Agriculture, Ecosystems and Environment 188 (2014) 256–263 Contents lists available at ScienceDirect Agriculture, Ecosystems and Environment j ourna l h omepage: www.elsevier.com/locate/agee Soil organic carbon and total nitrogen stocks under different land uses in a semi-arid watershed in Tigray, Northern Ethiopia Aweke M. Gelaw a,∗ , B.R. Singh a , R. Lal b a Norwegian University of Life Sciences, P.O. Box: 5003, 1432 Ås, Norway b Carbon Sequestration and Management Center, the Ohio State University, Columbus, OH 43210, USA a r t i c l e i n f o Article history: Received 29 October 2013 Received in revised form 21 February 2014 Accepted 24 February 2014 Available online 26 March 2014 Keywords: Land use change Carbon sequestration Soil Organic Carbon or Total Nitrogen concentrations Soil Organic Carbon or Total Nitrogen Stocks Ethiopia a b s t r a c t In Ethiopia, massive deforestation of natural forests and extensive use of agricultural lands have resulted in soil degradation. Soil organic carbon (SOC) quantity and quality are crucial to soil quality. However, knowledge on the effects of land use change on soil carbon storage in semi-arid northern Ethiopia is very limited. To address this problem, a study was undertaken within a semi-arid watershed in eastern Tigray, Northern Ethiopia, to estimate SOC and total nitrogen (TN) concentrations and stocks in 0–5, 5–10, 10–20 and 20–30 cm soil layers for five land uses: rainfed crop production (RF), agroforestry based crop production (AF), open communal pasture (OP), silvopasture (SP) and irrigation based fruit production (IR) each with five replications. Generally, both magnitude and difference in SOC and TN concentrations showed a decreasing trend with depth within and among most land uses. SOC and TN concentrations were highly correlated in all land uses and depths. Total stocks in 0–30 cm layer were 25.8, 16.1, 52.6, 24.4 and 39.1 Mg ha -1 for SOC compared with 2.7, 1.6, 4.9, 1.9 and 3.5 Mg ha -1 for TN in AF, RF, OP, IR and SP land uses, respectively. With RF as baseline and the duration of 50 years since land use conversion, the average rate of accumulation was 0.73, 0.46, and 0.19 Mg C ha -1 yr -1 in comparison with 0.065, 0.038, and 0.022 Mg N ha -1 yr -1 for OP, SP and AF, respectively. Soils under IR also accumulated 0.56 Mg C ha -1 yr -1 and 0.019 Mg TN ha -1 yr -1 in the 0–30 cm layer and in comparison with the RF land use system on an average of 15 years. The results of this study revealed that conversion of croplands to grasslands or integration of appropriate agroforestry trees in cropping fields in the region has large technical potential of SOC and TN sequestrations. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Ethiopia is one of the largest countries in Africa both in terms of land mass and population. The vast majority of the popu- lation (>80%) depends on agriculture which contributes around 42% of its national gross domestic product (GDP) (Barrow et al., 2011). Despite its strategic importance for the country’s economic development, the agricultural sector suffers from low efficiency, population pressure, ineffective land management, and unfavor- able land use practices leading to widespread land degradation and loss of environmental quality (Ashagrie et al., 2005; Amare et al., 2013). As a result of this human interference on natural forest and grazing lands, soil organic matter (SOM) has declined to low level especially in cultivated soils of the highlands (Chibsa and Ta, 2009). ∗ Corresponding author at: Aweke M. Gelaw, Norwegian University of Life Sci- ences, P.O. Box: 5003, 1432 Ås, Norway. Tel.: +47 944 28655; fax: +47 64965601. E-mail addresses: aweke.gelaw@nmbu.no, awekegelaw@gmail.com (A.M. Gelaw). Global climate change caused by rising levels of carbon diox- ide (CO 2 ) and other greenhouse gases is recognized as a serious environmental issue of the twenty-first century (Kumar and Nair, 2011). Agriculture is the human enterprise that is most vulnera- ble to climate change. Tropical agriculture, particularly subsistence agriculture is particularly vulnerable, as smallholder farmers do not have adequate resources to adapt to climate change (Verchot et al., 2007). The role of land use systems in stabilizing CO 2 levels and increasing carbon (C) sink potentials of soils have attracted consid- erable scientific attention in the recent past (Kumar and Nair, 2011). Soil organic matter (SOM), which contains more reactive SOC than any other single terrestrial pool, plays a major role in determining C storage in ecosystems and moderating atmospheric concentra- tions of CO 2 (Post et al., 1982). Soil C sequestration is the process of transferring CO 2 from the atmosphere into the soil in a form that is not immediately reemitted, and this process is being considered as a strategy for mitigating climate change (Sundermeier et al., 2005; Lal et al., 2007; Chen et al., 2009). It is a natural, cost-effective, and environment-friendly process (Lal, 2004). Once sequestered, C remains in the soil as long as restorative land use, no-till farming, http://dx.doi.org/10.1016/j.agee.2014.02.035 0167-8809/© 2014 Elsevier B.V. All rights reserved.