Inputs of biologically fixed nitrogen derived from the symbiotic relationship between legumes and their root-nodule bacteria into terrestrial ecosystems amount to at least 70 million metric tons per year. It is obvious that this enormous quantity will need to be augmented as the world's population increases and as the natural resources that supply fertilizer nitrogen diminish. This objective will be achieved through the development of superior legume varieties, improvement in agronomic practice, and increased efficiency of the nitrogen fixation process itself by better management of the symbiotic relationship between plant and bacteria. This paper considers ways and means by which populations of root-nodule bacteria, established and introduced, can be manipulated ecologically, agronomically, edaphically and genetically to improve legume productivity and, as a consequence, soil fertility.

Manipulation of rhizobia microflora for improving legume productivity and soil fertility: a critical assessment

This paper 1) reviews improvements and new approaches in methodologies for estimating biological N2 fixation (BNF) in wetland soils, 2) summarizes earlier quantitative estimates and recent data, and 3) discusses the contribution of BNF to N balance in wetland-rice culture.

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Biological N2 Fixation in wetland rice fields: Estimation and contribution to nitrogen balance

The positive role of green manuring in crop production has been known since ancient time. Importance of this soil ameliorating practice is increasing in recent years because of high cost of chemical fertilizers, increased risk of environmental pollution, and need of sustainable cropping systems. Green manuring can improve soil physical, chemical, and biological properties and consequently crop yields. Furthermore, potential benefits of green manuring are reduced nitrate (NO3 −) leaching risk and lower fertilizer N requirements for succeeding crops. However, its influence may vary from soil to soil, crop to crop, environmental variables, type of green manure crop used, and its management. Beneficial effects of green manuring in crop production should not be evaluated in isolation; however, in integration with chemical fertilizers. The objective of this article is to review recent advances in green manuring practice, in the context of potential benefits and drawbacks in use of this practice for ann...

Green Manuring in Crop Production

In recent years there has been a great surge of interest and activity in the field of biological nitrogen fixation (BNF). The legume-Rhizobium symbiosis is estimated to account for 40% of the world’s fixed nitrogen. Because the ultimate goal of BNF research is to increase crop yields on farmers’ fields, researchers’ attention must focus mainly on the understanding, improvement, and agronomic use of legume-Rhizobium symbiosis. Rightly so, since the increased cost of energy and the growing demand for N fertilizers are expected to cause dramatic increases in the price of this commodity in the years ahead. Rice (Oryza sativa L.) is the world’s most important food crop as more than 40% of the world’s population depend on rice as a major source of calories. Most of the rice in tropical developing countries is produced in lowland areas and by subsistence farmers whose resource base is so small and whose physical access to supply markets is so limited that they cannot take full advantage of technologies that are overdependent on purchased inputs. The BNF in legumes, used appropriately, reduces production constraints in low-input agricultural systems in the tropics. However, most traditionally used green manure (GM) species cannot tolerate prolonged periods of flooding that occur in most lowland rice-farming systems. A new and promising alternative may be the use of nitrogen-fixing, stem-nodulating legumes.

Stem-Nodulating Legume- Rhizobium Symbiosis and Its Agronomic Use in Lowland Rice

Rhizobia can establish a nitrogen-fixing symbiosis with plants of the Leguminosae family. They elicit on their host plant the formation of new organs, called nodules, which develop on the roots. A ...

Stem nodulation in legumes: diversity, mechanisms, and unusual characteristics

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Stem-nodulating legumes as green manure for rice in West Africa

The effect of ammonium nitrate on nodulation and nitrogen fixation (acetylene reduction activity) was investigated usingSesbania rostrata, a tropical legume with the ability to nodulate both roots and stems. Plants were grown in plastic pots on silica sand and gravel, inoculated one month after sowing and then continuously irrigated with an automatic irrigation system. Four nitrogen treatments were applied: 0, 1.5, 3.0 and 6.0mm NH4NO3. Related symbiotic parameters were evaluated 20 days later. With 3.0mm NH4NO3, root nodulation was inhibited. At that concentration, stem nodulation was not affected, but related nitrogenase activity decreased 85% and was completely inhibited at 6.0mm NH4NO3. Increasing NH4NO3 concentration resulted in a diminution of stem nodule size. Stem and root acetylene reduction were strongly inhibited by high mineral nitrogen concentrations; however root nitrogen fixation was more affected than stem nitrogen fixation.

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Adako Moudiongui

Papers

Stem-nodulating legumes as green manure for rice in West Africa

Effect of ammonium nitrate on nodulation and nitrogen fixation (acetylene reduction) of the tropical legume Sesbania rostrata