The significance of soil organic matter (SOM) in sustaining agriculture has long been recognized. The rate of change depends on climate, cropping system, cropping practice, and soil moisture. A 3-yr on-farm study was conducted in two major agro- ecologies (hills with warm-temperate 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 texture. Farmers' predominant cropping systems were selected from different cultivation length in addition 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, KMnO₄- oxidizable soil C, C storage, and C/N ratio in two climatic scenarios: warm-temperate and subtropical. A large difference in KMnO₄-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 KMnO₄-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 KMnO₄-oxidizable C but not for TC, confirming our earlier work on the usefulness of the KMnO₄ oxidized fraction for detecting a relatively short-term increase or decrease in soil C pool. The TC storage in uncultivated, <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.