Soil organic matter dynamics after deforestation along a farm field chronosequence in southern highlands of Ethiopia

Depletion of soil organic matter (SOM) is a major component of soil degradation that threatens the sustainability of smallholder farming systems in Ethiopia. In this study, soil organic carbon (SOC) and total nitrogen (N) dynamics following deforestation and subsequent cultivation were evaluated using natural abundance of 13C and 15N along a farm field chronosequence cleared from tropical dry Afromontane forest in south central Ethiopia. Soil samples were collected from five closely located farm fields cultivated for 7, 10, 26, 34 and 53 years after clearance and from an adjacent natural forest soil. All the soils in the study were Mollic Andosols/Humic Haplustands. The δ13C values of the SOC of the farm fields in the 0-10 and 10-20 cm layers were significantly elevated compared to the natural forest soil. In the 0-10 cm layer, the lowest δ13C value of −23.17‰±0.36 was recorded from the natural forest soil and the highest value of −15.7‰±1.50 from the 53-year-old farm field. Fractionation of the SOC of the bulk soil based on the δ13C values showed that in the 0-10 cm layer the SOC of the forest origin declined by 74.6%, which is equivalent to 54.1 Mg C ha−1 or 740 kg ha−1 year−1, where as SOC input from the agricultural crops was low (240 kg ha−1 year−1). However, part of the SOC of the forest origin lost from the surface 0-10 cm soil layer was translocated to the 10-20 cm layer. The SOC derived natural forest approached steady state around 30 years after cultivation while the SOC derived from agricultural crops reached steady state already after 10 years. The results show that the remaining forest derived SOC is recalcitrant while the SOC derived from agricultural crops has a relatively short mean residence time. The soils of the farm fields also showed increasing δ15N values indicating a loss of N from the system. The increasing δ15N values were consistent with the loss of total N along the chronosequence. A continued loss of N combined with insufficient input of fertilizer will result in a decreased productivity of the studied farming systems.