The economic and environmental costs of the heavy use of chemical N fertilizers in agriculture are a global concern. Sustainability considerations mandate that alternatives to N fertilizers must be urgently sought. Biological nitrogen fixation (BNF), a microbiological process which converts atmospheric nitrogen into a plant-usable form, offers this alternative. Nitrogen-fixing systems offer an economically attractive and ecologically sound means of reducing external inputs and improving internal resources. Symbiotic systems such as that of legumes and Rhizobium can be a major source of N in most cropping systems and that of Azolla and Anabaena can be of particular value to flooded rice crop. Nitrogen fixation by associative and free-living microorganisms can also be important. However, scientific and socio-cultural constraints limit the utilization of BNF systems in agriculture. While several environmental factors that affect BNF have been studied, uncertainties still remain on how organisms respond to a given situation. In the case of legumes, ecological models that predict the likelihood and the magnitude of response to rhizobial inoculation are now becoming available. Molecular biology has made it possible to introduce choice attributes into nitrogen-fixing organisms but limited knowledge on how they interact with the environment makes it difficult to tailor organisms to order. The difficulty in detecting introduced organisms in the field is still a major obstacle to assessing the success or failure of inoculation. Production-level problems and socio-cultural factors also limit the integration of BNF systems into actual farming situations. Maximum benefit can be realized only through analysis and resolution of major constraints to BNF performance in the field and adoption and use of the technology by farmers.

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Biological nitrogen fixation for sustainable agriculture: A perspective

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N2, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N2 fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope 15N, it has been possible to reliably measure rates of N2 fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N2 fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.

Optimising biological N2 fixation by legumes in farming systems

Soil-plant interactions in agroforestry systems

Nitrogen (N2) fixed by Rhizobium bacteroids in the legume nodule is excreted as ammonia to the surrounding host cell where it is efficiently assimilated into the amide group of glutamine. Generally glutamine is a minor exported solute of nitrogen, being further metabolised to asparagine in temperate species and to the ureides, allantoin and allantoic acid in tropical species. These solutes serve as the principal translocated forms of nitrogen in xylem. Compartmentalisation of the pathways of nitrogen metabolism and the role of ammonia in regulation of their activity is examined in nodules of both asparagine-forming (Lupinus albus L.) and ureide-forming (Vigna unguiculata L. Walp) symbioses.

Metabolism and translocation of fixed nitrogen in the nodulated legume

Nitrogen fertilization of alfalfa (Medicago sativa L.) at establishment has been a controversial practice. A review of the literature was conducted to determine conditions when preplant N fertilization of alfalfa resulted in increased yields in the seeding year. Field and lab studies demonstrated that when the soil nitrate levels were greater than 15 ppm and conditions were favorable for effective nodulation (soil pH from 6.2 to 7.5 and high populations of appropriate Rhizobium bacteria present), using preplant N fertilizer in alfalfa establishment did not result in economically important yield increases. Under field conditions that inhibit effective nodulation, alfalfa yield in the seeding year increased when a small amount of N fertilizer (10-50 Ib N/acre) was applied []

Nitrogen Fertilization in Alfalfa Production

Although the recent fall in the price of oil will ultimately be reflected in some reduction in the price of nitrogenous fertilizers the cost of the latter will still be sufficient to maintain inter...

C. A. Atkins

Papers

The Legume/Rhizobium Symbiosis: Limitations to Maximizing Nitrogen Fixation: