Microstructure characteristics of soils with different land use types in the Yellow River Delta

Study on changes in microscopic structure of saline-alkali soil induced by different land usage in the Yellow River Delta is important to understand their consequence for soil mechanical and hydrological processes and improve soil management and remediation. This paper presents the results of an experimental study on microscopic structure of soils taken from farmland, grassland and beach at different soil depths in this region. The particle size and mineral composition of each soil sample were measured using the particle size distribution analysis and X-ray diffraction analysis, respectively. The pore-size distribution was analyzed using mercury intrusion porosimetry, and soil particle arrangement and pore characteristics were measured using scanning electron microscopy (SEM). The results showed that the main particles of the three soils were silt, accounting for more than 60% of the total particle composition. The clay content in the three soils increased in the order of farmland > beach > grassland. Primary minerals such as quartz, calcite and albite were dominant in the three soils, accounting for more than 85% of their total mineral composition. The clay minerals in the three soils were dominate by illite, and the proportion of the clay minerals was less than 15% of the total mineral composition. The clay mineral content was higher in the farmland soil than in the grassland and beach soils and, as such, the mineral composition alone cannot explain the low permeability of the soils. The porosity of the farmland, beach and grassland soils was 25%, 32.58%-39.94% and 40.84%-46.16% respectively. With increasing depth, the porosity of the grassland and beach soil increased, whereas the change in farmland soil porosity did not show an identifiable trend. The pore characteristics of the grassland and beach soils were similar, while the farmland soil was dominated by micropores and ultra-micropores with pore diameter less than 2 μm. The specific pore surface area of the farmland soil was considerably larger than that of grassland and beach soils, with the adsorption capacity of the former much higher than that of the latter. Farmland soil was densely structured; it was dominated by micro-fissures, with disordered structures formed between the skeleton particles; its basic structural units were flat and flaky particles; the skeleton particles ware dominantly surface contact while the particles were cemented by clay minerals. The microstructure of the grassland soil was closely inlaid particles with a "brick-wall" arrangement, and the particle connection was mainly surface contact; it was composed of closely inlaid massive particles and overhead pores; there was no cementation between its skeleton particles. The beach soil was composed of closely accumulated granular particles and intergranular pores; there was no cementation between skeleton particles. These results improved our understanding of the changes in microscopic structure of saline-alkali soils in response to land usage in the Yellow River Delta. They provide guidance for remediation, utilization and development of coastal saline-alkali soil in the Yellow River Delta. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.