Agriculture Reference
In-Depth Information
4.9.3 m
ItIGatIon
of
Co
2
r
elease
In
the
a
tmosphere
The atmospheric CO
2
concentration has gained much attention in recent years due to its potential con-
tribution to global warming (Robinson et al., 1996). Soil is an important sink of CO
2
released to the
atmosphere. Many studies have shown that soil CO
2
originates primarily from microbial oxidation of
OM and root respiration (Hanson et al., 2000; Pumpanen et al., 2003). Approximately twice as much
C is stored in the soil as in the atmosphere (Lal et al., 1995; Qualls et al., 2003). Brady and Weil (2002)
reported that OM in the world's soils contains about 3 times as much carbon as is found in all worlds'
vegetation. SOM plays a critical role in the global carbon balance that is thought to be the major factor
affecting global warming (Brady and Weil, 2002). Similarly, Qualls et al. (2003) reported that the fac-
tors that control the storage of C in the soil are among those that play an important role in regulating
the concentration of CO
2
in the atmosphere. Qualls (2004) reported that humic substances are inher-
ently difficult for microbes to mineralize, and this property can contribute to the sequestration of C in
soil. Schlesinger (1990) and Post and Kwon (2000) reported that currently there has also been an added
interest in the role of SOM as a potential sink for atmospheric CO
2
. Hence, the OM content of the soil
can be considered as one of the vital components of CO
2
mitigation in the atmosphere.
The types of crop residues play important roles in C sequestration and soil aggregation because
of the C:N ratios or quality of the residues (Potter et al., 1998; Wright and Hons, 2004). The degra-
dation of fresh crop residues is often governed by C:N ratios (Oades, 1988; Chesire and Chapman,
1996), but as the residue undergoes decomposition, it becomes more recalcitrant and degradation 10
is controlled by the lignin content or lignin/N ratios (Tian et al. 1992). Hence, the ability of soils to
sequestration is closely related to N (Wright and Hons, 2004).
4.9.4 a
dsorptIon
of
h
erBICIdes
Herbicides are commonly used in modern agriculture to control weeds. If agricultural lands are
having low OM content, applied herbicides may leach easily and contaminate the surface or ground-
water. For example, atrazine (2-chloro-4-ethylamino-6-isopropylaminos-triazine) is a common her-
bicide applied to agricultural lands worldwide to control weeds (Ben-Hur et al., 2003). Atrazine is
one of the most widely detected herbicides in surface and groundwater in the United States (Poinke
et al., 1988; Ben-Hur et al., 2003). SOM can be divided into solid- and water-dissolved fractions,
both of which can associate with herbicides (Ben-Hur et al., 2003). Adsorption of herbicide on
SOM should decrease its transport in the soil profile (Moorman et al., 2001). Ben-Hur et al. (2003)
reported that the atrazine-dissolved OM complex decreases the mobility of atrazine if the lower
horizons are lower in OM than the upper horizons (the more typical field case) because of the dis-
solved OM adsorption on the solids.
4.10 SOM VERSUS CROP YIELDS
The importance of soil physical, chemical, and biological properties in crop production should be
evaluated by their effect or relationship with crop yield. If the association is positive, that means it
is highly important in improving or maintaining crop productivity. Some studies have related SOM
and SOM-associated parameters with crop productivity (Lucas and Weil, 2012). Positive correla-
tions between SOM content and crop yields have been observed (Kravchenko and Bullock, 2000;
Majchrzak et al., 2001; Alvarez et al., 2002). Stine and Weil (2002) found SOM, macroaggregate
stability, and soil porosity to be related to crop productivity under different tillage regimes.
Alvarez et al. (2002) reported light-fraction OM and mineralizable N to be related to wheat yield
variability. Other studies found that gains in SOM coincide with crop yield gains (Bauer and Black,
1994) and that losses in SOM coincided with crop yield losses (Diaz-Zorita et al., 1999). When crop res-
idues were returned to the surface of degraded soils, Bruce et al. (1995) found the restoration of soil pro-
ductivity to coincide with an increase in SOM. Lal (2006) found that increasing total SOC by 1 Mg ha
−1
