Biological Nitrogen Fixation by Legumes - Nitrogen Management in Crop Production - page 318

FIGURE 7.12 Lablab root growth at different N treatments. Left to right 0mgNkg −1 ,
0 mg N kg −1 + Bradyrhizobial inoculants, 100 mg N kg −1 + Bradyrhizobial inoculants, and 200 mg N kg −1 .
(Adapted from Fageria, N. K. et al. 2014. Commun. Soil Sci. Plant Anal . 45:111-125.)
The fixation is carried out by certain species of bacteria (mainly rhizobia and bradyrhizobia) in
association with legume roots.
The key to biological N fixation is the enzyme nitrogenase , which catalyzes the reduction of
dinitrogen gas to ammonia. The ammonia, in turn, is combined with organic acids to form amino
acids and, ultimately, proteins. There are several techniques available for measuring N 2 fixation in
legumes. However, the 15 N technique can measure N 2 fixation in legumes with reasonable precision
in less time. The growth of N 2 -ixing bacteria is influenced by many environmental factors. These
factors are related to climate, soil, and plants. The important climatic factors that affect biologi-
cal N fixation are temperature and soil moisture. Soil factors that influence dinitrogen fixation are
soil pH and soil fertility. The deficiency of P, S, Mo, and Fe in the growth medium or soil strongly
inhibits biological N fixation by legumes. Similarly, high levels of both NH 4 + and NH 3 − can also
limit N 2 fixation. Plant factors such as legume species and/or genotypes within species also influ-
ence biological N fixation. Without doubt, biological N fixation improves the N economy of soils.
Hence, strategies to improve environmental factors in favor of the growth of legume crops will
ultimately improve biological N fixation. Selection of crop genotypes with higher N fixation capac-
ity is the most important strategy from an economic and environmental point of view. In addition,
oil and crop management practices such as conservation tillage and appropriate crop rotation can
improve the biological N fixation of legumes. Molecular biology can be used to improve biological
N fixation in legumes, and this area needs to be explored in the future. In addition, it is hoped that
microbial genetics can transfer N-fixing genes to rhizobium strains, to make elite strains for use
in practical agronomy.
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