## 1 Introduction  
Soil organic matter (SOM) is the basis of soil fertility, and not only reflects the quality and health of the soil but also plays an important role in improving the soil's physical and chemical properties, in addition to plant growth [1]. The content and spatial distribution pattern of soil organic matter is affected by both natural factors and human factors, which have certain spatial variability regardless of scale. Natural factors include soil type, landform, meteorological climate, parent material type, soil texture, etc., while human factors include land use methods, tillage management measures, etc. [2; 3; 4]. Because of the high heterogeneity in soil organic matter, studying the spatial variability of SOM and its influencing factors can effectively elucidate the current spatial distribution of SOM in the region, which is of great significance for improving soil fertility, environmental protection, and sustainable agricultural development. At present, most of the research on the spatial variation of soil organic matter has focused on only a certain type of influencing factor, and most of them only focusing on a small scale. However, soil organic matter is affected by many factors, and the influence of all factors on organic matter content is scale dependent. In this present study, our spatial scales covered different ranges, including county, province, regional scales [5; 6; 7] (such as hilly red soil region in the Songliao Plain of China, or the small watershed of Loess Plateau), in addition to national scales, etc. There are many studies based on the county scale. For example, slope has been found to be the main controlling factor of the spatial distribution of organic matter on the Loess Plateau in Hengshan county [8]. In some counties of eastern China, it has been found that topographic factors such as elevation and slope direction have the most significant influence on the spatial distribution of organic matter [9; 10]. [PERSON] et al. found that the main controlling factors of soil organic matter in the Pinggu County of Beijing were terrain, soil type, texture, and land use type, but that texture and tillage methods had more significant effects at the township scale [11]. In Huzhu County, Qinghai Province, it was found that soil type, average annual precipitation, and altitude each play a leading role in the formation of organic matter spatial distribution [12]. There are few studies on the spatial distribution of soil organic matter at provincial scale in China [13; 14; 15]. At larger regional scales, topographic factors and climate seem to have more influence on organic matter. For example, the influence of climatic factors on soil organic matter is very obvious in dryland soil of the north-east region [16]. The SOM variation between different geographical regions in China is controlled by temperature, precipitation, and altitude, with the level of variance ranging from 41.5% to 56.2% [17]. At the national scale, studies have shown that 45% of the variation in soil organic carbon in the Spanish peninsula can be explained by climate variables and land use [18]. In Irish grassland soils, precipitation is the main controlling factor upon soil organic matter [19]. In summary, the main controlling factors affecting the spatial distribution of SOM vary from region to region, due to different geographical and environmental conditions. Most of the recent studies have focused on a limited range influencing variables or factors on SOM content in small and medium scales, such as City and County. In particular, there are few studies that comprehensively consider the effects of numerous natural and human factors at large scales, such as the provincial scale and beyond. The factors influencing SOM are different at different scales, so the different types of influencing factors should be considered comprehensively. Shandong Province is a major agricultural province in eastern China and has a significant role in the whole country. Its SOM content and distribution directly affect agricultural production.