Effects of dynamic factors of erosion on soil nitrogen and phosphorus loss under freeze-thaw conditions

Soil erosion directly leads to the decline of soil fertility, and under hydraulic erosion, the soil nitrogen and phosphorus released by hydrolysis enter the adjacent water body along with surface runoff and soil erosion. This causes potential hazards such as eutrophication and river siltation in the watershed, seriously affecting the safety of the ecological environment. The mechanism of action of the dynamic factors of erosion on nitrogen (N) and phosphorus (P) loss remains unclear. In this study, a series of laboratory experiments were carried out to characterize the N and P loss and its influencing factors under freeze-thaw conditions. Two treatments (i.e., LC: loess control and FT: freeze-thaw treatment) and five soil water contents on a gravimetric basis (SWCs) (i.e., 10%, 15%, 20%, 25% and 30%) were considered. The results showed that the total runoff was higher under 30% SWC and lower under 20% SWC for the LC and FT treatments. The freeze-thaw action caused higher sediment loss under low water content (10% and 15%). The runoff-associated total nitrogen (RTN), runoff-associated total phosphorus (RTP), and sediment-associated total phosphorus (STP) loss rate showed a larger fluctuation for FT than for LC. Freeze-thaw action not only caused the instability of the N and P loss behavior but also caused increased diversity among individual samples. The soil erodibility, runoff energy and runoff power were important dynamic factors associated with erosion, and the freeze-thaw action had a very large impact on these factors. Under freeze-thaw action, the effect of dynamic factors on phosphorus and nitrogen loss was significantly enhanced. For the LC treatments, the SWC could explain 60% of the variation in RTN loss and 63% of the variation in RTP loss; the runoff explained 90% of the variation in STN loss and the runoff time explained 97% of the variation in STP loss. For the FT treatments, the runoff time explained 63% of the variation in STN loss and 53% of the variation in STP loss. The results enable us to understand further the relationship between dynamic factors of rainfall erosion and nitrogen and phosphorus loss under freeze-thaw conditions.