Soil is a carbon (C) reservoir of the terrestrial C cycle next to biomass C. The soil C stock represents the balance between inputs of organic matter to soils and the loss of C through decomposition, leaching, and erosion of organic matter. About 50% of the soil C is stored in forests (Tremblay et al., 2006), which include dead organic matter and soil organic matter (IPCC, 2006). Soil under natural forest and protected forestland, could store high soil organic carbon as compared to other land uses (Wang L. et al., 2012). Thus, conversion of forest land to other land uses can contribute to disturbance of the natural ecosystem and decline of soil organic matter (SOM) which may result in the loss of soil carbon and its emissions to the atmosphere (Lawler JJ. et al., 2014; McGill., 2015).
Land use dynamics and management have profound influences on the quantity and changes of soil organic matter and in turn, it governs the fate of SOC storage potential on global basis there by determining the state of climate change (Jackson et al., 2000; Liu et al., 2012). It has contributed to soil degradation and soil loss, leading to a decrease in soil C storage (Eaton et al., 2008). C stocks in tropical forests are susceptible to land use changes (Le Quéré et al., 2015). Forestland conversion to other land-uses in the tropics is among the major factors leading to losses in C stocks. This contributes to global warming; which occurs from increased concentrations of greenhouse gases (GHG) in the atmospheric leading to the net increase of the global mean temperature (Bhartendu, 2012). The proximate and underlying causes of forest conversion include pressures from increased demand for forest resources; selective exploitation or destruction of tree species (IPCC, 2002). Several studies (Woldeamlak and Solomon, 2013; Tesfahunegn, 2016; Eric R., 2017) reported that removing vegetation cover reduces recycling of organic carbon to the soil. Thus, there is a need for developing sustainable systems to maintain and improve soil carbon content while mitigating GHG emissions.
Ethiopia is the cradle of worldwide Arabica coffee and named as the birthplace of Coffea arabica (Woldemichael, G. et al., 2013 and Kumar, 2015). In this country, coffee (Coffea arabica L.) occurs in the undergrowth of moist afromontane forests (Schmitt and Grote, 2006). Because of the expansion and management of this coffee plant, moist afromontane forest of the country, particularly that of south west and western parts has been currently under constant threat. In these areas, farmers grow annual crops in fields and collect coffee from semi-forested coffee systems, which are forests that are managed for coffee production by selective removal of trees and shrubs. Most of these stands have a spectral signature similar to undisturbed forests in low-resolution satellite imagery and are likely to be mapped as forests. However, this situation is eventually leading to forest degradation which may result in forest and soil carbon loss and its emissions to the atmosphere. Senbeta and Denich (2006) and Hundera K. et al. (2013) show that semi-forest coffee management system decreases the forest basal area, tree regeneration, reduces tree density and eventually leads to the disappearance of the forest tree species. On the other hand, this may affect forest carbon stock.
The study area, Anfilo district is one of the top 18 coffee producers’ districts from Oromia regional state (Warner et al., 2015) and characterized by a broad gradient of coffee forest management. Of the 11 districts of Kellem Wollega Zone, Anfilo is also predominantly known by a potential natural forest area, which is one of the remnants moist evergreen Afromontane forests in western Ethiopia and the forest has been playing a crucial role in mitigating climate change. Despite this importance, this natural forest has been subjected to degradation in the past few decades for coffee expansion and its management. Cutting trees for timber extraction, fuel wood collection and house construction were also common in the study area.
For sustainable utilization of natural resources, assessment of human made landscape modification is therefore very important. Even though, there are some studies conducted in the Anfilo district, no scientific information was available in the district about changes in soil carbon stock resulted from such conversion of natural forest to coffee- based forest. Therefore, a more refined C accounting system is clearly needed in the area to clarify changes in soil carbon storage, and preferably the one that includes different land cover types, whether natural or man-made. Therefore, this study was conducted to assess soil carbon stock changes as a result of conversion of natural forest to coffee-based forest in the Anfilo district in order to generate important information for the conservation, management and sustainable utilization of this forest to enhance the forest’s soil carbon stock by reducing deforestation and forest degradation.