Effects Of Freeze-thaw On Farmland Wind Erosion In Chinese Thick Mollisol Region

In Chinese thick layer Mollisol region,the windy weather coincides with soil freeze-thaw in the spring,which leads to an important impact of freeze-thaw on farmland wind erosion.However,there are few studies on the effects of freeze-thaw on wind erosion in black soil region.Therefore,based on simulated freeze-thaw and wind tunnel experiments,combined with the soil anti-erodibility indices and the soil erodibility index,this thesis studied the effects of the freeze-thaw action on farmland wind erosion,quantified the contribution of soil water content,freeze-thaw cycle and their interaction to wind erosion,analyzed the effects of freezethaw on wind erosion with total crop straw returning and ploughing into soil,and revealed the mechanism of the influence of freeze-thaw action on farmland wind erosion preliminarily.It provides important scientific basis for the prevention of soil erosion and the protection of black soil resources in Northeastern China.The main conclusions are as follows:(1)Previous soil freeze-thaw action increased farmland wind erosion intensity and sediment delivery rate significantly.At the experimental conditions,wind erosion intensity increased by 23.5%-404.2%(P<0.05),and the average sediment delivery rate increased by 59.1%-305.3%(P<0.05),compared with the control treatment.Moreover,the effects of previous soil freezethaw on wind erosion intensity and sediment delivery rate were related to initial soil water content and wind speed.In the experimental treatments with previous soil freeze-thaw action,wind erosion intensity and sediment delivery rate were the largest when the initial soil water content was 16.5%.In addition,the previous soil freeze-thaw action increased the sediment delivery height by up to 8 cm.(2)The freeze-thaw cycle has an important effect on wind erosion intensity and sediment delivery rate.As the number of freeze-thaw cycle increased,wind erosion intensity and the average sediment delivery rate increased and the increments gradually slowed down.When the number of freeze-thaw cycle increased from 1 to 7,the wind erosion intensity increased by 8.3%-47.3% and 40.1%-156.0%,respectively,and the average sediment delivery rate increased by 5.3%-17.3% and 33.2%-49.0% when the initial soil water content was 16.5% and 33.0%,respectively.(3)The contribution of initial soil water content,wind speed and their interaction to wind erosion intensity were significantly influenced by previous freeze-thaw action.The contribution rate of wind speed to wind erosion intensity decreased from 74.6% to 34.8% due to previous soil freeze-thaw action,while the sum of the soil water content and the interaction of soil water content and wind speed contribution rates increased from 17.7% to 62.8%.The contribution rate of soil water content to sediment delivery rate increased from 23.1% to 39.8% by previous soil freeze-thaw action,while the contribution rates of wind speed and the interaction of wind speed and soil water content decreased from 39.2% to 25.4% and 37.4% to 33.9%.In addition,the contribution rate of freeze-thaw cycle to wind erosion intensity increased from 12.6% to 28.8%,while the contribution rate of wind speed and the interaction of freeze-thaw cycle and wind speed to wind erosion intensity decreased from 71.2% to 64.5% and 13.4% to 3.2% with the increase of soil water content.And the contribution rate of wind speed to sediment delivery rate increased from 73.8% to 82.9%,while the contribution rate of freeze-thaw cycle and the interaction of freeze-thaw cycle and wind speed to sediment delivery rate decreased from 13.3% to 7.1% and 10.8% to 7.9%.(4)With the treatment of total crop straw returning and ploughing into soil(the proportion of straw was 4% of the dry soil),the freeze-thaw effect increased the wind erosion intensity by 18.6%-283.3%(P<0.05)and increased the average sediment delivery rate by 48.9%-347.9%(P<0.05).In addition,the increment of wind erosion intensity with total crop straw returning and ploughing into soil was smaller than that without straw returning after freeze-thaw.Compared with the no straw returning treatment,the wind erosion intensity and sediment delivery rate increased basically and the increments were 5.3%-159.8% and 32.5%-763.8% with total crop straw returning and ploughing into soil.(5)The previous soil freeze-thaw action decreased the soil anti-erodibility,and it increased the soil erodibility,leading to the increase of wind erosion intensity.Compared with the treatment without previous freeze-thaw action,soil anti-erodibility indices such as soil bulk density,soil shear strength and soil hardness reduced by 5.9%-7.3%,17.5%-39.1% and 46.3%-76.7% respectively with seven freeze-thaw cycles.The proportion of soil dry aggregate at the size of >5 mm decreased by 11.7%-30.4%,while the proportion of soil dry aggregate at the size of <1 mm increased by 0.7%-12.7%;the mean weight diameter of soil water stable aggregates decreased by 1.3%-41.7%,while the soil disintegration rate of soil erodibility index increased by 1284.6%-3780.0%.In addition,with the increase of freeze-thaw cycles,the decrement of soil anti-erodibility indices also showed a downward trend,which was also the main reason that the increment of soil wind erosion intensity decreased with the increase of freeze-thaw cycles.With total crop straw returning and ploughing into soil,the soil bulk density decreased by 13.3%-30.3%(P<0.05),the soil shear strength decreased by 2.9%-31.3% and the soil hardness decreased by 20.0%-74.4% compared with no straw returning treatment.Therefore,farmland wind erosion tends to be strengthened with total crop straw returning and ploughing into soil.