Aims This study was conducted to assess the effects of shifting cultivation and its conversion to mono-cropping on soil organic carbon (SOC) and total nitrogen (STN). Methods We compared soil pH, texture, bulk density and SOC and STN contents and stocks (0–100 cm) in natural forest (NF), adjacent shifting cultivation (SC) areas (> 100 years old) having three (SC-3Y), five (SC5Y) and seven (SC-7Y)-year-old fallowing, and 10 year-old mono-cropping field (MCF) converted from shifting cultivation in Western Ethiopia. Results There was no significant difference in soil pH in NF and all shifting cultivation areas. However, MCF had lower soil pH compared to SC-3Y and SC-5Y. There was no or very little difference in soil texture and bulk density across the study sites. Shifting cultivation did not affect SOC and STN stocks. However, conversion of shifting cultivation to mono-cropping decreased SOC (45–50% over 10 years; loss of 11.6 ± 0.2 Mg C ha−1 yr.−1 ) and STN stocks (18–45% over 10 years; loss of 0.6 ± 0.1 Mg N ha−1 yr.−1 ). Conclusions While shifting cultivation maintained SOC and STN, its conversion to mono-cropping decreased them, potentially contributing to global warming and decreasing soil fertility.