Abstract: In the Ningxia Irrigation Zone, 39 samples of soil profiles (0-100 cm) and 241 samples of surface layers (0-20 cm) were collected in 2009, and soil data attributing to 102 samples of profiles and 147 samples of surface layers were collected, which were obtained from the Second National Soil Survey of China in 1980. Then, soil organic carbon (SOC) densities in the surface layer and profile depth were calculated for the analysis of SOC stock change during the past 30 years. The results showed that SOC density of surface layer had increased significantly (P<0.01) from 16.55 Mg C·hm^-2 in 1980 to 21.44 Mg C·hm^-2 in 2009, with an annual increase of 0.98%. The SOC density in profile increased (P>0.05) from 55.70 Mg C·hm^-2 to 58.49 Mg C·hm^-2 with an annual increase of 0.17% during the past 30 years. Soil type was a major factor driving the change of SOC stock in the Ningxia Irrigation Zone. A strong relationship was found between SOC density and irrigation duration, indicating that irrigation had a prominent effect on SOC density change, the longer duration of irrigation, the higher density of SOC in the surface layer and profile. Fertilizer input, crop type and yield also affected the change of SOC density in the irrigated cropland. Non-significant relationship was found between SOC density and climate index such as air temperature and precipitation. Irrigation with Yellow River water plays a positive role in increasing SOC stock and decreasing greenhouse gas emission in Ningxia Irrigation Zone.

Key words: suitable distribution, Maxent, climate change., Elaeagnus mollis