Abstract: Enhancement of the carbon sequestration capacity and carbon storage of crop land soil is one of the key measures to reduce the greenhouse gas emission and increase the soil productivity. Soil samples were taken from the 0～40 cm soil layers in the plots of fallow, monocroppings of maize, wheat and alfalfa, and rotations of pea-wheat-wheat-millet and maize-wheat-wheat-millet in a 23 year long-term field experiment. Effects of different crops and cropping systems on different carbon forms were studied in dryland soil. Results showed that compared to fallow, soil carbon contents of monocropping or rotation of different crops all increased, with total carbon increased by 35%～74%, organic carbon increased by 47%～139%, inorganic carbon increased by 20%～26%, and light fraction organic carbon increased by 3～11 times. Over different cropping systems, the alfalfa monocropping had the highest contents of total carbon, organic carbon and light fraction organic carbon in 0～15 cm soil layers, then was the rotation of the cereals-millet and the pea-cereals, and the monocropping of maize and wheat had the lowest contents of these carbon forms. However, alfalfa monocropping only increased the inorganic carbon contents in the 0～5 cm soil layer. With the increase of soil layer depth, the contents of total carbon, organic carbon and light fraction organic carbon were all observed to decrease, and the greatest decrease occurred in the alfalfa monocropping. This indicated that long term monocropping of alfalfa was an effective measure to increase soil carbon in dryland soil, especially for the organic carbon and the light fraction organic carbon. However, this increase effects are found only in the layers above 15 cm soil depth. Rotations show greater potential in increasing soil carbon than monocropping of different crops, while no significant difference is observed between the two different rotations.