158
H. Elbasiouny and F. Elbehiry
microbial biomass C (MBC), dissolved organic C (DOC), and easily oxidizable C
(EOC). As those fractions respond relatively rapidly to land use and soil management
practices, it is suggested to be used as early and sensitive indicators of SOC changes
[11, 25].
4.2 Soil Inorganic Carbon (SIC) Stock
The SIC is a large stock too; nevertheless, the national or global studies on SIC
storage and content have been more tentative and have only focused on local or
regional assessments than estimates of SOC stocks [33]. The SIC stock consists of
elemental C and carbonate minerals (either primary carbonate which formed because
of weathering; or secondary carbonate which formed because of CO 2 dissolution in
soil water) [34, 35]. Most of the SIC exists as carbonates and is supposed to occur in
soils of arid and semiarid regions [33]. Although the role of inorganic carbon (IC)
in the C sequestration process is less important than OC, SIC stock determination is
very important for assessing the role of soil as a sink for CO 2 [36]. Therefore; carbon
stock, either organic or inorganic, is not only important to soil productivity and soil
functions, but also to C sequestration and global C cycle [37].
5 Soil Nitrogen (N) Stock and Its Relation to Soil Organic C
The SOM is a primary indicator of soil quality because its influence on soil physical,
chemical, and biological properties and functions, subsequently soil productivity.
Therefore, quantitative assessment of SOM is important for better understanding
the dynamics of SOM and providing valuable information for decision making to
determine management practices for maintaining or increasing productivity. The
measurements of SOM quality and quantity such as soil total N, SOC and soil C/N
ratio are the most widely recommended indicators of SOM quality. The labile fraction
of SOM such as microbial biomass, water extractable organic N, and light fraction
and particulate organic matter N are more sensitive to management-induced changes
and therefore can serve as early indicators of possible changes in SOM quality [38].
The global N stock in SOM is estimated by 1.33 × 10
5 Tg N (1 Tg = teragram =
10
12 g = 1 million tons) compared with 3.9 × 10
12 Tg N in atmosphere and 3.5 ×
10
3 Tg N in terrestrial biomass. Nitrogen in soil is mainly stored in organic form
(90%) and the rest 10% is stored as inorganic N (NH
4+ and NO
3− ). Soils vary in
their capacity to fix N, for example; the sandy soil has a little capacity to fix N than
clay soil [39, 40]. The N stocks also are strongly influenced by the land use and
management practices, OM additions, topographical position and climatic condition
[26].
H. Elbasiouny and F. Elbehiry
microbial biomass C (MBC), dissolved organic C (DOC), and easily oxidizable C
(EOC). As those fractions respond relatively rapidly to land use and soil management
practices, it is suggested to be used as early and sensitive indicators of SOC changes
[11, 25].
4.2 Soil Inorganic Carbon (SIC) Stock
The SIC is a large stock too; nevertheless, the national or global studies on SIC
storage and content have been more tentative and have only focused on local or
regional assessments than estimates of SOC stocks [33]. The SIC stock consists of
elemental C and carbonate minerals (either primary carbonate which formed because
of weathering; or secondary carbonate which formed because of CO 2 dissolution in
soil water) [34, 35]. Most of the SIC exists as carbonates and is supposed to occur in
soils of arid and semiarid regions [33]. Although the role of inorganic carbon (IC)
in the C sequestration process is less important than OC, SIC stock determination is
very important for assessing the role of soil as a sink for CO 2 [36]. Therefore; carbon
stock, either organic or inorganic, is not only important to soil productivity and soil
functions, but also to C sequestration and global C cycle [37].
5 Soil Nitrogen (N) Stock and Its Relation to Soil Organic C
The SOM is a primary indicator of soil quality because its influence on soil physical,
chemical, and biological properties and functions, subsequently soil productivity.
Therefore, quantitative assessment of SOM is important for better understanding
the dynamics of SOM and providing valuable information for decision making to
determine management practices for maintaining or increasing productivity. The
measurements of SOM quality and quantity such as soil total N, SOC and soil C/N
ratio are the most widely recommended indicators of SOM quality. The labile fraction
of SOM such as microbial biomass, water extractable organic N, and light fraction
and particulate organic matter N are more sensitive to management-induced changes
and therefore can serve as early indicators of possible changes in SOM quality [38].
The global N stock in SOM is estimated by 1.33 × 10
5 Tg N (1 Tg = teragram =
10
12 g = 1 million tons) compared with 3.9 × 10
12 Tg N in atmosphere and 3.5 ×
10
3 Tg N in terrestrial biomass. Nitrogen in soil is mainly stored in organic form
(90%) and the rest 10% is stored as inorganic N (NH
4+ and NO
3− ). Soils vary in
their capacity to fix N, for example; the sandy soil has a little capacity to fix N than
clay soil [39, 40]. The N stocks also are strongly influenced by the land use and
management practices, OM additions, topographical position and climatic condition
[26].