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

Freeze-thaw cycles (FTCs) influence soil nitrogen dynamics and soil aggregate stability. However, the driving mechanism affecting aggregate stability from the combined perspective of nitrogen (N) components and nitrogen 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 of soil nitrogen components and aggregate stability across five freeze-thaw frequencies (1, 3, 5, 9, and 17 cycles) under three freeze-thaw temperatures (-9 ℃/5 ℃, -18 ℃/5 ℃, and -26 ℃/5 ℃) using 15N tracing technology. Results demonstrated that freeze-thaw frequency and temperature both influenced aggregate stability in Mollisols. Specifically, with the increase of 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 TN content of silt + clay fractions and ¹⁵N-labeled TN content of macroaggregates, under higher freezing temperature (-9 ℃/5 ℃). In contrast, for lower freezing temperature (-18 ℃/5 ℃ and -26 ℃/5 ℃), 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 and ¹⁵N-labeled TN contents of microaggregates and silt + clay fractions. Therefore, in regions with higher freezing temperature, conservation tillage practices are recommended to maintain soil aggregate stability and preserve soil structure by minimizing exposure to additional freeze-thaw cycles. In regions with lower freezing temperature, it is advisable to limit the application of excessive nitrogen fertilizers to mitigate potential nitrogen losses. These findings are essential for developing strategies to mitigate the adverse effects of FTCs on soil quality and ecosystem functions in cold regions.