Global inputs of biological nitrogen fixation in agricultural systems

  • Herridge D
  • Peoples M
  • Boddey R
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Biological dinitrogen (N2) fixation is a natural process of significant importance in world agriculture. The demand for accurate determinations of global inputs of biologically-fixed nitrogen (N) is strong and will continue to be fuelled by the need to understand and effectively manage the global N cycle. In this paper we review and update long-standing and more recent estimates of biological N2 fixation for the different agricultural systems, including the extensive, uncultivated tropical savannas used for grazing. Our methodology was to combine data on the areas and yields of legumes and cereals from the Food and Agriculture Organization (FAO) database on world agricultural production (FAOSTAT) with published and unpublished data on N2 fixation. As the FAO lists grain legumes only, and not forage, fodder and green manure legumes, other literature was accessed to obtain approximate estimates in these cases. Below-ground plant N was factored into the estima- tions. The most important N2-fixing agents in agri- cultural systems are the symbiotic associations between crop and forage/fodder legumes and rhizo- bia. Annual inputs of fixed N are calculated to be 2.95 Tg for the pulses and 18.5 Tg for the oilseed legumes. Soybean (Glycine max) is the dominant crop legume, representing 50% of the global crop legume area and 68% of global production. We calculate soybean to fix 16.4 Tg N annually, representing 77% of the N fixed by the crop legumes. Annual N2 fixation by soybean in the U.S., Brazil and Argentina is calculated at 5.7, 4.6 and 3.4 Tg, respectively. Accurately estimating global N2 fixation for the symbioses of the forage and fodder legumes is challenging because statistics on the areas and productivity of these legumes are almost impossible to obtain. The uncertainty increases as we move to the other agricultural-production systems—rice (Oryza sativa), sugar cane (Saccharum spp.), cereal and oilseed (non-legume) crop lands and extensive, grazed savannas. Nonetheless, the estimates of annual N2 fixation inputs are 12–25 Tg (pasture and fodder legumes), 5 Tg (rice), 0.5 Tg (sugar cane),

Author-supplied keywords

  • Associative
  • Cyanobacteria
  • Dinitrogen (N2) fixation
  • Endophytic
  • Free-living
  • Global
  • Legumes
  • Nitrogen (N)
  • Oilseed legumes
  • Pulses
  • Rhizobia
  • Soybean

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