三峡库区紫色土植被恢复过程的土壤团粒组成及分形特征

Abstract

The quantity and quality of soil aggregates are important parameters of soil physical properties and plays an important role for sustaining soil porosity,improving soil infiltration,preventing soil erosion and so on. The Three Gorges Reservoir Region is regarded as an important region to secure the ecological safety of the mid-upper reaches of Yangtze River of China. Thus,big attention has been always paid to prevent the soil and water losses in this region,and many projects of vegetation restoration were carried out. Purple soil is the important soil resource easy to be eroded in this region. This study tries to understand and assess the changes of purple soil structure in different phases of vegetation restoration. Cupressus funebris is one of the most important tree species used for forestation and widely distributes in the low mountain and hill areas of this region. Using the method of taking space instead of time,the structure characteristics of purple soil aggregate during the vegetation restoration processes were investigated. Results showed that the dry-sieved soil aggregates in all soil layers at different restoration phases were mainly composed of particles with size ≥2 mm. Their ratio to the total was above80%,but the proportion of particles with size of 0. 5—0. 25 mm was the least. Compared with abandoned land,the quantity of dry-sieved soil aggregates( 5—2 mm) in surface soil layer for new forest land,sapling forest and mid-mature forest of Cupressus funebris was increased by 6. 03%,10. 32% and 10. 97% respectively,but the dry-sieved soil aggregates of 0. 25 mm was decreased obviously. The water stable soil aggregates at different restoration phases were mainly composed of particles with sizes 0. 25 mm and 3—1 mm. The quantity of water stable soil aggregates of ≥0. 25 mm obviously increased with the vegetation restoration processes in all soil layers,but decreased with increasing soil depth. Their fractal dimension( D) decreased significantly with the vegetation restoration processes. Mean weight diameter( MWD) and geometric mean diameter( GMD) of dry-sieved aggregates were positively related with the bigger size particles( ≥3 mm)and negatively related with the smaller size particles( 3 mm),but they all had a significant relation with ≥10 mm and2—1 mm. The D was influenced significantly by the amount of particles with size 1 mm,especially the particles 0. 5 mm( P0. 01). The MWD and the GMD of water stable aggregates presented a significantly positive relation( P0. 01) with the amount of particles with size ≥5 mm and negative relation( P 0. 01) with particles 0. 25 mm,inversely,the D presented a significantly positive relation( P0. 01) with particles 0. 25 mm. Whether the dry-sieved or the water stable soil aggregates,their relation between D and soil particle sizes reflected that the increase of small aggregates would improve soil fractal dimension,but weaken the structure stability of aggregates; and the increase of big aggregates could decrease the soil fractal dimension,but strengthen the structure stability of aggregates. Thus,the purple soil aggregates structure can be effectively improved after vegetation restoration in this region studied,and the improvement increases with the vegetation restoration time. However,the mid-mature artificial forest of Cupressus funebris with higher canopy density shall be thinned properly.