3.4 Soil organic carbon and soil organic carbon stock
Soil organic carbon influences a wide range of physical, chemical and
biological properties and is considered as the most important indicator
of soil quality (Larson and Pierce, 1994). Soil organic carbon content
was higher with thicker organic carbon rich surface soil layer in the
forest ecosystems than coffee plantation ecosystems (Fig.2). The higher
organic carbon content is attributed to the dense forest and surface
cover of Western Ghats and deposition of plant litter. Several studies
have arrived at similar results that soil organic carbon under
plantation was lower than under adjacent (original) native forest
(Turner and Lambert, 2000). There is more litter and root in forest and
grassland soils than arable land and consequently the accumulation of
soil organic carbon is more in forest and grassland soils (Zhang et al.,
2012). The influence of conversion from natural vegetation to cultivable
land on soil organic carbon content differs among regions. A decrease in
soil organic carbon is observed as the impact of conversion from natural
forest to coffee plantation (Afandi et al., 2002; Dariah et al., 2008).
In mid sub-tropical mountainous areas, a reduction in soil organic
carbon content on transition from native forest to arable land (Yang et
al., 2009). Several factors like clay content and type, pH, climate,
hydrology, organic matter inputs determine the stabilization of carbon
in soils. For a given climate, increased erosion and decreased organic
matter input account for a decline in soil organic carbon (Anil Kumar et
al., 2014).
Thus a reduction in the soil organic carbon content was resulted as the
land use changed from natural forest to coffee plantation. The
decomposition of organic matter under coffee plantation could occur more
intensively as the land was relatively more open than forest. Soil
disturbances involved in establishment of plantations resulting in
changes in the rates of accumulation, turn over and decomposition of
soil organic carbon, losses of carbon occurred at different rates in
different parts of soil profile might have led to the decreased content
of soil organic carbon in coffee plantation ecosystem. This result gives
us a hint that plantation management systems need to be modified to
reduce carbon losses.
Soil organic carbon stock in the surface also recorded a noticeable
reduction from 9.1 kg m-2 in the forest ecosystem to
7.3 kg m-2 in the coffee plantation, indicating a
lower contribution from leaf litter (Fig.3). Soil organic carbon stock
was considered as an important index of soil fertility, traditionally,
due to the nutrient supplying capacity and its positive effects on soil
structure and soil biological properties. But as land use influences
soil organic carbon stock, it provides a better indication on land
quality considering C sequestration into the pedosphere. Thus soil
organic carbon stock was assessed to understand the soil quality as it
is one of the important indicators. Though there was a reduction in per
cent soil organic carbon content in comparison to the natural forest
soils, soil organic carbon stocks remained higher in the coffee
plantations. The near zero tillage of coffee soils, complete canopy
cover of the plantations and negligible soil erosion were primarily
responsible for the higher organic carbon status in coffee soils (Nair
et al., 2016).The soil organic carbon stock was consistent with that of
SOC content over the 0-150 cm profile among all the pedons studied. Soil
organic carbon storage was higher in forest profiles, which is related
to the deep dense roots of forest trees whereas soil organic carbon
stock was higher in coffee plantations in Wayanad and Idukki. The
degraded nutrient status of soil could influence the soil carbon
sequestration potential. The amplitude of change in SOC stock was higher
at the surface than at the deeper layers in the profile. Soil organic
carbon stock, most reliable indicator of land quality, provides an idea
on the sustainability of land use. Thus it forms the basis to understand
the restorative properties of coffee soils. From this study it was
observed that the conversion of tropical forests into land for coffee
plantation had positive impacts on soil properties and the soil carbon
stock. Conversion of forest to coffee plantation caused an appreciable
change in organic matter content resulting in increased concentration of
soil carbon stock and exchangeable bases, hence a better land quality.
Coffee, mainly grown as agro-forestry system, has also benefitted
ecosystem services, in ecological restoration even after conversion from
natural forest.