Changes in the physiological and agricultural characteristics of peat-based Bradyrhizobium japonicum inoculants after long-term storage

Commercial soybean inoculants processed with sterilised peat and stored at 20 °C for 1–8 years were used as experimental materials to assess the changes in the physiological activity of Bradyrhizobium japonicum after storage. Viable counts decreased and physiological characteristics of the bacterium changed during storage, with an increase in the time taken for colony appearance on a medium without yeast extract, an increase in the lag time for nodule appearance on soybean grown in glass tubes and a decrease in survival on seeds. All the inoculants produced a significant increase in grain yield in a field experiment. The percentage of efficient cells in the field (relative to the plate counts) decreased as the length of storage increased. These results suggest that the physiological activity of B. japonicum cells changes after storage. Practical implications for inoculant quality control are discussed. Received: 20 September 1999 / Received revision: 3 March 2000 / Accepted: 6 March 2000     

Increase of Bradyrhizobium japonicum numbers in soils and enhanced nodulation of soybean (Glycine max (L) merr.) using granular inoculants amended with nutrients

Abstract: Mineral microgranules, amended with nutrients and inoculated with either peat or liquid Bradyrhizobium japonicum inoculants, increased the growth and recovery of the bacterium during laboratory incubation in unsterilized soil. Increases in the range of 1 log unit per g or ml inoculant used were observed in different soil types. B. japonicum showed better survival with nutrient-amended granules than in unamended ones, in soil undergoing desiccation. In a growth chamber experiment, the number of nodules per plant were significantly increased by nutrient-amendment of the granules, but only under suboptimal conditions for nodulation. Nutrient-amended granules significantly enhanced early nodulation of soybean and increased N content of the grain at harvest in four field trials. All these effects were obtained using an average of 10 kg granules amended with 1.14 kg glycerol and 0.16 kg sodium glutamate per hectare. The possible use of nutrient-amended granules to improve efficacy and reliability of microbial inoculation is discussed.     

Extension of Murray's law using a non-Newtonian model of blood flow

Background  

So far, none of the existing methods on Murray's law deal with the non-Newtonian behavior of blood flow although the non-Newtonian approach for blood flow modelling looks more accurate.     

Response of Rhizobial Populations to Moderate Copper Stress Applied to an Agricultural Soil

The use of pesticides in agricultural soils may affect the soil microbiota. The effect of repeated application of copper sulfate in soil on indigenous populations of rhizobia was assessed in a medium-term field experiment. Copper sulfate was applied over 8 years at two different rates, 12.5 and 50 kg of CuSO₄ ha^–1 year^–1, in the field. The concentrations of total copper in soil varied between 14.0 (control plots that did not receive copper sulfate) and 91.0 mg kg^–1 (the most contaminated plots) at the time of sampling, 3 years after the end of the copper treatments. All the other physicochemical parameters were similar among the plots that also shared the same cropping history. The target rhizobia were monospecific populations of Rhizobium leguminosarum bv. viciae nodulating Vicia sativa and communities of rhizobial species nodulating Phaseolus vulgaris. The size of the vetch rhizobial populations was significantly reduced in the soils with the higher Cu content, whereas the size of the Phaseolus rhizobial populations was not significantly affected. However, the number of nodules formed on both vetches and common beans were reduced for the plants grown in the most contaminated soils, suggesting an additional toxic effect of copper on plant physiology. The diversity (Simpson's indices) of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism of 16S–23S rDNA intergenic spacer (IGS), was not influenced by copper application. Also, the genetic structure of the R. leguminosarum bv. viciae populations was not modified by copper treatments. By contrast, a shift was observed in the composition of the Phaseolus-nodulating communities in relation to soil copper content. The communities were composed of three 16S rDNA haplotypes: one corresponding to the R. leguminosarum (biovar phaseoli) species, the two others forming a new lineage of Phaseolus rhizobia based on 16S rDNA sequence analysis. The reduced frequency of the R. leguminosarum species in the Phaseolus-nodulating communities from the copper-treated soils was linked to its higher sensitivity to copper as compared to the higher tolerance of isolates belonging to the other rhizobial lineage. The new lineage was functionally efficient for symbiotic nitrogen fixation with P. vulgaris. Our results suggest that functional redundancy among species exhibiting variability for copper tolerance preserved the size of Phaseolus-nodulating communities. In contrast, the abundance of the vetch-nodulating rhizobia, which was a monospecific functional group mainly constituted by copper-sensitive genotypes, was adversely affected by repeated application of copper sulfate.     

Trends in rhizobial inoculant production and use

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

Improved enumeration of Bradyrhizobium japonicum in commercial soybean inoculants using selective media

Two selective media proposed for the enumeration of Bradyrhizobium japonicum were tested using six strains of different origin and eight different commercial soybean inoculants. These media contained tetracycline, rifampicin and chloramphenicol to control bacterial contaminants, and cycloheximide and pimafucin to control fungal contaminants. They were compared with previously described selective media and plant infection technique counts. The proposed media provided better control of contaminants than previously described media, gave counts of B. japonicum similar to those obtained by the plant infection technique, and so may be used for quality control of commercial inoculants.     

Comparative effect of inorganic N on plant growth and N<Subscript>2</Subscript> fixation of ten legume crops: towards a better understanding of the differential response among species

Aims

A better understanding of how plant growth, N nutrition and symbiotic nitrogen fixation (SNF) are influenced by soil inorganic N availability, for a wide range of legume species, is crucial to optimise legume productivity, N₂ fixation, while limiting environmental risks such as N leaching.

Methods

A comparative analysis was performed for ten legume crops, grown in a field experiment and supplied with four N fertiliser rates. Dry matter, N concentration and SNF were measured. In parallel, root elongation rates were studied in a greenhouse experiment.

Results

For most species, N fertilisation had little effect on plant growth and N accumulation. SNF was reduced by soil inorganic N available at sowing but with large differences in the magnitude of the response among species. The response varied according to plant N requirements for growth and plant ability to retrieve inorganic N. Accordingly, root lateral expansion rate measured in RhizoTubes was highly correlated with plant ability to retrieve inorganic N measured in the field experiment.

Conclusion

Combining SNF response to soil inorganic N, shoot N and plant ability to retrieve inorganic N, allowed a robust evaluation of differential response to soil inorganic N among a wide range of legume species.