbiotic gene background but not the genomic background of its microsymbionts. In addition, biogeographic patterns existed in the Rhizobial populations associated with S. cannabina, which were mainly correlated with pH and salinity, as well as the mineral nutrient contents. This study provided novel information concerning the interaction between soil conditions, host plant and Rhizobia, in addition to revealing the diversity of S. cannabina Rhizobia in saline–alkaline soils.
-
abundance and diversity of soybean nodulating Rhizobia in black soil are impacted by land use and crop management
Applied and Environmental Microbiology, 2014Co-Authors: Jun Yan, En Tao Wang, Xiao Zeng Han, Zhi Hong Xie, Wen Feng ChenAbstract:To investigate the effects of land use and crop management on soybean Rhizobial communities, 280 nodule isolates were trapped from 7 fields with different land use and culture histories. Besides the known Bradyrhizobium japonicum, three novel genospecies were isolated from these fields. Grassland (GL) maintained a higher diversity of soybean bradyRhizobia than the other cultivation systems. Two genospecies (Bradyrhizobium spp. I and III) were distributed widely in all treatments, while Bradyrhizobium sp. II was found only in GL treatment. Cultivation with soybeans increased the Rhizobial abundance and diversity, except for the soybean monoculture (S-S) treatment. In monoculture systems, soybeans favored Bradyrhizobium sp. I, while maize and wheat favored Bradyrhizobium sp. III. Fertilization decreased the Rhizobial diversity indexes but did not change the species composition. The organic carbon (OC) and available phosphorus (AP) contents and pH were the main soil parameters positively correlated with the distribution of Bradyrhizobium spp. I and II and Bradyrhizobium japonicum and negatively correlated with Bradyrhizobium sp. III. These results revealed that different land uses and crop management could not only alter the diversity and abundance of soybean Rhizobia, but also change interactions between Rhizobia and legume or nonlegume plants, which offered novel information about the biogeography of Rhizobia.
-
diversity and biogeography of Rhizobia isolated from root nodules of glycine max grown in hebei province china
Microbial Ecology, 2011Co-Authors: En Tao Wang, Xin Hua Sui, Chang Fu Tian, Wen Feng Chen, Yan Ming Zhang, Yunzeng Zhang, Wen Xin ChenAbstract:A total of 215 Rhizobial strains were isolated and analyzed with 16S rRNA gene, 16S–23S intergenic spacer, housekeeping genes atpD, recA, and glnII, and symbiotic genes nifH and nodC to understand the genetic diversity of soybean Rhizobia in Hebei province, China. All the strains except one were symbiotic bacteria classified into nine genospecies in the genera of Bradyrhizobium and Sinorhizobium. Surveys on the distribution of these Rhizobia in different regions showed that Bradyrhizobium japonicum and Bradyrhizobium elkanii strains were found only in neutral to slightly alkaline soils whereas Bradyrhizobium yuanmingense, Bradyrhizobium liaoningense-related strains and strains of five Sinorhizobium genospecies were found in alkaline–saline soils. Correspondence and canonical correspondence analyses on the relationship of Rhizobial distribution and their soil characteristics reveal that high soil pH, electrical conductivity, and potassium content favor distribution of the B. yuanmingense and the five Sinorhizobium species but inhibit B. japonicum and B. elkanii. High contents of available phosphorus and organic matters benefit Sinorhizobium fredii and B. liaoningense-related strains and inhibit the others groups mentioned above. The symbiotic gene (nifH and nodC) lineages among B. elkanii, B. japonicum, B. yuanmingense, and Sinorhizobium spp. were observed in the strains, signifying that vertical gene transfer was the main mechanism to maintain these genes in the soybean Rhizobia. However, lateral transfer of symbiotic genes commonly in Sinorhizobium spp. and rarely in Bradyrhizobium spp. was also detected. These results showed the genetic diversity, the biogeography, and the soil determinant factors of soybean Rhizobia in Hebei province of China.
-
characterization of Rhizobia isolated from albizia spp in comparison with microsymbionts of acacia spp and leucaena leucocephala grown in china
Systematic and Applied Microbiology, 2006Co-Authors: Feng-qin Wang, Yongfa Zhang, En Tao Wang, Wen Xin ChenAbstract:This is the first systematic study of Rhizobia associated with Albizia trees. The analyses of PCR-RFLP and sequencing of 16S rRNA genes, SDS–PAGE of whole-cell proteins and clustering of phenotypic characters grouped the 31 Rhizobial strains isolated from Albizia into eight putative species within the genera Bradyrhizobium, Mesorhizobium and Rhizobium. Among these eight Rhizobial species, five were unique to Albizia and the remaining three were shared with Acacia and Leucaena, two legume trees coexisting with Albizia in China. These results indicated that Albizia species nodulate with a wide range of Rhizobial species and had preference of microsymbionts different from Acacia and Leucaena. The definition of four novel groups, Mesorhizobium sp., Rhizobium sp. I, Rhizobium sp. II and “R. giardinii”, indicates that further studies with enlarged Rhizobial population are necessary to better understand the diversity and to clarify the taxonomic relationships of Albizia-associated Rhizobia.
P. S. Poole - One of the best experts on this subject based on the ideXlab platform.
-
Transport and Metabolism in Legume-Rhizobia Symbioses
Annual Review of Plant Biology, 2013Co-Authors: Michael Udvardi, P. S. PooleAbstract:Symbiotic nitrogen fixation by Rhizobia in legume root nodules injects approximately 40 million tonnes of nitrogen into agricultural systems each year. In exchange for reduced nitrogen from the bacteria, the plant provides Rhizobia with reduced carbon and all the essential nutrients required for bacterial metabolism. Symbiotic nitrogen fixation requires exquisite integration of plant and bacterial metabolism. Central to this integration are transporters of both the plant and the Rhizobia, which transfer elements and compounds across various plant membranes and the two bacterial membranes. Here we review current knowledge of legume and Rhizobial transport and metabolism as they relate to symbiotic nitrogen fixation. Although all legume-Rhizobia symbioses have many metabolic features in common, there are also interesting differences between them, which show that evolution has solved metabolic problems in different ways to achieve effective symbiosis in different systems.
K E Giller - One of the best experts on this subject based on the ideXlab platform.
-
are the rates of photosynthesis stimulated by the carbon sink strength of Rhizobial and arbuscular mycorrhizal symbioses
Soil Biology & Biochemistry, 2009Co-Authors: Glaciela Kaschuk, Mariangela Hungria, Thomas W Kuyper, P A Leffelaar, K E GillerAbstract:Abstract Rhizobial and arbuscular mycorrhizal (AM) symbioses each may consume 4–16% of recently photosynthetically-fixed carbon to maintain their growth, activity and reserves. Rhizobia and AM fungi improve plant photosynthesis through N and P acquisition, but increased nutrient uptake by these symbionts does not fully explain observed increases in the rate of photosynthesis of symbiotic plants. In this paper, we test the hypothesis that carbon sink strength of Rhizobial and AM symbioses stimulates the rates of photosynthesis. Nutrient-independent effects of Rhizobial and AM symbioses result in direct compensation of C costs at the source. We calculated the response ratios of photosynthesis and nutrient mass fraction in the leaves of legumes inoculated with Rhizobial and/or AM fungi relative to non-inoculated plants in a number of published studies. On average, photosynthetic rates were significantly increased by 28 and 14% due to Rhizobial and AM symbioses, respectively, and 51% due to dual symbiosis. The leaf P mass fraction was increased significantly by 13% due to Rhizobial symbioses. Although the increases were not significant, AM symbioses increased leaf P mass fraction by 6% and dual symbioses by 41%. The leaf N mass fraction was not significantly affected by any of the Rhizobial, AM and dual symbioses. The rate of photosynthesis increased substantially more than the C costs of the Rhizobial and AM symbioses. The inoculation of legumes with Rhizobia and/or AM fungi, which resulted in sink stimulation of photosynthesis, improved the photosynthetic nutrient use efficiency and the proportion of seed yield in relation to the total plant biomass (harvest index). Sink stimulation represent an adaptation mechanism that allows legumes to take advantage of nutrient supply from their microsymbionts without compromising the total amount of photosynthates available for plant growth.
-
agricultural intensification soil biodiversity and ecosystem function in the tropics the role of nitrogen fixing bacteria
Applied Soil Ecology, 1997Co-Authors: J H P Kahindi, P L Woomer, T George, F M De Souza Moreira, N K Karanja, K E GillerAbstract:Abstract Among the nitrogen (N2)-fixing bacteria, the Rhizobia in symbiosis with legumes are generally the most important in agriculture, although Frankia, cyanobacteria and heterotrophic free-living N2 -fixers may fix significant amounts of nitrogen under specific conditions. The taxonomy of N2-fixing bacteria is undergoing substantial revisions due to the advent of molecular methods for phylogenetic analysis, and in certain cases this has proved useful in unravelling ecological relationships among confusing groups. Molecular methods are also proving useful in studies of biodiversity within populations of Rhizobial species. Rhizobia are surprisingly competent free-living bacteria, although few fix nitrogen in the free-living state, and the major factors that determine their population sizes in the absence of legume hosts are environmental stresses (such as soil acidity factors), protozoal grazing and some factors associated with agricultural intensification such as increases in salinity or heavy metal pollution of the soil. Rhizobial populations generally increase in response to the presence of the host legume. Due to the high degree of host-specificity between legume hosts and Rhizobial species, loss of a single Rhizobial species can result in loss of N2-fixation by that legume, although many legumes can be nodulated by several species of Rhizobia. However, as only a single, compatible Rhizobial genotype or strain is necessary for establishment of effective N, -fixation (i.e. the basis of the Rhizobial inoculant industry), it is questionable whether biodiversity within species is necessary to ensure function, although this may confer resilience in the face of further environmental stresses.
-
enumeration of indigenous rhizobium leguminosarum biovar trifolii in soils previously treated with metal contaminated sewage sludge
Soil Biology & Biochemistry, 1993Co-Authors: A M Chaudri, K E Giller, S P Mcgrath, Egbert Rietz, Dieter SauerbeckAbstract:Abstract The effects of heavy metals from metal-contaminated sewage sludge on the indigenous population of Rhizobium leguminosarum biovar trifolii in the soils of two well controlled field experiments at Braunschweig in northeast Germany are reported. Both experiments were in the same field, but one was on an old arable soil and the other on an ex-woodland soil. The following treatments were applied to both experiments: inorganic fertilizer at 180 kg N ha −1 yr −1 (control soils); uncontaminated sludge at 100m 3 ha −1 yr −1 or 300m 3 ha −1 yr −1 ; or metal-contaminated sludge at the same two rates. In the ex-woodland experimental soil, Rhizobial numbers decreased by an order of magnitude in plots treated with 100 m 3 yr −1 contaminated sludge compared to the control plots and plots receiving the same amount of uncontaminated sludge. The total Zn and Cd concentrations in these plots were close to the German limits, but were well below the U.K. and E.C. upper limits. Even with 300m 3 yr −1 uncontaminated sludge Rhizobial numbers decreased by several orders of magnitude compared to the control soils, in both field experiments: at both sites one of the four replicate plots had no Rhizobia, whereas the other three plots had decreased numbers. Metal concentrations in these plots were well below the U.K. and E.C. upper limits, except Zn which was close to the German limit, and ranged from (mg kg −1 ): Zn, 200–250; Cu, 46–62; Ni, 16–23 and Cd, 0.9–1.6. In plots receiving 300m 3 yr −1 contaminated sludge, in both field experiments, Rhizobial numbers were further decreased compared to plots receiving uncontaminated sludge at the same rate and were several orders of magnitude smaller than in the control plots. For example, in the old arable site, three out of four plots had no Rhizobia, with one plot containing 20 cell g −1 soil. In the ex-woodland site, two plots out of four had no Rhizobia, whereas, the other two had −1 soil. Metal concentrations in these plots were above the U.K. and E.C. upper limits for Zn, but were still below the corresponding limits for Cd. Copper concentrations in one old arable plot and three ex-woodland plots were slightly above the U.K. limits for soils of pH 5–6. The German limits for Zn, Cu, and Cd, but not Ni, were considerably exceeded in these plots. The major factor influencing Rhizobial numbers, in both field experiments, was not the soil pH, nor the organic carbon content, but rather the metal concentrations in the soil. The smaller N contents, chlorosis and stunting of clover plants grown in soils from plots containing no, or very few, Rhizobia was not due to phytotoxicity, but was shown by the addition of nitrogen fertilizer to be due to lack of N 2 -fixation. Our results indicate that although several metals were accumulated simultaneously in both field experiments there was a strong Zn effect on the numbers of Rhizobia in these soils. Significant reductions in Rhizobial numbers occurred even at metal concentrations well below the current U.K. and E.C. upper limits for all metals, but close to the much more strict German limits for Zn and Cd. These results are discussed in relation to the setting of safe limits for the long-term protection of soils.
