## 4 Discussion  
### Analysis of Factors Affecting SOM Content  
There are five natural soil-forming factors, and the influence that three major soil-forming factors (parent material, climate, and topography) had on SOM content was analyzed in this study. Soil parent material is the basis for soil formation, and soils developed under different parent materials have different material bases [32]. The SOM content of different parent materials in the study area was ordered as follows: lacustrine deposits > aeolian deposits > deluvium > littoral sediment > diluvial deposits > residual deposits > loess parent material > river alluvium. Soils developed from lacustrine deposits were mainly distributed in the north-west Shandong Plain, the Yellow River alluvial plain, and the southern shore of Laizhou Bay, where the richness of organic matter brought about by lake deposition has led to the highest SOM content. In addition, wind transported nutrients also leave rich organic matter. The SOM content of deluvium and diluvial deposits were higher than that of residual deposits [25]. This is because deluvium deposits tend to accumulate nutrient-rich weathered material that has moved down from higher elevations, and diluvial deposits can collect clay particles during the transportation of running water. While residual deposits migrate very little and accumulate products in situ, flowing water can weather away certain soils and easily cause nutrient loss, so the resulting level of organic matter is relatively low.  
Topography influences the process of soil formation by redistributing surface material and energy. Elevation and slope affect microclimates by influencing the water andsoil balance of microtopography and solar radiation, which leads to differences in SOM levels. Correlation analysis showed that SOM was positively correlated with elevation and slope. Higher elevation and slopes are less affected by human activities, and the vegetation was dense, so the soil surface was easily enriched with organic matter. The organic matter level is lower in areas with low elevation and slope, which is due to the fact that these areas are greatly influenced by human farming activities and high-intensity land use. This is consistent with results of small watershed scale research in the Dabie Mountains and the south-western Yunnan province (in China) [33; 34].  
Climate factors, such as temperature and precipitation, are affected by topography and also have a certain impact on soil organic matter, but their explanatory ability was less than 1%. The influence of climate variables on organic matter has two parts. Firstly, the microbial decomposition of SOM is temperature dependent, as the complex molecular structure of SOM is sensitive to temperature [35]. The SOM content in this area increased slightly with the increasing MAT. This was similar to resultsin agricultural fields in the Bavaria region of Germany. This is probably related to the intensive utilization of farmland soil that makes up for the adverse climate conditions and balances out the loss of organic matter caused by high-temperature decomposition. Earlier studies suggested that climate had a greater impact on organic matter in soils with a high intensity of management [36]. Secondly, the sensitivity of SOC to temperature is related to precipitation [37]. Precipitation as an important climate-related component, controlling the above (ANPP) and belowground (BNPP) net primary productivity, and thus influencing the input of organic matter into the soil. However, ANPP and BNPP are also influenced by irrigation condition, fertilization,the type of crop, etc., which collectively reduce the impact of precipitation on organic matter [36].