Assessment of Organic Carbon Status in Indian Soils - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 335

of the experiment. The comparison be-
tween the reported and the modelled yield
for the three crops (rice, wheat and sorghum)
showed a reasonably good correlation ( r = 0.8)
(Bhattacharyya et al ., 2010). Earlier experi-
ence of evaluation of the Century model in
the IGP showed overestimation by Century
at the humid site (Bhattacharyya et al .,
2007c). More recently, Milne et al . (2008)
suggested that Century might be less suited
to estimate the carbon dynamics of soils
under rice flooded every year. In contrast,
in the present study at Mohanpur, flooded
rice (flooded for 6 months over a period of
19 years of experimentation) yielded a good
Century output from the humid bioclimatic
system. It appears that further development
of the Century model is needed to improve
its performance when modelling carbon
return to the soil under extremely clayey
soils (>50% clay), as in the BSR (Akola).
The crop files obtained along with the
Century model were modified to suit the
tropical conditions (Table 28.5) . In general,
Century appeared to simulate crop yield
reasonably well for all the crops ( Fig. 28.2 ).
With the proper relationships, Century
simulation of measured yields is a gauge of
the model's performance in replicating the
actual crop residue returns of carbon and
nitrogen to the soil and in replicating crop-
water relations.

crop yield, while the application of fertil-
izers in combination with organic materials
brought an increase in TOC as well as in
crop yield. Usually, the harvesting of annual
crops is done from almost ground level.
Grain and straw are both used, although for
different purposes. Stubble is removed be-
fore sowing the next crop. This gives very
little scope of returning the biomass back to
the soil. This happens in cases when higher
fertilizer is added: yield increased but SOC
did not change much. This is in contrast to
the treatments where organic amendments
are applied along with inorganics. The ma-
nures, since they are mixed with the soil,
help to increase the SOC content. This model
has been found useful as a tool to arrive at
different threshold values of rainfall, which
influence the decomposition rate modifier
and also the rate of organic carbon sequestra-
tion in various bioclimatic systems ( Fig. 28.3 ).
The rate modifier for the moisture sum grad-
ually decreases from a subhumid moist to an
arid bioclimatic system, with a clear distinc-
tion at Sarol, Akola and Zarifa Viran. Vari-
ous other details of these sites vis-à-vis the
rate modifiers indicate that rainfall of nearly
850, 550 and 500 mm during the wet months
appears to be the three threshold limits
in these five bioclimatic systems in decid-
ing the rate modifier for the moisture sum.
This means that below these wet months'
rainfall, the organic carbon turnover rate
will be reduced, causing less organic carbon
storage in soils. While explaining inorganic
carbon sequestration and its consequences
on soil sodicity, a threshold limit of 850 mm
mean annual rainfall (MAR) was reported
( Bhattacharyya et al ., 2000, 2004), below
which the soils became more calcareous,
alkaline and sodic. RothC model output can
thus help to find the threshold limits of
the climatic parameters that influence SOC
decomposition and its content, which can
serve as a good indicator for soil quality and
health. The model showed that the addition
of fertilizers only should maintain the TOC
level in soils. To increase the level of or-
ganic carbon sequestration for posterity, the
addition of organics along with fertilizers
may be recommended. Recent findings on
the effects of organics on shrink-swell soils

RothC model experience

RothC could simulate changes in total or-
ganic C (TOC) in two contrasting ecosites.
Observed trends in TOC consist of an in-
crease in the subhumid sites, while manures
alone or in combination increase TOC ap-
preciably in arid and semi-arid sites. TOC
remained, however, almost similar over
years for the control (no fertilizer or manure)
and nitrogen, phosphorus and potassium
(NPK) treatments in all the four sites. In the
subhumid site, TOC increased marginally
when a less than recommended dose of fer-
tilizer was added. The addition of a higher
dose of fertilizer and manure brought a rapid
increase in TOC. The addition of fertilizer
only did not increase TOC but increased the