Carbon Sink Capacity and Agronomic Productivity of Soils of Semiarid Regions of India - Principles of Sustainable Soil Management in Agroecosystems - page 444

agronomic productivity is strongly related to soil quality. Therefore, curtailing deple-
tion and enriching SOC stock are essential to adapting and mitigating climate change,
buffering agroecosystems in harsh climates against extreme events (drought), and sta-
bilizing agronomic productivity by ensuring some returns even during the bad seasons.
Crop cultivation adversely affects the distribution and stability of soil aggregates
and reduces SOC stock (Kong et al. 2005). The magnitude of reduction in SOC due to
cropping, however, varies among climates and cropping systems (Lal 2004). Because
of the prevailing high temperatures, soils of the tropics generally emit more oxidative
products (i.e., CO 2 ) per unit SOC stock than those of temperate and cooler regions.
However, crop species also play an important role in maintaining quantity and quality
of SOC stock despite the diverse nature of crop residues (CRs) with highly variable
turnover or the mean residence time (MRT) in the soil (Mandal et al. 2007).
The duration and timing of “fallowing” within a cropping system can also affect
the magnitude of SOC stock (Halvorson et al. 2002) because of the differences
in cropping intensity and specific management practices. Once the pathways of C
sequestration in soils are identified, suitable agricultural strategies may be identified
that have the potential to enhance SOC stocks, offset CO 2 loading into the atmo-
sphere, and mitigate global warming (Lal 2009). A large proportion of the research
done thus far on SOC sequestration in soils of agroecosystems is confined to cold
and temperate regions. The effects of soil moisture regime on SOC decomposition
(and hence, long-term SOC storage) are highly variable. In general, the rate of SOC
decomposition is high when precipitation equals evapotranspiration (ET) (Parton et
al. 1987). Thus, the SOC dynamics must be different in arid and semiarid rainfed
conditions where precipitation always exceeds the ET.
Soils of the rainfed regions are characterized by low SOC and N stocks despite
large variations in the cropping system, soil type, rainfall, temperature, and soil/crop/
water management practices such as manuring and fertilization. Low crop yields and
low or no biomass residue retention, coupled with long fallow periods, which extend
up to 9 months in a year, result in adverse environments that do not sustain SOC lev-
els. However, the magnitude of change of SOC due to continuous cultivation depends
on the balance between the loss by oxidative forces during tillage, the quantity and
quality of CRs that are returned, and the amount of organic amendments added to the
soils. Therefore, crop and soil management practices have to be fine-tuned to ensure
long-term sustainability. The use of plant nutrients and organic amendments and the
inclusion and cultivation of legumes support SOC and its sustainability.
16.9 CARBON SEQUESTRATION
It is widely recognized that atmospheric CO 2 concentrations are steadily increas-
ing and leading to global climate change (IPCC 2001, 2007). The Kyoto Protocol
negotiated a framework for reducing the emissions of greenhouse gases (GHGs) in
December 1997. The protocol also recognized that some terrestrial ecosystems have
the potential to sequester large amounts of C and thus slow down the increase of
atmospheric CO 2 concentrations. An increase in SOM and the biomass pools could
buy time while reducing fossil fuel-related emissions of CO 2 . The SOM contains
~50% C on a weight basis.