Soil Aggregate Stability Under Freeze–Thaw Cycles in Mollisols as Evidenced by 15N Distribution

Freeze–thaw cycles (FTCs) influence soil nitrogen (N) dynamics and soil aggregate stability. However, the driving mechanism affecting aggregate stability from the combined perspective of N components and N distribution by 15N tracing technology in both bulk soils and soil aggregates remains worth exploring. This study took the farmland Mollisols in Hailun City, Heilongjiang Province, as the research object, and investigated the variations in soil N components and aggregate stability across five freeze–thaw frequencies (1, 3, 5, 9, and 17 cycles) under three freeze–thaw temperatures (−9 °C/5 °C, −18 °C/5 °C, and −26 °C/5 °C) using 15N tracing technology. The results demonstrated that freeze–thaw frequency and temperature both influenced aggregate stability. Specifically, with the increase in freeze–thaw frequency, soil aggregate stability was reduced through decreasing the proportion of macroaggregates (2–0.25 mm), increasing the proportion of silt + clay fractions (<0.053 mm), and reducing the total N (TN) content of silt + clay fractions under higher freezing temperature (−9 °C/5 °C). In contrast, for lower freezing temperature (−18 °C/5 °C and −26 °C/5 °C), the increased freeze–thaw frequency enhances soil aggregate stability by decreasing the proportion of silt + clay fractions, increasing the proportion of microaggregates (0.25–0.053 mm), and reducing the TN contents of microaggregates and silt + clay fractions. These findings are essential for developing strategies to mitigate the adverse effects of FTCs on soil quality and ecosystem functions in cold regions.