Chapter 2. Transformation and Transport Processes of Nitrogen in Agricultural Systems

Authors

Ronald Follett, USDA-ARSFollow

Date of this Version

2008

Comments

Published in Nitrogen in the Environment: Sources, Problems, and Management, Second edition, ed. J. L. Hatfield & R. F. Follett (Amsterdam, Boston, et al.: Academic Press/Elsevier, 2008).

“Copyright protection is not available for any work prepared by an officer or employee of the United States Government as part of that person's official duties.”
United States Code, Title 17, §105.

Abstract

This chapter discusses the transformation and transport processes of nitrogen (N) in agricultural systems and provides information on overall reservoir sizes for N. Nitrogen is ubiquitous in the environment and is required for the survival of all living things. It is also one of the most important essential nutrients and is central to the production of all crop plants. The most abundant form of N in the environment is elemental dinitrogen (N₂) gas that accounts for 78% of the atmosphere. N₂ gas is inert and is not directly available for plant uptake and metabolism. The atmospheric reservoir is estimated to contain ~4 X 10⁺⁹ Tg N₂ with a turnover time of 10⁺⁷ year (Reeburgh, 1997). However, some of the most mobile substances found in the soil-plant-atmosphere system contain N and the need to understand N transport and transformations in the environment has been the subject of many reviews and/or books (Keeney. 1982. 1989; Hallberg. 1987. 1989; Follett. 1989: Power and Schepers. 1989: Follett et al.. 1991; Galloway et al.. 1995: Mosier et al.. 1998: Laegreid et al.. 1999: Follett and Hatfield. 2001: and SCOPE 2004). '"Natural" fixation of atmospheric N₂ is estimated to be ~ 100 TgN/year. globally (Galloway et al.. 1995) primarily by lightning and biological processes. Once in fixed or "'reactive" form. N can be rapidly incorporated into living tissue. Conversion of relatively inert N₂ gas to biologically available forms is limited by the microbially mediated rate of N-fixation. The estimate of the N contained in the terrestrial biomass reservoir is 3.5 x 10⁺⁴ TgN with a turnover time of 50 years, while the soil reservoir is estimated at 9.5 X 10⁺⁴ TgN with a turnover time of 2000 years. The estimated sizes of the global reservoirs of dissolved N₂ and inorganic N in the oceans are 2.2 x 10⁺⁷ and 6 x 10⁺⁵ TgN respectively. Sediments are estimated to contain 4 x 10⁺⁸ TgN and marine biomass is estimated to contain 4.7 x 10⁺² TC TgN (Reeburgh. 1997). Estimates of the N in soil show it to contain 2.7 times more N than does above ground plant biomass, but only a fraction of the amount of N contained in the atmospheric reservoir. Microbially mediated denitrification (i.e .. conversion back to N₂ gas) completes the N cycle. Natural terrestrial and ocean denitrification amounts are estimated at 147 and 30 TgN/year. respectively.