The effect of soil water content and erodibility on losses of available nitrogen and phosphorus in simulated freeze-thaw conditions

The effects of soil water content (SWC) and erodibility on available nutrient losses, as well as the influence of freeze-thaw on available nitrogen (AN) and phosphorus (AP) losses from loess soils have rarely been considered. We report on a series of laboratory simulation experiments conducted to determine SWC and soil erodibility effects on AN and AP losses under freeze-thaw conditions. Effect of freeze/thaw compared to unfrozen, two treatments were set (LS: Loess that unfrozen; FTS: Freeze-thawed a loess), and we studied five SWCs, between 10% and 30%, and we divided SWCs into two groups: lower water content areas (10% and 15%), higher water content areas (20%, 25%, and 30%). Overall, significant differences in runoff/sediment associated AN and AP concentrations of different SWCs for two treatments (p < 0.05) were noted, while the relationship between SWC and AN and AP loss show a quadratic function change trend (R2 > 0.8). Largest runoff-associated AN and AP losses were found when the SWC was 30% and the largest sediment-associated AN and AP losses were found when the SWC was 10% in the two treatments. The soil erodibility factor (K) of the LS was less than that of the FTS when the SWC was low, and greater in a high water content area. The influence of runoff on sediment was positively linear. The absolute slope of the regression line between runoff rate and sediment yield rate is shown to be suitable as a soil erodibility indicator, while runoff-associated AN losses are mainly controlled by runoff rate, and are weakly affected by soil erodibility (p > 0.05). However, soil erodibility significantly influenced sediment-associated AP losses (p < 0.01), best described by a positive logarithmic relationship. Since the sediment-associated AP losses dominated the total AP losses for the two treatments, soil erodibility also exhibited a significant influence on total AP losses (p < 0.01). The freeze-thaw effect increased the total AN loss when the SWC was 15%, 20%, and 30%, and increased the total AP loss when it was 10% and 15%. The results of this study provide a better understanding of soil and available nutrient loss mechanisms under freeze-thaw conditions.