Characteristics and Development of Agricultural Soil in Hetao Irrigation District, Inner Mongolia

Yang YANG, Liming LAI, Li MEI

Abstract Hetao Irrigation District is located in the cold and arid region of Hetao Plain， Inner Mongolia， where the agricultural soil has unique characteristics. Although the agricultural soil properties in Hetao Irrigation Districts have been reported， the overall characteristics remain unclear. Through literature review and investigation， the overall characteristics， development patterns， and related reasons were explored， consequently  providing theoretical support for enhancing soil utilization and formulating sustainable soil development strategies. The results showed that the agricultural soil in Hetao Irrigation District originated from the sedimentary layer and anthropogenic mellowing produced by the diversions of the Yellow River. The soil has periodic secondary salinization characteristics， accompanied by a slightly increasing pH value over time. It has low soil organic contents with a stable changing trend， low nitrogen， and phosphorus contents but high potassium and sulfur content， uneven nutrient distribution， diverse production performance， weak but stable ecological performance， and heterogeneous soil quality with a stable change trend. These findings indicate that this kind of soil can be used to plant diverse crops tolerant to different saline-alkali and requiring various nutrients. This agricultural soil is sustainable， but it is also faced with the problems of increased saline-alkali， nutrient loss， and pollution.

Key words Agricultural soil; Secondary salinization; Organic matter; Soil nutrients; Heterogeneity; Ecological features; Hetao Irrigation District

Received： April 9， 2022  Accepted： Jun 12， 2022

Supported by the Science and Technology Department Project of Inner Mongolia Autonomous Region; the Talent Introduction Startup Project of Hetao College （No. HYRC2019006）; the Science and Technology Research Project of Hetao College （No. HYZX201952）.

Yang YANG （1987-）， female， lecturer， master， devoted to the research on soil fertility.

*Corresponding author. Liming LAI （1965-）， male， Henan Province， Ph.D.， professor， mainly devoted to the research on the impact of agricultural production activities on soil and environment， E-mail： liming.lai@qq.com.

Located in the west of the Inner Mongolia Autonomous Region， Hetao Irrigation District （belonging to Bayan Nur City） is in the middle and upper reaches of the Inner Mongolia section of the Yellow River （106°10′-109°30′ E， 40°12′-41°20′ N）， with an altitude of 1 007-1 015 m. It faces the Yellow River in the south， Yinshan Mountain in the north， UlanBuh Desert in the west， and Baotou City in the east. With flat terrain[1-2]， it has a total land area of about 11 200 km2[3].  Since the Holocene （about 11 700 years ago）， Hetao Irrigation District has been a typical cold and arid region， with an average annual precipitation of 159.8 mm， and an average daily temperature of 7.56 °C， and annual evaporation of about 2 200 mm[4]. Hetao Irrigation District is dependent on irrigation for agricultural production， making it ranked the first among the gravity irrigation districts in Asia. As the major irrigation water source， the Yellow River has an average transit water volume of 28 billion m3. The introduction of the Yellow River for irrigation could be traced back to the year 2 BC， having a history of 2 000 years and a current irrigated acreage of approximately 680 000 hm2[5-6].

In Hetao Irrigation District， the agricultural soil belongs sub-type of the cumulated irrigated soil[7]. The silting layer is usually 30-70 cm in thickness， with some exceptions of more than 1 m thick. With the A11-Ab-c profile， the soil is homogeneous from top to bottom， commonly having cultural relics at the bottom and a moderate lime reaction[7]. There are numerous investigations and studies on the soils in Hetao Irrigation District[8-9]， but they are all scattered or based on certain soil properties， such as soil nutrients[10-12]， soil salinization[13-16]， water and salt migration law[17-20]， soil pollution[21-22]. However， the overall analysis of the soil characteristics in the Hetao Irrigation District is still lacking.

In this study， the overall soil characteristics and development patterns， as well as the causes in Hetao Irrigation District were studied from the agricultural and ecological perspectives， aiming at providing theoretical support for formulating sustainable development strategies and enhancing soil utilization there.

Origin of the Agricultural Soil in Hetao Irrigation District

The sedimentary layer， caused by the multiple diversions of the Yellow River， is the origin of the agricultural soil in the Hetao Irrigation District. Geologically， the Hetao Plain is a long-term subsidence faulted basin. During the lengthy geological period， Hetao Irrigation District was dominated by lake water and later formed a lake-based， salt-rich sedimentary layer. The course of the Yellow River goes toward the Hetao Irrigation Region but is blocked by the Yinshan Mountains （Fig. 1）， causing multiple divisions. Resultantly， the Yellow River alluvium formed and covered the lacustrine sedimentary layer. Under the action of the swept area of the ancient Yellow River channel and the freshwater introduced from the Yellow River for irrigation， the lacustrine saltwater area is covered with a freshwater layer， forming a complex and changeable saline-fresh water layer in the Hetao Plain[23]. This layer becomes the parent material of soil in the Hetao Irrigation District. Accordingly， this soil is mainly formed by local factors， particularly the cumulated irrigated soil[12].

Fig. 1 The distribution of 16 sampling points in the Hetao Irrigation Area

The sediment-containing Yellow River water was introduced into the soil via long-term irrigation. This， coupled with the dry farming mellowness measures like artificial fertilization and farming， makes the cumulated irrigated soil turn into agricultural soil （about 50 cm in thickness） （Fig.2）. The Yellow River water irrigation introduces sediment into the farmland， which thickens the soil layer and changes the original sandy soil texture. The higher groundwater level leads to the alternate occurrence of oxidation-reduction reaction in the middle and lower parts of the profile， which affects the conversion and migration of materials as well as the development of soil profiles during agricultural soil formation， and controls the form and availability of soil elements[24-25]. The products of producer activities （livestock manure， charcoal， cinder， brick， and ceramic debris） can thicken the cultivated soil layer. In addition， the application of farmyard manure， irrigated silt， stubble， and litter of crops increases the content of organic matter and nutrients such as nitrogen， phosphorus， and potassium in the soil[24]. Agricultural activities such as plowing， harrowing， and cultivating make the warp， fertilizer， and stubble mixed with the cultivated layer uniformly， breaking the silting layer and thus creating a good cultivated layer structure[24].

Fig. 2 Soil profile at 0-50 cm depth of sunflower field in Linhe District， Bayan Nur， Hetao Irrigation District， Inner Mongolia （photoed on July 30， 2019）

Secondary Salinization Characteristics and Development Analysis

Secondary salinization is the most prominent feature of agricultural soil in the Hetao Irrigation District. The groundwater level rises due to canal leakage and flood irrigation. Under drought conditions， the salty groundwater rises to the surface through capillary action to evaporate and accumulate salt on the soil surface， resulting in secondary salinization. Accompanying soil salt accumulation， Na+ exchanges with Ca2+ and Mg2+ of colloid complex， resulting in secondary alkalization of soil （in some depressions， due to high groundwater levels， the organic matters and sodium sulfate in soil are converted into sodium carbonate by biological action， accelerating soil alkalization）[16]. Soil secondary salinization is also known as soil secondary salination[26]. The soil salinity fluctuates annually[27]. In this study， the test data of 16 sampling points （Fig. 1） across the whole irrigation area after the autumn harvest in 2019 and 2020 showed that the average soil total salt content in the 0-20 cm depth was 1.49 and 2.18 g/kg， respectively. However， in general， the total salt content of the cultivated soil layer in the Hetao Irrigation District was less than 10 g/kg[28]， which decreased significantly downward， and the average salt content in the 0-100 cm soil layer was 2-3 g/kg[29]. Related to the annual climate and irrigation amount， the total salt content increases with the rise of temperature[28]， and decreases with the increase of precipitation and irrigation amount[29].

There are three trends in the development of soil secondary salinization in the Hetao Irrigation District over time. （1） The soil secondary salinization in Hetao Irrigation District has the characteristics of inter-annual periodicity with an irreversible trend. The salt content is the highest in April， followed by June， and the lowest in July （Table 1）[30]. This feature determines the importance of utilizing salinized soil. For example， planting sunflowers can make 1/3 of the saline and alkaline land produce as many economic benefits as well cultivated land （Fig. 3）. （2） The soil secondary salinization area in Hetao Irrigation District has a decreasing trend. From 1991 to 2005， the total acreage of saline-alkali land decreased from 1 019 km2 to 814 km2， with an average annual reduction of 14.6 km2， and in detail， the heavy， moderate and mild saline-alkali land decreased by 55， 34， and 116 km2， respectively[31]. From 2006 to 2014， the overall saline-alkali degree continued to decrease[32]. In addition to the discharge of salt caused by the improvement in irrigation and drainage system， the decline of the average groundwater level in the Hetao Irrigation Area[32-34] is also conducive to reducing the salinization acreage. （3） The soil pH value in the Hetao Irrigation District increases slightly over time （Table 2）. There are two reasons， increased soil secondary alkalization and neglected prevention and control of soil alkalization.

Uneven Nutrient Distribution， Low Organic Matter Content， and Development Analysis

The soil nutrients distribute unevenly at the spatial scale in Hetao Irrigation District. From 2005 to 2008， the monitoring and investigation of soil nutrients in the Hetao Irrigation District showed that the average value of soil organic matter content was 13.9 g/kg， with a coefficient of variation of 21.58%. The average content of total nitrogen was 0.83 g/kg， having a similar coefficient of variation （20.48%） with soil organic matter. The average available phosphorus content was 12.7 mg/kg， with the coefficient of variation （50.55%） much higher than total nitrogen. This indicates that the distribution of available phosphorus is more uneven than total nitrogen. The average available potassium content was 185 mg/kg， with the coefficient of variation （26.53%） higher than that of total nitrogen while lower than that of available phosphorus[12]. The monitoring results of 16 sampling points in the Hetao Irrigation District in 2020 showed that the average available sulfur content in 0-20 cm soil depth was 159.92 mg/kg， with the coefficient of variation （112.5%） higher than that of organic matter， total nitrogen， available phosphorus， and rapidly available potassium. This indicates that available sulfur distributes quite unevenly. The main reason is that the parent material of the soil in the irrigation area is the Yellow River alluvium， which is faced with a varying deposition environment during the soil formation， resulting in the formation of the sand-clay interbedding soil structure and ever-changing soil bodies[23]. According to the nutrient classification standard for the 2nd National Soil Census， the agricultural soil in Hetao Irrigation District is low in organic matter， lacking in nitrogen and phosphorus， and rich in potassium and sulfur in general.

In the past 35 years （1986-2019）， the average organic matter content in the 0-20 soil depth was stable and slightly increased over time （10.9-13.9 g/kg） in Hetao Irrigation District （Table 3）. The reasons for the loss of agricultural soil organic matter in the Hetao Irrigation District were explained by the following aspects： the activities of soil microorganisms in the growing season of crops promoted the decomposition of soil organic matter[37]; washing salt by irrigation may lead to partial organic matter leaching[38]; and producers tended to apply chemical fertilizers and less organic fertilizers[39]. However， soil organic matter content could be increased by the silt and sediments introduced by the irrigation water of the Yellow River[40]， the application of farmyard manure （in winter， producer in the Hetao Irrigation District generally grazed sheep on the farmland， leaving sheep manure）， a large number of crop residues and stubble， straw returning to the field （the return of straw to the field in the Hetao Irrigation District was already a common phenomenon）， etc. Such a steadily increasing trend provides a solid foundation for maintaining the sustainability of agricultural soils[41].

Soil Quality Heterogeneity and Development Analysis

Soil quality， as a comprehensive concept， refers to the sustainable capacity of soil as an important ecosystem in sustaining plants， animals， and humans[43]. For the biosphere， soil quality assessment plays an important role not only in food and fiber production， but also in maintaining local， regional， and global ecosystem functions[44-45].

As a kind of cumulated irrigated soil， the cultivated layer of agricultural soil in the Hetao Irrigation District has a greatly varied clay weight， roughly dividing into the red soil， loess， foam soil， sandy soil， sand-covered soil with lower clay and upper sand， and sand-covered sand with upper clay and lower sand or leaky sand soil. The varied clay weight of the cultivated layer endows the soil with a diverse production capacity. This indicates the diversity of soil quality， called soil heterogeneity in this study. Soil heterogeneity in Hetao Irrigation District determines crop diversity. More than 30 crops have been planted in the Hetao Irrigation District， including 3 major crops （maize， sunflower， and spring wheat） and other featured crops （Table 4）[15， 23， 46-47]. For example， Cucumis melo L. was introduced from the United States to Gansu Province for trial cultivation in the 1940s[48]， but it became a specialty of Dengkou County in the Hetao Irrigation District[49]. The fundamental reason is that the soil and climate in Dengkou are the most suitable for the growth of this melon[50].

The agricultural soil quality in Hetao Irrigation District maintains a stable development trend over time. Having a history of 2000 years （due to the war， the cultivation has been interrupted several times）[51]， the fundamental cause of the agricultural soil in the Hetao Irrigation District is based on the irrigation of the Yellow River. The contemporary agricultural irrigation in Hetao Irrigation District is all achieved by "grid （furrow） field flooding irrigation"， which avoids soil erosion. Mulching with plastic film avoids the loss of agricultural soil in this area caused by wind erosion. The steadily increasing trend of soil organic matter （Table 3） maintains the stability of soil fertility. The decrease of the saline-alkali land area[31-32] ensures the stability of soil salinization degree in this region. The annual "washing salt with irrigation" （irrigation system of autumn and spring irrigation） ensures the stability of the degree of secondary salinization[52]. The negative effects of excessive fertilization， inappropriate application of pesticides， and plastic film residues on soil quality and the environment have attracted widespread attention[53-54]. Currently， the measures "water control， fertilizer control， pesticide control， and mulch control" are being taken， which have a controlled impact within a certain range. However， little attention has been paid to the possible consequences of a gradual increase in soil pH.

Soil Ecology and Development Analysis

Another major function of soil quality is its role in maintaining local， regional， and global ecosystem functions[44-45， 55]， which is called soil ecology characteristic in this study. The agricultural soil in the Hetao Irrigation District is ecological since its development and change are stable over time. Soil production performance， buffer performance， and purification performance are the keys to maintaining the stability of soil ecology. Affected by natural factors and human activities， these three properties are characterized by dynamic changes. Specifically， irrigation， application of chemical fertilizers and pesticides， mulching film， and agricultural machinery affect soil productivity. The process of irrigation and salt washing leads to the leaching of nutrients and ions， and the decrease of microbial biomass[52， 56-57]， thus affecting the soil buffering ability. Soil purification depends on the one hand on type， quantity， and activity of microorganisms in the soil， and on the other hand on the physical and chemical properties such as soil structure， soil organic matter content， temperature and humidity， and ventilation conditions[42]. The over-application of chemical fertilizers， as well as plastic film residues， can lead to soil pollution to various degrees， which leads to the decline of soil purification capacity. However， these effects are not enough to destroy the ecology of the soil in the Hetao Irrigation District. The reasons are as follows： （1） Normal production of about 667 000 hm2 of farmland carries on the ecological service functions of absorbing carbon dioxide in the air， releasing oxygen， resisting sand and dust storms， blocking desert， purifying soil， and alleviating groundwater pollution during the diverse growing seasons of crops （May-October）. Agriculture contributes to the conservation and sustainable use of biodiversity， having an extremely important ecosystem service function[58]. （2） Afforestation in Hetao Irrigation District is another important factor to stabilize soil ecological performance. The farmland shelterbelt with an area of about 579.85 km2 is laid along the water courses， field ditches， and sublateral canals， with main forest belt spacing of 100 m and main sub-forest belt spacing of 1 000 m[59-60] （Fig. 4）. The main tree species for shelterbelt are poplar， willow， jujube， and elm.

Conclusion

To sum up， the agricultural soil in the Hetao Irrigation District is a kind of cumulated irrigated soil， which originated from the sedimentary layer and its artificial maturation caused by the multiple diversions of the Yellow River. Characterized by secondary salinization of interannual periodicity， the soil shows decreased heavy salinization， increased pH value， low soil organic matter content but with stable change trend， low nitrogen and phosphorus content， high potassium and sulfur content， uneven nutrient distribution， diversified production performance， weak but stable ecological performance， heterogeneous soil quality but with stable change trend. Overall， the agricultural soil in the Hetao Irrigation District is sustainable， but some problems need to be further investigated， such as low soil buffer capacity， increased alkalinity， low purification capacity， weak ecosystem stability， soil nutrient loss， soil compaction， pollutants from chemical fertilizers and plastic film residues.

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