Changes in soil TN and TP induced by freeze-thaw effect and the driving mechanism of environmental factors in a small watershed

Freeze-thaw significantly affect soil total nitrogen (TN) and total phosphorus (TP), yet their responses and driving mechanism at the small watershed scale remain unclear. We analyzed 184 soil samples (0-20 cm) from a typical Mollisols watershed (1.47 km 2 ) before and after freeze-thaw in Northeast China to assess spatial variations in TN and TP and to identify the key environmental factors driving their dynamics. Results indicated that TN and TP showed high spatial heterogeneity. After freeze-thaw, TN and TP changes ranged from -0.57 to +0.31 g kg -1 and -0.49 to +0.35 g kg -1 , respectively. Overall, TN decreased significantly by 12.5%, while TP increased significantly by 10.5% (p<0.001). TN loss was highest in farmland, while TP increased significantly in farmland and gullies. Moreover, changes in TN and TP increased with distance from gullies. The combined effects of gully distance, topography, and soil properties mainly drove these changes, with gully distance and soil properties independently explaining 7.9% and 7.0% of the variation, respectively. However, the driving factors of TN and TP variations differed. Gully distance was the primary factor negatively affecting TN change (path coefficient = -0.42) by limiting its migration, while soil erodibility contributed significantly as well. TP change was driven by its content before freeze-thaw (path coefficient = -0.78) and jointly regulated by gully distance, snow cover and soil aggregate stability. Land use indirectly influenced the changes in TN and TP through changing soil properties. The results provide a scientific basis for managing soil nitrogen and phosphorus during freeze-thaw.