Recent Evolution of Soil Salinization in China and its Driving Processes.
Jingsong Yang, Institute of Soil Science, Chinese Academy of Sciences, 71 Beijing Donglu Road, Nanjing 210008, China
Salt affected soils spread extensively in China. Those soils cover wide area from tropical to temperate zones, from the coast to the inland and from the semi-arid to desert regions in the nation. Currently the total area of salt affected soil is about 36 million hectares, which occupies 4.88% of usable land in whole country. About nine million hectares of arable land is salt affected, accounting for 6.62% of total land in the country. The salt affected soils mainly distribute in the Northwest China, the North China, the Northeast China and coastal regions. Area of the soil in six provinces of western part of China accounts for 69.03% of whole country. Current hot spots of soil salinization evolution almost involve all distribution areas of the salt affected soils, including Northeast China, Northwest China, the Inner Mongolia and the coastal region. In the Songnen Plain and the Great Band region of the Yellow River, salinization trend towards more severe at two aspects: expansion in area and enhancement in extent. Area of salt affected soils reaches 3.5 million hectares currently in the Songnen plain. Annual rising rate of salinization is as high as 1-1.4% and about 45% of salt affected land in the area has been degraded into abandoned severe saline land. Due to irrigation maladjustment, area of salinization increased 1%-3% annually in last three decades in the Great Band region of the Inner Mongolia. Secondary salinization also occurred in the West Corridor of Gansu Province by similar reason. Irrigation with brackish water has been an important cause of salinization evolution in north part of China. According to a monitoring data from Guyuan region of Ninxia Province, soil salt content increased to 2.3 g/kg after 5 years of brackish water irrigation and up to 8.3 g/kg after 14 years. Shallow ground in most River irrigation districts of Ninxia, Inner Mongolia and Xinjiang Provinces has been observed and salinization occurred as the result. Salt-water regime is changed under drip irrigation practices in some region of Xinjiang, which causes sat accumulation consequently. Generally, 35% of irrigation land in Gansu, Xinjiang and Ninxia, and 50% irrigation land in inner Mongolia is current facing the hazard and threat of salinization. As another hot spot, salt-water movement is unbalance in coastal regions such as the Yellow River delta, and salinization is also developing in such area. Driving factors of evolution of salinization in China mainly consist of irrationalities in water resource management and land management, and of climate change. Irrational water use is the dominant cause of soil salinization evolution in irrigation districts, which are mainly represented as great seepage of irrigation canal and low efficiency of the irrigation water, as unnecessary high irrigate norm, poor drainage system and blockage of drainage ditches, and as raise of ground water table by construction of plain reservoir. Micro irrigation practice such as drip irrigation in arid region has potential dangers on change of salt-water regime and therefore may cause salt accumulation. Local salinization has already been found in some areas with such irrigation practice and such phenomena trend to an expansion. Irrational land management in semi-arid and arid regions is also a driving factor in salinization accelerating, including cutting down the area for salt disposal basin by overexploitation of waste land in irrigation districts, worsening of soil physical and chemical properties owing to poor fertility management, and increasing of surface evaporation and weakening of drainage due to destroy of plantation by over-grazing, deforestation and irrational land exploitation. Salt affected area is generally located in the region influenced by climate change in China. Climate change is considered as another significant driving factor on salinization evolution. Currently climate in North China and Northeast China has been turning to a trend of warm-and-dry process. Such climatic change has promoted the evolution of modern salinization process in the region. Climate change can also cause the sea level raise. Salt leaching of soil in coastal region including the Yellow River and Changjiang River deltas is therefore subject to a disadvantage owing to blockage of salt outflow and seawater intrusion. Consequently, Desalinization in the region is staved and even local salinization expansion is observed.