Research On Rocky Desertification Of Karst Mountainous Region In Northern Guangdong Province

Karst rocky desertification is a land degradation process widely occurred under the fragile karst environmental of subtropical region, which is characterized by severe soil erosion, percentage of bare rock area increasing, vegetation degenerating, and land productivity declining. It restricts the local economic development, and is ranked as one of the severe eco-environmental problems in China. Karst mountainous region of northern Guangdong locates in the watershed of Beijiang River-- the branch of Zhujiang River. Rocky desertification occurred in this region may threaten the ecological safety of the catchment area of Zhujiang River.By interpreting remote images, detecting the 137Cs activity and its loss rate in soil, observation of rainfall erosion as well as the artificial rainfall experiment, this paper discusses the temporal and spatial pattern features of rocky desertification land in karst mountainous region in northern Guangdong, the soil erosion rates and erosion characters in rocky desertification land of different degrees.(1) Based on remote images of research region in 1974, 1988 and 2004, maps of the distribution of karst desertification land are portrayed. Area of rocky desertification land and other landscape pattern indices are obtained according to the information in the maps. In 2004,the area of rocky desertification land in research region is 534.56 km2, which covers 3.85% of the total area of research region. It is in a state of reversion in recent 30 years and the area reduced by 68% than that in 1974. Its reversion speed from 1988 to 2004 is higher than that from 1974 to 1988. The ratios of moderate and severe rocky desertification lands keep rising, and their fragmentation degrees increase, which shows that moderate and severe rocky desertification lands have strong development potential and expansion tendency.(2) Rocky desertification is synthetically influenced by multi-factors such as lithologic characters, geomorphy, slope, soil features, vegetation, precipitation and human activities. In northern Guangdong, the rocky desertification lands distribute around carbonatite range, and are mostly superposed the distribution region of limestone soil. Moderate and severe rocky desertification lands are closely related with the distribution of slope-farmland. The irrational human activity is the most important factor which caused the rocky desertification.(3) The long-time average annual value of soil erosion modulus of karst mountainous region in northern Guangdong estimated by 137Cs technique is 2433.2 t km-2 a-1, which is much higher than the allowable erosion amount of karst region. Cultivated soil has a higher erosion modulus than uncultivated soil, and the soil erosion modulus of slope-farmland gets to 4968.3 t km-2 a-1. Along with the development of rocky desertification, soil erosion rate increased in early stage. Soil on upper position was kept on washing out and depositing in the low lying land. Because of the limitation of soil, the amount of soil erosion reduced gradually and ultimately stopped. The rudimental soil substance appeared in small patches and is the base of maintaining the damaged karst ecosystem.(4) According to the observation data in soil erosion sample plots from May 2004 to April 2005, the yearly amounts of soil erosion are: untouched natural grassland, 67.92 t km-2 a-1; cropland, 1756.96 t km-2 a-1, which is about 25 times higher than that in untouched natural grassland; nudation, 2455.37 t km-2 a-1, which is nearly 36 times of that in untouched natural grassland. Though the particularity of soil distribution in karst region limited its absolute amount of soil erosion, in certain samll sections where the soil distributed continuously and soil layer is thick, the soil erosion modulus may be higher than that in non-karst region where it has the same conditions with the former.(5) Soil erosive quantity on untouched natural land and cropland is most related to single rainfall, while the erosive quantity on bare-land is more related to the maximum intensity of continuous rainfall in 30 minutes. Vegetation can effectively protect land from soil erosion by short-term strong rainfall. Percentage of vegetation coverage is the main factor that influenced the amount of soil erosion: soil erosive quantity on bare-land is about 60 times higher than that on untouched natural grassland, and that the speed of soil loss on farmland 15 times higher than that on untouched natural grassland.(6) The runoff in research region has the characters of super-infiltering runoff pattern according to the results of artificial rainfall experiments. When rainfall intensity is low, the runoff yield and soil erosive quantity increased slightly along with the delay of precipitation time while they increased strongly under high rainfall intensity. The change of erosive quantity on bare-land is related to soil erodibile amount. The loss of soil on bare-land raised swiftly in short period, surface soil was soon washed away and the area of soil reduced. In later period, the change rate of soil erosive quantity minished because of the lack of soil. The change of soil loss on grassland is opposite to that on bare-land: in earlier stage of rainfall soil erosive quantity increased slowly but rised sharply in posterior stage. For the water erosive energy exceeded the vegetation protection. The change of soil loss on farmland is not obvious.(7) Along with the development of rocky desertification, the loose sand grains with low bonding capacity in surface soil runaway easily with runoff. The ratio of silt and clay ascended slightly. Soil bulk density raised, porosity reduced, permeability descended. The soil structure deteriorated. At the same time, nutrient in soil lost swiftly due to the soil erosion and land productivity declined severely.Rocky desertification results in the deterioration of physicochemical property of soil, which restrained the growth and development of vegetation and enhanced the possibility of the generation of surface runoff. It accelerated the development of rocky desertification at the same time and formed an infernal circle.