Effect of soil erodibility on nitrogen and phosphorus loss under condition of freeze-thaw

Abstract: The freeze-thaw processes affect an area of 46.3% in China. The process of soil nutrient loss under freezing and thawing was seldom been studied. Under the condition of rainfall simulation, the characteristics of soil and nutrients loss under different soil water content (SWC) conditions were studied. The effects of freeze-thaw and erodibility on total nitrogen (TN) and total phosphorus (TP) losses on the loess slope were analyzed. Loess slope (LS) and freeze-thawed slope (FTS) were set, and we studied five SWCs, between 10% and 30%. The results showed that there was a significant difference in runoff/sediment associated TN and TP concentrations under different SWCs for two slopes (P<0.05). Largest runoff-associated TN and TP losses were found when the SWC was 30% and the largest sediment-associated TN and TP losses were found when the SWC was 10% in the two slopes. The sediment-associated nutrient losses dominated the total nutrient loss in all treatments, and when the SWC was 20%, average sediment-associated TN and TP losses occupied 99% of totals in the LS and FTS. The K values decreased firstly and then increased in both LS and FTS and in low SWC area. FTS soil erodibility was greater than that of the LS with an average ratio of 1.4. The influence of runoff on sediment was positively linear. The absolute slope of the regression line between runoff rate and sediment yield rate was suitable as a soil erodibility indicator. The runoff-associated and sediment-associated total TN and TP loss rates increased linearly with runoff rate and sediment yield rate under different SWCs for the two slopes. The runoff-associated TN and TP losses were mainly influenced by runoff rate, and were weakly affected by soil erodibility (P> 0.05). However, soil erodibility significantly influenced sediment-associated TN and TP losses. Since the sediment-associated TN and TP losses dominated the total TN and TP losses for the two slopes, soil erodibility also exhibited a significant influence on total TN and TP losses. The freeze-thaw effect increased total loss of TN by 1.6 times when the SWC was 10%. It increased total TP losses by 5 and 1.9 times when the SWC was 10% and 15%, respectively. Considering the SWC in the loess region was generally no more than 15%, the freeze-thaw would promote the loss of nitrogen and phosphorus in the loess region. The effect of soil erodibility on nitrogen loss was relevant to freezing and thawing. The nitrogen loss increased first and then decreased with the increase of soil erodibility before freezing and thawing. The nitrogen loss increased with the soil erodibility increased after freezing and thawing. While the effect of soil erodibility on phosphorus loss showed no relationship with freeze-thawing. The loss of phosphorus always increased with the soil erodibility increased. Therefore, a series of ecological construction measures should take to control soil erosion and reduce soil erodibility in order to reduce the nutrients loss in the loess region. The results provide a better understanding of soil and nutrient loss mechanisms under freeze-thaw conditions in the loess slope.