Agriculture Reference
In-Depth Information
of use (Fageria and Baligar, 2005). N cycle in soil-plant system is very dynamic and complex due
to the involvement of climatic, soil and plant factors, and their interactions. The N cycle is defined
as the sequence of biochemical changes undergone by N, wherein it is used by a living organism,
transformed upon the death and decomposition of the organism, and ultimately converted into its
original oxidation state (Soil Science Society of America, 2008). A simple definition of the N cycle
is interacting biological processes in the soil are termed the N cycle. Addition of N in the soils and
uptake by plants is also a part of the N cycle. In agroecosystems, the N balance between the input
and output determines the fate of N in these systems (Ghoshal, 2002; Singh et al., 2011). Therefore,
N dynamics holds the key to designing suitable management strategies to achieve sustainable crop
production.
Silgram and Shepherd (1999) reported that the N cycle in soils is largely microbially medi-
ated, and a major component involves the transformation of organic N into the available mineral
forms, primarily nitrate (NO
3
-N) and ammonium (NH
4
-N). The opposing process of immobiliza-
tion essentially involves the conversion of mineral N into organic N by microorganisms, with the
balance between mineralization and immobilization processes (net mineralization) determining the
effect on the magnitude of the soil mineral N pool. The mineralization-immobilization balance is
of pivotal importance as it controls the supply to and magnitude of the plant available mineral pool
(Silgram and Shepherd, 1999).
One factor contributing to the low efficiency of N fertilization is the highly dynamic nature of
the N cycle. A considerable part of the N available to crops may originate from the mineralization
of organic materials such as soil organic matter, manure, or crop residues. Transformations from
one N form into another, including mineralization, are mainly mediated by soil microorganisms,
which are affected by a number of factors, including temperature, water content, oxygen availabil-
ity, pH, supply of nutrients, soil texture, as well as organic matter content and quality (Robertson
and Groffman, 2007; Geisseler et al., 2012). These dynamic interactions make it difficult to esti-
mate the amount of N mineralized from organic sources and the temporal pattern of mineralization
(Geisseler et al., 2012).
Figure 2.1 shows a simplified model of N cycle in a soil-plant system. The main components of
N cycle are the N transformation process. In addition, it can be seen from this figure that the main
N input sources of N to a soil-plant system are chemical fertilizers, organic manures or residues,
biological N
2
fixation, and atmospheric N
2
. Similarly, the main N depletion sources in a soil-plant
system are leaching, denitrification, volatilization, surface runoff, and plant uptake. A detailed dis-
cussion of N cycle or processes in agricultural soils can be found in Stevenson (1986).
Soil organic matter plays a key role in the global C and N cycles (Curtin et al., 2012). Soils con-
tain more than twice as much C as the atmosphere and three times the amount stored in living plants
(Schlesinger, 1997). Organic N and its mineralization in soil-plant system is also part of N cycle.
The mineralization of soil organic N is microbially mediated, with the rate of mineralization being
strongly dependent on temperature and soil moisture. In addition, ammonified N may be fixed on
the soil colloides depending on cation exchange capacity. The N immobilization also has temporary
influence on N uptake to plants. The N immobilization is defined as the transformation of inorganic
N compounds (
NH
4
+
,
NO
3
−
,
NO
2
−
, and NH
3
) into the organic state. Soil organisms assimilate inor-
ganic N compounds and transform them into organic N constituents of their cells and tissues, and
the soil biomass (Jansson and Persson, 1982).
In the surface mineral soils, N content ranges from 0.2 to 5.0 g kg
−1
with an average value of
about 1.5 g kg
−1
(Brady and Weil, 2002). More than 90% of the N in most soils is in the form of
organic matter. The organic form of N protects the N from loss; however, it is also not available to
crop plants. This organic form of N should be mineralized to
NH
4
+
and
NO
3
−
before its uptake by
plants. Mineral N seldom accounts for more than 1-2% of the total N in the soil (Brady and Weil,
2002). Mineralization is the conversion of an element from an organic form to an inorganic state
as a result of microbial activity (Soil Science Society of America, 2008). Soil microorganisms play
a key role in the mineralization of organic substances, which they decompose to obtain mineral
