Soil Quality Characteristics And Comprehensive Evaluation Of Uncultivated Black Soil In Typical Black Soil Area Of Songnen Plain

As an important grain producing area,the black soil area Northeast China is seriously degraded by soil erosion and strong human activities.At present,a lot of work has been carried out on soil protection and land restoration effect evaluation in black soil area.However,most of the studies take natural restoration land or leisure land disturbed by different degrees as control,and rarely involve the background characteristics of uncultivated black soil quality,which cannot accurately quantify the level and degree of soil quality improvement.Therefore,in order to clarify the current situation of soil quality of uncultivated black soil,this paper takes Nenjiang,Keshan,Bei’an and Hailun in the typical black soil area of Songnen plain as the research area,and takes the uncultivated black soil（natural secondary forest and tessellated meadow）in each area as the research object.The methods of field sampling and indoor analysis were used to measure and analyze the soil physical,chemical and biological quality related indexes in the depth of 0-100 cm of uncultivated black soil,and the background characteristics of soil quality of uncultivated black soil in different regions were clarified.The improved analytic hierarchy process was used to assign weights to each index and calculate the comprehensive scores of soil quality in different regions.The soil quality of different regions was quantified,and the soil quality of uncultivated black soil in different regions was comprehensively evaluated.The main results of this study are as follows:（1）The physical quality characteristics of uncultivated black soil:the soil bulk density ranged from 0.51 to 1.55 g·cm^-3,which increased with the increase of soil depth,and the difference of soil bulk density in different regions below 10 cm was obvious.The total porosity,capillary porosity,saturated water holding capacity and capillary water holding capacity of soil decreased gradually with the increase of soil depth.The total porosity of surface layer in each area was 60～70%,and the porosity in deep soil did not change significantly.The saturated water holding capacity decreased significantly in the vertical direction,and there were significant differences in soil saturated water holding capacity in the same soil layer in different regions.Among them,Bei’an natural secondary forest and Nenjiang tessellated meadow have higher porosity and water holding capacity.The total content of silt and clay did not change much,and the overall content of sand in Bei’an natural secondary forest and Hailun tessellated meadow was significantly higher than that in other areas of the same vegetation type.The content,average weight diameter and geometric mean diameter of>0.25 mm water-stable aggregates in each area were generally larger in the topsoil and lower in the depth of 40～80 cm.（2）The chemical quality characteristics of uncultivated black soil:the soil p H ranged from5.20 to 6.44,showing strong to weak acidity.The contents of total nitrogen,total phosphorus,available nitrogen and organic carbon decreased gradually with the increase of soil depth,while total potassium,available phosphorus and available potassium were unevenly distributed in the vertical direction,and there was no obvious change rule;The soil total nitrogen content ranged from 0.60 to 10.55 g·kg^-1,and there were significant differences in total nitrogen between different regions;The soil total phosphorus content ranged from 0.34 to 1.41 g·kg^-1,and the content levels of Bei’an natural secondary forest and Nenjiang tessellated meadow were relatively high;The soil total potassium content in each region was relatively rich,ranging from17.21 to 31.25 g·kg^-1,among which the total potassium content in Helen natural secondary forest and Keshan tessellated meadow was higher.The soil available nitrogen content was the highest in Nenjiang,and the average contents of natural secondary forest and tessellated meadow were254.11 mg·kg^-1 and 296.66 mg·kg^-1,respectively;There were significant differences in soil available phosphorus content in different regions,among which the content levels of Bei’an natural secondary forest and Keshan tessellated meadow were relatively high;The content of available potassium in each region was the highest in Keshan,and the overall range from 108.41to 407.33 mg·kg^-1.The content of soil organic carbon in different regions ranged from 3.70 to125.48 g·kg^-1,with the highest content in Nenjiang and the lowest in Hailun;The proportion of easily oxidized organic carbon in the active components is the highest,and the proportion of each component in the total organic carbon does not change significantly in the vertical direction.（3）Soil biological quality characteristics of uncultivated black soil:The activities of soil invertase,urease,acid phosphatase and catalase were high in the surface layer and low in the deep layer,and there were significant differences between different regions.The soil invertase activity in each region showed a decreasing trend with the increase of soil depth.The activity of soil acid phosphatase had no obvious regularity in the vertical direction of each study area,and the activity of acid phosphatase in Keshan was higher as a whole.The soil catalase activity in each region decreased with the increase of soil depth,and the maximum value appeared in the0-10 cm topsoil,which was significantly higher than that in other soil layers.Keshan has higher catalase activity in topsoil and deep soil than other regions.（4）The improved analytic hierarchy process was used to quantitatively evaluate the uncultivated black soil in different regions.It was found that the soil quality score of each region decreased with the increase of soil depth,and the soil quality score of surface soil was significantly higher than that of deep soil.There was a certain gap in soil quality scores between different regions of the same soil layer.The order of soil quality comprehensive scores in natural secondary forests in each region was Keshan（0.47）,Nenjiang（0.45）,Bei’an（0.39）,and Hailun（0.36）;The order of comprehensive scores of soil quality in tessellated meadow was Keshan（0.47）,Nenjiang（0.45）,Beian（0.28）and Hailun（0.26）.According to the grading standard of soil quality in the study area,the soil quality of Keshan and Nenjiang is grade III,which is moderate,and the soil quality of Bei’an and Hailun is grade IV,which is relatively poor.In summary,the soil quality of uncultivated black soil in different regions of Songnen typical black soil area was significantly different.The soil physical,chemical and biological quality of each region decreased with the increase of depth in 0-100 cm soil depth,and the soil quality of Nenjiang and Keshan was significantly better than that of Bei’an and Hailun.Therefore,when carrying out land management and land restoration in the typical black soil area of Songnen plain,it is necessary to fully understand the differences in soil quality between different regions,formulate diversified standards and management measures,so as to improve the protection effect of black soil and the efficiency of land restoration,and strengthen the sustainable use of black soil.