## 1 Introduction  
Grasslands make up nearly 20% of the land surface area [1] and 12% of the Earth's organic matter [2]. Ecosystems of grasslands are crucial to the cycling and neutralization of carbon dioxide [3, 4]. China's grassland area accounts for about 7% of the world's grassland area [5] and its carbon storage accounts for about 9-16% of the world's carbon storage [6]. Therefore, changes in soil organic carbon (SOC) stocks in Chinese grasslands are important for earth climate change and carbon cycle impact [7, 8]. In general, SOC stability is directly related to endogenous soil properties (e.g., texture, clay minerals, organic matter, oxides) and is also influenced by exogenous factors (e.g., climate, land use, vegetation type, etc.) [9, 10, 11].  
SOC is generally classified as reactive organic carbon and mineral-associated organic carbon (MOC) [12, 13] linked to particles [14, 15]. MOC is organic carbon complexed with soil mineral particles [16]. It is more stable [17] and has a longer turnover time. It accounts for a larger proportion of SOC [18]. Numerous factors, including the soil nutrient content, nutrient input, and land management practices, affect the concentration of MOC [19, 20, 21, 22]. Among the main influencing factors are soil texture and mineral types [23, 24]. Soil minerals have a large specific surface area and cation exchange capacity and soil texture is considered to be the dominant factor controlling MOC dynamics [17]. The soil microstructure and pore system affect the stability of SOC [25]. In addition, clay minerals as soil skeletons [26] can influence the stability of SOC through surface interactions [27; 28]. Different clay minerals have different abilities to adsorb and complex SOC [29]. Layered silicate minerals are effective in protecting SOC [30], with 2:1 minerals being more advantageous for SOC adsorption compared to 1:1 minerals [31]. The combination of clay minerals and oxides can promote the formation of aggregates [32] and improve the adsorption capacity of SOC [33]. A high soil pH can reduce the binding of minerals to SOC by affecting the electrical charge [30]. The effect of different grassland types on MOC varies, especially in the temperate zone and alpine grasslands, where there are differences in hydrothermal conditions and vegetation types [34; 35]. The temperate zone and alpine grasslands provided the best environment to explore the influence of layered silicate mineralogy on MOC [36].  
Gradient studies are an effective way to explore the influence of environmental conditions (i.e., climate, vegetation, soil) on the ecosystem [37]. While previous studies have focused on changes in a single grassland type or grassland SOC stocks, studies of temperature and precipitation gradients need to cover a large area. It is difficult to draw broad conclusions from small-scale field studies. Large-scale soil samples were gathered for this investigation along a gradient of temperature and precipitation. Natural gradients were used to explore the characteristics of MOC under different climates and its relationship with clay minerals. The influence of clay minerals on the stability of MOC under natural gradients was revealed. The relative contributions and interactions of different influences at large scales were considered. We hypothesized that soil clay minerals had an important impact on MOC content under large-scale studies. The correlation between soil clay minerals and MOC content was the result of multiple environmental variables. The purpose of the study was the following: (1) to determine the characteristics of the clay mineral composition and MOC of natural grassland transects in China; (2) to determine the correlation between clay minerals and MOC content in grassland soil on a large scale; (3) to quantitatively assess the important effects of environmental variables (climate, soil, bacteria) on clay minerals and MOC.