| Soil erosion and severe ecological environment of Loess Plateau are paid more attention to by Chinese government and academic society. Study on soil ecological functions and characteristics of Pinus tabulaeformis carr. (P.T.C.) and Robinia Pseudoacia L. (R.P.L.) plantation can reveal interaction mechanism between forest and soil can clarify forest protective function to environment and can assess the quality of ecological environment in west-area, so it has essentially theoretical and practical value. The following finding and main innovative points are based on the systematic research on forest stands characteristics and the time-space distribution of soil hydro-physical property, chemistry, nutrient and microorganism. 1. It is deduced of the time-space dynamic regularity of soil hydro-physical property of P.T.C. and R.P.L. plantation. Soil bulk density of P.T.C. plantation and that of R.P.L. plantation have similar dynamic property. Their changes with tree age have not been significantly found. They increase with the soil depth by power function. The spatial gradients of them decay with the soil depth by power function and trend to be steady. But in the surface layer gradients vary to a degree. Soil bulk density of P.T.C. plantation and that of R.P.L. plantation are less than that of wasteland. Additionally the effect on bulk density of P.T.C. plantation is more obvious than that of R.P.L. plantation. Soil capillary prosity of P.T.C. and R.P.L. plantation have irregular change with tree age and fluctuate with soil depth. Compared with soil capillary prosity of P.T.C. that has not been found in obvious variation, that of R.P.L. plantation is found increased in different depth. The result represents that the effect of R.P.L. plantation on increasing capillary prosity is more remarkable than that of P.T.C. plantation. Soil non-capillary prosities of P.T.C. and R.P.L. plantation have still not been found in regular change. Non-capillary prosity of P.T.C. plantation is logarithmically decayed with soil depth and it's spatial gradient is exponentially increased. The spatial gradient in surface layer changes sharply while gradients in other layers are steady. Non-capillary prosity of R.P.L. plantation is fluctuant. As a whole, P.T.C. and R.P.L. plantation all increase the non-capillary prosity of soil. Furthermore, P.T.C. has stronger effect than R.P.L. Soil total prosities of P.T.C. and R.P.L. plantation have no clear regularity and fluctuate with soil depth. Either P.T.C. or R.P.L. plantation can increase the total prosity of soil. Soil max.moisture capacity, capillary capacity and moisture capacity of P.T.C. and R.P.L. plantation have not been explored in regular increase and differ from each other with soil depth. Corresponding with soil depth, max.moisture capacity and capillary capacity of P.T.C. plantation decrease with power function and it's spatial gradient increase with power function. Except for that of surface layer spatial gradients of other layers are comparatively stable, unlike that of R.P.L., which has fluctuant change. Moisture capacity of P.T.C. and R.P.L. plantation also has fluctuant change with soil depth. In the three objective lands, capillary capacity of P.T.C. is biggest, that of R.P.L. takes the second and that of wasteland the smallest. As a result, P.T.C. and R.P.L. can significantly promote the hydro-physical property of soil. 2. It is gained of the fractal characteristics of soil particle of P.T.C. and R.P.L. plantation forestland. The soil particle content of P.T.C. and R.P.L. plantation in different soil particle-size varies more greatly, but changes little with tree age; In the R.P.L. plantation forestland, the clay content tends to decrease with tree age increases, which expresses that the R.P.L. plantation can strengthen the ability of soil aggregate. The clay, silt and sandy content of P.T.C. and R.P.L. plantation varies more dramatically with soil depth, this shows that the difference of them are produced by the process of soil formed, the litters and root system. The fractal dimension of soil particle in P.T.C. and R.P.L. plantation forestland tends to be equal on different side soil and it is smaller than the same soil texture on the other place. These shows that the soil aggregate ability can be strengthened and the soil dimension had reduced with the help of the litters and root system of P.T.C. and R.P.L. The fractal dimension of soil particle can reflect the distribution degree of soil particle in forestland; therefore it is taken as assessing index. 3. It is obtained of the time-space effect of soil nutrient and chemical characteristics of P.T.C. and R.P.L. plantation forestland. Soil organic matter, total N, available N and available P of P.T.C. and R.P.L. plantation forestland changes irregularly with tree age, they decrease with the soil depth by power function. The spatial gradients of them increase with the soil depth by power function and trend to be steady. As a whole, P.T.C. and R.P.L. plantation all increase the organic matter, total N, available N and available P of soil. Furthermore, P.T.C. has stronger effect than R.P.L. Available N of R.P.L. plantation all increase in different soil layer, which prove the effect of fixed N by R.P.L. plantation. Soil pH value of P.T.C. plantation varys irregularly with tree age that of R.P.L. plantation is fluctuant. Soil pH value of P.T.C. and R.P.L. plantation forestland increase with soil depth on the basis of power function. Its spatial gradients decrease with the soil depth by power function and trend to be steady. P.T.C. and R.P.L. plantation all decrease the soil pH;Additionally P.T.C. has stronger effect than R.P.L. Soil total P of P.T.C. plantation varys irregularly with tree age that of R.P.L. plantation is fluctuant. The content of total P changes irregularly with soil depth. Soil available K of P.T.C. plantation varys irregularly with tree age, that of R.P.L. plantation is fluctuant, that of R.P.L. plantation changes irregularly with soil depth, that of P.T.C. plantation changes with soil by power function, therefore The spatial gradients of them increase with the soil depth by power function and trend to be steady. But Soil available K of P.T.C. plantation in surface layer gathers more than other layer. Soil exchangeable Ca and exchangeable base capacity of P.T.C. plantation varies by Logistic model, that of R.P.L. plantation changes irregularly; Soil exchangeable Mg of P.T.C. plantation increases by exponential function, that of R.P.L. plantation is fluctuant; Soil exchangeable Ca, exchangeable Mg and exchangeable base capacity P.T.C. and R.P.L. plantation varys irregularly with soil depth. P.T.C. and R.P.L. plantation all decrease them, it is proved that Plantation can improve the condition of soil nutrient effectively. 4. It is obtained of distributive feature of soil microorganism of P.T.C. and R.P.L. plantation forestland. The quantity of soil bacterium of P.T.C. and R.P.L. plantation forestland varies irregularly with tree age, but it changes with soil depth by Quadratics Ratio model. Compared with the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest and wasteland, the quantity of soil bacterium of P.T.C. and R.P.L. plantation forestland is more than wasteland, but less than the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest. The quantity of actinomyces of P.T.C. and R.P.L. plantation forestland varies irregularly with tree age, that of P.T.C. trend to smaller. Compared with the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest and Secondary bush forest and wasteland, the quantity of soil actinomyces of P.T.C. and R.P.L. plantation forestland is smaller than Secondary bush forest; it is proved that plantation productivity is a actor affecting quantity of actinomyces. The quantity of actinomyces of P.T.C. and R.P.L. plantation forestland decreases with tree age by exponential function, its spatial gradients increase with the soil depth by power function and trend to be steady. The quantity of fungi of P.T.C. and R.P.L. plantation forestland varies irregularly with tree age, it decreases with tree age by exponential function, and its spatial gradients increase with the soil depth by power function and trend to be steady. Compared with the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest and wasteland, The quantity of soil fungi of P.T.C. and R.P.L. plantation forestland is more than the mixed stands of Pinus tabulaeformis and Amorpha fruticoca L. forest and wasteland, it is proved that plantation canimprove environment of fungi. 5. A method of accelerated velocity for analyzing and assessing the dynamic regularity of soil ecosystem has risen. At first, the dynamic regularity of ecological factor has obtained according to traditional method; then the velocity goes forward one by one; the rate of ecological factor by time, the rate of the rate of ecological factor by time, spatial gradient of ecological factor, the rate of spatial gradient of ecological factor and their feature have gained. This method is a new way to researching the dynamic regularity of soil ecosystem. 6. For fitting to use this method, four concepts are introduced. That is, takes the first and the second-order derivative of time-dynamic function respectively as variational rate and accelerated variation rate function, and takes the first and second-order derivative of space-dynamic function as gradient and accelerated gradient function. Among them, velocity function and accelerated velocity function indicate dynamics mechanism explicit. A gap for researching dynamic mechanism of ecosystem has found.
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