Effects of shell sand content on soil physical properties and salt ions under simulated rainfall leaching

Highlights

• Shell sand content can significantly affect soil particle size composition and soil texture.

• The mixed soils with shell sand-fluvo-aquic soil ratios of 1:1 and 1:2 can improve soil water storage capacity.

• The mixed soils with shell sand-fluvo-aquic soil ratios of 1:1 and 1:2 can inhibit the accumulation of soil salt.

• The mixed soil with shell sand-fluvo-aquic soil ratio of 1:1 has the best effect on water storage and salt suppression.

Abstract

To reveal the influence of shell sand content on soil physical properties and salt ion changes under simulated rainfall leaching conditions, various ratios of shell sand to fluvo-aquic soil were simulated to change the particle composition of shell sand. Basic physical and chemical properties, such as soil particle-size composition, soil bulk density, soil porosity, soil pH, soil electrical conductivity, and soil salt ions were determined and analyzed. Fuzzy mathematics was used to evaluate the potential for soil water storage and salt suppression by altering the shell sand-to-fluvo-aquic soil ratio. The results showed that with decreasing shell sand content, the soil particle composition with different ratios of shell sand to fluvo-aquic soil changed significantly. With decreasing shell sand content, bulk density and capillary porosity of the soils with different ratios increased, and the total porosity and noncapillary porosity decreased. The soil pH first decreased and then increased, following the order 0:1 > 1:0 > 1:1 > 1:2, while the electrical conductivity increased. Cl⁻, NO₃⁻, Na⁺, Ca²⁺, and Mg²⁺ showed a trend of first decreasing and then increasing with decreasing shell sand content, following the order 0:1 > 1:0 > 1:2 > 1:1. The mixed soil with a 1:1 ratio of shell sand and fluvo-aquic soil has the largest leaching loss of salt segregants. A comprehensive evaluation of the soil improvement performance with respect to the ratio of shell sand to fluvo-aquic soil revealed a trend of 1:1 > 1:0 > 0:1 > 1:2, and the mixed soil with a 1:1 ratio of shell sand to fluvo-aquic soil showed the greatest improvement. The shell sand content can significantly affect soil water and salt transport, and a suitable shell sand content can effectively improve soil water storage and salt inhibition effects.

Introduction

Shell dikes are unique ecosystems that form from shells of shellfish, whose debris accumulates in the intertidal zone under wave erosion. Shell dikes are mainly composed of marine shells and fragments and a thin layer of fine sand, silt, peat, and silt clay (Xia et al., 2020). There are two shell dikes distributed in Wudi County, Binzhou City, in the Yellow River Delta (YRD). These dikes are connected with the shell dikes in Tianjin and Hebei to form a large-scale and unique domestic shell beach ridge coast, which is similar to the shell banks in St. Louis, USA, and Suriname, South America. Known as one of the world’s three major ancient shell dikes, the YRD shell dike occupies an extremely important position in the study of global Quaternary geology and coastal geomorphology. Shell dikes can withstand waves and storm surges, provide protection for the survival of coastal residents, and are transfer stations for bird migration, wintering, habitation, and reproduction (Zhao et al., 2010). A shell dike is a ridge type of silty or silty coast; it is widely distributed in North America and the Gulf of Mexico coast, the Suriname Coastal Plain, the Blot Bay of Australia, and the west coast of Bohai Bay, Laizhou Bay, northern Jiangsu and the Yangtze River Delta in China (Xie et al., 2012, Li et al., 2016). Shell sand has important ecological functions and scientific research value in terms of the particularity of landform types and biodiversity protection.

Water and salt conditions are the main factors affecting the vegetation restoration of the shell embankment ecosystem in the YRD (Xia et al., 2014, Dong et al., 2016). Shell sand has a large porosity, poor water holding capacity, and low organic matter and available nutrients. Under the influence of vegetation coverage, temperature, and the physicochemical properties of shell sand soil, large rainfall events result in rapid water penetration into the deep layer of shell sand soil and a rapid decrease in soil surface moisture (Xia et al., 2016). A large area of shell sand suffers severe wind erosion due to no surface vegetation coverage. Improving the grain structure of shell sand to improve the water storage and soil conservation function of shell sand can provide suitable growth conditions for plants. The current research on shell dike soil in the YRD focuses mainly on the content and morphological characteristics of trace elements in shell sand (Liu et al., 2009a), the soil distribution characteristics of salt in shell dikes (Dong et al., 2016), the soil moisture transport of shell sand (Chen et al., 2019) and the fractal characteristics of soil particles (Xia et al., 2020). However, few studies examine on the effect of shell sand content on soil particle-size composition and salt ion changes under rainfall conditions in this area, resulting in a lack of knowledge regarding the appropriate water and salt conditions for vegetation restoration in the distinct habitat of shell dikes.

The soil particle size distribution (PSD) has fractal characteristics. These fractal characteristics not only characterize the size composition and pore distribution of soil particles (Gao et al., 2014, Sun et al., 2016) but also reveal soil hydraulic characteristics (Xia et al., 2017, Xiao et al., 2020), soil texture uniformity, air permeability (Ju et al., 2021, Wang et al., 2011a), and soil fertility (Lv et al., 2011). The composition of soil particles affects physical properties such as soil bulk density and porosity, as well as the distribution of soil moisture and salt ions (Montero, 2005). In saline-alkali soil mixed with yellow diluted sediment, the content of large particles is significantly reduced, the content of fine particles is significantly increased, and the soil structure is improved (Mao et al., 2009). After mixing a certain amount of sand into coastal clay saline soil, the very fine sand particles in the sediment can influence the particle composition of the clay saline soil, and the soil salt content decreases with an increasing amount of sand (Zheng et al., 2019). In the process of tidal flat soil leaching, the order of desalination rate is SO₄²⁻>Ca²⁺>Mg²⁺>Cl⁻>Na⁺>K⁺, and both the HCO₃⁻ content and pH increase (Liu et al., 2011).

Therefore, we must adopt plant measures (increase vegetation coverage) to reduce the wind erosion of shell dikes. Given the high content of shell sand and low capillary porosity in shell dike soil, the effective rainfall stored in the soil after rainfall is low, and the water storage capacity is poor. Fluvo-aquic soil in the muddy coastal zone next to the shell dike contains a higher content of silt and clay, and the soil is dense with poor permeability. We hypothesize that adjusting the content of shell sand can change the pore structure, water holding capacity and salt conditions of shell sand, and build a shell sand habitat suitable for plant growth. Adding fluvo-aquic soil to reduce the content of shell sand in shell dike soil will increase the soil content of fine grains or silty clay, thereby increasing the soil capillary function, and the soil water storage capacity, while possibly also increasing the salt content. Therefore, the appropriate ratio of shell sand to fluvo-aquic soil is a key issue to be solved in this study. The appropriate ratio can not only improve the water storage capacity of shell sand soil but also prevent an increase in soil salinity. To verify the above hypothesis, this study uses soils treated with different ratios of shell sand to fluvo-aquic soil to simulate the impact of rainfall leaching on the physical properties of the soil and salt ions. We investigated the influence of different shell sand contents on the physical properties of the soil and its water and salt contents. In addition, the impact of shell sand content on soil water and salt content was clarified. The development of a good shell sand-to-fluvo-aquic soil ratio has an important guiding role in the selection of suitable water and salt habitats for the growth of shell embankment vegetation in the YRD.