Effects of cropping and short-natural fallow rotation on soil organic carbon in the Eastern Province of Zambia

•COM-C and POM-C were affected by the composition and amount of input.•COM-C and POM-C decreased even with a short cropping period.•POM-C recovered only in 2-year fallow after 3-year cropping.•Woody biomass was lower in fallow after 2-year than 1-year cropping.•A 2-year fallow after 3-year cropping rotation could retain SOC and POM-C. Recent shortened fallow in miombo woodland in southern Africa may affect the restoration process of soil organic carbon (SOC). This has not yet been clarified under controlled field experiments so the objective of this study was to evaluate the changes in SOC under different cropping periods followed by short fallow through changes in the composition and the amount of vegetation as input returned to soil. The treatments consisted of a cropping period for 1, 2, 3, 4, 5, 10 and 40 years, and a fallow period for 1, 2, 3 years after cropping for 1, 2, 3, 10 and 40 years. The SOC (2000μm), particulate organic matter C (POM-C, 53–2000μm), and the composition and the amount of plant materials returned to soil were measured. Results showed that SOC decreased significantly after 10-year cropping in response to a low amount of input. However, COM-C decreased after only 1-year cropping, and COM-C and POM-C decreased gradually during cropping for 2–5 years. The rapid changes of those labile fractions even with short cropping partially reflected the decrease in input and proportion of woody weeds that were decomposed slowly. Although the amount of input decreased after returning to fallow, SOC did not decrease because leaving plant litter on the soil surface retarded decomposition. Only in 2-year fallow after 3-year cropping, did POM-C recover to the level at the beginning of cropping. This was attributed to the large input from the growth of herbaceous plants which was accelerated due to the decreased competition with woody biomass which decreased with the increase in the previous cropping period. Therefore, 2-year fallow after 3-year cropping is a relatively sustainable management in terms of POM-C and SOC although woody biomass decreased compared to 1-year cropping.