Abstract: 【Objective】to better understanding on the mechanisms in crop rotation and fertilization on SOC in cropping soils in semi-arid regions. 【Method】Changes in soil organic C (SOC) (0-20 cm) and aboveground biomass from 1984 to 2002 was investigated in ten cropping systems in a long-term field experiment established in 1984 on the Loess Plateau. 【Result】For the continuous winter wheat cropping system (W/W), the above ground biomass without fertilization (W/W+CK) was 3.7 t hm-2, the above ground biomass in W/W+NP and W/W+NPM was 2.2 and 3.4 times that in W/W+CK. For the wheat- wheat+millet-pea rotation system (W/WM/P), the above ground biomass without fertilization (W/WM/P +CK) was 3.2 t hm-2, the above ground biomass in W/WM/P +NP and W/WM/P +NPM was 2.3 and 2.7 times that in W/W+CK. The above ground biomass in W/W/S +NP, G/G +CK, and G/G +NPM was 8.6, 25.5, and 30.5 t hm-2, respectively. After 18 years SOC in continuous winter wheat without fertilization was near to the value (6.5 g kg-1) at the experiment beginning. SOC in treatments applied with N and P fertilizers increased by 7-13% after 18 years, compared with control (the without fertilizer). SOC in the treatments applied with farmyard manure, N, and P fertilizer increased by above 47% after 18 years. Compared with continuous wheat, SOC with wheat-grass rotation or continuous alfalfa increased by 10-25% after 18 years. There was a highly significant correlation between SOC and soil total N ( ，R2＝0.88＊＊). 【Conclusion】The amount of returned root-C to soil plays an important role in SOC in the semi-arid region. Incorporation of crop straw into soil, crop rotation with grass, and conversion of cultivated land into grassland could promote SOC accumulation and sequestration of CO2 in farmlands in the semi-arid region.

Key words: semi-arid region, fertilization, crop rotation, SOC