| Black soil is a kind of leached soil with thick humic substance layer, loosening texture and high natural fertility. Black soil in our country mainly distributes in Song-Nen Plain in the North East, the main commodity grain production base of our country. However, because of the influence of unreasonable farming method and natural factors, nutrient in black soil runs off little by little, and the average thickness falls down to 20-30cm, which develops at a faster speed annually, causing the explosion of soil-forming rock in partial areas. As a kind of non-renewable resources, it is difficult to recover for black soil after the disappearance. Therefore, there is important theoretical and practical significance to study the degradation and protection of black soil.Although amount of work of black soil has been done and abundant research productions have been achieved, there still are some problems. First, there are different understanding on degradation existing state and tendency of black soil. It is difficult to synthetically and quantificationally study the degradation degree because of the shortage of unitized appraisal system of degradation. Secondly, the important requirement of black soil to keep high production and fertility is the good structure. With the prolong of farming time of agricultural black soil, the granular in soil is destroyed, forming the nutty, mealiness and massive structure, which is the main influencing factor on crop yield. In existing research findings, the degradation of black soil is studied mainly from the aspects of physics, chemistry and biology, while the mechanism and influence of degradation in structure isn't paid enough attention and the structural indexes are still not used in the quality evaluation of black soil fertility. All the problems impede the understanding on black soil degradation and the scientific decision on degradation protection.In this paper, the agricultural black soil in Jilin province is taken as study subject. The quantitative structural indexes are extracted by using of microstructure quantitative analysis software on the base of indoor physical, chemical and structural tests. The present degradation situation of agricultural black soil in Jilin province is studied firstly from the aspects of nutrient, physics, chemistry and structure feature. Then the structural quality evaluation system is established to praise the structure quality of different sample synthetically on the basis of gray fuzzy clustering method. The microstructure feature of different degradation degree of black soil is studied and the influence mechanism of component, organic matter, chemistry property on structure degradation is analyzed. Finally, the fertility evaluation system taking structure parameter into account is established, and the extension and fuzzy synthetical evaluation method are used to appraise the fertility quality of black soil, to determine the fertility situation of black soil in Jilin. The main achievements and conclusions are obtained as followings:1. On the basis of structure feature, the problem of structure degradation of black soil is synthetically discussed first time. Nine parameters such as water-stable aggregate larger than 0.25mm, micro aggregate larger than 0.01mm, dispersion rate, structure failure rate, aggregate degree, pore equivalent area, capillary tube pore content, particle fractal dimension and pore fractal dimension are selected combined with the factual condition of black soil in Jilin province and former study, the grade standard of all parameters is established. Additionally, gray fuzzy clustering method is applied to evaluate the structure quality of black soil. The results show that more heavy degradation exits in 53 per black soil in Jilin, and the degree is medium-heavy.2. The fertility evaluation system taking structure parameter into account is established for the first time. In this system, 11 evaluation parameters are selected such as the nutrient indexes including organic matter, total nitrogen, rapidly available phosphorus and rapidly available kalium, the environmental indexes including bulk density, pH value, exchange cation and clay particle content, and three structure indexes including water-stable aggregate larger than 0.25mm, dispersion rate, and pore equivalent area. Using this system, fertility quality of 15 black soil samples in Jilin is evaluated by the means of extension and fuzzy synthetical appraisal method. Results show that there 3 IV grade in fertility, which is better in quality, such as G5(Siping); 3 II grade in fertility, which is worse in quality , such as G9(Songyuan dry land); 4 samples in the quality of medium to bad, such as G3(Xinglong Mountain); 5 samples in the quality of medium to good, such as G7(Fuyu). So the total quality of Jilin black soil is medium (about 60%).3. The micro mechanism of different degree structure degradation is studied qualitatively and quantitatively. Study demonstrates that there is obvious difference in microstructure of black soil with different degree structure degradation. The main structure types of black soil with medium-heavy degradation are skeleton structure, laminated structure and conglobation structure, and the structure units mainly are simple grain, coagulum and lamination lacking of aggregate. There are an illogical proportion of pores which are large or small ones. And cementation and contact connection are the main connection styles. The main structure types of black soil with slight degradation are conglobation structure and transition of skeleton to conglobation structure, with the main units of conglobation, near round or oval and cavernous body. The structure is loose, capillary pore accounts for a large part. All size pores exit in this structure, and fill matter connects the conglobation. There is a wide prospect of microstructure study, so it can be used to evaluating soil degradation.4. As to evaluation methods, the extension theory is better than fuzzy mathematics. It is more subjective in the selection of membership function and calculation model of fuzzy evaluation method, what's more, the phenomenon of information losing is obvious during the progress of calculation, which can be avoided in the extension theory that is feasible and easy for programming. Additionally, extension theory can be used not only to determine the fertility quality of black soil, but also to arrange in the grade.5. The study of unit connection type in different degradation structure shows that: the units are formed by the cementation function of clay and oxide of Fe or Al in the black soil with medium-heavy structure degradation. The combined water connections of clay or cementation are the main connection styles, forming the units with worse porosity, retentiveness and water stability. At the same time, with the change of water content the soil becomes denser, the porosity and arability reduces. The stronger cementation of oxide of Fe or Al makes the poor porosity of units, which is disadvantageous to the form of coagulum structure.6. The study of organic matter component in black soil with different structure degradation degree shows that: there is a direct relationship between structure degradation, reduction of water-stable aggregate and the fall of humic acid content. The reduction in the ratio of humic acid to fulvic acid may be the main reason for black soil structure degradation. The mechanisms of cementation function between humic and fulvic acid and clay is studied. Results demonstrate that because of the structural difference between humic acid and fulvic acid, there is a larger clay adsorption capacity in humic acid, and a more stable chelate complex of humic acid and metal ion, so the aggregate formed by humic acid is more stable too.7. The analysis of relationship between particle fractal dimension and all particle size contents show that: there is an obvious positive correlation between fractal dimension and clay content, and negative correlation between fractal dimension and sand content, which proves that the particle fractal dimension is a good index to represent soil quality. On the other hand, the relationship between particle fractal dimension and nutrient content demonstrates that there is an obvious positive correlation between fractal dimension and organic content and an obvious negative correlation between fractal dimension and total phosphorus content.8. The study on structure fractal features shows that particles and pores in black soil have obvious self-similar fractal character, and there are intimate relationship between particle and pore shape fractal dimension and soil structure and fertility, which can disclosure the degradation tendency of black soil, so it can be used as a synthetical index in the evaluation of black soil structure degradation.9. The exchange cation in black soil with structure degradation is studied. Results show that there is a high content of exchangeable K and Na in some heavy degraded black soil. The exchangeable K and Na can result in the expansion and dispersion of soil, destroy the connection between organic matter and clay, enhance the disperse rate of aggregate and reduce the structure stability. So the increase in exchangeable K and Na is one reason of causing structure in black soil.10. The study on nutrient feature of Jilin black soil shows that the organic content changed slightly compared with 20 years ago, and the content of total and rapidly available N increased in different degree, the content of rapidly available P and K reduced greatly.
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