Soil Aggregate Stability and Organic Carbon Content among Different Forest Types in Temperate Ecosystems in Northeastern China

In this study, the AAOC levels of five forest types decreased with increasing soil depth. This trend can be attributed to the natural state of the study area as a forest with minimal human disturbance and abundant understory vegetation. Most fallen branches and leaves accumulate in the topsoil, providing a material basis for microbial activity through decomposition and the formation of a significant amount of humus. This leads to the surface accumulation of SOC. A higher AAOC, SOC, and SOCS were observed in birch and black birch forests (Figure 3, Table 2). This may be due to the fact that compared to coniferous forests and mixed forests, the microbial secretion in broad-leaved forests and the soil beneath them exhibit higher substance secretion and easier litter decomposition, resulting in a higher organic matter content in the topsoil and a greater carbon content in the soil [39]. This indicates that due to their strong carbon sequestration capabilities, birch and black birch species contribute to SOC accumulation, providing more carbon sources for soil aggregates. Our findings demonstrate that there is a highly significant positive correlation between the AAOC level of fine macro-aggregates (0.25–2 mm) and the corresponding class content, indicating that the class composition of soil aggregates has a greater influence on the AAOC. The majority of SOC is concentrated in the fine macro-aggregates (0.25–2 mm), suggesting that these aggregates have the strongest ability to accumulate SOC and serve as the main carrier of soil organic carbon. This may be attributed to the forest ecosystem offering a favorable habitat for soil organisms and microorganisms; their increased activity range leads to an accelerated decomposition rate, and the presence of a greater abundance of fungal hyphae in macro-aggregates compared to other aggregate classes enhances the concentration of organic carbon within these aggregates during decomposition [40]. The prevailing dominance of fine macro-aggregates (0.25–2 mm) in the studied area indicates their potential superiority in carbon sequestration over micro-aggregates. Additionally, fine macro-aggregates (0.25–2 mm) offer a larger surface area for organic matter attachment, resulting in a higher SOC. Dorji et al. and Wu et al. also reported that macro-aggregates had a higher organic carbon content and stronger carbon sequestration capabilities than micro-aggregates in different forest types [2,41], which is consistent with the results of our study. On the other hand, micro-aggregates form macro-aggregates through the adhesion of organic matter, resulting in a lower SOC compared to macro-aggregates. As the size of soil aggregates decreases, the turnover rate of organic carbon gradually slows down. Larger soil aggregates have a greater capacity for the microbial decomposition of organic carbon, as microorganisms can only access the aggregated structure for carbon decomposition through the secretion of extracellular enzymes and the expenditure of significant energy [42]. Some studies have indicated that the AAOC levels of silt and clay fractions (43]. Mangalassery et al. also suggested that micro-aggregates have more classes and stronger adsorption capabilities and they can more efficiently protect soil organic matter by increasing the inaccessibility to microorganisms and through mineral interaction [44]. Additionally, their aggregate structure is less influenced by external conditions [45]. Micro-aggregates contain a higher mineral content, thereby providing stronger physical protection for the stability of organic carbon [46]. Indeed, different climate and soil conditions can indeed lead to variations in the decomposition characteristics of soil microbial communities [47], thereby affecting the changes in organic carbon content within different aggregate classes. By studying the distribution of SOC among different aggregates classes, further insights can be gained into the protective mechanisms of the AAOC. In this study, the AAOCCR from different forest types exhibited similar patterns to the variation in aggregate classes within the soil profile. At the 0–10 cm soil depth, the AAOC of coarse macro-aggregates (>2 mm) was comparable to that of fine macro-aggregates (0.25–2 mm). However, the AAOCCR of the coarse macro-aggregates (>2 mm) was significantly lower than that of the fine macro-aggregates (0.25–2 mm). The influence of different soil depths on the stability of aggregates and the SOC is likely to depend on the input of carbon. As surface litter enters deeper soil layers, the input of carbon will decrease.