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.