Soil Organic Matter Dynamics in the Subhumid Agroecosystems of the Ethiopian Highlands

We used natural 13 C abundance coupled with particle‐size fractionation to evaluate soil organic carbon (SOC) dynamics following deforestation and subsequent cultivation in the subhumid Ethiopia highlands. Surface soil (0–10 cm), leaf, root, and litter samples were collected from natural forest and fields cultivated for 25 yr (Wushwush) and from Podocarpus dominated natural forest and 30 yr cultivated fields (Munesa) and C, N and δ 13 C signature were measured. Total SOC declined by 55% (32.0 Mg ha −1 ) at Wushwush and by 63% (40.2 Mg ha −1 ) at Munesa following cultivation, while losses of N amounted to 52% (2.8 Mg ha −1 ) and 60% (3.1 Mg ha −1 ) at the two sites, respectively. δ 13 C values of bulk soils of natural forests at Wushwush (−24.3‰) and Munesa (−23.4‰) were significantly lower than those from the corresponding cultivated fields (−19.9‰, Wushwush and −15.5‰, Munesa). Deforestation and continuous cultivation at Wushwush and Munesa resulted in depletion of 80 and 96% of the initial forest‐derived SOC in sand, while 73 and 85% of C 3 SOC was lost from silt fraction of the two sites, respectively. These results suggest that SOC in sand was a very labile component of SOM and is a more sensitive indicator to changes in soil C storage in response to land use changes. However, the substantial amount of forest‐derived SOC lost from silt indicates that SOM associated with silt was also quite susceptible to management changes, and that at least in the soils under study represents a moderately labile SOM pool, which is generally not the case in temperate soils. Forest‐derived SOC in clay declined by 48 and 61% at Wushwush and Munesa, respectively, suggesting that clay retained C 3 derived SOC more effectively and that SOM bound to clay was more stable than SOM associated with sand and silt fractions.