Biological nitrogen fixation for sustainable agriculture: A perspective

Abstract

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

Composition of coffee shade tree species and density of indigenous arbuscular mycorrhizal fungi (AMF) spores in Bonga natural coffee forest, southwestern Ethiopia

“…The nodulation status of some of these legumes by root nodule bacteria of the genera Rhizobium and Bradyrhizobium has been reported (Granhall, 1987;Assefa and Kleiner, 1998). The significance of tree and herbaceous legumes in many agricultural production sectors is well addressed (Granhall, 1994;Snoeck et al, 2000) and nitrogen-fixing systems often offer economically attractive and ecologically sound means of reducing external inputs and improving soil fertility (Bohlool et al, 1992). More precisely, the addition of nitrogen to coffee systems by leguminous shade trees has been reported in many countries (Babbar and Zak, 1994).…”

Comparative genomics of the core and accessory genomes of 48 Sinorhizobium strains comprising five genospecies

“…These bacteria are among the largest fixers of atmospheric N 2 gas in the biosphere and account for the deposition of nearly 100 to 195 teragrams per year. The effective use of biological nitrogen fixation via application of rhizobia leads to sustainable cropping systems with a net positive impact on the environment [1]. Most currently recognized legume-nodulating bacteria belong to the α-proteobacteria and are members of the genera Allorhizobium , Azorhizobium , Mesorhizobium , Rhizobium , Sinorhizobium (renamed Ensifer ), or Bradyrhizobium [2,3].…”

Legumes in the reclamation of marginal soils, from cultivar and inoculant selection to transgenic approaches

“…For decades, nitrogen fertilizers have been continuously used to improve crop yield. However, the use of nitrogen fertilizers accelerates the depletion of large amounts of fossil, nonrenewable energy resources, and it contributes substantially to environmental pollution through atmospheric emission and leaching of nitrogenated compounds to ground or surface water (Bohlool et al 1992;Peoples and Crasswell 1992;Velthof et al 2009). …”

“…Biological nitrogen fixation, that is, the assimilation of atmospheric nitrogen in form of organic compounds, is a sustainable source of nitrogen in cropping systems, as fixed nitrogen can be used directly by the plant, and it is less susceptible to volatilization, denitrification, and leaching (Jensen and Hauggaard-Nielsen 2003;Garg and Geetanjali 2007). Thus, biological nitrogen fixation in agrosystems can mitigate the use of fertilizers and consequently reduce global warming and water contamination (Bohlool et al 1992). Nitrogen fixation (diazotrophy) is an exclusive property of prokaryotic organisms containing the nitrogenase enzyme complex.…”