Biological N Inputs

peter J. Bottomley David D. Myrold

Global N Inputs Biological Nitrogen Fixation Free-Living N2-Fixing Bacteria Associative N2-Fixing Bacteria Phototrophic Bacteria

Symbiotic N2-Fixing Associations between Legumes and Rhizobia

Biotechnology of BNF

Acknowledgments

References and Suggested Reading global ninputs

The nonmetallic element N is essential for life. Although a huge amount of N (4 X 1021g) exists in the atmosphere, soils, and waters of Earth, more than 99% of this is in the form of N2 and also is unavailable to almost all living organisms (Table 14.1). To transform N2 into "reactive N," the triple bond must be broken so that N can bond with C, H, and O and form the building blocks of life. In the prehuman world, N2 was transformed into reactive N via lightning discharge (5 tera-grams (Tg) N year-1; 1 Tg equals 1012 g) and by biological N fixation (BNF) (100-140 Tg N year-1). Very few organisms possess the "N-fixing" enzyme complex necessary to carry out BNF. In the past, reactive N has not accumulated globally, because the microbial process of denitrification (whereby NO- is converted to N2) occurs at approximately the same rate as BNF. During the past 100 years, however, the annual input of anthropogenically created reactive N has increased dramatically due to (a) modest increases in legume use in agriculture (15 to 30 Tg N year-1), (b) large increases in fossil fuel combustion (1 Tg N year-1 in 1860 vs

TABLE 14.1 Global Nitrogen Pool Sizes"

Nitrogen pool

Pool size (g of N)

Lithosphere

1.0 x 1023

Atmosphere

3.9 x 1021

Coal

1.0 x 1017

Hydrosphere

2.3 x 1019

Soil organic N

1.0 x 1017

Soil fixed NH4

2.0 x 1016

Biota N

3.5 x 1015

Microbial N

1.5 x 1015

"Reprinted from Paul and Clark (1996) by permission of the publisher.

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