| As the main type of cultivated land resource in southwest China,purple soil slope cultivated land is mainly distributed in the hills and surrounding mountainous areas of the Sichuan Basin.It is an important production base for grain and agricultural products in southwest China,and also an important erosion zone in the Yangtze River basin.The slope farmland in purple soil is frequently disturbed by human activities,and has the characteristics of periodicity and high intensity.The production process of slope farmland has a strong impact on the physical and chemical properties of soil.In addition to the impact of artificial farming activities,the soil properties of the topsoil have decreased,the risk of erosion has gradually increased,and the crop yield is low and unstable,which seriously threatens the quality of the topsoil of purple soil farmland,restricting the sustainable utilization and development of slope farmland resources and regional agriculture.This paper takes the topsoil of typical purple soil slope farmland in the Three Gorges Reservoir Area as the research object.Through setting up five different erosion levels(erosion 0 cm(S-0),erosion 5 cm(S-5),erosion 10 cm(S-10),erosion 15 cm(S-15),erosion 20 cm(S-20)),and two soil management methods（non fertilization（CK）,chemical fertilizer（F））),the field positioning shovel soil erosion simulation plots were established,The impact of soil erosion on the degradation of topsoil quality and the restoration effect of F measures on the physical and chemical properties of topsoil at different erosion levels were analyzed.The interannual variation characteristics of the physical and chemical properties of eroded topsoil were revealed.Based on the degradation process of soil quality at the eroded topsoil on slope farmland,restoration approaches were proposed based on the degradation process of soil quality at the eroded topsoil.The main conclusions are as follows:（1）The physical properties of the topsoil on purple soil slope farmland gradually deteriorate with the intensification of erosion,and F measures have a certain restoration effect on the physical properties of the eroded topsoil on purple soil slope farmland.Under CK,the maximum decrease in saturated hydraulic conductivity was observed,with S-5 to S-20 decreasing by 33.86%to 48.95%compared to S-0.Under F measures,the clay content,silt content,porosity,and saturated hydraulic conductivity of eroded topsoil increased by 2.00%to 27.30%compared to CK,respectively,while the silt content and unit weight decreased by 0.18%to 27.72%compared to CK;Under CK,the saturated hydraulic conductivity in the profile of 30-40 cm is significantly lower than that of the surface layer,and the S-5～S-20 is 73.39%～79.41%lower than that of S-0.Under F measures,the silt content,clay content,porosity,and saturated hydraulic conductivity in the profile of 0-20 cm of eroded topsoil have changed by-7.19%～34.56%compared to CK,respectively.The sand content and soil bulk density profile distribution have changed by-34.72%～16.66%compared to CK;After four consecutive years of management,the sand content of S-20 soil under CK increased the fastest,with an annual average growth of 3.66%.Under F measures,the saturated hydraulic conductivity of eroded topsoil increased by 4.67%to 6.22%annually,and the annual variation range of other indicators was distributed between-10.48%and 7.30%;Under CK,soil bulk density increased by3.19%to 5.49%,saturated hydraulic conductivity decreased by 33.92%to 48.99%,and crop yield decreased by 7.52%to 47.37%.Under F measures,soil bulk density decreased by 0.18%to 3.78%compared to CK,saturated hydraulic conductivity increased by 20.03%to 48.95%,and crop yield increased by 15.52%to 55.71%.Overall,as the degree of erosion intensifies,the phenomenon of coarsening and ossification of topsoil increases,while the bulk density increases,porosity and saturated hydraulic conductivity decrease,and the infiltration performance of topsoil gradually decreases;However,F measure has a recovery effect on the physical properties of eroded topsoil on purple soil slope farmland,and has a good recovery effect on the soil texture and saturated hydraulic conductivity of 0-20 cm of the profile.（2）The chemical properties of the topsoil on purple soil slope farmland gradually deteriorate with the intensification of erosion,and F measures have a certain restoration effect on the chemical properties of the eroded topsoil on purple soil slope farmland..Under CK,the soil total nitrogen decreased the most,with S-5～S-20 decreasing 10.63%～41.38%compared to S-0.Under F measures,the soil organic matter,total nitrogen,total phosphorus,and available phosphorus in the eroded topsoil increased 12.69%～46.66%compared to CK,respectively;Under CK,the soil available phosphorus saturated hydraulic conductivity decreased the most at 30-40 cm of the profile compared to the surface layer,with a decrease of 56.56%to 61.33%from S-5 to S-20 compared to S-0.Under F measures,the soil organic matter,total nitrogen,total phosphorus,and available phosphorus in 0-20cm of the eroded topsoil profile changed by-11.38%to 50.02%compared to CK,respectively;After four consecutive years of management,the total potassium in the S-20 soil under CK increased the fastest,with an annual average growth of 0.63%.Under F measures,the organic matter in the eroded topsoil increased by 4.73%to 10.05%annually,and the annual change range of other indicators was distributed between-1.62%and 7.69%;Under CK,soil organic matter,total nitrogen,and available phosphorus decreased by 6.86%to 28.70%,10.63%to 41.38%,and 9.54%to 31.00%,respectively,and crop yield decreased by 7.52%to 47.37%.Under F measures,soil organic matter,total nitrogen,and available phosphorus increased by 20.20%to 46.66%,13.52%to 34.50%,4.34%to 16.22%,respectively,compared to CK,and crop yield increased by 15.52%to 55.71%.Overall,as the degree of erosion intensifies,nutrients in the topsoil continue to lose,and the phenomenon of soil impoverishment worsens;However,the F measure has a recovery effect on the chemical properties of the eroded topsoil in the slope farmland of purple soil,and has a good recovery effect on the organic matter,total nitrogen,total phosphorus,and available phosphorus in the 0-20 cm soil profile.（3）The erosion topsoil quality and crop yield of purple soil slope farmland decrease with the intensification of erosion,and the restoration of erosion topsoil quality can be carried out from two perspectives:topsoil structure restoration and topsoil function restoration.The soil saturated hydraulic conductivity of purple soil slope farmland degrades most along the tilth profile,and decreases by 2.23%to 3.01%per 1 cm of erosion on average;The soil attribute area（SSQI）at each profile level of purple soil slope farmland decreased with the aggravation of erosion,with an average decrease of 1.87%to 4.10%per 1 cm of erosion;The quality index of tilth layer on slope farmland is S-0（0.41）>S-5（0.35）>S-10（0.31）>S-15（0.28）>S-20（0.22）.Among them,S-0 tilth layer belongs to a more reasonable tilth layer,while S-5～S-20 tilth layer belongs to an unreasonable tilth layer.The soil bulk density,saturated hydraulic conductivity,organic matter content,penetration resistance,and shear strength of S-0～S-20 tilth layer on slope farmland are not within the suitability threshold range;With the intensification of erosion,the topsoil quality index under CK decreased by 14.23%to46.37%,and the crop yield decreased by 7.52%to 47.37%;Under the F measure,the topsoil quality index increased by 20.00%to 48.95%compared to CK,and the crop yield increased by 15.52%to55.71%.The topsoil quality index on slope farmland matched the crop yield with the highest R2（R2=0.8752）,and the topsoil quality index had the best fitting effect on the crop yield;The restoration of eroded topsoil quality should be based on the restoration of topsoil structure or function,and the selection of subsoil tillage,soil reconstruction,and biochar and straw return should be made respectively. |
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