Soil carbon sequestration under different exotic tree species in the southwestern highlands of Ethiopia

At Belete forest in southwestern Ethiopia (7°33′N, 36°35′E), tree plantations were established on abandoned farmland, which was previously mainly used for maize cultivation. Total carbon and 13C analyses were used to evaluate the changes in soil organic carbon (SOC) pools associated with land use changes using a comparative approach. Forest clearing followed by continuous cultivation of crops caused a loss of 43% (75.4 Mg ha− 1) total SOC and 73% (128.4 Mg ha− 1) forest derived SOC after nearly 75 years. The net loss of SOC was lower because of addition of 53.0 Mg ha− 1 of SOC of C4 crop origin (mainly maize) to the farmland. On the other hand, afforestation of farmland led to a net accretion of SOC of 69.6 and 29.3 Mg ha− 1 after 20 years under Cupressus lusitanica and Pinus patula stands, respectively. The SOC accretion of plantation origin amounted to 63.3 Mg ha− 1 under C. lusitanica and 24.2 Mg ha− 1 under P. patula. Contrary to the results obtained in some other studies, the SOC of C4 origin did not decline in these stands. This could be attributed to pasture grasses of C4 origin that took over after land abandonment and continued to grow under the tree canopies. The grasses could thus have compensated for the SOC loss. SOC might also have been close to a steady state under the pre-plantation period. Based on the SOC amount found in a reference stand of native forest, afforestation with Eucalyptus grandis during 20 years, preceded by 20 years of cultivation and 35 years of pasture, returned the total SOC to nearly pre-deforestation levels. SOC accumulation rates of 1–3.2 Mg ha− 1 y− 1 are apparently possible 20 years after afforestation of an abandoned farmland but the accumulation rate is species dependent.