Agriculture Reference
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SOC to TCSs is higher under rice-based systems, while the highest SIC contribution
to TCSs is in soils supporting the cotton-based system.
On a regional scale, aboveground and below-ground biomass production is prob-
ably the major determinant of the relative distribution of SOC with depth (Jobbagy
and Jackson 2000). The aboveground biomass has probably only a limited effect
on SOM levels compared to the below-ground biomass as has been demonstrated
by several long-term residue management studies (Clapp et al. 2000; Reicosky et
al. 2002). The dominant role of root C and a greater relative contribution of root
versus shoot biomass to the SOC stock is widely recognized (Rasse et al. 2004).
Root-to-shoot ratio is 0.21 to 0.25 for corn, 0.23 for soybean, and 0.50 for barley
(Allmaras et al. 2004; Bolinder et al. 1997). In addition to the root biomass, its
composition also has a strong impact on SOC sequestration. In general, leafy plants
decompose faster than the woody plants, and leaves decompose faster than roots
(Wang et al. 2004). The second most abundant compounds after proteins are lignins,
which largely contribute to terrestrial biomass residues. These compounds exhibit
a higher resistance to microbial degradation as compared to celluloses (Martin and
Haider 1986). Suberin is mostly found in root tissues and is a major contributor to
SOM (Nierop et al. 2003). Among dryland crops, concentration of lignin is 4% in
sorghum, 8% in soybean, 10% in maize, 9% to 13% in millet, 11% to 13% in rice,
and 6% to 16% in legumes like alfalfa (Scheffer 2002; Fernandez et al. 2003; Bilbro
et al. 1991; Devevre and Horwath 2000; Clement et al. 1998). Corn roots also contain
a wide range of fatty acids beside carbohydrates, lignin, lipids, and alkyl-aromatics
(Gregorich et al. 1996).
The effect of a cropping system on SOC concentration varies with soil type. In
Vertisols and associated soils, cotton- and sorghum-based systems have larger SIC
stocks, while soybean and groundnut systems have higher SOC stock. In general,
legume-based systems have higher SOC stocks than cereal-based systems practiced
on Vertisols in the tropics (Wani et al. 1995, 2003). In Inceptisols, maize-based sys-
tems have high SOC and SIC concentrations. In Alfisols, rice-based systems (Ranchi
and Phulbani) have relatively higher SOC concentration, while groundnut-based
(Anantapur) systems have larger SIC concentrations. These trends may be due to
larger carbonate deposits observed in the subsoil layers of the profile and frequent
addition of gypsum to groundnut along with differences in rainfall, parent mate-
rial, and other management practices adapted at these locations. In Aridisols, pearl
millet-based systems at Sant Kabir (SK) Nagar have higher TCSs than those in Hisar,
Haryana.
16.7.3 c
aRBon
S
tockS
in
R
elation
to
R
ainfall
In general, SOC stocks increase with an increase in the mean annual rainfall (
Figures
16.9
and 16.10,
r
= 0.59,
P
< 0.05). In contrast, however, the SIC stocks decrease with
the increase in mean annual rainfall, from 156.4 Mg ha
-1
for rainfall of <550 mm to
26.0 Mg ha
-1
for >1100 mm. As the SIC stocks exceed those of SOC, TCS decreases
with increase in mean annual rainfall from 183.8 Mg ha
-1
in the arid environment
(<550 mm) to 70.2 Mg ha
-1
in subhumid regions (>1100 mm). However, cation
exchange capacity (CEC) is significantly and positively correlated (
r
= 0.81 [highly
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