Agriculture Reference
In-Depth Information
Soil microbes use SOM as their food source, and by the respiration
process can release the stored energy and nutrients available for their own use
and for plant use. Through the mineralization process the humification of
organic compounds also allow the production of humus, which is the resistant
part of SOM. Humus, as previously mentioned, can influence different soil
properties including water holding capacity, soil porosity, CEC, bufferic
capacity, soil structure, and soil nutrients availability (Zech et al., 1997).
Immobilization is the process by which mineral nutrients are used by soil
microbes and become organic in their biomass. During the immobilization
process, C/N ratio of organic matter is a very important factor determining the
rate of immobilization and mineralization and plant access to nutrients. A high
C/N ratio, for example equal to 80 for wheat straw, results in N deficiency in
soil, because the available N in soil is immobilized by soil microbes. In other
words soil microbes utilize soil mineral N for their activities and in cases of
high C/N ration N deficiency appears in plants. Hence, it is important to
alleviate the deficiency by for example adding N fertilizer. Using plant residue
with lower C/N such as legumes (C/N=20) is also another way of adjusting
nutrient ratio in soil (Janssen, B. 1996; Bengtsson et al., 2003).
The process of organic N mineralization is also performed by some
specific soil microbes, namely
Azotomonas
and
Azetobacter
, respectively
resulting in the production of NH
4
+
and NO
3
-
(Gonzalez et al., 2006).
Denitrification of N is also common under oxygen deficient conditions, for
example in compacted soils or saturated soils. Under such conditions soil
microbes use NO
3
instead of oxygen as electron receiver resulting in the
emission of N oxides as well as molecular N
2
(Parton et al., 1996).
N
2
emission is not considered favorite, because it results in N loss from
soil. However, under the following condition N
2
emission may be favored.
NO
3
leaching as a result of excess N fertilization causes the pollution of
surface or ground waters. Excess N in water is not favorable for human health,
increases the growth of aquatic plants and hence decreases the availability of
oxygen for the use of other aquatic organisms. Hence, it is likely to decrease
the amounts of N in water by directing the polluted water into water canals and
allow the process of denitrification (Nuhoglu et al., 2002).
Soil microbes can also fix nutrients in symbiotic or non-symbiotic
association with their host plants. There are arbuscular mycorrhizal fungi in
soil, developing symbiosis with most of the terrestrial plants. In their
symbiosis the host plant supplies the fungi with carbon in the exchange for
nutrients. For the development of such a symbiosis the presence of the host
plant is necessary, although the fungal spores are able to germinate in the