The Research Of Soil Structure And Macro-pores Of Forestlands And Grasslands Under Different Vegetation Restoration Years In Loess Hilly Area

To explore the effects of grain for green on soil quality further on Loess Hilly Area, it is extremely essentialto study soil structure and macro-pores in response to vegetation restoration. Taking 10, 25-year vegetation restorationforestlands and grasslands on the Guanshan Forest Farm, which is located in Jingchuan County, Gansu Province,as the sites, by analyzing the basic physical properties, root system characteristics, soil aggregates characteristics and soil macro-pores characteristicsin the 0-30 cm depth, the resultswere obtainedas followed:（1） With the restoration years increasing from 10 to 25 years, soil clay contentincreased, and the sand contentreduced; the soil particle fractal dimension of25 yearswas higher than the 10-year sites; the particle fractal dimension of forestlandwas less than the grassland at 25-year restoration, and the fractal dimension of grassland increased rapidly from 10 years to 25 years. With the soil depth increasing, soil moisture and soil bulk density increased, soil saturated water content, soil water stable aggregates and organic matter content decreased; with the restoration yearsincreasing, soil bulk densityreduced, and soil moisture, soil saturated water content, organic matter content as well aswater stable aggregate content significantly increased, namely, the roots activity in the soil and the change of root-zone environmentcontributed to the improvement of soil properties; by comparing different restoration ways, soil moisture content and saturated water contentof forestlands were higher than the grasslands; the soil organic matter content of forestlands was significantly higher than the grasslandsin the 0-20 cm depth, but in the 20-30 cm the organic matter of grasslandswas slightly higher than the forestlands; the soil water stable aggregates of grasslands was significantly higher than forestlands.（2） Root biomass, root length density, and root area indexrapidly reduced with soil depth increasing; with the restoration yearsincreasing, root biomass, root length density, and root area indexof forestlands and grasslandstended to increase. In the 0-10 cm and 20-30 cm depth, root biomass of F25 awas the highest, while in the 10-20 cm G_25awas the highest; root length androot area indexof grasslands weregreater than the forestlands.（3） The order of soil aggregates clay contentwas F_25a>G_25a>F_10a>G_10aunder dry sieving, while the sand content wason the contrary; with the grain size of aggregate structure increasing, clay content increased, and sand contentdecreased. The organic carbon content of aggregateswasF_25a>G_25a>G_10a>F_10ain descending order; with the soil depthincreasing, organic carbon content in aggregates decreased; and with the aggregate grain size increasing, the organic carbon content increased, andthe organic carbon content of aggregates above 5mm was the highest.With vegetation restoration years increasing, soil aggregate stability enhanced, mainly shown as followed: water stable aggregates contentincreased, water stable aggregates fractal dimension reduced, MWDand GMD increased, structure damage rate reduced. Thewater stable aggregateslarger than 5mm mainly caused the increase of water stable aggregates on the forestlands, while water stable aggregates was affected mainly by the increase of >1mm aggregateson the grasslands. With the soil depthdescending, water stable aggregates fractal dimension,the MWD and GMD under dry sievingand the structuredamage rate of soil aggregates showed an increasing trend, additionally, there was inconsistent performanceunder wet sieving; the fractal dimension and structuredamage rate of C25 awere the lowest. Theincreaseof organic carbon content and clay content could help improve soil aggregate stability, at same time, the soil aggregates also help the soil organic carbon stored in the soil pool.（4） With soil depthincreasing, the CT number（CTN） mean of each image increased, and CTN standard deviation decreased; the order of CTN mean wasG_25a<G_10a<F_25a<F_10afrom low to high amongsites; the CTN standard deviation of forestlands was above the grasslands, and increased with the restoration yearsincreasing; there was significantly positive linear correlation betweenCTN and soil bulk density, soil water content. In addition to G25 a, the soil macro-porosity of other three siteschanged gentlywith soil depth; the descending order ofmacro-porosity was: G_25a>F_25a>F_10a>G_10a, and the macro-pores number was: F_25a>G_25a>G_10a>F_10a; with the restorationyearincreasing, macro-porosity increased, and the macro-pores numberalso increased. CTN mean and macro-pores characteristics were significantly negatively correlation; and CTN standard deviation and macro-pores characteristics were significantly positively correlated.In conclusion, the vegetation restoration made the soil basic propertiesimprove andthe root system grow and develop, then, the stability of soil aggregatesenhanced and soil porositywas improved. With the restoration yearincreasing, the improvement was more remarkable. Within the range of 0-30 cm soil depth, the soil particle distribution, root development, soil structural stability and porosity characteristics of grasslands were superior to forestlands, and the abilities to store water and organic carbon of forestlandswere better than grasslands. Thefutureecological constructionshould adhere to the combination of forest and grass to improve the soil quality and the ecological environmentin Loess Hilly Area.