Dynamic accumulation of soil organic carbon of terrace changed from slope cropland in the hilly loess plateau of eastern Gansu Province

In order to reveal spatial-temporal distribution characteristics of soil organic carbon (SOC) under different influencing factors (terrace age, soil type, altitude, slope aspect, etc) after slope cropland changed into terraces in the hilly loess plateau of eastern Gansu Province, the terraces with different ages were chosen based on the way of interaction of time and space. The results demonstrate that the SOC content in 0-60 cm soil layer of the loessial soil in terraces is continuously accumulated in nearly 50 years after the terraces building. Compared with the SOC content of slope cropland, the contents of SOC in 20-40 cm and 40-60 cm soil layers in 50 years later are increased by 54.6% and 52.4% respectively, while the increasing range of 0-20 cm soil is only 33.7% (P0.05). The content of SOC is increased significantly and exceeds the SOC content of slope cropland with the increase of terrace ages during the initial 8 years（P0.01）. Significant increase of SOC is found after 24 years but there is no significant change later (P0.05). The contents and accumulation rates of SOC are different in different soil types and terrains with the terrace age. The SOC accumulation in 0-20 cm layer of the loessial soil is the most obvious compared with the deeper layers, whereas, there are significant increases of SOC in 20-40 cm and 40-60 cm layers in the red clay soil (P0.05). The SOC content in 0-20 cm layer in the northward slopes is higher than that in the southward slopes. Compared with the southward slopes, the average of SOC in the northward slopes is increased by 22.1% after 25 years. The spatial-temporal variance of SOC is mainly influenced by soil types which could explain 62.0% of the organic carbon variance. The slope aspect and altitude could explain 20.4% of the variance in all, and the terracing age and organic fertilizer could explain 11.7% and 5.8% of the variance respectively. These results indicate that the terrace soil has a weaker SOC accumulation effect under the current land management conditions.