Distribution characteristics of soil aggregates and organic carbon in the frost-thaw affected regions of the Yangtze River source area, Tuotuo River, Qinghai-Xizang Plateau

Soil aggregates are the fundamental structural units of soil. The stability of soil aggregates and the content of organic carbon (Corg) are key indicators of the physical protection of Corg within soil aggregates. Freeze-thaw cycles can alter the structure and composition of soil aggregates, reducing their stability. This study investigates the distribution patterns, stability, and changes in Corg content within soil aggregates, as well as their interrelationships, in the frost-thaw affected regions of the Yangtze River source area, Tuo River, Qinghai-Xizang Plateau. The study focuses on four soil types: alpine meadow soil (AMS), alpine desert grassland soil (ADGS), alpine marsh meadow soil (AMMS), and alpine desert sandy soil (ADSS). The dry sieving method was employed to determine the size distribution of soil aggregates. Stability indices of aggregates and the contribution of Corg in aggregate size fractions to total soil Corg were analyzed to assess aggregate stability. Correlation analysis was conducted to reveal the relationships between soil aggregates, Corg in aggregate size fractions, and soil stability. The results indicate that AMS and ADGS had the highest proportions of the largest aggregate fraction F6 (>2 mm), accounting for 34.42% and 33.65%, respectively. AMMS exhibited the highest proportion of the smallest aggregate fraction F1 (R0.25) were highest for ADGS, indicating the best aggregate stability, whereas ADSS exhibited the lowest stability. The Corg content in soil aggregates showed a distribution pattern of decreasing, then increasing, and finally decreasing again with increasing aggregate size. For AMS and ADGS, large aggregates (>0.25 mm) had the highest Corg contribution, while for the other two soil types, micro-aggregates (R0.25, MWD and GMD. Long-term freeze-thaw cycles in the study area caused a transformation of large aggregates into smaller ones, leading to significant changes in aggregate stability and the distribution of Corg content within aggregates.