Saline–Alkali Soil Reclamation Contributes to Soil Health Improvement in China

Abstract

Soil salinization is a significant threat to soil health, especially to the agricultural ecosystem; it reduces vegetation biomass, destroys ecosystem diversity, and limits land use efficiency. This area of investigation has garnered extensive attention in China, especially in the arid and semi-arid areas, totaling 7.66 × 10⁶ ha. A variety of theoretical research and technology developments have contributed to soil water and salt regulation and the screening of salt-tolerant varieties to improve nutrient utilization efficiency and microbial control and reduce ecological problems due to saline-based obstacles. These techniques can be classified into physical treatments, chemical treatments, biological treatments, and combined treatments; these different measures are all aimed at primarily solving saline–alkali stress. In general, the improvement and utilization of saline–alkali soil contribute to soil health improvement, concentrating on high-quality development, food security, ecological security, cultivated land protection, and agricultural upgrading. However, the risks of various technologies in the practical production process should be highlighted; green and healthy measures are still expected to be applied to saline–alkali land.

1. Introduction

The term “saline–alkali soil” describes a range of soil types that contain abnormally high concentrations of soluble salt ions. These ions have a negative impact on the physical, chemical, and biological properties of the soil and plant growth features. Saline soil mainly refers to soil anions from chloride and sulfate, and alkaline soil mainly refers to soil anions from carbonate and bicarbonate [1]. Soil salinization and alkalization have already reduced soil health, and the FAO describes “salinization” as one of the biological chemical indicators of soil health [2], characterized by high pH, soluble salts, and nutrient holding capacity and availability. Furthermore, soil salinization and alkalization reduced functional microbial biomass and activity in farmland soil [3]. Alarmingly, the global extent of saline and sodic soils had expanded to 424 million ha of topsoil (0–30 cm) and 833 million ha of subsoil (30–100 cm) in 2021 [4], having changed from approximately 800 million ha in 2010 [5], indicating a rising trend over the past decade that is anticipated to persist [4]. This has emerged as a pressing global environmental concern, particularly in arid and semi-arid regions [6]. Fortunately, in China, the area of saline–alkali soil shows a decreasing trend, which is a testament to the effective improvement and utilization efforts undertaken in recent decades. According to the second national soil survey, the area of saline–alkali soil is 3.69 × 10⁷ ha, accounting for 4.88% of the total available land [7,8]. The third national soil survey revealed a reduction down to 7.66 × 10⁶ ha of saline–alkali soil in China, with this occupying 5.99% of the total available land [9]. Saline–alkali soil is predominantly distributed in three major regions. Firstly, the arid and semi-arid areas of the mid-west, such as Xinjiang, Qinghai, Inner Mongolia, Ningxia, and other areas, account for 96.1% of the total saline–alkali land, which is classified as the chloride–sulfate type [10,11]. Secondly, the northeast regions, including Jilin, Inner Mongolia, Heilongjiang, and other areas, are mainly composed of soda saline–alkali land dominated by carbonates, accounting for 3.2% of the total saline–alkali land [12,13]. Lastly, the eastern coastal region, encompassing Shandong, Jiangsu, and Hebei, which is dominated by chloride-type saline–alkaline soil, accounts for less than 1% of the total saline–alkali land [14,15]. This proves that there is great potential for saline–alkali soil improvement and utilization in China.

Saline–alkali soil improvement and utilization has always been a significant scientific and commercial problem and is of great importance for soil health. Saline–alkali soil is also of great importance to the environmental protection and sustainability of agriculture in China [16]. There is a high population but a limited quantity of land resources; meanwhile, the demand for land resources for economic development is increasing [17], which causes pressure and challenges to sustainable agricultural development. Therefore, salinity and alkalinity reduction play an important role in agricultural production development, land productivity improvement, food security, and cultivated land expansion in China [18]. Meanwhile, saline–alkali soil is an important part of the terrestrial ecosystem [19], and its physical, chemical, and biological properties, as well as the ecosystem material and energy cycling processes, are different from those of other soils [20].

Generally, the improvement and utilization of saline–alkali soil is of great importance in China due to the limited arable land resources and the growing demand for agriculture, and its great significance for improving soil health. In fact, saline–alkali farmland has already been confirmed as one of the main types of low- to medium-yield farmland in China, which is crucial for national food security. In recent years, a large number of studies have been conducted to explore appropriate improvement measures, which is the first step in the reclamation of saline–alkali soil. Further research is required to elucidate the laws of water and salt transport, finally revealing the quality and productivity level of the improved saline–alkali soil. Therefore, in this review, we have summarized the improvement measures for saline–alkali soil, the soil water and salt transport processes, and the fertility and yield of saline–alkali soil. In the last section, we put forward some expectations for the improvement and utilization of saline–alkali soil in China.

Therefore, the summary of the relevant research on saline-alkali land in China is of great significance for guiding sustainable development and utilization. We used Web of Science and China National Knowledge Infrastructure to search peer-reviewed publications between January 2001 and December 2023, including research papers, reviews, and meta-analyses, which are related to saline–alkali land in China. Different combinations of terms were used, such as “saline–alkaline soil”, “saline soil”, “alkaline soil”, “soil salinization”, and “salt-affected soil” in the title, abstract, or keywords. We only selected papers reporting field and laboratory studies on the saline–alkaline soil of China, which could contribute the soil health improvement. We mainly discussed improvement measures, the soil water and salt transport process, nutrient change, and yield improvement in saline–alkali soil. Although there are many related research articles and reviews, a comprehensive analysis specifically focused on the improvement measures and their effects on soil water, salt, nutrients, and yields for agriculture is yet to be conducted, especially the different changes in saline–alkali soil caused by different measurements, the water and salt transport process, and the enhancement mechanism of nutrients and yields. Based on the review of research on saline–alkali soil, prospects for saline–alkali soil research in China are proposed.

2. Improvement Measures for Saline–Alkali Soil

In general, the improvement and utilization of saline–alkali soil can be divided into three parts: physical measures, chemical measures, and biological measures; these are important ways to improve soil health, corresponding to soil health indicators to a high degree [2].

2.1. Physical Measures

Physical measures are always used to change the boundary and/or soil profile conditions, as shown in Figure 1, and this is highly related to the saline–alkali soil physical properties, which can reflect the flow patterns of mass and energy in the soil environment, finally determining plant development and microbiological activity [21]. Its essence is to change the upper and lower boundary conditions of the soil, and it is related to soil water-gas parameters. First, the upper boundary conditions, such as agronomic measures (including film mulch, buried layer, and irrigation) and drainage management (e.g., flood irrigation and drip irrigation), are used to change the soil water and heat flux between the soil and air. Second, the cultivation layer conditions in the soil profile, including tillage (e.g., rotary tillage and deep vertically rotary tillage) and agronomic measures (e.g., buried layer), are typically applied to improve the physical soil properties of the topsoil. The bottom boundary condition, such as subsurface drainpipes, shafts, and drainage ditches, controls the groundwater level. New methods for the physical control of salinization have emerged, such as degradable liquid films, biomass materials, and porous adsorption materials, which have developed alongside technological progress.

Based on the upper boundary conditions, there are many studies regarding the effect of mulch on soil water and salt. Mulch significantly reduces soil–water evaporation, controls and reduces soil moisture loss through evaporation, limits the upward migration of highly mineralized groundwater under capillary action, and inhibits surface salt accumulation. Mulch also shows a significant effect on soil water storage and salt suppression [22,23,24]. Plastic film significantly increases soil moisture and decreases salinity in the top 20 cm, especially during the seedling stage [25]. Furthermore, plastic mulching improves plant height and leaf area index, and the yield with mulch is also significantly higher than that without mulch [26]. Mulch types, such as plastic mulch, sand mulch, and straw mulch, have also been introduced for saline soil reclamation, and different mulch types show varying degrees of reduction in electrical conductivity; they can also increase the soil temperature and nutrient availability [27].

Flood irrigation, drip irrigation, saline water, and brackish water irrigation are the most commonly used irrigation methods. Flood irrigation has already been used in the Hetao Irrigation District of Inner Mongolia, China. Autumn irrigation is a traditional salt reduction method in this area, which is not only related to the amount and duration of irrigation water but is also closely linked to the soil freeze–thaw process [28]. Drip irrigation can effectively alleviate soil water shortages [29], and it is used to create a favorable soil water and salinity content in the root zone [30]. Saline water irrigation is recommended in arid and semi-arid areas where freshwater is limited [31], but it requires effective drainage measures; otherwise, it may increase the mineralization of groundwater and cause secondary salinization. Spring irrigation maintained the yield of spring maize when the salinity of irrigation water was <3 g/L, but it was also found that brackish water irrigation may lead to salt accumulation in the soil profile [32].

Plow layer regulation is also important for saline–alkali soil reclamation. Deep vertical rotary tillage management is one of the main tillage techniques widely used in saline soil, and it has the ability to efficiently lower pH, bulk density, and soil solution conductivity, and enhance soil structure [33]. In coastal saline zones, tillage—such as rotational tillage to a depth of 15 cm and deep tillage to a depth of 25–30 cm—is an effective management strategy for enhancing soil qualities, encouraging plant productivity, and raising financial advantages [34]. Straw is one of the most commonly used materials as the buried layer. A buried maize straw layer significantly decreases salt accumulation in the offseason [35], as the buried layer contributes to breaking the continuity of upward capillary movement and salt accumulation [36]. However, we notice that soil water content decreases significantly above the buried layer, so the buried layer must be combined with mulch [37].

Field research in Xinjiang, China, found that subsurface pipe drainage combined with drip irrigation greatly lowered the soil salt content, and the soil salt content decreased as the subsurface pipe spacing decreased after drip irrigation, which was conducted at a soil depth of 0–200 cm [38]. Shafts are always combined with subsurface pipes to collect infiltrated water. Traditional methods of reducing soil salinity include drainage ditches; open ditch drainage treatment was shown to have a stronger desalination impact than subsurface pipe drainage [39], although other research suggested the ability of ditch drainage to reduce soil salinity was only moderate [40].

In general, physical measures require lots of different agricultural machinery and are usually used to intervene in the process of water and salt transport, including soil water infiltration, surface runoff, water–heat exchange between air and soil, etc. Physical measures are the most important methods applied in saline–alkali soil reclamation, which has been widely used, and it is one of the important ways to improve saline–alkali soil health through ameliorating the soil structure, infiltration rate, water holding capacity, etc. Different areas with soil salinization call for different physical measures, one or two measures as the main approach, supplemented by supporting measures, such as water-saving irrigation combined with tillage, film mulch plus buried layer, and or deep tillage.

2.2. Chemical Measures

As shown in Figure 2, there are many soluble ions in saline–alkali soil, and they can be divided into two categories according to the characteristics of salt ions. One is saline soil, which consists of Na⁺, K⁺, Cl⁻, SO₄²⁻, etc., and the other mainly contains Na⁺, HCO₃⁻, CO₃²⁻, etc., and is defined as alkali soil with a high pH value. Soil conditioners are used to improve ion exchange, acid–base neutralization, and ionic balance. These are referred to as chemical measures [41]. The balance of the K⁺/Na⁺ ratio in soil plays an important role in maintaining crop growth [42]. The most commonly used soil conditioners mainly include calcium-containing compounds such as gypsum, which works more efficiently in alkali soil, helping to decrease pH [43], as well as acidic materials, such as potassium dihydrogen phosphate, ferrous sulfate, and aluminum sulfate, and organic acids such as humic acid and furfural residue. In the saline–sodic soil of China’s Songnen Plain, flue gas desulfurization gypsum has been widely used due to its effects on pH, electrical conductivity, and the reduction in exchangeable sodium percentage [44]. The dissolved Ca²⁺ after gypsum application can exchange Na⁺ in soil colloids, reducing the exchangeable sodium adsorbed by soil colloids [45]. Many studies have confirmed that gypsum application can improve soil yield [46,47,48]; however, the excessive use of gypsum can inhibit plant growth and reduce yield [49].

Acidic materials are used to improve the neutralization reaction; they mainly affect HCO₃⁻, then Na⁺ and Cl⁻, and the pH is reduced by 10–20% [50,51]. A substantial phosphate precipitate was produced in alkaline soil [52], but the addition of acid phosphate suppressed alkaline stress through the neutralization reaction. Humic acid fertilizer may also have an impact on bacterial and fungal community structures, particularly at the harvest stage when soil nutrient availability and root nutrient absorption are enhanced in saline soil. [53]. The application of humic acid amendments improved the yield and quality of sugar beets in saline–alkali soil, with the yield and sugar production of sugar beets increasing by 11.29% to 32.54% and 13.50% to 38.61%, respectively [54].

Biochar is one of the most used soil conditioners in saline–alkali soil. Quite apart from improving soil structure, biochar addition can replace excess exchangeable sodium in saline–alkali soil by increasing soil organic carbon and cations, thereby reducing the soil’s electrical conductivity and salt content [55]. However, biochar with a high pH can increase soil pH [56]. Three different pH levels of corn straw biochar (pH = 8.01), wheat straw biochar (pH = 6.93), and peanut shell biochar (pH = 7.71) were applied in saline–alkali soil. The application of wheat straw biochar with the lowest pH significantly reduced saline soil pH, confirming that the pH difference between biochar and saline–alkali soil may be the main reason for soil pH changes [57]. Furthermore, the carboxyl and other functional groups of biochar alleviate saline–alkali stress on saline–alkali plants [56]. Additionally, biochar altered the C:N ratio in saline–alkali soil [58,59]. The addition of peanut shell biochar in coastal saline–alkali soil reduced the absorption of Na⁺ by Kosteletzkya virginica, enhanced the absorption of K⁺, and improved potassium use efficiency [60]. Soil K⁺ content increased by 34.1% and 70.2% when 2% and 2.5% woody biochar was applied to saline–alkali soil, respectively [61]. Biochar also affects physical processes, and a low biochar application rate reduces soil water evaporation, but not at a high biochar application rate. A high application rate clearly demonstrates an increase in the soil’s ability to store water while decreasing the surface soil’s salinity and sodium adsorption ratio [62].

In addition, some polymer materials are also applied to the improvement and utilization of saline–alkali soil. Polyacrylamide ameliorates saline soil structure, especially the macroaggregate (>0.25 mm) content in coastal areas, improves soil hydraulic properties, and alleviates salt stress [63]. Overall, chemical measures have an obvious effect on soil improvement over a short time, but chemical amendments examine a wider range of potential problems for secondary pollution, and a lifecycle assessment is recommended to evaluate their safety, economy, and long-term effectiveness [41]. A pot experiment confirmed that applying chemical amendments improved soil health, and actually, the combined application of chemical and organic materials was better at improving saline–alkali soil health [64].

Above all, chemical measures aim to change the chemical processes in saline–alkali soil, and chemical amendments would improve ion content and reduce pH, but the environmental effects and influence characteristics should be evaluated and quantified because the exogenous materials may pollute soil, with these affected by the addition of compounds to saline–alkali soil. The implementation of chemical measures depends on the main chemical and nutrient characteristics in the soil, such as calcium-containing compounds usually applied to alkali soil, humic acid, and furfural residue are widely used in saline–alkali soil, which suffers from low soil organic matter.

2.3. Biological Measures

Biological measures refer to salt tolerance improvement and adaptive planting in saline–alkali soil. Root growth and root exudation improve soil physicochemical properties, increase the dry matter accumulation of plants, and remove salt from the soil through crop harvesting. There are three parts of biological measures: plant salt tolerance, soil fertility improvement via plant growth, and desalination via plant harvesting [60]. There are many salt-tolerant plants in the ecological environment, such as Spartina anglica, Phragmites australis, Suaeda salsa, Salicornia europaea, Sesbania cannabina, Tamarix chinensis, Imperata cylindrica, Pennisetum alopecuroides, Setaria viridis and Cynodon dactylon [41,65].

These salt-tolerant plants have been used as pioneer plants for the reclamation of saline–alkali soil, especially in heavily saline–alkali soil. In fact, Tamarix, Siberian white thorn, and sand jujube can reduce soil salinities, pH, and bulk densities. Their root exudates increase soil micro-organisms and enhance the activities of soil cellulase, urease, and dehydrogenase [66]. Planting salt-tolerant trees improves the physical and chemical properties of the soil to varying degrees. The results showed that, after planting, the electrical conductivity was reduced by 70–80%, the content of soil microaggregates (0.25–0.053 mm) was improved by 5.0–5.9%, the bare soil particle size was less than 0.053 mm, and electrical conductivity was 1.65 dS/m [67]. The dry salt discharge and plant salt accumulation theories were proposed to alleviate the stress of saline–alkali soil. This involves retaining low-lying areas near saline–alkali farmland for excess irrigation water and high-salinity groundwater accumulation, allowing the salt to accumulate in low-lying areas or be absorbed by plants [68,69]. In dry salt discharge systems, halophytes promote soil water and salt transport through transpiration, dispersing soil salt in open spaces and effectively improving the efficiency of salt accumulation in saline–alkali wasteland [70,71]. The results showed that Salicornia europaea and Salicornia salsa can remove 4.7 Mg/ha and 5.2 Mg/ha of salt from heavy saline soil, respectively. The accumulation efficiency of Salicornia europaea plants for Na⁺ and Cl⁻ is 2.2 to 2.3 times that of Salicornia salsa [72]. Plant salt accumulation contributed to the biological adaptability improvement of saline soil, but further observations of long-term effectiveness are needed due to the frequent salt exchange between soil and groundwater in saline soil areas.

A review concluded that saline–alkali soil mostly inhibited plant growth and development through pH and ionic osmosis [73]. Alkaline pH stress limited root growth by reducing the ethylene and auxin content [74]. Salt-tolerant plants’ root and/or leaf cells have the ability to regulate osmotic pressure; some plants’ root exudation could isolate the plants from the salt stress [42], and the metabolites, such as sucrose, amino acids, alkaloids, flavonoids, and carotenoids, could also help to deal with the stress of saline–alkali soil [75]. According to metabolome research, the halophytes Suaeda salsa and Salicornia europaea have significant quantities of branched-chain amino acids, which may help them withstand high saline–alkali stress [76]. In fact, the transgenic technique has already been applied to improve the salt tolerance of plants; for instance, the OsLOL5 gene increases transgenic rice’s tolerance to saline–alkali soil by upregulating OsAPX2, OsCAT, Os-Cu/Zn-SOD, OsRGRC2, and ROS detoxification [77]. The decreasing cost and increasing efficiency of genomics have revolutionized the understanding of the mechanism of biological processes and brought new possibilities and options to the salt tolerance of plants and high-yielding crops [42]. The P. tenuiflora genome improves the salinity and drought tolerance of cereals [78]. The manufacture of secondary metabolites, the hormone signal transduction system, and antioxidant enzymes may play a role in the tolerance of saline–alkaline soil, which is associated with differentially expressed genes [79].

Soil micro-organisms are the main participants in soil nutrient cycling; moreover, bacteria are one of the most important micro-organisms in saline soil, which affects soil health [80] because the micro-organisms contribute to soil ecological function stability. The reclamation of saline–alkali soil using micro-organisms has been confirmed as a useful method [81]. In the fertilizer market, microbial fertilizers are mainly divided into three categories: agricultural microbial agents, composite microbial fertilizers, and bio-organic fertilizers [81]. Agricultural microbial agents are fertilizer complexes with special species and porous substances such as peat [82]. Composite microbial fertilizers mainly consist of two or more effective bacterial strains [83]. Bio-organic fertilizers refer to the fermentation products of effective bacterial strains and decomposed organic matter, such as manure from poultry and livestock. Microbial fertilizers significantly improve the excretion of Na⁺ and uptake of K⁺ by plants, thereby increasing the K⁺/Na⁺ ratio in soil and improving plant nutrient absorption [84]. For instance, microbial fertilizers secrete extracellular polysaccharides on the surface of plant roots, which form bacterial biofilms that protect the roots from sodium chloride stress [85]. Volatile organic compounds released by microbial fertilizers cause a number of physiological changes in plants and increase their resistance to salt and alkali [86]. Moreover, inoculation of indigenous microalgae has been recommended as an eco-friendly and sustainable method [87].

In conclusion, biological measures contribute to salt removal, plant growth, and the integrated effects of micro-organisms, which are closely related to soil ecosystems, and it is important for keeping and/or improving saline–alkali soil health, but there are limited mature and promotable biological measures, for they are governed by biological adaptability and environmental friendliness; moreover, the efficiency of biological measures requires co-operation with other measures.

2.4. Advantages and Disadvantages of Different Measures

Based on the above content, we have summarized the advantages and disadvantages of different measures. As shown in

Table 1. The advantages and disadvantages of different measures.

Measures	Advantages	Disadvantages
Physical measures	Strong operability, simple method, Strong applicability, etc.	Water source demand, large engineering quantity, costly, etc.
Chemical measures	Material variety, producing effects quickly, etc.	Short duration, secondary pollution, etc.
Biological measures	Eco-friendly, sustainable, etc.	Time-consuming, regional specificity, etc.

, physical measures are widely used in various areas, and their principles and methods are simple and mature; however, they have highly relied on agricultural machinery, so their promotion and application are still limited. Chemical measures are more effective, especially in moderately and severely saline soils with poor soil structure and nutrient depletion; however, there are problems such as having only a single effect, short duration, and their use may result in secondary pollution. Theoretically, biological measures have been recommended as eco-friendly and sustainable methods as the plants and soil micro-organisms are important parts of the ecosystem; however, different species of salt-tolerant plants and/or micro-organisms have different climatic and environmental requirements, and inappropriate operation may reduce their improvement effect, meaning their efficiency requires co-operation with other measures. In general, film mulch and irrigation widely used in the arid and semi-arid areas of central and western China; gypsum added in alkali soil of northeast China; subsurface pipe drainage applied in the eastern coastal saline–alkali soil, which is determined by their saline–alkali characteristics (Table S1).

3. Soil Water and Salt Changes in Saline–Alkali Soil

Water and salt transport in saline–alkali soil is an essential issue during improvement and utilization (Figure 3), and soil water and salt are fundamental indicators of soil health. However, high soil water and salt stress deteriorate soil quality and inhibit the growth of plants and soil biological properties [88,89], so the regulation of soil water and salt is included in the management of soil health. Based on the decision regarding water and salt database management, soil water and salt monitoring and simulation proceed from the initial data analyses to any final decision-making. There is a common Chinese saying: “Salt comes with water, and salt goes with water”, which indicates that water is the driving force of salt. Numerous elements, including tillage, soil conditioner, irrigation, soil texture, groundwater, and climate, all have an impact on it. Ion exchange between soil particles and soil solution, diffusion in soil solution, and movement with bound water and pore water in soil are the four methods through which salt is transported with water in soil [90]. The potential energy theory is applied to study water and solute transport problems in saturated and unsaturated soils. Based on the water–salt balance theory, a simple soil–water–salt dynamic prediction model is proposed, which is characterized by avoiding the details of unsaturated water movement as much as possible when studying the salt movement process in unsaturated zones [91]. Furthermore, saline–alkali soil reclamation involves the control and management of water and salt movement [90]. Hence, the central issue of saline–alkali soil is soil water and salt transport under various factors. Below, we summarize some of the aspects involved in soil water and salt transport, such as models, irrigation, buried layers, freezing, and thawing, which are highly discussed in China.

3.1. Soil Water and Salt Simulation and Monitoring

In order to gain a better understanding of the continuous changes in water and salt, model simulation is an effective method, such as HYDRUS, SWAT (Soil and Water Assessment Tool), DRAINMOD (A Hydrological Model for Poorly Drained Soils), SHAW (Simultaneous Heat and Water Model), and COMSOL (COMSOL Multiphysics). HYDRUS is widely used in the study of saline–alkali soil; it can effectively simulate the transformation of water, multi-component solutes, and heat in both saturated and unsaturated zones [92], such as coastal saline soil under film mulch and buried layers [93], the unsaturated root zone of raised land [94], saline water irrigation under subsurface drainage conditions [95], root water uptake calculations [96], and subsurface pipe drainage in reclaiming coastal areas [97]. There is much research on soil solute transport in agricultural systems, while soil solute transport in other ecosystems is limited. Moreover, the root uptake of soil water and nutrients is a key factor affecting the transport of solutes in saline–alkali soil, and the coupling relationship between crop growth and the absorption of water, nutrients (solutes), and solute transport by crop roots are rarely considered. Moreover, almost all models need lots of measured soil parameters, which determine the accuracy of the model, and the accuracy and effectiveness of model applications vary depending on the characteristics of climate, hydrological conditions, and topography. In conclusion, improving simulation accuracy based on a coupling relationship is an urgent problem that needs to be solved, especially in the theory of water and solute transport in the future.

Remote sensing and near-earth sensing technology are important methods for the multi-element, multi-scale, and integrated monitoring of soil salinization [41]. Examples include aviation/satellite optical/microwave remote sensing images [98], magnetic earth conductivity meters [99], ground penetrating radar [100], time-domain reflectometers (TDR), and frequency-domain reflectometers (FDRs) [101,102]. For instance, a remote sensing monitoring model analyzed the spatial dynamics and main factors of soil salinization in the agricultural areas of northern Xinjiang [103]. The combination of the SWAP (Soil–Water–Atmosphere–Plant), GIS (Geographic Information System), and RS (Remote Sensing) models was used to explain the distribution of soil water and salt spatiotemporal evolution at the regional scale under water-saving irrigation conditions [104]. A linear regression model was constructed between magnetic- induced earth conductivity and soil salinity, quantitatively evaluating the spatiotemporal evolution of soil salinization in the Yangtze River Estuary region over the past 10 years [105]. Landsat imagery and magnetic conductivity meter (EM38) data were combined for the study of the spatial variability of regional soil salinity [106]. A soil salinization monitoring model was constructed using unmanned aerial vehicle multispectral remote sensing and GF-1 satellite remote sensing data, and a further-improved TsHARP scale conversion method was applied to soil salinization monitoring through unmanned aerial vehicle and satellite remote sensing upscaling [107]. Overall, model simulation, remote sensing, and near-earth sensing technologies are important ways to accurately quantify soil water and salt transport in saline–alkali soil, and future research should strengthen the collection of observation data at different scales and construct universal scale conversion functions, especially under spatiotemporal hydrological processes, changing climates, special topographic and geological conditions, etc.

3.2. Soil Water and Salt Dynamics under Irrigation

Irrigation is the driving force of soil water and salt transport in saline–alkali soil, as shown in Figure 3. In northwest China, 225 and 300 mm of winter irrigation are advised in order to desalinate the soil, promote cotton growth, and save water [108]. Film mulching plus irrigation also contribute to salt reduction in the root zone, especially in heavily saline soil [109]. It was observed that throughout the irrigation period, the soil salt distribution was more consistent below the top of the ridge than it was below the furrow. Subsequent research using saline water and mulched furrow irrigation revealed that the soil salt content of the surface soil layer beneath the top of the ridge was lower during the irrigation period than that below the bottom of the furrow [110]. Micro-sprinkler irrigation was confirmed as a potential method for alleviating soil salinization, especially in coastal saline soil in northern China [111]. A meta-analysis indicated that drip irrigation generally decreased soil salt content in the root zone by 37.7% relative to flooding irrigation, and a flow rate of 2–4 L/h was recommended in drip emitters, which could have a positive effect on salt control. Furthermore, the salinity of irrigation water should be lower than 2 dS/m [18].

Saline water irrigation is very important for agriculture in the arid regions of northwestern China [32]. It was demonstrated that saline irrigation with a salinity below 10.6 dS/m can lessen freshwater shortages during a lengthy 15-year period of saline water irrigation [112]. After 2 years of saline water irrigation under a subsurface drainage system, the soil salinity reduced annually, and there was no salt buildup in the topsoil [113]. The ridge planting system was more effective under drip irrigation with saline water when planting small shrubs and herbaceous plants in the coastal saline soils [114]. Nevertheless, a study also discovered that following saline irrigation, soil salinization did not increase (<1.0 dS/m) in the 40–60 cm soil depth, where a large number of lateral roots also germinated and spread horizontally, but it did significantly accumulate in the topsoil (crust and 0–10 cm soil layers) [115]. Regardless of the salinity of the irrigation water, heavily saline soil changed to a weakly salinized or even non-saline profile at 0–1 m [116]. The average soil salt concentration in the 1.0 m profile was 336% and 547% of the initial level after 3 years of irrigation with moderately salted and highly salted water, respectively [117]. Generally, various irrigation methods were applied to soil desalination and soil water improvement in the root zone, especially in the arid and semi-arid regions of China.

3.3. Soil Water and Salt Dynamics under Freezing and Thawing

An essential process in saline–alkaline soil is freezing and thawing. Water migrates towards the freezing front mostly due to temperature gradients in the water [118]. In the course of cooling, salt and water go toward the cold end [119], and the area with a high salt content will melt first, followed by the low-salt-content area. This phenomenon is caused by the different freezing points, resulting in salt leaching in the surface layer [120]. However, there are different results regarding the thawing process and salt reduction. It was suggested that soil salinization can occur because more salt is collected in the top soil during the freezing phase than salt leached during the warming period [121]. Particularly in coastal saline soil, the initial water content and bulk density of the soil can also have an impact on the water infiltration and desalination of melting salty ice water. As the original soil water contents and bulk densities diminish, the top soil’s salt content also decreases [122]. It was also found that meltwater from saline ice can contribute to the successive infiltration of water with different salinity, resulting in the desalination of coastal saline soil [123]. In the topsoil layers, the soil water content in the upper layer is greatly under the effect of salt-free ice compared to saline ice, but at the deeper soil layers, the situation is reversed [124]. Furthermore, the infiltration rate is faster with saline ice meltwater compared to salt-free ice meltwater when infiltrating into saline soils, and this effect increases with ice salinity level and decreases with ice sodium adsorption ratio [125]. An essential component of soil water and salt transfer occurs when ice and salt undergo phase transitions in saline soils. The findings demonstrated that salt-free soils have less liquid water content and a higher concentration of salt solution as a result of the phase transition between ice and water [126].

3.4. Soil Water and Salt Dynamics under Buried Layers

Previous research has mainly focused on the continuous soil layer, and salt accumulation in the surface layer is the main reason for salt stress, which is caused by capillary action. In order to address this problem, a buried layer is an effective method that has been used in recent years to separate the surface and subsoil, which breaks the continuity of capillary movement and changes soil water and salt migration between the upper and lower layers of soil [36]. The buried layer mainly aims to prevent soil water evaporation and salt accumulation [127]. There are many materials that are used as buried layers, such as straw, wood fiber, biochar, and peat, and straw is the most commonly used buried material. Different buried materials, amounts, and depths contribute to the different characteristics of water and salt transport. A buried straw layer can improve soil water storage in the topsoil by retarding the infiltration process [35] due to the overburden weight compaction effect caused by the buried layer [128,129]. Both a buried straw layer and a buried straw layer plus film mulch could significantly decrease the salt salinity of the top 0–40 cm, especially at the sowing stage of sunflower, and a buried straw layer plus film mulch effectively reduced salt content throughout the growth period [35]. They discovered that adding a layer of straw and gypsum from flue gas desulfurization decreased the salinity and alkalinity of the soil [130]. In comparison to the other methods, the combination of straw layer burial (6.0 t/ha) and surface mulch (3.0 t/ha) was demonstrated to be an effective method of returning straw [131]. A buried wood fiber layer plus plastic film mulch was also applied in coastal saline soil, which decreased water stress and increased efficiency in water utilization throughout the growth season, controlling soil salt content to below 2 g/kg [93]. A buried peat layer reduced the infiltration rate by 68.3% compared with the control, and the buried peat layer increased by 11.9% the 0–20 cm soil water content at the end of the infiltration stage [132]. Buried depth is another factor that affects soil salt and water. For buried layer depths of 30 cm and 50 cm, researchers discovered that when the buried layer was 30 cm deep as opposed to 50 cm deep, the amount of soil water in the 0–30 cm depth decreased more quickly. The wood fiber and biochar layers may both prevent surface soil from becoming too salted [36]. A buried layer is a proper method for soil salt reduction; however, the buried layer limits water’s upward movement, and it has difficulties in its operations. Finding effective ways to achieve salt inhibition is one of the topics for the future.

4. Nutrients in Saline–Alkali Soil

Nutrients are important indicators of soil health, including the chemical and biological aspects of nutrient holding capacity, chemical availability, the C:N ratio, and the organic matter in the biological aspect, which are defined by FAO [2]. Saline–alkali soil improvement and utilization are limited by poor nutrients. Considerable research has been conducted on soil fertility in salinized land. Soil organic carbon, nitrogen, and phosphorus are important nutrient components in soil. However, studies have shown that soil organic carbon content in the most saline soil is less than 1% [133]. The nitrogen utilization efficiency of urea in saline farmland is 14 to 29%, while the phosphorus utilization efficiency is 10 to 25%, which is lower than that of conventional farmland [134]. Generally, for saline–alkali soil as one of the main types of low-to-medium-yield farmland in China, soil fertility and/or nutrient storage capacity improvement is of great significance for grain yield increase, carbon sequestration, and increasing soil organic matter. However, the primary factors limiting agricultural productivity are soil salinity and alkalinity [135].

Improving soil fertility and nutrient storage capacity is of great significance to increasing grain production and reducing carbon emissions, especially in saline–alkali soil, as it is severely lacking in nutrients. We find that advances in soil salinity, alkalinity, and nutrients will soon follow, especially regarding the inhibitory mechanism of salinization on soil nutrient storage, the mutual feedback mechanism between soil structure adjustment and nutrient storage capacity expansion, the carbon and nitrogen stabilization mechanism of agricultural and livestock waste resource utilization, and the principle of organic carbon regulation and carbon sink capacity enhancement in saline farmland soil. We also notice that limited studies explain whether to reduce salt first or increase soil nutrients first; theoretically, soil nutrients could be improved after salinity reduction, but there are many studies that confirmed that exogenous organic matter addition contributed to salinity reduction. In this study, we think there is a positive interaction between salinity reduction and soil nutrient improvement when one of them first comes into play, along with the other.

4.1. Soil Organic Carbon and Its Improvement in Saline–Alkali Soil

The soil organic carbon (SOC) content in most saline soils is lower than 1% due to high soil salinity [133]. Soil salinization significantly reduces soil organic carbon and microbial biomass carbon by 20.6% and 36.5%, respectively [136]. The mean soil organic carbon density of natural saline–alkaline wetlands is generally lower than that in other wetlands in China [137]. It has been confirmed that saline–alkali soil amelioration can sequester more carbon [45], and micro-organisms and rhizosphere enzyme activity are what propel the conversion of organic and inorganic carbon in saline–alkaline soil [138].

In saline–alkaline soil, fertilization techniques are useful for controlling soil organic carbon [139]. Combining chemical fertilizer with sheep dung, corn straw, fodder grass, and granular corn straw increases the amount of organic waste applied to the soil, particularly in the free light fraction and organic carbon in the occluded fraction [140]. While adding nitrogen boosts aboveground biomass and encourages plant development, it has little effect on soil organic carbon stores [141]. Under organic supplements, such as humic acid plus organic amendments and biofertilizer plus organic amendments, the soil organic carbon and total C content of the 0–40 cm soil layer increase by 9–40% when compared to chemical fertilizer [135]. Furthermore, researchers concluded that soil pH was a decisive factor over the soil organic carbon in saline–alkali grassland [141]. Sludge-based vermicompost and other organically modified soils were achieved by reducing pH and salinity and increasing soil organic carbon content [140]. When there is enough carbon supplied, fungal diversity decreases, resulting in limited CO₂ emission and ensuring the input of carbon [142]. In the salinization areas of Inner Mongolia, China, the application of conditioners, such as those containing marlstone and enzymes, increased the dissolved organic carbon and fractionated organic carbon content, as well as the number of aggregates of size >0.25 mm, when compared to soil planted with Jerusalem artichoke alone [143]. Deep tillage reduced soil organic carbon accumulation, but the addition of vermicompost compensated for this reduction, significantly increasing soil organic carbon content in the saline–alkali soils [144]. Using biochar is an important way to increase soil carbon. Mineral-associated organic carbon and soil organic carbon increased with the duration of biochar application, but the particulate organic carbon content did not [145]. Rice planting contributed to an increase in soil organic matter by enriching soil microbiome diversity [146]. Planting duration also improved soil organic carbon content. It was confirmed that the carbon pool management index and carbon pool index increased with the increase in rice planting duration in the saline–alkali paddy fields in western Jilin, China [147]. However, not all soil amendments could improve soil organic carbon. Gypsum addition significantly reduced dissolved organic carbon by about 36–47%, whereas gypsum and biochar amendments could enhance the stability of soil organic matter in saline–alkaline paddies [43].

There are other factors that can further influence soil organic carbon in saline–alkali soil, such as irrigation, freezing and thawing, mulching, and the number of reclamation years. While salty water irrigation at low concentrations had no effect on soil carbon sequestration, high concentrations of saline water were detrimental to soil carbon sequestration [148]. The features of the saline–alkali soil in wetlands and the distribution of carbon may be impacted by freeze–thaw cycles. With repeated cycles, the amounts of water-soluble organic carbon, microbial biomass carbon, and rapidly oxidized organic carbon increased, decreased, and then eventually achieved a steady state [149]. Mulching with polyethylene decreased soil organic carbon by 16% and 6%, respectively, with and without organic amendment [150]. Mulching needs to be carried out in conjunction with subsurface organic amendment. Although reclamation time did not increase the amount of carbon in the soil, it did convert particle organic matter into organic matter linked with minerals [151].

4.2. Soil Nitrogen in Saline–Alkali Soil

Nitrogen is important in saline–alkali soil. Saline–alkali soil yield and economic benefits are highly related to N application and the soil salt content. When soil salinity is more than 3.5 g/kg, planting is not advised; however, N treatment might boost crop production and economic advantages when soil salinity is between 1.8 and 2.9 g/kg. When the soil salinity is between 2.9 and 3.5 g/kg, the net economic benefit is negative [152]. The nitrogen use efficiency of cotton was 26.1–47.2%, and the residual N in the soil was 7.7–14.9%, which was contributed via ¹⁵N-labeled fertilizer addition [153]. The soil nitrogen content in saline–alkali soil is influenced by multiple factors, and salinity, soil amendments, and irrigation are concluded below.

Soil salt mainly affects the migration and transformation process of nitrogen, such as nitrogen leaching and ammonia volatilization, and it restrains the root uptake of nitrogen [154]. Nitrate leaching occurs when the soil matric potential thresholds are higher than −20 kPa; excessive N fertilizer contributes to nitrate leaching [155]. Soil salinity promoted NH₃ volatilization by about 19.50–22.78% of the N input, and nitrate leaching was 32.89–43.84% in the saline soil of the Yellow River Delta, as calculated using Hydrus-2D [156]. One study found that N loss in the top soil is highly determined by anammox and linked to denitrification in the deep layer, especially in coastal saline–alkali soil. Furthermore, anammox took up 40–87.5% of total N within 0–50 cm soil, and it declined at lower than 50 cm [157]. A field experiment showed that the cumulative emissions of N₂O increased with the increase in soil salt content in the Hetao Irrigation District of Inner Mongolia, China [158]. The transformation of soil nitrogen is mostly fueled by soil microorganisms, and there is a considerable positive correlation between urease activity and soil nitrogen [159]. It was found that reducing nitrogen fertilizer application could retain more NH₄⁺-N, contributing to limited nitrate leaching, which is very important in reducing environmental pollution [160].

Different soil amendments also affect nitrogen migration and transformation. Biochar application decreased denitrification rates by 23.63–39.60%; polyacrylamide reduced denitrification and ammonia volatilization rates by 9.87–29.08% and 11.39–19.42%, respectively, whereas the ammonia volatilization rates rose by 9.82–25.58% [161]. Biochar considerably raised the NH₄⁺-N contents by 80.08% in saline–alkaline soil and encouraged the conversion of NH₄⁺-N into NO₃⁻-N [162]. Biochar could reduce NH₄⁺-N leaching by improving crop nutrient absorption and utilization [163]. Moreover, there were increases in total nitrogen (9–198%) and available nitrogen (12–49%) after biochar application [164]. Vermicompost and humic acid fertilizer showed a trend of reducing N₂O emissions in saline–alkali soil, which contributed to increasing nosZ gene copy numbers, especially in the macroaggregate microbial community [165]. Nitrogen and straw addition significantly reduced the soil salt content and increased the nutrient contents [166]. Legumes are suitable for saline soil reclamation, as they can increase the soil nutrient content due to nitrogen-fixing bacteria improvements [167]. Applying humic acid and gypsum in tandem at a weight-based ratio of 1:3 did not result in an increase in nitrogen leaching [168]. Earthworms and arbuscular mycorrhizal fungal hyphae contributed to a reduction in soil salinity and an increase in NO₃⁻-N, NH₄⁺-N, microbial biomass nitrogen, and the abundance of nifH and AOB amoA genes [169].

Irrigation affects soil mineralization rate. The organic nitrogen mineralization rate was lowered by using brackish water irrigation; the nitrogen mineralization rate and net nitrogen mineralization rate were lowered by 41.07% and 11.62%, respectively [170]. A 10-year field experiment concluded that long-term saline water irrigation significantly increased soil NH₄⁺-N concentration and inhibited N₂O emission, and it decreased soil NO₃⁻-N and total nitrogen content due to the effect of saline water irrigation on the denitrifying bacteria community [171].

4.3. Soil Phosphorus

Phosphorus plays a crucial role in plant nutrition and soil fertility and is a key element in photosynthesis and respiration, as well as nucleic acid and membrane synthesis [172,173]. The deficiency of available phosphorus limits plant production and the development of agriculture [174,175], especially in saline–alkali soil with a high pH and low phosphorus solubility, where plants suffer from salt stress and phosphorus deficiency [176].

Saline–alkali soil affects the effectiveness of phosphorus in two ways. Firstly, soil salt reduces the effectiveness of phosphorus by enhancing the adsorption of phosphorus, and this varies depending on the soil texture [177]. There is a chemical reaction between Ca²⁺ or Mg²⁺ and phosphate ions, and they then combine with hydroxyl and oxy groups to change into hydroxyphosphates and fluorophosphates that are not available to plants. Secondly, salt ions affect the absorption of phosphorus via the cell membrane, especially Cl^- [178]. Salt ions also restrict the mineralization and decomposition of organic phosphorus and reduce the effectiveness of organic phosphorus [179]. The exogenous addition of phosphorus is a promising strategy to improve plant salt tolerance. After phosphorus application, the plant K⁺/Na⁺ ratio in the leaves, stems, and roots of two alfalfa varieties significantly increased, which indicated that phosphorus fertilizer improved the salt tolerance of the plants [180]. However, it was also found that whether phosphorus addition can enhance plant salt tolerance depends on the water use efficiency and salt content [181]. Therefore, the comprehensive effects of salt stress and phosphorus deficiency will ultimately determine the plant’s salt tolerance.

Strategies for improving the effectiveness of phosphorus are of great importance in saline soil, and they can also be divided into three methods, according to Section 2. Cultivation measures can improve soil physical structure, reduce the toxicity of salt to plant roots, and promote plant phosphorus absorption. Moreover, physical measures strengthen phosphorus cycling by improving the soil structure of saline soil, enhancing ventilation and permeability, and increasing plant species diversity [182]. Soil amendments also promote phosphorus availability. Phosphogypsum could reduce the pH of the surrounding soil, which is beneficial for the dissolution and release of inorganic phosphorus [183]. The application of mineral phosphorus and organic fertilizer not only significantly increases the available phosphorus content but also improves the yield [184]. However, the chemical fertilizer of calcium superphosphate with organic fertilizer and amendment (CaSO₄) did not improve the effective phosphorus content in moderately saline soil due to the formation of calcium phosphate [185]. Reducing urea addition also improves available phosphorus content [186] and improves alkaline phosphatase activity in the 0–10 cm soil layer [187] and reducing 30% of the nitrogen content via fulvic acid addition improves the activation and cycling of phosphorus by reducing Ca₂-P and Ca₈-P [188]. Biochar addition also increases the effective phosphorus content by 40.72–84.8% compared with conventional phosphorus application [189]. The inoculation of marine phosphate-solubilizing bacteria increased soil available phosphorus content by 12.5%, and it increased by 61.2% when marine phosphate-solubilizing bacteria were combined with organic amendments [190]. Phosphorus leaching was reduced, and phosphorus availability increased via the combined use of phosphorus-accumulating and phosphorus-solubilizing bacteria [191]. Paenibacillus sp. C1, a saline–alkaline-tolerant bacterial strain, was found to have good characteristics in terms of acid production and phosphorus dissolution, especially in high salinity and alkalinity soil [192].

In general, the application of phosphorus fertilizer is an effective way to improve soil phosphorus availability and crop yield, but the application amount in saline–alkali soil should be adjusted according to the soil’s physical and chemical properties and any amendment.

5. Yield Improvement in Saline–Alkali Soil

The ultimate goal of improving and utilizing saline–alkali soil is increased production, and different improvement measures and regions have shown varying results. We summarized the results related to yield under different improvement measures in three main saline–alkali regions; it was clearly shown that crop yields in saline–alkali soil increased under effective improvement measures. Many studies have confirmed that a high yield is contributed via the combined effect of multiple measures, such as mulching, irrigation, tillage, and soil amendments.

Many studies have confirmed that yield is improved by mulch plus irrigation; the details are shown in

Table 2. Yield under mulch combined with other measures.

Measures	Yield	Location	Crop	Reference
Brackish water irrigation	6.0–8.0 Mg/ha	37°31′ N, 116°30′ E	Winter wheat	[195]
Brackish water irrigation	3.0–9.0 Mg/ha	37°31′ N, 116°30′ E	Summer maize	[195]
Brackish water irrigation	11.7–15.5 g/plant	41°35′ N, 86°10′ E	Lint yield	[196]
Drip irrigation	5100–6200 kg/ha	40°53′ N, 86°56′ E	Seed cotton	[197]
Drip irrigation with saline water	1250–3100 kg/ha	44°19′ N, 85°59′ E	Seed cotton	[198]
Cattle manure	5124–6197 kg/ha	38°46′ N, 117°13′ E	Maize	[194]
Wood fibre layer	1000–1232.7 kg/ha	37°45′ N, 118°59′ E	Seed cotton	[93]

. Mulch plus irrigation was the main reason for yield improvement due to salt leaching under irrigation; mulch reduced soil evaporation, which could reduce salt stress and improve soil water content, thus ensuring crop growth. It was found that salt stress limited cotton yield by 5.2% for every unit increase in soil EC when EC was above 7.7 dS/m [193]. Brackish water and/or saline water have been applied in saline–alkali soil due to the limiting fresh water; although the yield was improved by using these methods, there is a high risk of mineralization in groundwater and secondary salinization. Obtaining a sufficient supply of fresh water is an urgent problem in this area; maybe economic brackish water and/or saline water desalination is the proper method. Moreover, increasing soil organic matter and reducing salt accumulation by using cattle manure [194] and a buried wood layer [93] could also contribute to yield improvement.

Table 3. Yield under tillage combined with other measures.

Measures	Yield	Location	Crop	Reference
Traditional tillage and mulch	4655–5331 kg/ha	36°46′ N, 117°13′ E	Maize	[199]
Deep tillage	10,168–12,288 kg/ha (shoot biomass)	41°04′ N, 108°00′ E	Sunflower	[200]
Conventional tillage	2700–5100 kg/ha	79°25′ N, 40°01′ E	Seed cotton	[201]
Tillage with irrigation amount	45–908 kg/ha	38°47′ N, 106°20′ E	L. barbarum L.	[202]

shows yield improvement under tillage plus mulch or irrigation; it indicates that tillage methods, such as traditional tillage, deep tillage, and smash ridge tillage, represent an important way to affect crop yield in saline–alkali soil. These methods are used to loosen soil; deep tillage and smash ridge tillage can break the plow bottom layer, which contributes to plant root development and improves salt leaching. Tillage is indispensable in saline–alkali soil. Due to the uneven distribution of salt ions and rough ground, no-tillage recommended by some studies cannot be adopted in saline–alkali soil reclamation. The details show that traditional tillage with straw mulching could ultimately increase grain yield by 11.3% when compared to no-tillage or mulch [199]. Deep tillage with straw mulch could also contribute more to yield improvement than deep tillage with no mulch [200]. Moreover, it was found that crop yield increases with an increase in smash ridge tillage depth, and when compared to conventional tillage, the yield rose dramatically from 65.24% to 84.14% when using smashing ridge tillage at 60 cm [201]. Ridge planting Lycium barbarum L. with drip irrigation in saline soil guarantees a yield close to the local farmland (~900 kg/ha), which was applied in the saline–sodic wasteland of Ningxia [202]. Breaking the plow bottom using a subsoiling technique in alkali soil can effectively reduce salt content and increase crop yield [203].

Soil amendment has been widely used in saline–alkali soil, which also contributes to yield improvement. This mainly improves soil structure, increases soil organic matter, and exchanges water-soluble ions. Biochar has been widely used in saline–alkali soil, but the addition of different amounts and types of biochar may result in different yields (

Table 4. Yield of saline–alkali soil under soil amendments combined with other measures.

Measures	Yield	Location	Crop	Reference
Biochar	19–35 t/ha (aboveground biomass)	33°33′ N, 120°22′ E	Wheat	[205]
Biochar	20–39 t/ha (aboveground biomass)	33°33′ N, 120°22′ E	Maize	[205]
Corn straw biochar	5.0–7.8 t/ha	44°50′ N, 123°35′ E	Maize	[206]
Biochar	11–14 t/ha	46°37′ N, 125°11′ E	Maize	[207]
K-rich biochar	3–3.5 t/ha	37°55′ N, 118°48′ E	Wheat	[208]
K-rich biochar	5.5–7.5 t/ha	37°55′ N, 118°48′ E	Maize	[208]
Gas desulfurization gypsum	776–1428 t/ha	40°15′ N, 110°50′ E	Sunflower	[209]
Polyacrylamide	842–1531 kg/ha	38°19′ N, 117°23′ E	Cotton	[210]

). As a porous material, biochar can enhance the porosity and water-holding capacity of saline–alkali soil and reduce soil bulk density due to its large surface area and pore structure. The inorganic components contained in biochar, such as Ca²⁺ and Mg²⁺, could reduce the content and relative proportion of soluble Na⁺, thus reducing soil EC and improving yield. The organic carbon and nutrients of biochar can also increase the soil’s organic carbon and nutrient content in saline–alkali soil. Gypsum is usually used in alkali soil for a high yield (Table 4). Desulfurized gypsum has been suggested as a useful soil amendment, particularly in northeast China, west of the Songnen Plain [204]. The dissolution of desulfurized gypsum produces Ca²⁺, which displaces exchangeable Na⁺ in the soil, thereby reducing the pH value and soil alkalinity and improving soil physicochemical properties. Desulfurized gypsum can enhance the ion adsorption capacity of soil. Chemical polymer materials, such as polyacrylamide, could improve the formation of soil aggregates and enhance soil water retention capacity, ultimately increasing crop yield (Table 4).

6. Perspectives on the Improvement and Utilization of Saline Alkali Land

Based on the above contents, we also find the following: (1) Technological innovation in this area is in a bottleneck period. The existing technology mainly involves updates and optimizations of previous technologies, such as irrigation, mulching, and subsurface pipe drainage, but this also reflects the reliability and durability of such technologies. (2) The theory and technology of the green reduction of obstacles in saline–alkali land is limited by practical application. More attention is paid to agriculture; however, the ecological value of saline–alkali soil reclamation using ecological restoration methods is underestimated and/or ignored. Ecological restoration methods should be developed alongside agronomic measures. (3) The technology for improving soil fertility and expanding nutrient storage in saline–alkali land is not mature enough, and its long-term effectiveness needs to be strengthened. (4) The development and application of improved and new materials still face a challenge in saline–alkali soil reclamation to achieve increased efficiency, economy, and environmental friendliness.

7. Conclusions

Various soil improvement measures have been developed and applied to saline–alkali soil. These measures are closely related to soil health based on physical, chemical, and biological indicators. Practical measures depend on regional characteristics, such as salinity and alkalinity, soil ion composition, pH, hydrological processes, groundwater levels, soil texture, and regional policies. The technical modes of the three major saline–alkali areas of China are different due to regional differences. However, with the maturity of technology and the spreading of information, compound measures have been widely promoted in different saline–alkali soils.

With the development of saline–alkali soil research, we have also found that nutrient regulation is also very important for saline–alkali soil production. Soil organic carbon enhancement is a key factor in improving soil fertility, and it is confirmed that there is great potential for carbon sequestration in saline–alkali soil. Improvements in soil nitrogen and phosphorus content are highly related to yield, especially in the plow layer. However, the utilization efficiency of nitrogen and phosphorus is lower than that of general farmland soil due to salt and alkali stress. Overall, soil water and salt regulation represent essential steps in improving saline–alkali land and soil nutrient improvement ensures saline–alkali soil production.

Above all, the diverse types, abundant resources, and vast geographical area of saline–alkali soil in China provide unique research conditions for soil scientists and agriculturists. Research is needed to help improve saline–alkali soil health, which is of great importance to soil health in China and in the world. In the future, we look forward to strengthening the following research areas: (1) the efficient and precise control of salt, as well as safe and economical water use theory and technology; (2) reducing the risk of secondary pollution; (3) promoting nutrient storage capacity and improving nutrient efficiency; and (4) comprehensively considering the development prospects of saline–alkali land in agriculture, resources, ecology, and the environment. More work should be conducted on saline–alkali soil improvement and utilization based on the perspective of soil health.