Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

Preface. I: Introduction. 1. The History of Agroforestry. 2. Definition and Concepts of Agroforestry. II: Agroforestry Systems and Practices. 3. Classification of Agroforestry Systems. 4. Distribution of Agroforestry Systems in the Tropics. 5. Shifting Cultivation and Improved Fallows. 6. Taungya. 7. Homegardens. 8. Plantation Crop Combinations. 9. Alley Cropping. 10. Other Agroforestry Systems and Practices. III: Agroforestry Species. 11. General Principles of Plant Productivity. 12. Agroforestry Species: the Multipurpose Trees. 13. Component Interactions. IV: Soil Productivity and Protection. 14. Tropical Soils. 15. Effects of Trees on Soils. 16. Nutrient Cycling and Soil Organic Matter. 17. Nitrogen Fixation. 18. Soil Conservation. V: Design and Evaluation of Agroforestry Systems. 19. The Diagnosis and Design (D&D) Methodology. 20. Field Experiments in Agroforestry. 21. On-Farm Research. 22. Economic Considerations. 23. Sociocultural Considerations. 24. Evaluation of Agroforestry Systems. 25. Agroforestry in the Temperate Zone. Glossary. SI Units and Conversion Factors. List of Acronyms and Abbreviations. Subject Index.

An introduction to agroforestry

A comprehensive report of the acetylene reduction assay for measurement of N2 fixation is presented. The objective is to facilitate the effective use and identify some potential limitations of the method. The report is based on more than 200 accounts of the use of this technique in 15 countries during the last 5 years. Methods of measurement of N2 fixation are compared. Nomenclature, e.g., N2[C2H2] fixed, is introduced to identify values of N2 fixation determined by C2H2-C2H2 assay. The biochemical basis of the assay is described along with relevant characteristics including Km, C2H2/N2 conversion factor, and specific N2[C2H2]-fixing activities obtained with various systems. Effects of combined nitrogen, temperature, light, pO2, N2, pC2H2 and water on activity are summarized. Available methods for sample preparation, assay chamber, gas phase, assay condition, termination of reaction, C2H4 analysis and expression of results are compared. The many uses of the C2H2-C2H4 assay for investigations of the biochemistry of nitrogenase and physiology of N2-fixing organisms, definition of N2-fixing organisms and measurement of field N2 fixation by legume, non-legume, soil, marine, rhizosphere, phylloplane and mammalian samples are tabulated.

Applications of the acetylene-ethylene assay for measurement of nitrogen fixation

The principal objections to the proposition that organic agriculture can contribute significantly to the global food supply are low yields and insufficient quantities of organically acceptable fertilizers. We evaluated the universality of both claims. For the first claim, we compared yields of organic versus conventional or low-intensive food production for a global dataset of 293 examples and estimated the average yield ratio (organic:non-organic) of different food categories for the developed and the developing world. For most food categories, the average yield ratio was slightly 1.0 for studies in the developing world. With the average yield ratios, we modeled the global food supply that could be grown organically on the current agricultural land base. Model estimates indicate that organic methods could produce enough food on a global per capita basis to sustain the current human population, and potentially an even larger population, without increasing the agricultural land base. We also evaluated the amount of nitrogen potentially available from fixation by leguminous cover crops used as fertilizer. Data from temperate and tropical agroecosystems suggest that leguminous cover crops could fix enough nitrogen to replace the amount of synthetic fertilizer currently in use. These results indicate that organic agriculture has the potential to contribute quite substantially to the global food supply, while reducing the detrimental environmental impacts of conventional agriculture. Evaluation and review of this paper have raised important issues about crop rotations under organic versus conventional agriculture and the reliability of grey-literature sources. An ongoing dialogue on these subjects can be found in the Forum editorial of this issue.

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Organic agriculture and the global food supply

Plant growth promoting bacteria (PGPR) associations range in degree of bacterial proximity to the root and intimacy of association. In general, these can be separated into extracellular PGPR (ePGPR), existing in the rhizosphere, on the rhizoplane or in the spaces between cells of the root cortex, and intracellular PGPR (iPGPR), which exist inside root cells, generally in specialized nodular structures. The latter includes rhizobia and Frankia species, both of which fix nitrogen in symbiosis with higher plants. There has been considerable development in understanding signaling mechanisms of rhizobia (iPGPR) during the establishment of the rhizobia–legume symbiosis, and this may serve as a model of knowledge regarding cross-talk and plant growth promoting mechanisms. We provide a detailed review of this process, including plant-to-bacteria signal molecules, followed by bacterial perception and consequent production of bacteria-to-plant signals. A history of PGPR discovery is also provided, indicating progress in understanding each of the PGPR groups. Recent advances in understanding plant growth responses to microbial signals are reviewed, along with the research areas that require attention. Based on new understandings of signaling mechanisms in the iPGPR (rhizobia) and recent findings with ePGPR we are able to speculate regarding general patterns of signaling in the ePGPR.

Intracellular and extracellular PGPR: commonalities and distinctions in the plant–bacterium signaling processes

The only recorded instances of legume, species with aerial nodules are for Neptunia oleracea [ 11 and Aeschynomene indica [2,3 J . This latter, which grows in flooded soils, has two types of nodules: root nodules like other legumes, and stem nodules. Stem , nodules of A. indica usually are distributed sparsely along the lower stem and look more like small swellings than conventional Rhizobium nodules. We report here on a tropical legume, Sesbania 2.1. Techniques for structural studies

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_5/b_fdi_02-03/00864.pdf

Nitrogen-fixing nodules induced by Rhizobium on the stem of the tropical legume Sesbania rostrata

Thirteen Acacia species were classified into three groups according to effective nodulation response patterns with fast- and slow-growing tropical strains of Rhizobium. The first group nodulated effectively with slow-growing, cowpea-type Rhizobium strains; the second, with fast-growing Rhizobium strains; and the third, with both fast- and slow-growing Rhizobium strains. The Rhizobium requirements of the Acacia species of the second group were similar to those of Leucaena leucocephala.

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Nodulation of Acacia Species by Fast- and Slow-Growing Tropical Strains of Rhizobium

Field and cylinder experiments conducted in France and in Senegal showed that polyacrylamide, previously proposed as an entrapping gel for preparing Rhizobium inoculants, could be replaced by alginate (AER inoculant) or a mixture of xanthan and carob gum (XER inoculant). Semi-dried or dried AER and XER were used successfully provided that their storage time was less than 90 days. In soil inoculation trails, no marked differences were observed among semi-dried XER, dried AER, and dried XER. A number of seed inoculation experiments indicated that dried XER significantly outranked AER. Seeds preinoculated by up to 48 days with XER yielded plants which were comparable in nodulation and growth parameters to those derived from plant receiving peat inoculation at the time of planting.

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Polymer-entrapped rhizobium as an inoculant for legumes

Acetylene reduction activity was demonstrated in pure cultures of two actinomycete strains isolated from nodules of Casuarina equisetifolia. This activity was comparable to that of free-living Rhizobium strains, but appeared to be less sensitive to pO2 and more sensitive to the presence of combined nitrogen. Images

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In vitro nitrogen fixation by two actinomycete strains isolated from casuarina nodules.

Nitrogenase activity estimated in the rhizospheres of rice, maize and different tropical grasses grown under controlled laboratory conditions was shown to depend upon plant species. High nitrogenase activity (2000–6000 nmoles C 2 H 4 h −1 g −1 dry root) occurred in rice rhizosphere, this activity being only 10 times lower than that of symbiotic systems; in the rhizosphere of many other grasses grown in a similar way nitrogenase activity was as low as 10 nmoles C 2 H 4 h −1 g −1 dry root. The influence of soil type on nitrogenase activity was impressive; but the exact nature of the factors implicated could not be established. A rather weak flush of nitrogenase activity in the rhizosphere occurred in the early stage of the plant growth; it was probably due to the exudation of compounds from the seed and lasted 2 or 4 days according to the size of the seed. When the plant entered into its intense photosynthetic phase, the nitrogenase activity gradually increased. When the shoots were severed, nitrogenase activity in the rhizosphere ceased. Nitrogenase activity in the rhizosphere responded greatly to light intensity. Extrapolation of these laboratory findings to the field is discussed.

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Yvon Dommergues

Papers

Nitrogen-fixing nodules induced by Rhizobium on the stem of the tropical legume Sesbania rostrata

Nodulation of Acacia Species by Fast- and Slow-Growing Tropical Strains of Rhizobium

Polymer-entrapped rhizobium as an inoculant for legumes

In vitro nitrogen fixation by two actinomycete strains isolated from casuarina nodules.

Non-symbiotic nitrogen fixation in the rhizospheres of rice, maize and different tropical grasses