## 4 Discussion  
### Carbon Storage and Mineralization in Soil Aggregates  
Differences in environmental factors such as dry-wet and freeze-thaw cycles led to variations in aggregate formation and the stability of soil aggregates along the precipitation gradient. Regions with a more humid climate and higher temperatures were conducive to the formation and stabilization of macro-aggregates. Soil aggregates are the fundamental units of soil structure and their quantity and quality determine the physicochemical properties and coherence of the soil. In this study, the 1-2, 0.5-1, and 0.25-0.5 mm aggregate fractions decreased with a reduction in the precipitation gradient. In comparison, the <0.25 mm aggregate fraction increased, indicating that precipitation is a significant factor affecting aggregate formation [27]. It may be due to the provision of moisture required by the soil. Adequate moisture facilitates microbial activity and organic matter decomposition, key to soil aggregate formation. Additionally, moisture promoted the bonding of soil particles, aiding in aggregate formation and stability. Furthermore, temperature affects microbial activity in the soil; optimal temperatures can enhance microbial growth and organic matter decomposition [28], which is beneficial for soil aggregate formation. Precipitation and temperature together influenced plant growth and root development, thereby affecting soil aggregate formation. Plant roots can promote the binding of soil particles, while the decomposition of plant residues provides organic matter, increasing soil structural stability. Previous studies have shown that different soil types (such as sand, loam, and clay) have varying physical and chemical properties, which also affect the binding capacity of soil particles [29]. Additionally, organic matter was one of the key factors in soil aggregate formation, with a high organic matter content contributing to enhanced soil structural stability [30]. In this study, we found that larger aggregates had a higher organic carbon content than smaller ones, consistent with previous research conclusions [31]. Large aggregates provided a microenvironment where organic matter was protected from microbial decomposition. This physical isolation reduced the chances of microbial contact with organic matter, slowing down the decomposition rate. The microenvironment in large aggregates was favorable for specific microbial communities that participate in the transformation and stabilization of organic matter. This microbial activity contributed to the accumulation of organic matter within aggregates. The growth of plant roots in soil leads to the secretion of organic substances that promote the formation of soil aggregates. Large aggregates often form in the rhizosphere; thus, they may contain higher levels of root exudates and, consequently, enriched organic carbon. Additionally, soil mineral particles and clay particles in large aggregates may have a strong adsorption capacity for organic matter, which helps to fix and retain organic carbon [32]. SOC mineralization is a crucial process in the balance of soil carbon pools and its study helps reveal the dynamics of soil carbon storage. The rate of mineralization indicates the speed of organic carbon decomposition. Overall, aggregates of different sizes exhibit a high rate of organic carbon mineralization in the initial stages of incubation, followed by a rapid decline. The mineralization rate slowed over time, becoming relatively stable, which was associated with soil microbial activity and the ease of carbon mineralization [33]. Organic carbon was decomposed quickly into less efficiently mineralizable parts; most organic carbon decomposed during 60 days of indoor incubation is the easily mineralizable type. Initially, soil and aggregate samples, moistened to 60% of field capacity, stimulated microbial activity. Easily mineralizable organic carbon, such as sugars and proteins, was abundant and rapidly decomposed by microbes, releasing large amounts of CO2.