Agriculture Reference
In-Depth Information
amounts, compared with the symbiotic N fixation (Arzanesh et al., 2011). The
process of N fixation is of environmental and economical significance, as it
can significantly reduce the use of chemical N fertilization and its subsequent
leaching. The amount of N fixed by symbiotic fixation ranges from a hundred
to a few hundred kg/ha (Galloway et al., 1995).
There are also the autotrophic green-blue algae (
Cyanobacteria
), which
are able to fix atmospheric N
2
in the rice fields (Jordan et al., 2001). The
chemotrophic soil bacteria such as sulfur oxidizing and phosphorous
solubilizing bacteria are able to provide their necessary energy by oxidizing
elemental sulfur and solubilizing mineral P. As a result of sulfur oxidation by
Thiobacillus
hydrogen ions are produced decreasing soil pH and resulting in
the production of sulfate ion (Miransari et al., 2007; Zabihi et al., 2011).
There are also some soil microbes, acting as pathogens and decreasing
plant growth and crop production. Through different processes such as
invading plant roots, production of cellulitic enzymes, feeding on plant tissues
and carbohydrates, feeding on soil beneficial microbes, etc, soil pathogens can
reduce plant growth and crop production (Sturz et al., 1999).
T
OPSOIL AND
E
ROSION
Topsoil is the most influential soil layer for plant growth and microbial
activities. Accordingly, the process of erosion, which is the removal of soil
surface layer, by wind or water adversely affects the potential of soil
production. Soil movement and precipitation into water sources such as dams
can also decrease their efficiency (Evans, 2005; 2006).
There are different parameters affecting the erosion trend (soil erodibility)
including soil, plant and climate properties as well as topography. Soils, with
higher amounts of organic matter and hence a more improved structure, are
less subjected to the erosion process. Soil erosion reduces water movement
into the soil and increases the amounts of runoff. Sediments are moved from
the slopes to the valleys. This is one of the reasons for the fertility of valleys
(Brayan, 2000; Papiernik et al., 2009).
Plant presence in soil can also significantly reduce erosion by controlling
the amounts of soil moved or leached away, because plants can decrease the
rate of wind and water movement. In addition, plant roots are able to keep soil
particles together and hence decrease the rate of erosion. The amounts of soil,
leached away by erosion are estimable using different equations such as world
erosion equation (Knapen et al., 2007a,b). Because for the formation of just 1
