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College of Environment & Planning, Liaocheng University, Liaocheng 252000, China

Abstract　This study aimed to provide a basis for the rational improvement of the microstructure of the soil that had been planted with crops for a long time. Conventional and micro-morphological methods were used to study the effects of planting grain and cotton crops on soil properties in Gaotang County, Shandong Province. The results showed that long-term cultivation of grain and cotton crops has a significant impact on soil micro-structure. The top soil (0-10 cm) of wheat-maize field has higher organic matter content, uniform aggregate size, high separation, and strong packing void connectivity, forming a type micro-structure that is conducive to agricultural production. The characteristics of soil matrix are mainly inherited from soil parent materials, and the impact of crop cultivation on soil matrix is very weak. The amount of formations in the soil is less, and only Fe nodules, Mn nodules and excrement are observed.

Key words　Crop, Topsoil, Soil micro-morphology, Gaotang County, Shandong

1　Introduction

Land use is generally considered as one of the important factors of global change. Large-scale transformation of natural vegetation into agricultural land and increased intensity of agricultural soil management have caused changes in climate, soil, water resources, vegetation and biodiversity. At present, soil effect studies have been widely reported in the literature, covering soil erosion[1], carbon-nitrogen circulation[2-3]and physical properties[4-5]. Among them, soil micro-morphological analysis can better help understand the process of soil structure formation and provide assistance for selecting and improving soil management activities. At the same time, some small changes that cannot be measured by physical and chemical methods can be observed directly in thin slices, so the soil micro-morphology has become a very valuable research method[6]. Researchers at home and abroad have studied the characteristics of soil micro-morphology under different land use types and management methods. Xia Yanlingetal.[7]quantitatively evaluated the soil micro-morphology of forest land in desert area, forest-grain intercropping and unused land of Culai Mountain in Shandong Province. Shi Yingetal.[8]analyzed the soil micro-morphological characteristics of forest land, garden land and vegetable plots on the representing alluvial fan in the foothills of Xiaoxishan Mountain in Beijing. He Haiyanetal.[9]studied the soil micro-morphological features of 30 years of arable land, two years of abandoned land, 20 years of man-made poplar forest land and natural meadow soil in the black soil region of Hailun City, Heilongjiang Province. Pang Jianglietal.[10-12]studied the micro-morphological features of the artificial orchards and arable land in Bailu Plain, irrigated and non-irrigated arable land in the Guanzhong region, and forest land returned from farmland in the eastern Guanzhong region of Shaanxi Province. Qin YSetal.[13]studied the effects of different fertilization methods on the micro-morphological characteristics of surface soil of Sichuan purple soil and found that organic-inorganic fertilizer application significantly improved soil structure. Sun Zenghuietal.[14-15]analyzed the soil micro-morphological characteristics of farmland and greenhouse vegetable field under the three planting patterns (regular planting, pollution-free planting and organic planting) in Quzhou, Hebei. Sun Jingweietal.[16]studied the eight years of different farmland restoration measures (tillage and organic material application) on the micro-morphology of newly formed soil aggregates of black soil parent materials in Hailun City, Heilongjiang Province. Jeffrey Letal.[17]studied the composition, micro-morphology and distribution of man-made objects in the soil of Detroit, and explored the systemic relationships with the land use history. Ryszard Metal.[18]studied the effects of human activities on the formation of soil and plough layers in urban areas through micro-morphological features. Olena Zetal.[19]applied micro-morphological methods to study the characteristics and decomposition of particulate organic matter in the forests and grasslands of Catalonia, and identified nine organic-related barriers. The studies above cover the soil micro-morphological characteristics under a variety of land use types and management measures. However, there are few studies on the differences in soil micro-morphological features under different crops. Therefore, with the fields planted with grain (wheat-maize) and cotton (widely planted during 1980s-1990s, now planted in local areas) crops as the research object, the micro-morphological features of soil under long-term cultivation of different crops were explored to provide a basis for reasonably improving the microstructure of soil that has been cultivated with crops for a long time.

2　Materials and methods

2.1OverviewofexperimentalsiteGaotang County is located in the northeastern part of Shandong Province (116°01′-116°28′ E, 36°39′-37°07′ N), with a total area of 960 km2. It belongs to the warm temperate continental monsoon climate with annual average temperature of 13℃, highest temperature in July of 32.2℃, lowest temperature in January of -7.8℃, and average annual rainfall of 589.3 mm. The rainfall in June-August accounts for 67% of the total throughout the year. The landform in the county is different types of slightly uplifted alluvial plain of Yellow River. The average elevation is 27 m. The soil is composed of fluvo-aquic soil and a small amount of sandy soil, and is suitable for the cultivation of various crops. The main crops are maize, wheat and cotton.

2.2SamplecollectionThe soil samples were collected from Gaotang County, Shandong Province. In October 2016, the fields that had been planted with grain (wheat-maize) and cotton crops for more than 30 years in Liubaihu Village, Gaotang County were selected. The profile of the soil was dug out, and the profile characteristics of each soil layer were observed and recorded. The samples were divided into two categories: whole rock samples for analysis of soil physical and chemical properties and original samples (PVC tube with outer diameter and height of 5 cm) for analysis of soil micro-morphology.

2.3ExperimentalmethodsThe samples were air-dried, ground and sieved in the room for use. (i) Determination of soil properties. The pH of the soil was measured using acidity meter. The content of organic matter was determined by potassium dichromate oxidation-external heating method. The composition of soil particles was analyzed with the straw method. The soil bulk density was determined with the ring knife method. (ii) Analysis of micro-morphology. The original samples were fixed in 198# unsaturated polyester resin + curing agent, sliced, ground roughly, ground finely, and stuck to slides with epoxy resin. Then, the other side was sliced, ground roughly, and ground finely. The slips were stuck with fir balsam. Finally, thin pieces of soil with a thickness of about 30 μm were made. They were observed under a polarizing microscope and photographed. The soil micro-morphology was described using the Georges Stoops terminology system[20].

3　Results and analysis

3.1SoilprofileandphysicochemicalpropertiesAs shown in Table 1, due to the influence of the alluvial parent material of the Yellow River, the soil layer of the experiment site is deep, and the particle composition is in a vertical change. The soil texture in the plough layer (0-20 cm) is sandy clay, in which the content of viscous particles is 15%-20% and the content of sand is more than 55%. The color is yellow, and it changes slightly with changes in texture and organic matter content. The plough layer is looser, with more roots and pores. Rust streaks and spots can be seen on the lower layer. The natural water content is 20%-30%.

Table 1　Profile characteristics and main properties of different soil layers of soils planted with different crops

3.2SoilmicrostructureandporesSoil microstructure refers to the size, shape and arrangement of particles, pores and aggregates,i.e., part of the soil barrier only associated with solids and pores[20]. As shown in Fig.1, the 0-10 cm and 10-20 cm soil layers of the wheat-maize and cotton fields are dominated by granular microstructure, and bulk and hole-like microstructures are observed occasionally. The development intensity and separation degree of aggregates of each soil layer are quite different. The aggregates in the surface soil (0-10 cm) of the wheat-maize field are the best, and they are coated with pores, which should be related to high organic matter content. The size of aggregates in the surface soil (0-10 cm) of the wheat-maize field ranges from 50 to 200 μm, more uniform. In other soil layers, the aggregates are mostly larger than 200 μm. The pores in each soil layer are mainly composite pores. In the monolithic structure, holes and planar pores are observed occasionally. The differences in the pore shape, size and anastomosis between the soil layers are obvious. In the surface soil (0-10 cm) of the wheat-maize field, the pores have a larger quantity and the best connectivity.

3.3MatrixandorganicsAs shown in Fig.2, when 2 μm is used as the boundary between coarse and fine particles, the coarse and fine particles (c/f) in each soil layer are distributed according to the pattern of open phenocryst embedding. The coarse particles are mainly quartz, of which the shapes are subrounded and rounded, with a small amount of plate-shaped and columnar feldspar and mica. There is more sand larger than 20 μm, consistent with the analysis result of soil particle composition, and the spatial arrangement is random. The fine particles are all gray siliceous clay, of which the red color of iron dyeing is more obvious. The type of fabrics is all rough spot double refraction. Due to the influence of straw returning in recent years, organic matter of different decomposition levels, different sizes exits in all soil layers, and its number accounts for about 5% of the slices (Fig.3). There are similarities in the types, shapes, sizes and related distribution of coarse and fine particles in all soil layers, indicating that they are prone to parental inheritance, and the impact of different crops is small.

Note: a. maize, soil of 0-10 cm depth; b. maize, soil of 10-20 cm depth; c. cotton, soil of 0-10 cm depth; d. cotton, soil of 10-20 cm depth. PPL, 10 × 10.

Fig.1Granularmicro-structureofdifferentlayersofsoilsplantedwithdifferentcrops

Note: a. maize, soil of 0-10 cm depth; b. maize, soil of 10-20 cm depth; c. cotton, soil of 0-10 cm depth; d. cotton, soil of 10-20 cm depth. XPL, 10 × 10.

Fig.2c/fdistributionandfinematerialsdoublerefractionofdifferentlayersofsoilsplantedwithdifferentcrops

3.4SoilformationsSoil formation is a non-continuous fabric unit, and it distinguishes itself from adjacent substances in the concentration of one or more components or internal fabrics[20]. Affected by frequent farming, the amount of formations in the plough layer of the experimental site is not large, dominated by Fe nodules, Mn nodules and excrement (Fig.4). The former reflects the moist environment of soil in summer and autumn, and the latter comes from earthworms and other soil animals.

Note: a. maize; b. cotton. PPL, 10 × 10.

Fig.3Organicmatterinthetop10cmofsoilsplantedwithdifferentcrops

Note: a. Mn nodule; b. Fe nodule. PPL, 10 × 10.

Fig.4Fe/Mnnodulesinthetop10cmofsoilsplantedwithdifferentcrops

4　Conclusions and discussions

Long-term planting of grain and cotton crops in the study area has a certain effect on some properties and micro-morphology of soil. It is mainly manifested in the high organic matter content and the well-developed microstructure of the surface layer of the wheat-maize field. Aggregates of uniform size and high separation degree and pores with strong connectivity are formed, conducive to agricultural production. However, the differences in the composition of the soil particles, the spatial distribution pattern of the coarse and fine particles, and the shape and type of the particles are insignificant, which mainly inherit the same parent material. The impact of crop cultivation is very weak. Affected by frequent farming, the amount of soil formations is small, and only formations caused by moisture conditions and animal activities are found.

Wheat-maize rotation has formed a soil micro-structure that is more conducive to agricultural production. The reason is that more high-density plant roots have accelerated the development of aggregate structure in the soil. After the crops are harvest, more crop roots, as well as the straw of crops are returned to the farmland, so the organic matter content of the soil is increased, further promoting the development of agglomeration structure[21-22]. Therefore, in the agricultural production, the types of crops should be increased as much as possible, and the multiple cropping index should be increased to improve the structure of the soil and promote the sustainable development of agricultural production.