Soil Organic Carbon and Total Nitrogen pools under Different Land Uses in Northern Ethiopia Aweke M. Gelaw1, B. R. Singh1 and R. Lal2 1Norwegian University of Life Sciences, NMBU, Aas, Norway; 2The Ohio State University, OSU, Columbus, OH, USA RF AF OP SP IR Introduction Land use and soil management affect both total and depth wise distribution of soil organic carbon and Total Nitrogen pools. Agricultural soils can be a source or a sink of atmospheric CO2 (Al-Kaisi et al., 2005; Chen et al., 2009). Objectives To determine SOC and TN concentrations and stocks in 0–5, 5-10, 10-20 and 20-30 cm soil layers for five land use systems, rain fed cropping (RF), agroforestry (AF), open pasture (OP), irrigation (IR) and silvopasture (SP) at Mandae watershed in Tigray, Northern Ethiopia Material and methods A total of 100 evenly distributed soil samples (four depths, five land uses, and five replications) were collected using a soil auger. Samples were taken to 30- cm as per the IPCC (2006) guidelines, and separated into increments of 0 – 5, 5-10, 10-20 and 20-30 cm depths. Soil cores within each replicate were well mixed and combined to a composite sample by depth. Samples were air-dried and passed through a 2 mm sieve and identifiable crop residues, root material, and stones were removed during sieving. Soil–bulk density samples were taken at the same soil depth intervals by the core method (Blake and Hartge, 1986). Concentrations of SOC and TN (% w/w) were determined at the Carbon Sequestration and Management Center Laboratory, OSU, USA; by the dry combustion method (Nelson and Sommers, 1996) Figure 1. A photo from the sampling sites Soil ρb value for each depth interval was used to calculate SOC and TN stocks (Mgha-1) as follows: SOC (or TN) Stock (Mgha-1) = [SOC (or TN) concentration (gkg-1) x ρb (Mgm-3) x depth (m)] /10 Data analysis The effects of the different land use systems on SOC and TN concentrations and stocks within each depth and each site were subjected to one-way ANOVA. SAS statistical package was used for the statistical analysis Results and Discussion Table 1. Magnitude and rate of soil organic carbon and total nitrogen stocks accumulation in four different land uses in 0-30 cm depth taking RF as a baseline. Land Use Soil Organic Carbon accumulation (Mg C ha-1) Rate of Soil Organic Carbon accumulation (Mg C ha-1yr-1) Total Nitrogen accumulation (Mg N ha-1) Rate of Total Nitrogen accumulation (Mg N ha-1yr-1) 0-30 cm AF 9.74(7.17)B 0.19(0.14) 1.10(0.64) AB 0.022(0.013) OP 36.53(16.99)A 0.73 (0.34) 3.25(1.96) A 0.065 (0.039) IR 8.34(8.73)B 0.56 (0.58) 0.28(0.55) B 0.019 (0.037) SP 23.01(19.70)AB 0.46(0.39) 1.91(1.86) AB 0.038(0.037)   NS Fig. 2 Distribution of SOC and TN concentrations across four depths and five land uses . Mean values followed by standard errors in the parentheses; values with different letters are significantly different. NS = not significant at P < 0.05 (Tukey’s α = 0.05). Soils under OP and SP land uses stored more SOC and TN than those under agricultural land uses, RF, AF, and IR due to mainly the negative effects of tillage Accumulations of SOC and TN stocks in reference to the control (RF) land use were in the order: OP > SP > AF > IR due to the effects of tillage and amount of organic inputs to the soil from the land uses Much of SOC and TN in grass- and-tree based land uses (OP and SP) concentrated in the top 0-5 cm depth Conclusion Land use change from RF to OP, SP, IR, and AF increased SOC and TN stocks which confirming our hypothesis that land use change from dry land cropping to irrigation and grass- and tree-based land use systems increases SOC and TN retention capacities of soils. The results of this study indicated that SOC and TN concentrations in soils of the region can be increased by converting arable lands to grass lands and silvopastures or adopting no-till and reduced tillage practices. SOC and TN in OP and SP land uses were concentrated in the top 0-5 cm depth, indicating the risks of large amounts of CO2 release from the surface soil if these land uses are converted into cultivated cropland. References Al-Kaisi, M.M., Yin, X.H., Licht, M.A., 2005. Soil carbon and nitrogen changes as influenced by tillage and cropping systems in some Iowa soils. Agric Ecosyst Environ 105: 635- 647. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute A (ed.) Methods of Soil Analysis. Part1. 2nd edn. Am Soc. Ag., Soil Sci. Soc. Am. Madison, WI, pp. 363–375. Chen, H., Marhan, S., Billen, N., Stahl, K., 2009. Soil Organic Carbon and Total Nitrogen Stocks as affected by different land uses in Baden-Wurttemberg (southwest Germany). J. Plant Nutr. Soil Sci. 172: 32-42. Nelson, D.W., Sommers, L.E., 1996. Total carbon, Organic carbon and Organic matter. Laboratory methods. In: Sparks DL et al. (eds) methods of soil analysis. Part 3. SSSA Book ser. 5. SSSA, Madison, WI, pp. 961-1010.