Present situation and change of soil desertification

I. Present situation of soil desertification

Soil desertification refers to the process of land degradation in arid and semi-arid areas, which is mainly marked by sandstorm activities and sand dunes activation in different degrees due to the influence of natural factors and human activities on the soil environment.

Land desertification in Songnen Plain is expanding. At present, the desertification land area in the whole region is 145.438+0× 104 hm2, accounting for 7.95% of the total land area. Among them, the lightly desertified area is 57.4729× 104hm2, the moderately desertified area is 78. 1653× 104hm2, and the severely desertified area is 10.0079× 104hm2. See Figure 8-4 for the proportion of light, medium and heavy desertification land. Desertification land is mainly distributed in the northern Songliao Basin, the low plains on both sides of Nenjiang River, the floodplains and terraces of the Second Songhua River, Taoer River and Huolin River. Climatic conditions belong to semi-arid areas or semi-humid to semi-arid transition areas. The degree of desertification gradually decreases from southwest to northeast and transits to semi-humid area. Sandy land is mostly fixed and semi-fixed sand dunes, and there are few moving sand dunes. The degree of desertification is mainly moderate and light desertification, followed by light desertification. Desertification land has thin soil layer and low cohesive soil content, while sandy soil or sandy soil has low organic matter content and poor nutrients. Therefore, the soil's fertility conservation ability is poor, and its ability to resist wind erosion and water erosion is poor. Slightly desertified land, with obvious small pieces of wind erosion or quicksand and a small amount of small sand piles near 1/4 area. The coverage of natural vegetation is 30% ~ 50%, the effective thickness of soil layer is generally 10 ~ 20 cm, and clay particles in soil account for about 20%. On the sand surface of 1/4, there are a few patches of mobile sand dunes, wind erosion pits or wind-driven shrub sand piles in the middle sandy land. The coverage of natural vegetation is 10% ~ 30%, the effective thickness of soil layer is less than 10 cm, and the clay particles in soil are less than10%. In severely desertified land, there are dense contiguous mobile sand dunes, parabolic sand dunes or blown sand piles on the sand surface of 1/2. The coverage of natural vegetation is less than 10%, and only a few centimeters of fine soil particles or light yellow sand grains are directly exposed on the surface. See Table 8-4 for the distribution of sandy land in major cities and counties.

Figure 8-4 Composition of Land Desertification Degree

Table 8-4 Distribution Statistics of Desertification Land in Major Cities and Counties

According to the regional differentiation of sandy land, sandy land can be divided into six major sandy lands: Wu Xiang Sandy Land, Fuyu Sandy Land, Sheli Sandy Land, Dumeng Sandy Land, Tailai Sandy Land and Qiqihar Sandy Land.

Wuxiang sandy land is the largest sandy land in Songnen Plain. It starts from Haiyu in Tongyu County in the west, extends to Taipingchuan in the southeast, and then turns to the east. After passing through Changling County, it turns to Wulan Tuga in the northeast, with a total length of about 150 km and a width of 50 ~ 80 km. It is a huge arc-shaped sand dune (mound) belt, with the arc top protruding to the south. The height difference of sand ridge is 10 ~ 25m, and the ridge top is flat and slightly undulating, with a maximum length of 20 ~ 30km and a maximum width of 3 ~ 5km. Most of the sand dunes are platform-shaped and step-shaped, with a round top, and the height difference is generally10 ~ 30 m. Most sand ridges and sand dunes are fixed and semi-fixed and covered with vegetation, but some sand ridges have been destroyed by vegetation, resulting in serious wind erosion, forming many wind erosion pits and gullies. Wu Xiang sandy land has a unique landform, with depressions, lakes and swamps alternately distributed between sand ridges (hills), especially in the sandy land east of Taiping River. The width of depressions between ridges varies from several hundred meters to several thousand meters, and some depressions or lakes are surrounded by alkali spots of different shapes.

Dumeng Sandy Land is located in the low plain on the east side of Nenjiang River, and it is the second largest sandy land in Songnen Plain, with the most concentration in Dorbod Mongolian Autonomous County, extending eastward to Daqing, Zhaoyuan West and the southern end of Lin Dian. The sandy land is composed of a series of northwest-southeast sand belts with a relative height of10 ~ 20 m. The northern part of the sandy land is mainly cultivated land, and the main forms of desertification are soil wind erosion and ground sand covering in spring. The central part is mostly grassland and woodland, and the grassland is seriously degraded, with quicksand in some areas; Most of the south and east are cultivated land. Although there are shelter forests, they are not perfect and soil wind erosion and desertification can be seen everywhere. There are a large number of wind-eroded sand pits and seriously degraded sandy grasslands in the west of Daqing, and there are also phenomena of desertification and abandonment of cultivated land.

Fuyu sandy land is located on the floodplain on both sides of the second Songhua River, concentrated in Fuyu area on the right bank. Sandy land is mainly dense sand dunes, mostly fixed sand covered by forest land, and bare sand dunes are mostly concentrated in the windward side and tuyere of the sand belt. At present, there are many shelterbelts on the sandy land, but there are obvious wind erosion and small quicksand on the imperfect forest network or tuyere. The degree of desertification is mainly mild, and sandy land and developing sandy land account for about 4.3% of the total sandy land area.

Sheli Sandy Land is located in the south of Zhenlai on both sides of Sheli Town and Taoer River in Da 'an City, with the highest concentration in Sheli area, forming a series of sand belts extending from northwest to southeast in the valley and low plain in the lower reaches of Taoer River. Sand dunes are mostly fixed or semi-fixed, and the degree of land desertification is mainly mild desertification caused by soil wind erosion. In recent years, desertification has been increasing in areas where shelterbelts have been seriously damaged.

Tailai sandy land is distributed along the west side of Nenjiang River in Tailai County and the north-south direction in the northeast of Zhenlai County, which is a serious desertification area in Songnen Plain. Because the shelterbelt network is not perfect, soil wind erosion is strong in spring, and there are small pieces of quicksand and wind erosion pits. The sand moves from the top of the mountain to the top of the mountain, the sand layer thickens and the wind erosion pit expands continuously. There are mobile sand dunes along the Nenjiang River in Jiangqiao Town, with an area of 167 hm2, accounting for 0.65,438+05% of the total sand area.

Qiqihar sandy land is located in Qiqihar and Fuyu areas, on both sides of Nenjiang River north of Binzhou Railway. Because the farmland shelterbelt is well protected and the degree of desertification is light, the desertification methods are soil wind erosion in spring and local surface sand covering.

Second, the dynamic changes of land desertification in recent 20 years.

(1) The total area of desertified land has expanded and some areas have decreased.

Figure 8-5 Area and ratio of land desertification degree

The change of sandy land area in Songnen Plain is the increase of moderate and severe sandy land, especially the increase of moderate sandy land and the decrease of light sandy land. In the past 15 years, the land desertification area increased by 8.793 1× 104hm2, in which the light desertification area decreased by 0.1615×104hm2 and the moderate desertification area increased by 7.2hm2. The change of sandy land with different landforms is that the moderate and light desertification in low plain and piedmont inclined plain increases rapidly, while the sandy land in high plains shows a decreasing trend. The areas with serious desertification in Tongyu County, Dorbod County, Changling County and Guo Qian County in Inner Mongolia have not changed much, but the desertification areas in Zhaodong City and Zhaozhou County have decreased, while the desertification areas in Daqing, Qiqihar, Tailai and Zhaoyuan County have increased more, and the increased desertification areas are mainly moderate and mild desertification.

(2) Areas with severe desertification have been effectively controlled, and areas with mild desertification in the north have a tendency to expand.

Judging from the geographical distribution of the development of desertification areas, the desertification areas in the south, such as Tongyu, Dulbert, Taonan and Fuyu, have been basically controlled after decades of governance. Although it has increased slightly, the annual expansion rate is less than 0.3%. Changling County decreased slightly, while Daqing, Qiqihar and Tailai in the north increased significantly. Daqing and Qiqihar increased by 65438+ in the past.

Three. Influencing factors of land desertification

Comprehensive analysis of the causes of land desertification in Songnen Plain includes both natural factors and human factors. Natural factors mainly include: dry climate, strong wind, low forest and grass coverage and abundant sand sources; Human factors mainly include deforestation, over-cultivation, over-grazing and over-burning firewood, and large-scale development and utilization of water resources.

natural causes

Drought, lack of rain and frequent gale are the main climatic factors for the formation of desertification land in Songnen Plain. The western part of Songnen Plain is an arid and semi-arid area, and the annual precipitation is less than 450 mm, especially in spring, the precipitation only accounts for about 10% of the annual precipitation, and the humidity index in most areas is below 0. 1, which is the driest season in a year. There are strong winds as well as droughts in spring. The annual average wind speed in the west of Songnen Plain is 4.0m/s, and the wind speed in spring is 5. 1 m/s. The annual average number of windy days is 26.3 d, and the spring average is 17. 1d, accounting for more than 60% of the whole year. In spring, vegetation has just entered the growing period, with low coverage and bare sand surface. Under the frequent action of strong winds, strong sandstorms are prone to occur. The forest coverage rate of sandy land in western Songnen is only about 10%, and the distribution is uneven. There is basically no forest shelter forest in the sandy area. Low forest coverage and perennial gale are the direct factors that cause land desertification in Songnen Plain.

The sand source of Songnen Plain Sandy Land comes from the valley plains of Songhua River, Nenjiang River, Taoer River and Huolin River and Horqin Sandy Land in the southwest. The Quaternary loose sediments in Horqin Sandy Land are about 100 m thick, with the maximum thickness exceeding 180 m, and the grain size is concentrated in 0.5 ~ 0.065,438+0 mm, mainly medium and fine sand, and directly exposed to the surface. Sandy land is distributed in Songhua River, Nenjiang River, Taoer River, Huolin River and other valley plains. Thick, and its upper particle size is mostly 0.25 ~ 0.1mm. The existing sandy soil layer is thin, with low clay content, sandy or sandy soil, and its wind erosion resistance is poor. These two sandy lands and the existing fixed and semi-fixed sand dunes provide sand sources for the development of sandy land in western Songnen.

(B) Human factors

Human activities have promoted the development of land desertification. Songnen sandy land used to be a forest and grassland environment with dense rivers and lakes and lush trees in historical period. The western part of Songnen Plain was developed late, starting in the middle of 20th century. With the rapid increase of population, excessive reclamation, overgrazing and excessive firewood burning, sandy land has been destroyed in a large area, the wind erosion area of cultivated land has been increasing, and land desertification has expanded rapidly. For example, in Qiqihar City, there was 200× 104hm2 grassland in 1960, but at present there is only113.3×104hm2. While the grassland area has been reduced by nearly half, the number of livestock has nearly doubled, from 240× 104 to 450. Overgrazing leads to serious grassland degradation and desertification, such as the "North Woods" in Houxin Village, Yaoxin Township, Dorbod Mongolian Autonomous County. Before 1960, the vegetation was good, the forest and grass were lush, the vegetation was destroyed by cutting and farming, the fixed sand dunes were revived, and quicksand was everywhere. At present, the sandy land coverage has reached more than 1 m, and the surrounding farmland has been greatly harmed.

Although the "Three North" shelter forest has been built for nearly 30 years, the average tree diameter has been 20 ~ 30 cm. Driven by economic interests, in recent years, the cutting speed in some areas is very fast, and the trees in front of the house and behind the house in rural areas and the woodland in state-owned forest farms are cut down in large quantities, which has not kept up with the update and supplement. For the shelterbelt with sparse forest network and imperfect protective barrier, the protective ability is further reduced, which leads to the sandstorm driving straight in and the resurgence of sand dunes.

The large-scale exploitation of water resources is another man-made factor in the development of desertification. Water retaining works in the upper reaches of the Nenjiang River, such as Chaersen Reservoir, Xianghai Reservoir, Hongshan Reservoir and Nierji Reservoir. , intercepting the water from the upper reaches of the river, so that the sandy substances in the downstream riverbed and flood plain are often exposed to the surface and become the "sand source" of land desertification.

The development of groundwater leads to the decline of groundwater level, which in turn reduces soil moisture and humidity, and the surface vegetation degenerates and withers, providing favorable conditions for the occurrence of land desertification. If the groundwater map in 2005 is superimposed with the land desertification map, it will be found that the groundwater level in the areas with serious land desertification has dropped greatly. Among them, the groundwater level in Daqing, Qiqihar and Tailai counties, where land desertification developed rapidly, was mostly 2 ~ 5 m in the 1970s, and was less than 2 m in many places, while it was mostly 5 ~ 10 m in May 2005, and the groundwater level generally decreased by 3 ~. Shallow water level, long-term maintenance of soil moisture and humidity, and lush growth of surface vegetation can inhibit the occurrence of land desertification.