| The liquid water in the soil freezes into ice in cold climates,and melts when the temperature rises.This freezing and melting process makes the wedge pressure of the water film in soil fine particles and mineral micro-fractures change,which leads to the soill aggregates and minerals were destroyed.The freeze-thaw cycle causes the colloid and soil clay aggregate into soil micro-aggregate,and the dispersed micro-particles re-aggregate into large particles.Soil aggregat’es and soil particles were destroyed and then re-aggregated.This physical properties change affects soil chemical behavior,causing soil microbial activity and organic matter changes,and resulting in soil nutrient transport and transformation processes changes.In order to reveal the soil nitrogen and phosphorus migration process and its action mechanism under freezing-thawing conditions,the simulated rainfall experiment was conducted to study the rainfall-runoff,sediment yield and infiltration characteristics on loess slope(LS)and frozen-thaw slope(FTS)under different soil water content conditions.The nitrogen and phosphorus migration process and its loss pathway was analyzed.As the same time,combined with erosion dynamics factor and soil erodibility,the action mechanism of the nitrogen and phosphorus migration was analyzed under freezing-thawing conditions,and clarified nitrogen and phosphorus migration arnd their response time on runoff and sediment.The main research results as follows:(1)The runoff and sediment yield characteristics was been clear during rainfall time,and the effects of soil moisture and freeze-thaw on the runoff and sediment yield process and the critical water content were determined.Under freeze-thaw effect,the initial runoff increase rate was slow,and fluctuation degree of sediment yield rate was reduced.There was a critical value for the influence of soil moisture on runoff and sediment yield.The calculation shows that the critical water content was 19%on LS,and 16%～21%on FTS.The f,reeze-thaw effect changed.the infiltration process,therefore,after freeze-thaw,the runoff time,runoff and sediment yield loss also changed.When the water content was 10%,the FTS runoff time was earlier 3.17%.than LS,the runoff loss increased by 7.47%,the sediment yield increased by 42.55%;when the water content was 25%,the FTS runoff time later 9.73%than LS,the runoff loss decreased by 7.29%,and the sediment yield decreased by 28.19%.(2)The nitrogen loss pathway along with runoff and sediment,and the effects of soil moisture and freeze-thaw on nitrogen loss characteristics were determined.The peaks time of nitrogen loss rate on the LS and FTS were different under different water contents.The freeze-thaw effect increased the peaks appear frequency and prolong the time in sediment-nitrogen loss,and increased critical water content which soil moisture affected nitrogen loss.The critical soil water content on the LS was 11%～15%,and the freeze-thaw slope was 17%～19%.The freeze-thaw effect increased total nitrogen(TN)loss by 4%to 20%,and decreased the ammonium nitrogen(AN)loss by 6%to 21%.The nitrogen loss with runoff and sediment was mainly organic nitrogen,and the proportion of inorganic nitrogen was small.Especially in sediment-nitrogen loss,the inorganic nitrogen loss is less than 5%.The different nitrogen components loss pathways were different on the slope.The total nitrogen(TN)was mainly loss with sediment,and the loss rate accounts for 78%to 90%;the nitrate nitrogen(NN)was mainly loss with runoff,and accounts for 86%～96%.However,the ammonium nitrogen(AN)was mainly loss with sediment,and accounts for 52%to 66%of the total loss when the soil water content<the field-moisture capacity;and the AN mainly loses with runoff,and accounts for 52%to 81%of the total loss when the soil water content>the field-moisture capacity.(3)The phosphorus main loss pathways with runoff and sediment,and the effects of soil moisture and freeze-thaw on phosphorus loss were clarified.The freeze-thaw effect makes the fluctuation of the RTP loss rate smaller,and the difference in peak value in different water contents was more significant.The influence of soil moisture on the phosphorus has a critical water content.It is found that the critical water content was 13%-18%on LS,while was 17.5%-22.7%on FTS.The phosphorus loss with runoff was mainly water-soluble available phosphorus,accounting for more than 55%of phosphorus loss.However,when the water content was 20%,the phosphorus loss was mainly in the form of granular mineral phosphorus,and accounting for more than 63%of the total phosphorus loss.The total phosphorus(TP)was mainly loss with sediment,and accounts for 93%to 97%of the total loss.When the water content is 10%,the freeze-thaw effect increases the phosphorus loss by 12%to 56%.However,when the water content>the field water holding capacity,the freeze-thaw effect will reduce the risk of phosphorus loss and reduced by 13%.(4)Freeze-thaw effect changes the relationship between runoff rate and sediment yield,but has no significant effect on the relationship between cumulative runoff and cumulative sediment yield.At high water content,the freeze-thaw effect accelerates the RAN loss rate with runoff loss,and at low water content,it slows down.Under Freeze-thaw effect,the linear relationship between RTN loss rate and runoff loss rate changed from positive correlation to negative correlation.The phosphorus loss rate on FTS was greater than that of LS;and at low water content,the correlation between RAP and runoff rate become worse,but at high water content the correlation between them getting better.The effect of soil moisture on the relationship between runoff and sediment yield and nitrogen loss can be characterized by quadratic function,but this relationship is not applicable under freeze-thaw action.(5)There exist break point in nitrogen and phosphorus loss process,and there has hysteresis effect on the response of nitrogen and phosphorus migration process to soil and water loss.Under non-freeze-thaw conditions,the break point in nitrogen and phosphorus loss was occurred in 25～30 min,their relationship changed also in 25～30 min.The response of nitrogen migration to runoff was sensitive,but to sediment yield was slowness and exist 5～30 min lag time.The response of phosphorus migration to sediment yield was sensitive,but to runoff was slowness and exist 30～40 min lag time when the soil water content>the field-moisture capacity.The break point in nitrogen and phosphorus loss was occurred in 25～3 5 min,and the relationship between nitrogen-phosphorus loss and runoff-sediment loss was changed in 25～45 min.The response of nitrogen migration to runoff and sediment yield was both slowness and exist 30～45 min and 10～30 min lag time,respectively.The response of phosphorus migration to sediment yield was sensitive,but to runoff was slowness and exist 15～20 min lag time when the soil water content<the field-moisture capacity.Therefore,under non-freeze-thaw conditions,the mid-rainfall time was a critical period for nitrogen and phosphorus loss on the slope;but the critical period under freeze-thaw conditions was middle and late rainfall time.The nitrogen and phosphorus loss not only related to the runoff and sediment yield,but also closely with the micro-topography.In addition,the micro-topography variation more diversified and it had more complicated impact on the nitrogen and phosphorus migration process under the freeze-thaw conditions than non-freeze-thaw conditions.(6)The freeze-thaw action affected the relationship between erosion dynamics and soil nitrogen and phosphorus loss.The soil erodibility increased by 43%when the soil water content was 10%.The freeze-thaw action effect increases the runoff energy by 8.7%and the runoff power by 16.42%,while reduced soil erodibility by 28.19%when the water content was 25%.After freezing and thawing,the effects degree of runoff,infiltration,runoff energy and runoff power on the runoff-nitrogen loss increased,while soil water content on it decreased.At the same time,the effects degree of runoff time,runoff energy and runoff power on sediment-nitrogen loss increased,while the infiltration decreased.The effect of soil water content on sediment-phosphorus decreased,but the runoff power and runoff energy on it increased.Therefore,after freezing and thawing,the effects of runoff power and runoff energy on nitrogen and phosphorus was enhanced. |
PDF Full Text Request