Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Biological nitrogen fixation with non-legumes : An achievable target or a dogma?

Abstract: Nitrogen is most often the limiting nutrient for crop production, since only a fraction of atmospheric ni- trogen is made available to the plants through biologi- cal nitrogen fixation (BNF). Extending the BNF ability to non-legumes would be a useful technology for in- creased crop yields among resource-poor farmers. The idea that genetic manipulation techniques might be used to engineer crop plants to fix nitrogen is, of course, not new. However, the more we understand about the biochemistry and physiology of BNF, the less likely it seems that this goal will be achieved by 'simply' transferring the genes for nitrogen fixation to suitable crop species. Induction of nodulation has therefore been the main target of researchers over the past few years. This review briefly describes the pro- gress made towards nodular symbiosis between rhizo- bia and other free-living atmospheric nitrogen fixers and non-legume crops. NITROGEN is an essential plant nutrient. It is the nutrient that is most commonly deficient, contributing to reduced agricultural yields throughout the world. Molecular nitrogen or dinitrogen (N2) makes up four-fifths of the atmosphere, but is metabolically unavailable directly to higher plants or animals. It is available to some microorganisms through biological nitrogen fixation (BNF) in which atmospheric nitrogen is converted to ammonia by the enzyme nitro- genase. According to statistics by FAO (2001), about 42 million tons of fertilizer N is being used annually on a global scale for the production of three major cereal crops, i.e. wheat, rice and maize (17, 9 and 16 million tons res- pectively). Crop plants are able to use about 50% of the applied fertilizer N, while 25% is lost from the soil-plant system through leaching, volatilization, denitrification and due to many other factors causing not only an annual economic loss of US$ 3 billion but also cause pollution to the environment. Some of the adverse environmental effects of excessive use of nitrogenous fertilizers are: (i) metheamoglobinemia in infants due to NO3 and NO2 in waters and food, (ii) cancer due to secondary amines, (iii) respiratory illness due to NO 3, aerosols, NO2 and HNO3, (iv) eutrophication due to N in surface water, (v) material and ecosystem damage due to HNO3 in rainwater, (vi) plant toxicity due to high levels of NO 2 and NH4 in soils, and (vii) excessive plant growth due to more available N, depletion of stratospheric ozone due to NO and N2O. If a BNF system could be assembled in the non-legume plants, it could increase the potential for nitrogen supply because fixed nitrogen would be available to the plants directly, with little or no loss. Such a system could also enhance resource conservation and environmental security, besides freeing farmers from the economic burden of purchasing fertilizer nitrogen for crop production. Thus, a significant reduction in the relative use of fertilizer N can be achieved if atmospheric N is made available to non-legumes directly through an effective associative system with some of the characteristics of legume symbiosis. Recently, several ap- proaches using techniques developed in the area of bio- technology have raised new hopes that success in this secondary objective may yet be realized. It is the authors opinion that there are now sound reasons to anticipate that at least some non-leguminous field crops may also become independent of soil nitrogen. We intend to ex- plain the reasons for this renewed optimism, against the background of knowledge accumulated in the past century that will be relevant to any ultimate su ccess in exploiting these new approaches.

Seasonal and spatial variation of nitrogen fixation in the barrow, alaska, tundra*

Abstract: A seasonal study of nitrogen fixation was carried out on the arctic tundra in the vicinity of Barrow, Alaska. Nitrogen fixation rates were measured on tundra cores at selected sites at 10-day intervals throughout the summer. The acetylene reduction technique was the principal tool. Significant nitrogen fixation was found throughout the mid-summer months. Indirect evidence suggests that blue green algae of the genus Nostoc were the principal agents along with lichens of the genus Peltigera since isolated specimens of these reduced acetylene with high efficiency. A survey of nitrogen fixation was also carried out on subarctic alpine tundra in the

[49] Nitrogen fixation in cyanobacterial mats

Abstract: Publisher Summary This chapter focuses on nitrogen fixation in cyanobacterial mats. Intertidal sediments, hot springs, salt ponds, salt marshes, and mangrove forest sediments are often characterized by dense populations of cyanobacteria. Benthic filamentous cyanobacteria may form rigid structures, called cyanobacterial mats. Cyanobacterial mats usually contain a very high biomass, and in many environments, especially intertidal sediments, the availability of combined nitrogen alone is too low to allow development of the mat. Therefore, biological nitrogen fixation in many mats will be of paramount importance. Many cyanobacteria are known to fix nitrogen. Not only cyanobacteria that differentiate heterocysts, but also several unicellular and filamentous cyanobacteria without heterocysts have been shown to fix nitrogen. The majority of cyanobacterial mats are built by filamentous, nonheterocystous cyanobacteria. Therefore, it can be assumed that in many mats nitrogen fixation is performed by nonheterocystous organisms. In cyanobacterial mats, two different methods of nitrogenase measurement can be used. These are the bell-jar method for in situ measurements and the cork-borer sampling technique.

Nitrogen fixation and transfer in grass-clover leys under organic and conventional cropping systems

Abstract: Symbiotic dinitrogen (N2) fixation is the most important external N source in organic systems. Our objective was to compare symbiotic N2 fixation of clover grown in organically and conventionally cropped grass-clover leys, while taking into account nutrient supply gradients. We studied leys of a 30-year-old field experiment over 2 years in order to compare organic and conventional systems at two fertilization levels. Using 15N natural abundance methods, we determined the proportion of N derived from the atmosphere (PNdfa), the amount of Ndfa (ANdfa), and the transfer of clover N to grasses for both red clover (Trifolium pratense L.) and white clover (Trifolium repens L.). In all treatments and both years, PNdfa was high (83 to 91 %), indicating that the N2 fixation process is not constrained, even not in the strongly nutrient deficient non-fertilized control treatment. Annual ANdfa in harvested clover biomass ranged from 6 to 16 g N m−2. At typical fertilizer input levels, lower sward yield in organic than those in conventional treatments had no effect on ANdfa because of organic treatments had greater clover proportions. In two-year-old leys, on average, 51 % of N taken up by grasses was transferred from clover. Both, organically and conventionally cropped grass-clover leys profited from symbiotic N2 fixation, with high PNdfa, and important transfer of clover N to grasses, provided sufficient potassium- and phosphorus-availability to sustain clover biomass production.