Soil organic carbon, total nitrogen stocks and CO2 emissions in top- and subsoils with contrasting management regimes in semi-arid environments.

This study aims to investigate soil organic carbon (SOC) and total nitrogen (TN) contents and stocks, CO2 emissions and selected soil properties in croplands, grazing lands, exclosures and forest lands of semi-arid Ethiopia. Sampling was done at 0-30, 30-60 and 60-90 cm soil depths and concentration and stocks of SOC, TN and selected soil properties were determined using standard routine laboratory procedures. There were variations in distribution of SOC and TN stock over 90 cm depth across land use types and locations, decreasing from topsoils to subsoil, with average values ranging from 48.68 Mg C ha-1 and 4.80 Mg N ha-1 in Hugumburda cropland to 303.53 Mg C ha-1 and 24.99 Mg N ha-1 in Desa'a forest respectively. Forest sequestered significant higher SOC and TN stock, decreasing with depth, compared with other land use types. In Desa'a and Hugumburda, the conversion of forest to cropland resulted in a total loss of SOC stock of 9.04 Mg C ha-1 and 2.05 Mg C ha-1, respectively, and an increase in CO2 emission of 33.16 Mg C ha-1 and 7.52 Mg C ha-1 yr-1, respectively. The establishment of 10 years (Geregera) and 6 years (Haikihelet) exclosures on degraded grazing land increased SOC stock by 13% and 37% respectively.