## 4 Discussion
### Soil Organic Carbon and Its Mineralization  
This study found that during a 60-day incubation period, the highest SOC content and cumulative mineralized carbon were observed in MS, followed by TS, with the lowest in DS. The reason for this outcome is that the MS region received higher rainfall, resulting in generally higher soil moisture content, which provided an optimal environment for microbial survival, thereby enhancing microbial activity. Active microbial communities can more effectively decompose and transform organic matter in the soil, accelerating the SOC mineralization process [35]. Additionally, ample rainfall not only supplies necessary moisture but also improves soil physical properties, such as soil porosity and structural stability. These improvements aided in increasing oxygen diffusion in the soil, providing conditions for aerobic decomposition of organic carbon. In areas with higher rainfall, plant growth was usually more vigorous, leading to increased organic matter inputs in the soil, such as fallen leaves and root exudates [36]. These inputs of organic matter provided more substrates for SOC mineralization. Areas with higher rainfall were often accompanied by specific climatic patterns, such as higher humidity and lower temperature fluctuations, which directly impacted the mineralization and stability of SOC [37]. We found that for profile soils, both the SOC content and its cumulative mineralization significantly decrease with increasing soil depth [38]. Typically, the topsoil had higher SOC content due to more abundant plant residues and root exudates, as well as a more active microbial community. As soil depth increases, the input of these organic materials diminishes and microbial activity decreases, resulting in relatively lower SOC content in the subsoil. The subsoil usually receives less organic matter, as most plant debris and organic wastes accumulate in the topsoil [39]. Microbial activity in the subsoil was generally lower than in the topsoil due to a limited supply of oxygen and moisture, which were essential for microbial growth and activity. Additionally, the temperature in the subsoil was usually lower than that of the topsoil. The cooler temperatures slowed down microbial activity, making the organic matter in the subsoil more stable and less susceptible to microbial decomposition, thereby reducing the rate of organic carbon mineralization. Additionally, precipitation had a more significant impact on the mineralization of organic carbon in the topsoil than in the subsoil. The organic carbon mineralization in the 0-50 cm soil layer showed a highly significant positive correlation with precipitation (\(p<0.001\)), while in the 50-100 cm layer, it is highly significantly correlated with SOC and POC (\(p<0.001\)). The reason for this may be that precipitation typically only affects soil layers ranging from a few centimeters to several tens of centimeters. In other cases, especially during prolonged heavy rains or in soils with good permeability, water can penetrate soil layers several meters deep. Precipitation directly increases the moisture content of the topsoil, providing necessary conditions for the survival and activity of soil microbes [40]. This increased moisture condition had a direct and significant promoting effect on the mineralization process of SOC, as microbial activity is the primary driver of SOC mineralization. In the subsoil, the sources and nature of SOC may differ from those in the topsoil. The SOC in the subsoil was more likely derived from harder-to-decompose organic matter, such as plant root residues, and its mineralization process was more closely related to soil POC [41]. Furthermore, Clay + Silt showed a significant positive correlation with SOC mineralization in the 50-100 cm soil layer (\(p<0.05\)). In the subsoil, SOC mainly came from hard-to-decompose organic matter, such as deeper root residues.