Agriculture Reference
In-Depth Information
higher is the denitrification rate (Aulakh et al., 1991). The relationship between SOC and denitrifica-
tion is discussed by Bremner and Shaw (1958), McGarity (1961), and Aulakh et al. (1992).
4.7.3 d
InItroGen
f
IxatIon
Dinitrogen fixation is the conversion of molecular nitrogen (N
2
) into ammonia and subsequently
into organic nitrogen utilizable in biological processes (Soil Science Society of America, 2008).
Although mixed cropping and crop rotation with legumes were practiced for centuries, the basis
of their benefit was not recognized until Boussingault (1838), a French scientist, presented evi-
dence that the legumes fixed nitrogen from the air (Burris, 1998). Rhizobia encompass a range
of bacterial genera, including
Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium,
Allorhizobium
, and
Azorhizobium
, which are able to establish a symbiosis with leguminous plants
(Sessitsch et al., 2002). Rhizobia form nodules on roots or stems of their hosts, in which they
reduce atmospheric nitrogen and make available to the plants. Biological nitrogen fixation is an
important component of sustainable agriculture, and rhizobium inoculants have been applied fre-
quently as biofertilizers (Sessitsch et al., 2002). In addition, rhizobium is a frequent rhizosphere
colonizer of a wide range of plants and may also inhabit nonleguminous plants endophytically.
In these rhizospheric and endophytic habitats, they may exhibit several plant growth-promoting
effects, such as hormone production, phosphate solubilization, and the suppressions of pathogens
(Sessitsch et al., 2002).
With the advancement of research on dinitrogen fixation, many aspects of the biochemistry of
the process were elucidated between 1930 and 1980, and it was shown that two catalytic proteins
plus a reductant and adenosine triphosphate were involved in the reduction of N
2
to ammonia in
all the systems studied (Burris, 1998). Biological nitrogen fixation had a significant economic and
environmental impact on crop production worldwide. Biological N fixation by legumes offers the
potential to reduce and sometimes eliminate the need for N fertilizers for the following crop (Singh
et al., 2004a). The quantity of nitrogen fixed by legumes varied from species to species and was also
influenced by environmental factors. However, Peoples et al. (1995) reported that various legume
crops and pasture species often fix as much as 200-300 kg N ha
−1
. Globally, symbiotic nitrogen
fixation has been estimated to amount to at least 70 million metric tons nitrogen per year (Brockwell
and Bottomley, 1995). SOM improved nitrogen fixation by improving the soil physical and chemi-
cal properties. SOM improves the soil structure and pH, and it has been reported by Moawad et al.
(1984), Palanipappan et al. (1997), and Howieson et al. (1992) that improved soil structure and pH
improves activities of dinitrogen-fixing bacteria in the soil.
4.7.4 m
YCorrhIzae
f
unGI
The mycorrhizal association is an important factor in the retention or loss of C in terrestrial ecosys-
tems as well as in plant nutrition (Calderon et al., 2012). One of the most important groups of soil
microorganisms is mycorrhizal fungi. Vesicular-arbuscular mycorrhizal (VAM) fungi are present in
nearly all-natural soils, and these fungi infect the greater majority of plants including the major food
crops (Fageria et al., 2011). Mycorrhizae fungi have been shown to improve the nutrition of the host
plants for nutrients that are diffusion limited, such as P, Zn, Cu, and Fe (Tinker, 1982; Marschner
and Dell, 1994; Smith and Read, 1997). Mycorrhizae fungi receive carbohydrates from the host
plant in return for the development of an extensive hyphal network that effectively provides the plant
with a substantial increase in the root surface area (Smith and Read, 1997; Richardson, 2001). The
symbiosis may also enhance the plant's resistance to biotic and abiotic stresses. Additionally, VAM
fungi develop an extensive external hyphal network, which makes a significant contribution to the
improvement of soil structure. Therefore, these fungi constitute an integral and important compo-
nent of agricultural systems (Harrier and Watson, 2003). An adequate amount of OM improves the
VAM association of plants due to the improved physical and chemical soil properties.
