Agriculture Reference
In-Depth Information
7
Biological Nitrogen Fixation
by Legumes
7.1 INTRODUCTION
The importance of legumes in agriculture is as old as the history of mankind. The family of
Leguminosae is as diverse as it is large, comprising more than 19,000 species of plants, ranging
from tiny herbs to huge trees (Ruiz-Diez et al., 2012). Despite this diversity (Lewis et al., 2005),
only about 100 legumes have any substantial agricultural importance, although they are undoubt-
edly as important as grasses in global terms (Howieson et al., 2008). The legumes of agricultural
importance grow on 12-15% of the earth's arable surface and account for 27% of the world's pri-
mary crop production, with grain legumes alone contributing 33% to the dietary protein nitrogen
(N) needs for humans (Graham and Vance, 2003; Ruiz-Diez et al., 2012).
Furthermore, legumes have played a crucial role in agricultural production throughout history.
Their success in N-deficient soils results from root nodules containing symbiotic
Rhizobium
bac-
teria that reduce N
2
to NH
3
(Phillips, 1980). Biological N fixation by legumes in association with
rhizobia is known as dinitrogen (N
2
) fixation. Dinitrogen fixation is defined as the conversion of
molecular nitrogen (N
2
) into ammonia (NH
3
) and subsequently into organic N utilizable in biologi-
cal processes (Soil Science Society of America, 2008). Rhizobium cells that reduce N
2
in root nod-
ules are termed bacteroids. Biological N fixation resulting from symbiosis between legume plants
and rhizobia provides a significant amount of N
2
to agricultural systems worldwide. It is one of the
most spectacular natural phenomena (next to photosynthesis) and does not cause any hazard to the
environment. In addition, the high costs of chemical fertilizers and developing agricultural systems
that are ecologically sustainable have renewed interest in the process of dinitrogen fixation in recent
years. Dinitrogen fixation is carried out by a variety of prokaryotic organisms that reduce atmo-
spheric N
2
gas (which plant cannot assimilate) to NH
3
, a form of N that plants readily incorporate
into organic N (Layzell and Moloney, 1994). Lopez-Garcia et al. (2009) stated that inoculation of
legume seeds with
Bradyrhizobium japonicum
strains selected to maximize atmospheric N
2
fixa-
tion has the potential of contributing to agricultural sustainability and conservation of this nutrient
soil resource.
The quantity of N
2
reduction by prokaryotes at the global level is immense. The most impor-
tant N
2
-ixing agents in agricultural systems are the symbiotic associations between crop and for-
age/fodder legumes and rhizobia (Herridge et al., 2008). According to Delwiche (1983), the total
world biological N fixation amounts to about 118 × 10
6
Mg year
−1
. However, Brady and Weil (2002)
calculated the total N fixation at about 139 × 10
6
Mg year
−1
by legumes, nonlegumes, meadows
and grassland, forest and woodland, and other vegetated land. Similarly, Brockwell and Bottomley
(1995) and Sessitsch et al. (2002) reported that, globally, symbiotic N fixation has been estimated to
amount to about 70 million metric tons per year. Herridge et al. (2008) reported that annual inputs
of fixed N are calculated to be 2.95 Tg for pulses and 18.5 Tg for oilseed legumes. Soybean is the
dominant crop legume, representing 50% of the global crop legume area and 68% of global produc-
tion. Herridge et al. (2008) calculated that the soybean to fix 16.4 Tg N annually represents 77% of
the N fixed by the crop legumes. Three of the largest soybean-producing countries are Brazil, the
United States, and Argentina.
Rhizobia are well known for their capacity to establish a symbiosis with legumes. They inhabit
root nodules, where they reduce atmospheric N and make it available to the plant (Sessitsch et al.,
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