Effects of earthworm casts on water and salt movement in typical Loess Plateau soils under brackish water irrigation
Introduction
Soil salinization results in severe land degradation and reduces agricultural sustainability in arid and semi-arid areas (Daliakopoulos et al., 2016). To ensure land resource security and to meet global food demand, issues relating to land use and soil salinization must be addressed. Multiple measures have been implemented to reduce soil salinization over the last few decades, including the application of soil improvers (Mahmoodabadi et al., 2013, Hasini et al., 2019) and drip irrigation, which leaches salt from the topsoil (Zhang et al., 2014). The biological and combined repair effects of desalinization plants in saline-alkali soils can also effectively prevent soil salinization (Nouri et al., 2017, Feng et al., 2019). Moreover, suitable soil remediation materials can minimize soil salinization, optimize soil structure, and promote plant growth. For example, the use of biochar in saline-alkali soils was found to stimulate plant growth by reducing the specific adsorption rate and surface area of sodium and lowering soil electrical conductivity (EC) (Saifullah et al., 2018). Altering the bulk density of soil can also promote soil salinity leaching; gypsum modifiers can reduce the total salt content in soil (Suhayda et al., 2010).
Several studies have also demonstrated the impact of earthworm casts on soil water content and salt ions (Mcinerney and Bolger, 2000). Earthworm casts have nonhomogeneous surfaces and large specific surface areas and are rich in both nutrients and beneficial microorganisms. Studies have shown that earthworm casts facilitate aggregation and water stability in soils, which promote soil insulation and water preservation (Molina et al., 2013, Aksakal et al., 2016). Additionally, earthworm casts contain more organic matter and nutrients than many other biological fertilizers and can increase the water-holding capacity of the soil (Van Groenigen et al., 2019). Earthworms decompose organic matter in the soil, which results in the decomposition of mineral elements (Lim et al., 2014). Earthworm casts are therefore rich in mineral elements and usually have high cation exchange capacities (CEC) (Clause et al., 2014). Moreover, the increase in soil porosity and permeability after adding earthworm casts is conducive to salt loss, which leads to a decrease in the soil EC (Elvira et al., 1998). The application of earthworm casts also increases the calcium content of soil, which can replace exchangeable soil sodium and ameliorate soil salinity (Hanay et al., 2004). Moreover, earthworm casts can effectively aggregate soil particles and improve soil structure (Tejada et al., 2008). The quantity and spatial arrangement of differently sized aggregates determine the distribution of soil pores, which in turn affects the hydraulic characteristics, aeration, retention, permeability, tillage, and water–salt movement in soils (Dexter, 1988, Bronick and Lal, 2005).
Due to limited water resources and over-exploitation of groundwater, brackish water is an important resource in farmland irrigation in North and Northwest China (Chen et al., 2018, Liu et al., 2019). Therefore, considerable research attention has turned toward studying groundwater quality and brackish water irrigation. Xu et al. (2013) estimated that the Loess Plateau in the northwest region of China contains 3 billion m3 of brackish water resources with a salinity of 2–5 g L–1. He et al. (2019) assessed the hydrochemical characteristics of groundwater in the Luohe aquifer, in Wuqi County on the Loess Plateau and found that the mean total dissolved solids were at 1311.0 mg/L, indicating brackish water. Analysis of 36 shallow groundwater samples, conducted by Li et al. (2019), showed total dissolved solids ranging from 123.10 to 1276.00 mg/L and total hardness of 123.10–1276.00 mg/L. Compared to freshwater irrigation, brackish water irrigation is more likely to cause salt accumulation in the soil. Appropriate measures, therefore, are required to prevent soil salinization. Previous studies have shown that bio-organic fertilizers, such as green manure and chicken manure, can effectively ameliorate soil salinization by increasing soil CEC, and promoting salt ion leaching (Chaganti and Crohn, 2015, Geng and Boufadel, 2015). However, it remains unclear whether, and to what degree, the addition of earthworm casts influences water and salt movement under brackish water irrigation in the northwest China region. Therefore, this study used two typical soils from the Loess Plateau as examples and investigated the effects of earthworm cast amendment under brackish water irrigation on soil physical and chemical properties, including (1) soil infiltration and evaporation, (2) soil salt distribution and movement, and (3) the sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) of the soil surface layer (0–20 cm).
Section snippets
Soil and earthworm casts
We analyzed two typical soil types from the Loess Plateau: (1) anthrosol (CS) and (2) cambisol (AS). CS (Earth-cumuli-Orthic Anthrosol) has a terric horizon derived from organic manure and loess materials (FAO, 2014). The CS and AS samples were collected from the upper 0–60 cm of soil in wheat fields of Yangling (108°2′ E, 34°4′ N) and Shenmu (110°52′ E, 38°83′ N), respectively. The basic physico-chemical properties of the soil samples are listed in Table 1. Undisturbed downcore soil samples
Effects of earthworm casts on soil infiltration and evaporation
The dynamic characteristics of the soil-wetting front over time are shown in Fig. 1. The soil type, earthworm cast treatment, and their interactions significantly influenced the movement of the wetting front (p < 0.05; Table 4). The application of earthworm casts reduced the migration velocity of the wetting front in all treatments, except for the 1% treatment in CS. In CS, the 3% and 5% treatments significantly increased the time taken for the wetting front to reach the column base, by 23.9%
Effect of earthworm casts on soil infiltration and evaporation
The infiltration capacity and evaporation intensity of soil are mainly affected by characteristics such as soil texture, structure, and initial water content (Gill and Jalota, 1996, An et al., 2018). The application of earthworm casts as an exogenous material affects soil water movement in two ways: (i) earthworm casts promote the formation of soil aggregates, reduce BD, and alter the water movement channels in soil (Manivannan et al., 2009, Li et al., 2021), and (ii) the pore structures of
Conclusions
The application of earthworm casts significantly influenced water and salt movement in soil, and its effects were closely related to soil type and cast treatment concentrations. Earthworm casts reduced both evaporation and the rate of water infiltration in both anthrosols and cambisols; in most cases, the reduction was enhanced with more concentrated cast treatments. Earthworm casts also significantly altered the soil structure, which facilitated the leaching of salt ions, reducing the salt
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, China (Grant No. XDA23070202).
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