Assessment of Organic Carbon Status in Indian Soils - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 329

the terrestrial ecosystem to the atmosphere,
of which 15- 17% C is contributed by the de-
composition of SOC (Houghton and Hackler,
1994). It is important to note that organic
matter (OM) preferentially accumulates in
submerged rice systems, which store rela-
tively more SOC as compared to their up-
land cropping system (Sahrawat et al ., 2005).
To sustain the quality and productivity
of soils, information on SOC in terms of its
amount and quality is essential. In recent
years, global warming has created aware-
ness on the role of C cycling in agroeco-
systems in storing atmospheric C in Indian
soils. The first comprehensive study on
SOC status in Indian soils was conducted
by Jenny and Raychaudhuri (1960), fol-
lowed by an estimation of soil organic C
stock of 24.3 Pg by Gupta and Rao (1994),
which was later revised by Bhattacharyya et al .
(2008) ( Table 28.1 ). The total C stock in In-
dian soils is low, as compared to tropical
regions and the rest of the world ( Table 28.2) .
Comprehensive research efforts on SOC in
Indian soils have provided interesting infor-
mation. For brevity, a few case studies from
the two important food-growing zones of
the country, namely the Indo-Gangetic Plains
(IGP) and the black soil region (BSR), are
discussed here.

(Bhattacharyya et al ., 2007a). Besides, the
high annual temperature in the central, west-
ern and the peninsular region of the country,
representing the BSR, does not permit SOC
build-up either (Bhattacharyya et al ., 2000).

Depletion of SOM in croplands
in history of agricultural
development of India

The IGP, with about 13% geographical cover-
age in India, produces nearly 50% of the
food grains for 40% of the total population of
India. However, recent reports of the land
use and soils of the IGP indicate a general
decline in soil fertility (Bhandari et al .,
2002). Soils that earlier rarely showed any
nutrient deficiency symptoms are now de-
ficient in many nutritional elements. Long-
term soil fertility studies have shown a re-
duction in soil organic matter content, as
well as in the other essential nutrients that
had higher levels of nutritional elements in
the earlier years (Abrol and Gupta, 1998;
Bhandari et al ., 2002). The biological activ-
ity of soils has declined gradually, resulting
in the reduced efficiency of the inputs ap-
plied (Abrol and Gupta, 1998). As a conse-
quence, parts of the IGP have an aridic envir-
onment at present (Eswaran and van den
Berg, 1992). The sustainability ratings of
some soils of the IGP for the R/W system in-
dicate many soil constraints, including low
SOC (Bhattacharyya et al ., 2004). It is in this
context that the soils of the IGP of the In-
dian subcontinent require focused attention.
The BSR, constituting nearly 74.6 million
hectares (Mha) in area, is known for low
average SOC content (Bhattacharyya et al .,
2008); however, the occurrence of high-
quality brown forest soils (mollisols), with
nearly 3- 5% SOC, is not rare (Bhattacharyya
et al ., 2006a). The current arid and semi-
arid environment prevailing in central and
southern peninsular India, representing the
major BSR, has been ascribed to the global
warming phenomenon (Eswaran and van den
Berg, 1992). This has been indicated as the
causative factor for the low SOC level in the
shrink-swell soils of India (Bhattacharyya
et al ., 2000).

Dynamics of SOM in India's
Croplands over the Past
Two Decades

Reduced productivity of the rice-wheat
(R/W) system in the IGP has been linked
with declining SOM (Bhandari et al ., 2002).
The earlier report on declining productivity
in Haryana and Punjab by Sinha et al . (1998)
hinted at a decrease in SOC from 0.5% in
the 1960s to 0.2% in 1998 in major R/W
regions of the Indian IGP. A key question
being raised often is whether the R/W sys-
tem of the IGP is sustainable in terms of soil
health, with SOC as the key soil quality par-
ameter. As compared to the IGP, the BSR has
been experiencing less intensive agriculture,
as is evidenced by relatively less potassium (K)
stock in the IGP due to excessive mining