Deforestation and land use strongly effect soil organic carbon and nitrogen stock in Northwest Ethiopia Dessie Assefa a, ⁎, Boris Rewald a , Hans Sandén a , Christoph Rosinger a , Abrham Abiyu b , Birru Yitaferu b , Douglas L. Godbold a a Institute of Forest Ecology, University of Natural Resources and Life Sciences (BOKU), Peter Jordan-Straße, 82 Vienna, Austria b Amhara Agricultural Research Institute, Bahir Dar, P. O. Box 527, Ethiopia abstract article info Article history: Received 11 November 2016 Received in revised form 25 January 2017 Accepted 4 February 2017 Available online xxxx Soil is the largest terrestrial organic carbon pool and can act as a source or sink for atmospheric CO 2 . Although reliable soil carbon (SOC) stock measurements of major ecosystems are essential for predicting the inﬂuence of advancing climate change, comprehensive data on SOC stocks is still scarce for most ecosystems in subtropical areas. In this study, SOC and N stocks of different land use systems were investigated along a climatic gradient in Northwest Ethiopia. The land use systems ranged from dry subtropical Afromontane forest, as the baseline, to cropland as the most degraded system. In addition, we investigated the changes of SOC stocks after interventions to recover vegetation cover; these were eucalyptus plantations and an exclosure to prevent grazing. Total SOC varied between land use systems and ranged from 3.1 kg C m −2 in croplands to 23.9 kg C m −2 in natural forest, and average N stock ranged from 0.4 kg N m −2 in croplands to 2.1 kg N m −2 in natural forest. In forests, there were a clear vertical gradient in SOC and N stock down the soil proﬁle, and 60% of the total SOC and N stocks were found in the upper 10 cm soil depth. Using the Sr/Ca and Ba/Ca ratios and the vertical distribution of the C/N ratio of the soil, the losses of SOC were shown to be due to loss of the of the upper soil layer. Afforestation of degraded croplands and grazing lands with eucalyptus increased SOC stocks to nearly 70% of the natural forest levels within 30 years. Exclosure, which removed grazing pressure and allowed regeneration of native vegeta- tion, increased SOC in the top soil only. © 2017 Published by Elsevier B.V. Keywords: Land use change Eucalyptus Grazing land Exclosure Strontium 1. Introduction Soil is a key factor in the global carbon cycle and can act either as a source or as a sink to atmospheric CO 2 . Worldwide, soils are estimated to hold 3150 Pg of carbon (C) which is more than four times the amount of carbon stored in terrestrial plant biomass (650 Pg C) or the atmo- sphere (750 Pg C) (Fan et al., 2016). The size of the pool of soil organic carbon (SOC) is determined by the input of plant-derived carbon, the potential to sequester carbon through physical and bio-chemical pro- cesses, and the loss of SOC through heterotrophic respiration, leaching, and erosion (De Deyn et al., 2008; Jobbágy and Jackson, 2000). Reliable SOC stock measurements of major ecosystems are essential to parameterize models estimating net C stocks and changes in different biomes (Powers et al., 2011; Zimmermann et al., 2009). While current models are used to estimate global and regional soil carbon pools and for predicting the inﬂuence of advancing climate change on those pools (Jones et al., 2005), comprehensive data on SOC stocks is still scarce for some ecosystems at scales relevant to local management as well as national carbon inventories (Victoria et al., 2012). Because the impact of land use conversion (LUC) varies depending on the land use type and the abiotic factors present, regional and ecosystem level stud- ies are important (Milne et al., 2007; Schrumpf et al., 2011; Schulp et al., 2008). In Ethiopia, extensive soil carbon surveys are especially scarce for the remnant natural forests of the NW Ethiopian highlands and the land use systems established on previously naturally forested area since 1950 (Zeleke and Hurni, 2001). Natural forests and woodlands cover b 9.5% of the Amhara region and about 60% of the total area is used as cropland and grazing land (Bekele, 2011; Desta et al., 2000). Eucalyptus globules (Labill.) has been planted in the central highlands of Ethiopia since 1895 (Pohjonen and Pukkala, 1990). It is the dominant exotic spe- cies planted in the highland areas because of its fast growth (Pohjonen and Pukkala, 1990), non-palatability to livestock, multiple use and high economic return . Soil carbon stocks are inﬂuenced by factors such as climate, geology and weathering history, and biotic variables such as species composi- tion and density (Fernandez et al., 2013; Vesterdal et al., 2013). The most important human effect on the rate of changes in SOC stock is at- tributed to LUC (Don et al., 2011; Guo and Gifford, 2002; Houghton and Goodale, 2004), and LUC is a signiﬁcant factor in global emissions of CO 2 Catena 153 (2017) 89–99 ⁎ Corresponding author. E-mail address: dessie.assafa@boku.ac.at (D. Assefa). http://dx.doi.org/10.1016/j.catena.2017.02.003 0341-8162/© 2017 Published by Elsevier B.V. Contents lists available at ScienceDirect Catena journal homepage: www.elsevier.com/locate/catena