Soil carbon sequestration under chronosequences of agroforestry and agricultural lands in Southern Ethiopia

摘要

When forest is converted to other land uses, it often may lead to loss of organic carbon and nitrogen in terrestrial ecosystems. Soil organic carbon (SOC) and total nitrogen (TN) concentration and stocks, and the rate of change in the chronosequencesof 12, 20, 30, 40, 50 years of agroforestry and agricultural lands in an Andic Paleudalf were investigated. A greater proportion of SOC and TN was concentrated in 0 to 20 cm depth and their concentration in agroforestry and agricultural lands was significantly lower than in the natural forest. The SOC stocks in all chronosequences of traditional agroforestry were higher than the corresponding chronosequences under agricultural lands. The loss of SOC stock under the chronosequence of 12 to 50 years of agroforestry and agricultural lands varied from 35 to 92 Mg/ha or 16 to 42 % of the stock under natural forest. The rate of SOC loss after 12 yrs of agroforestry was 5. 6 Mg /ha/yr, while it declined to 0.9 Mg /ha/yr after 50 yrs. The corresponding lossesunder agricultural lands were slightly higher. The trend of N losses, although of much lower magnitude (0.24 Mg /ha/yr after 12 yrs to 0.04 Mg /ha/yr after 50 yrs of agroforestry), was generally the same. These results show that the losses of SOC and TNstocks were higher in agricultural lands than in agroforestry but the stocks in both land uses increased with increasing chronosequences of 12 to 50 years.