Studies On Ecological Degradation Mechanism Of Representative Karst Mountains In Chongqing

The Karst area has been regarded as a kind of vulnerable eco-environment the same as the ecotone of the desert in 1983. But most existed researches have neglected the degraded processes affected by the land use patterns, restricted in the big district scale or single affected factors, and lacked of the illumination on the degradation and rehabilitation mechanism of Karst eco-environment from middle and small scale and the point of view of ecosystem. To study the processes, characteristics and key factors, make contribution to understand the run laws of Karst ecosystem, and provide theoretical guidelines for the restoration and reconstruction of degraded karst ecosystem and the optimizing of land utilization structure.The earth's surface of carbonatite in Chongqing area is about 32038.14 km2, makes up of 38.9% of the overall land, and distributes mostly at northeast and southeast regions of Chongqing. This paper chooses the Guanyinxia anticlinal mountains, Jinfushan mountains in Nanchuan, Qianjiang county and Wushan county as the typical representation, and chooses 10 land use patterns: primeval forests, secondary forests, man-made economic woodland(orchard), shrub grass land, old field (2a,5a,10a abandoned dry field ),upland, dry land, vegetable land and bare land. This paper has discussed the impact of different land use patterns on soil physical and chemical properties, the compositions of soil seed banks, bio-mass and bio-diversity by field investigation and lab analysis, hope to expound the relationships between land use patterns and the changes of soil fertility, soil seed bank's quality and quantity, bio-diversity and boy-mass. Analyze the key factors, processes and indices. The results show that:1. The particle size distribution is determined mainly by the lithologic characters of carbonate rocks, in addition determined by the vegetation and land utilizing in Karst environment. Long term cropping and soil erosion would affect the particle size distribution of surface soil. The sandification of surface layer is very general but distinctly different among different land use patterns. After the vegetation has turned into secondary vegetation or man-made vegetation or reclamation, the surface soil layer is characterized by sandification, and the sandification is more evident after the reclamation.The aggregates stability analysis shows that the contents of >0.25mm water-stable aggregates changes with the land use patterns obviously, therefore, the proportion of all levels water-stable aggregates may reflect the soil aggregate quality preferably, the ratio of structural destruction may express the aggregate's stability. Beibei, Qianjiang, Jinfushan and Wushan's ratio of structural destruction's range after water sifting is 2.9%~35.1%, 9.2-85.8%, 2.2%~48.8% and 2.2%~21.8% respectively, and the average value is 15.1%, 35.9%, 13.6%, 10.2%, this results show that the soil aggregates water stability have a biggish difference in different regions and under different land use patterns of the same area in Chongqing Karst mountains.The contents of >5mm, >2mm and >0.25mm water-stable aggregates of surface later change as follows: grass slope > primeval vegetation > orchard > secondary woodland > abandoned infield > cultivated land, primeval forests > grass slope > secondary woodland > orchard > abandoned field > cultivated land, grass slope > primeval forests > secondary woodland > abandoned field > orchard > cultivated land. The surface layer and sub-surface layer of woodland's and grass slope's soil are dominated by >2mm water-stable aggregates, but it is relatively less in orchard, abandoned field and cultivated land's soil. This results show that the cultivation activities influence the larger water-stable aggregates. The woodland and shrub grass land are reclaimed in Karst mountains, then, the soil's antierodibility and water retention capacity may be lowered and the Karst eco-environment's drought may be increased by the evident declineof the content of water-stable aggregates, after returning slope...