## 4 Discussion
### The Differences of Influence Factors on SOM Content  
The study area is rich in soil types and parent material types. Different types of soil develop from different parent materials, and have some attributes of their parent materials, thus there is a certain correlation between parent materials and soil types. The spatial variation of SOM content in Shandong Province was predominantly controlled by soil type and parent material, which is similar to results in Guangxi Province and Huzhu County of Qinghai Province (both located in China) [12; 13]. The effects of soil texture and land use pattern on the spatial variation of SOM are less than those of soil type and parent material. The difference in the effect of soil texture on SOM was reflected in both parent material and soil type; likewise, parent material, soil type, and soil texture exhibited great influences on land use pattern. Compared with the above factors, climatic and topographic factors are less able to influence the spatial variation of organic matter.  
The topography of the study area is mainly hilly and plain, with little topographic relief or variation in climate factors, which is not enough to play a significant role in the spatial distribution of organic matter. Similar results were shown in many areas, such as northern Jiangsu Province and the Hebei Province (in China), and North Central Iowa [15; 38; 39]. In some areas with large undulating topographic and rich landscape types, the spatial variation of organic matter was mainly determined by topography and climate factors, which was confirmed by many researches. For example, the combination of temperature, precipitation, and elevation could explain 41.5-56.2% of the variation in SOM for different geographical regions. Temperature and precipitation variables determined surface SOM concentration in northern, north-eastern and north-western China, while precipitation and elevation were the main factors controlling SOM concentration in eastern and southern China [17]. Studies have shown that SOC density is more sensitive to temperature in Northeast China. Temperature alone can explain 35.6% of the organic carbon distribution, while the explanatory ability of temperature and precipitation reached 42.7% [16; 45]. In the Luzhou tobacco farming area of south-eastern Sichuan Province, where the topography is dominated by mountains and hills, elevation was the most influential factor for SOM variation, which can independently explain 31.3% of SOM variation [46]. In general, compared with climate factors, topographical factors had a greater impact in this study area.  
In this area, climates factors have little explanatory ability compared with other factors, but the influence of temperature on the spatial variation of SOM was slightly greater than that of precipitation, which may have been due to the fact that irrigation can reduce the effect of MAP on SOM variation. Similar results were found in areas such as the Yunnan-Guizhou-Guangxi regions and the Jiangsu province (in China) [14; 31]. However, in some other countries, such as Spain [18] and India [47], precipitation has a greater impact on organic matter than temperature. Therefore, there are differences in the effect of climate variables on SOM content in different regions.