Carbon Sink Capacity and Agronomic Productivity of Soils of Semiarid Regions of India - Principles of Sustainable Soil Management in Agroecosystems - page 467

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1

y = -0.0004 x + 0.691
R 2 = 0.05427

0

400 500 600 700 800 900

Rainfall (mm)

1000 1100 1200

FIGURE 16.16 Relationship between mean annual rainfall and mean C depletion rate in
seven long-term experiments.

not filled. Therefore, these soils have a high C sink capacity. However, sink capacity
and/or storage rate cannot continue indefinitely (Six et al. 2002). Each soil with a dif-
ferent C loading may reach a new steady state of SOC stock over time. Assessment of
SOC stock for these treatments at periodic, perhaps decadal, intervals may provide
insights into the strategies of C management in these soils. Lal et al. (2007) estimated
that the rate of SOC sequestration in the United States, ranging from 100 to 1000
kg ha -1 year -1 , depends on climate, soil type, and site-specific management. The
global potential of SOC sequestration and restoration of degraded/desertified soils is
estimated at 0.6 to 1.2 Pg C year -1 for about 50 years, with a cumulative sink capac-
ity of 30 to 60 Pg (Lal et al. 2003, 2007), comprising 0.4 to 0.8 Pg C year -1 through
adoption of RMPs on cropland (1350 Mha); 0.01 to 0.03 Pg C year -1 on irrigated
soils (275 Mha); and 0.01 to 0.3 Pg C year -1 through improvements of rangelands
and grasslands (3700 Mha). Maintaining a constant level of SOC stock (zero change)
requires C input of 1.10 Mg C ha -1 year -1 in Vertisols under a winter sorghum system
to 3.47 Mg C ha -1 year -1 under a soybean-based system. The rate of C input required
for groundnut, finger millet, and winter sorghum systems is much lower than those
reported by Kong et al. (2005) (3.1 Mg ha -1 year -1 ) for Davis, CA; Majumder et al.
(2007) (4.59 Mg ha -1 year -1 ) for rice-wheat-jute systems; Majumder et al. (2008)
(3.56 Mg ha -1 year -1 ) for irrigated rice-wheat systems of the Indo-Gangetic plains;
and Mandal et al. (2007) (2.92 Mg ha -1 year -1 ) for rice-based systems in subtropical
India. The lower input of C needed to maintain a constant level in this study may
be due to lower initial SOC levels (1.4-3.9 g kg -1 soil) (Srinivasarao et al. 2006). In
the studies referred to above, the initial SOC concentrations were approximately
three to six times higher (>6-15 g kg -1 soil) than those in the present study. But in
the case of soybean, pearl millet, and upland rice, this rate is comparatively higher
(Srinivasarao et al. 2011e, 2012e,f). In the case of soybean-safflower sequences culti-
vated in Vertisols, initial SOC concentration of the soil was quite higher (6.2 g kg -1 ).
To maintain or improve this level, a significant amount of biomass C is required. In
the case of pearl millet-based systems in Entisol, soil was degraded and had low fer-
tility. Summer temperature of the experimental location is also quite high compared
to other locations. This may be the reason for requirement of higher critical C input.