## 4 Discussion
### Influence of Soil Texture, Soil Type and Land Use Patterns on SOM Content  
The variation of SOM content is affected not only by parent material, topography, and climate, but also by other factors, such as soil texture, soil type, and land use patterns. Soil texture is a comprehensive indicator of the soil's physical and chemical properties. There are differences in soil structure and fertilizer retention capacity between different soil textures, which also have an important influence on the SOM content. Previous studies have shown that loam and clay soils have high SOM content. Clay and slit have a strong ability to preserve water and fertilizer, so soils with high clay and silt content have a high organic matter content [28; 38]. In contrast, soils with high sand content have weak protective mechanisms, and therefore have a rapid decomposition of organic matter mineralization. The soils in the study area are predominantly clay loam soil, which consists of mostly clay particles. The clay particles combine with organic matter to form an organic-inorganic complex that ensures the stable uptake of SOM by reducing the mobility of SOM, and promotes the accumulation of SOM by reducing the rate of mineralization.  
Different types of soil have different soil-forming processes, soil development level, tillage-management measures, and fertilizer supply capacity, which result in differences in soil properties between different soil types. Studies have found that Lime concretion black soil and paddy soil have the highest SOM content. This is because their topsoil had more clay particles, which have strong ability to retain mineral nutrients [39]. In addition, soil-forming conditions also have a certain effect on the SOM content of Lime concretion black soils. The areas where Lime concretion black soils are distributed are flat and low-lying, and groundwater drainage is blocked. These environmental conditions allow a variety of wet and aquatic herbaceous plants to flourish, thus providing the formation of humus in the black soil layer. Paddy soils have different physio-chemical and biological properties compared with dry land soils, due to differences in water conditions and human management. Rice cultivation can increase SOM content more than dry land cultivation. The decomposition of SOM in water-saturated soils is faster than in well-earated soils due to the input of crop residues. The high input of organic matter also makes paddy soils more susceptible to carbon sequestration [40; 41; 42]. However, Aeolian sand soil and Fluvent soil are coarser in texture, both having little clay content and easily losing organic matter, and thus their SOM levels are relatively low.  
Compared with soil textures and soil types, land use pattern is more easily influenced by human activities. Therefore, the influence of land use patterns on SOM is also important. There were differences in soil organic matter content among different land use types. In this research, the SOM content of grassland and forest land was significantly higher than that of dry land and paddy fields. There is relatively little human activity in grassland and forest land, and the soil surface is covered with a large amount of fallen leaves, which accelerates the accumulation of organic matter [43]. Long-term tillage and intensive tillage can also lead to organic matter loss; as a result, the SOM content of dry land and paddy fields is relatively low [8; 19; 44]. SOM content is higher in paddy fields than in dry land, which is mainly because paddy fields are generally in flat areas and have favorable conditions for the accumulation of SOM. In addition, the waterlogging conditions in paddy fields create a relatively humid environment, which reduces the activity of microorganisms. The mineralization of organic matter in paddy fields was inhibited, and the rate of organic matter decomposition decreased. As a result, the SOM content of paddy fields is higher than that of dry land.