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25 Tg N year-1 in 2000), and (c) enormous increases in the use of Haber-Bosch process-derived fertilizer N for food production (zero, pre-20th century; currently —110 Tg N year-1) (Fig. 14.1). The input of reactive N has increased over 10-fold in 100 years, with about 85% being used by agriculture to support the ever-increasing human population during the same period. In addition, enhanced use of reactive N for food production has had secondary effects on the N cycle. For example, the intensification of agriculture and its expansion into forest and grasslands has released reactive N from long-term storage in soil organic matter. It has been estimated that the burning of forests and grasslands, draining of wetlands, and soil tillage liberate approximately 40 Tg N year-1 of reactive N (Vitousek et al., 1997).

Although yearly estimates of global BNF have not changed much across both pre-and postindustrial eras, the distribution of BNF has changed as a result of urbanization and intensive agriculture. Large areas of diverse natural vegetation, which once included N2-fixing species as part of their floral composition, have been replaced by monocultures of non-N2-fixing, high-yielding crop species that require addition of reactive N to achieve their yield potentials. In contrast, agricultural BNF is restricted to smaller areas of intensively managed crop and grazing lands. Unfortunately, increased use of N2-fixing grain legumes for human and domestic animal consumption often results in net export of N from agricultural soils. Another major consequence of the human-driven alteration of the N cycle involves the movement of reactive N from sites of application to remote sites. BNF occurs inside living organisms in which fixed N is quickly assimilated into cell constituents. In contrast, reactive N applied to non-N2-fixing crop species as fertilizer or animal waste has a less certain fate. For example, ammoniacal and urea-based N fertilizers can be converted to various gaseous or water-soluble N oxides (NOx) by the microbial processes of nitrification and denitrification and dispersed from the site of application. NH3 can also be dispersed to the atmosphere for deposition elsewhere (a) during application of anhydrous NH3 or animal waste or (b) after enzymatic conversion of soil-applied urea to NH3 by the enzyme urease. There are both positive and negative effects associated with the deposition of reactive N at sites remote from its source. Plant and microbe growth may be stimulated and N immobilized. Scientists have estimated that an extra 100-1000 Tg C year-1 may be fixed globally because of atmospheric transfer and remote deposition of reactive N. Too much N deposition, however, can result in soil acidification, greater NO- leaching, and loss of plant species diversity in terrestrial ecosystems.

It might seem somewhat ironic that the justification for studying BNF so intensively over the past 100 years lies in the need to increase inputs of reactive N via BNF into N-limited agroecosystems. These studies continue, however, against the backdrop of negative environmental consequences caused by the huge global excesses of reactive N. Nonetheless, scientists work to thoroughly understand the mechanism of BNF in order to increase the efficiency of manufacturing N fertilizer. In addition, there is an urgent need to learn how certain N2-fixing plants and specific microorganisms establish symbiotic and endophytic associations while others do not. There is hope that one day scientists might be able to create N2-fixing corn, wheat, and rice and reduce the use of fertilizer N.

biological nitrogen fixation

BNF is a process exclusively restricted to the prokaryotes of the domains Archaea and Bacteria. However, there are many examples of symbiotic associations that

TABLE 14.2 Examples of Genera of Diazotrophic Bacteria Arranged by Mode of Energy Generation and the Oxygen Sensitivity of Their Diazotrophy

Energy source

Sensitivity of N2 fixation to oxygen

Examples (Genera)

Heterotrophic

Aerobic diazotrophs

Azotobacter, Gluconacetobacter

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