Variability of Soil Organic Carbon stocks under different land uses: A study in an afro-montane landscape in southwestern Uganda R. Twongyirwe a, b, c, ⁎, D. Sheil b, d, e , J.G.M. Majaliwa c , P. Ebanyat f , M.M. Tenywa g , M. van Heist b , L. Kumar h a Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK b Institute of Tropical Forest Conservation, Mbarara University of Science and Technology, P.O. Box 44, Kabale, Uganda c Department of Geography, Geo-Informatics and Climate Sciences, School of Forestry Environmental and Geographical Sciences, College of Agricultural and Environmental Sciences, Makerere University, P.O. Box 7062, Kampala, Uganda d Center for International Forestry Research (CIFOR), P.O. Box 0113 BOCBD, Bogor 16000, Indonesia e School of Environment, Science and Engineering, Southern Cross University, P.O. Box 157, Lismore, NSW 2480 Australia f Department of Agricultural Production, College of Agricultural and Environmental Sciences, Makerere University, P.O. Box 7062, Kampala, Uganda g Makerere University Agricultural Research Institute, Kabanyolo, College of Agricultural and Environmental Sciences, Makerere University, P.O. Box 7062, Kampala, Uganda h School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia abstract article info Article history: Received 3 December 2011 Received in revised form 12 September 2012 Accepted 20 September 2012 Available online 21 November 2012 Keywords: SOC stocks Sampling depth Landscape position Bwindi Southwestern Uganda We explore and compare quantities and patterns of Soil Organic Carbon (SOC) in protected forest and neigh- boring land around Bwindi Impenetrable National Park (a mountain protected area in Southwestern Uganda). We assessed paired sites of natural forest and major land uses (potato, tea and grazing lands) converted be- tween 1973 and 2010. These pairings were replicated at three altitudinal zones. Plots (20 m by 50 m) were demarcated within each site. Five composite soil and core samples were obtained from 0 to 15 cm (top-soil) and 15–30 cm (sub-soil) at each plot. In total, 192 composite soil and core samples were collected. Within forest we found marked site to site variation in SOC from 54.6 to 82.6 Mg/ha. There was a tendency for higher SOC in converted land, associated with higher bulk density suggesting quality based land use selection with forest left on inferior soils. Cultivation, landscape position, slope and sampling depth were all significantly (P b 0.05) related to variation in SOC stocks following forest conversion but time since conversion had no de- tectable impact. Interestingly, there was no significant relationship between SOC in the top and sub-soils. Higher SOC is largely determined by higher bulk density. The large SOC stocks in these afro-montane soils are less predictable and more persistent than anticipated. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Forest vegetation and soils are important reserves and sinks of carbon, sequestering about 1200 Gt globally (Lewis et al., 2009; Luyssaert et al., 2008; Slik et al., 2009; Wang et al., 2004). Most terres- trial carbon is stored in soils, on average about three times more than the carbon in vegetation and in total about twice what is present in the atmosphere (Batjes and Sombroek, 1997). Conversion of forests to other land uses adds to greenhouse gas emissions (DeFries et al., 2007; Van der Werf et al., 2009) contributing to climate change (Gullison et al., 2007), but also affects soil properties although this re- mains poorly quantified (Cotler and Ortega-Larrocea, 2006). Carbon storage in forest soil is the balance between inputs, mainly from plant material, and losses from decomposition, erosion and other processes (Sun et al., 2004). Carbon enters the soil from litter fall, root and mycorrhizal turnover, plant exudates and microbial fixation (Feller and Beare, 1997). Under steady-state conditions, the carbon gain is matched by equivalent carbon losses (Kirschbaum, 2000). This balance is greatly affected by forest clearance and subse- quent land management practices (Korkanc et al., 2008). SOC is affected by environmental factors such as topography, parent material, soil depth, and land use (Fu et al., 2004; Johnson et al., 2000; Ollinger et al., 2002). Often the key relationships are indirect and potentially complex. Topography influences precipitation and temperature (Tsui et al., 2004), solar radiation, and relative humidity (Finney et al., 1962; Franzmeier et al., 1969). Aspect determines length of exposure to sun light and can influence weathering and veg- etation (Rech et al., 2001; Sidari et al., 2008; Yimer et al., 2006). With advances in climate change mitigation through Reducing Emissions from Deforestation and forest Degradation (REDD), much emphasis has been put on above ground carbon (Cerbu et al., 2010; Harris et al., 2008) but less attention given to below ground carbon (Cole and Ewel, 2006; Navar, 2009). But if SOC changes with forest loss, and varies with land use, such carbon may play a significant role in local, national and global carbon budgets. We therefore need more data on SOC stocks (Gibbs et al., 2007). Geoderma 193–194 (2013) 282–289 ⁎ Corresponding author at: Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UK. Tel.: +44 7552043333. E-mail addresses: rt369@cam.ac.uk, twongyirwe@gmail.com (R. Twongyirwe). 0016-7061/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geoderma.2012.09.005 Contents lists available at SciVerse ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma