Abstract The significance of soil organic matter (SOM) in sustaining agriculture has long been recognized. The rate of change depends on cli- mate, cropping system, cropping practice, and soil moisture. A 3-yr on-farm study was conducted in two major agro-ecologies (hills with warm-tem- perate climate and plains with subtropical climate) of Nepal. The soils in warm-temperate climate are Lithic subgroups of Ustorthents with well-drained loamy texture, and in subtropical climate are Haplaquepts with imperfectly drained loamy tex- ture. Farmers’ predominant cropping systems were selected from different cultivation length in addi- tion to a reference sample collected from adjacent virgin forest. The objectives were to examine the effect of cultivation length and cropping system on total carbon, KMnO 4 -oxidizable soil C, C storage, and C/N ratio in two climatic scenarios: warm- temperate and subtropical. A large difference in KMnO 4 -oxidizable soil organic C was observed due to the effect of cultivation length and cropping system. However, TC remained similar during the 3-year study. The decrease in KMnO 4 -oxidizable C due to cultivation was more in the surface layer (43–56%) than in the subsurface layer (20–30%). Total C in uncultivated, < 10-year cultivated, and >50-year cultivated soil was 22, 13, and 10 g kg –1 in warm-temperate climate and 10, 6, and 5 g kg –1 in subtropical climate, respectively. During the 3-year study period in both climates, large changes in soil C were observed for KMnO 4 -oxidizable C but not for TC, confirming our earlier work on the usefulness of the KMnO 4 oxidized fraction for detecting a relatively short-term increase or de- crease in soil C pool. The TC storage in unculti- vated, < 10-year cultivated, and >50-year cultivated soil was 38, 25, and 19 Mg ha –1 in warm- temperate climate and 22, 15, and 12 Mg ha –1 in subtropical climate, respectively. The rice–wheat and maize–potato cropping systems were good in storing soil C of 30 and 20 Mg ha –1 for 0–15-cm soil depth in warm-temperate climate. The rice–wheat cropping system was also good in storing soil C in subtropical climate (19 Mg ha –1 ) compared with other cropping systems studied. Keywords KMnO 4 -oxidizable carbon Æ Total carbon Æ Carbon storage Æ C/N ratio Æ Climate Æ Cropping system R. K. Shrestha (&) School of Natural Resources, Ohio State University, 210 Kottman Hall, 2021 Coffey Road, Columbus, OH 43210-1085, USA e-mail: shrestha.10@osu.edu J. K. Ladha IRRI-India, CG Block, NASC Complex, DPS Marg, New Delhi 110012, India S. K. Gami Department of Crop and Soil Sciences, Cornell University, Bradfield Hall, Ithaca, NY 14853, USA Nutr Cycl Agroecosyst (2006) 75:257–269 DOI 10.1007/s10705-006-9032-z 123 ORIGINAL PAPER Total and organic soil carbon in cropping systems of Nepal R. K. Shrestha Æ J. K. Ladha Æ S. K. Gami Received: 6 August 2005 / Accepted: 31 May 2006 / Published online: 1 August 2006 Ó Springer Science+Business Media B.V. 2006