Advances in Biological and Physical Salt-Reduction Technologies for Reclaiming Saline–Alkali Land: A Comprehensive Review with an Emphasis on China

Abstract

Soil salinization affects more than 954 Mha of arable land globally, with approximately 10–20 Mha abandoned annually. Conventional engineering and chemical remediation suffer from high water demand, salt re-accumulation, and secondary pollution risks. While this review draws primarily on the extensive body of research from China—where saline–alkali land covers approximately 99.13 Mha—it also incorporates key international case studies for comparative analysis. The review synthesizes biological and physical technologies for saline–alkali land reclamation, identifies critical challenges, and proposes an integrated remediation framework. A systematic search of Web of Science, Scopus, and CNKI databases (2000–2026) yielded 2847 records, of which 41 studies formed the systematic evidence base for the five technology clusters and 41 were retained as background references following the PRISMA framework. Because the search included the Chinese CNKI database and China contains one of the world’s largest saline–alkali land areas, particular emphasis is placed on Chinese case studies, complemented by representative international examples. Data were extracted on technology type, salt removal efficiency, crop yield, and application stage, and synthesized through quantitative cross-technology comparison. Five dominant technical clusters were summarized: (i) gene-based breeding (CRISPR/Cas, MAS) achieving 20–28% yield gains on sodic soils; (ii) halophyte phytoremediation removing 83–91% of soil salts over three growing seasons; (iii) microbial inoculants improving crop salt tolerance by 15–35% under controlled experimental conditions; (iv) agronomic rotations and straw amendment reducing topsoil salinity by 30–50%; and (v) solar-driven interfacial evaporation achieving 91.4% salt removal in a single proof-of-concept field trial at a material cost of approximately USD 0.004 per straw unit. Integrated bio-physical deployment, however, remains at the experimental scale. Combining rapid physical desalination with long-term biological remediation represents a promising research direction that requires systematic field validation before practical deployment. Key knowledge gaps include the long-term edaphic consequences of solar desalination, field-scale reliability of microbial consortia, and absence of regionally validated integrated protocols. We propose a structured roadmap with explicit timelines and policy recommendations to accelerate translation from research to practice.

1. Introduction

1.1. Background

According to the Food and Agriculture Organization of the United Nations (FAO, 2021), more than 954 million hectares (Mha) of arable land worldwide are affected by soil salinization [1,2,3], and an estimated 10–20 Mha of irrigated land are abandoned annually as a direct consequence [2,4]. In China, saline–alkali land covers roughly 99.13 Mha. The principal types include coastal saline soils along the eastern seaboard, sodic saline–alkali soils in the Songnen Plain of Northeast China, oasis saline soils in Xinjiang, irrigated alluvial saline soils in the Hetao Irrigation District, and salinized fluvo-aquic soils of the Huang-Huai-Hai Plain [5]. Following internationally accepted diagnostic conventions [4,6], saline soils are defined by an electrical conductivity of the saturation extract (ECe) ≥ 4 dS m⁻¹ at 25 °C; sodic soils by an exchangeable sodium percentage (ESP) ≥ 15 (or a sodium adsorption ratio, SAR, ≥ 13); and alkaline soils by pH ≥ 8.5. Throughout this review, “saline–alkali” (or “saline–sodic”) is used when salt accumulation and elevated sodicity/alkalinity co-occur, whereas “salt-affected” is used generically for all of these categories. Where the included studies report such parameters, they are noted; otherwise, the terminology of the original studies is retained. Global estimates of salt-affected land vary with the definition adopted (saline vs. sodic vs. saline–sodic) and the land-use denominator (total land, arable land, or irrigated land); the figures cited here follow the FAO (2021) assessment [4].

High salinity and alkalinity damage plant cell membranes, inhibit enzymatic activity, and trigger ion toxicity and osmotic stress; all of these effects curtail growth and can lead to complete crop failure [7,8]. The core mechanisms of salt perception and signaling have been extensively mapped at the molecular level, involving the SOS (Salt Overly Sensitive) pathway, the ABA-dependent signaling cascade, and ROS scavenging networks [6,9]. Interestingly, moderate salt stress enhances the accumulation of soluble sugars, organic acids, vitamin C, and lycopene in select crops such as tomato, opening a niche for premium-quality specialty agriculture on otherwise marginal saline–alkali land [10].

Existing remediation strategies fall into four broad categories: engineering, chemical, physical, and biological [11,12]. Here, “engineering” approaches denote measures requiring permanent infrastructure (e.g., subsurface drainage networks and irrigation systems), whereas “physical” approaches modify the soil surface or soil hydrology without major infrastructure (e.g., mulching, straw interlayers, and interfacial evaporation devices); some practices, such as drip irrigation combined with straw management, span both categories. Engineering approaches—principally subsurface pipe drainage and surface irrigation systems—can rapidly lower root-zone salinity. However, they consume large volumes of freshwater. They also require substantial capital investment (USD 1000–3000 ha⁻¹ for drainage installation). Furthermore, if drainage water is not properly managed, these approaches can trigger secondary salinization [5,13]. Chemical amendments, including gypsum (CaSO₄·2H₂O), phosphogypsum, humic acid, and synthetic polymers such as polyacrylamide (PAM), displace exchangeable sodium and improve soil structural stability; long-term use, however, can disrupt soil physicochemical equilibrium and accrue recurring costs of USD 100–500 ha⁻¹ yr⁻¹ [8,11]. Biological and green physical technologies are increasingly viewed as sustainable alternatives, offering the potential for lower long-term costs and a reduced environmental footprint. In a notable recent advance, Dong et al. [14] developed a straw-based, non-contact solar interfacial evaporation desalination device that achieved rapid desalination of heavily saline–alkali soil at a material cost of approximately USD 0.004 per unit.

In this review, biological remediation is defined as the deliberate use of plants, microorganisms, and their ecological interactions to reduce soil salinity or sodicity and to restore soil quality. For example, halophyte phytoremediation exports salts through harvested biomass, microbial secretion of extracellular polymeric substances, organic-acid-mediated pH reduction, and straw decomposition. It is important to distinguish such soil-remediation approaches from crop-adaptation approaches, which improve the ability of crops to grow and yield under saline conditions without necessarily altering the soil itself, such as breeding for salt tolerance, CRISPR-based genetic improvement, and inoculation that enhances plant physiological tolerance. Some approaches, notably microbial inoculation and halophyte cultivation, can contribute to both categories, and this distinction is applied consistently throughout the review.

1.2. Objectives and Scope

This review systematically synthesizes progress in biological measures and green physical desalination technologies for saline–alkali land improvement. Our specific objectives are: (i) to catalog and critically evaluate the five major technology clusters represented in the recent literature; (ii) to identify quantitative performance gaps and cross-technology synergies; (iii) to assess the economic, ecological, and scalability constraints that limit field deployment; and (iv) to propose a research roadmap that integrates technological innovation with policy mechanisms for accelerating translation. Geographically, this review places particular emphasis on China—which hosts one of the world’s largest saline–alkali land areas (≈99.13 Mha) and contributes a substantial share of the recent literature on this topic [5,15]—while integrating representative case studies from other regions (e.g., Australia, the United States, India, and the Middle East) to provide a global perspective. Figure 1 presents the conceptual architecture linking the five technology clusters into a unified “rapid desalination → bioremediation → sustainable production” coupling model.

This review makes three original contributions to the saline–alkali land remediation literature: (i) to our knowledge, it provides one of the first systematic quantitative comparisons of the five major technology clusters assessing cross-technology metrics of salt removal efficiency, time scale, cost, and maturity level; (ii) it proposes an integrated “rapid physical desalination → biological remediation → sustainable production” three-phase coupling model, grounded in ecological niche theory and functional complementarity, to overcome the temporal mismatch between biological and physical approaches; and (iii) it incorporates solar-driven interfacial evaporation desalination into the saline–alkali land remediation framework, critically evaluating its potential and limitations alongside established technologies.

2. Systematic Review Methodology and Search Strategy

This review followed a systematic search and screening protocol informed by the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses, Supplementary Materials) framework [16]. A systematic search was conducted across Web of Science, Scopus, and CNKI databases for publications from 2000 to 2026. The search strategy combined terms for saline–alkali land remediation (e.g., “saline-alkali land,” “salt-tolerant crops,” “halophyte,” “phytoremediation,” “PGPR,” “straw amendment,” “solar desalination”) using Boolean operators. Owing to the inclusion of the CNKI database and China’s prominent share of global saline–alkali land research, the retrieved literature is geographically weighted toward Chinese studies. This review therefore emphasizes Chinese case studies and technologies, while incorporating all eligible international studies to maintain a global perspective. The database-specific search strings were as follows. Web of Science Core Collection: TS = (“saline-alkali” OR “saline alkali” OR “salt-affected” OR “sodic soil *” OR “saline soil *” OR “alkaline soil *”) AND TS = (“reclamation” OR “remediation” OR “desalination” OR “phytoremediation” OR “halophyte *” OR “salt-tolerant crop *” OR “PGPR” OR “plant growth-promoting rhizobacteria” OR “straw amendment” OR “straw mulching” OR “solar evaporation” OR “interfacial evaporation”), timespan 2000–2026, document types = article, review, or patent. Scopus: TITLE-ABS-KEY((“saline-alkali” OR “salt-affected” OR “sodic” OR “saline”) AND (“reclamation” OR “remediation” OR “desalination” OR “halophyte *” OR “PGPR” OR “straw” OR “solar evaporation”)) AND PUBYEAR > 1999 AND PUBYEAR < 2027. CNKI: subject-term combinations of 盐碱地 (saline–alkali land), 盐渍化 (salinization), 改良 (amelioration), 修复 (remediation), 耐盐 (salt tolerance), 盐生植物 (halophytes) and 秸秆还田 (straw return). No language or document-type restrictions were applied at the search stage.

The initial search yielded 2847 records. After removing 747 duplicates, 2100 records underwent title and abstract screening, of which 1450 were excluded as irrelevant or lacking full-text access in English or Chinese. The remaining 650 full-text articles were assessed for eligibility. Of these, 82 references were retained, comprising 41 studies forming the systematic evidence base for the five technology clusters (14 gene-based breeding/MAS, 7 halophyte phytoremediation, 6 microbial remediation, 13 rotation/intercropping/straw, and 1 solar-driven desalination) and 41 background/supporting references used for definitions, context, and methodological framing. Figure 2 presents the PRISMA flow diagram documenting the screening process. Studies published in English or Chinese were eligible; records in other languages were excluded when no English or Chinese full text was available. Only primary research studies contributed quantitative performance data to Tables 1–3; reviews and background references were used solely for context and were excluded from quantitative comparisons, thereby preventing double counting of evidence. Although patents were included in the initial search, only one patent [17] was ultimately cited for its methodological relevance, as most relevant findings were published in peer-reviewed journals.

Data extraction and standardization. From each of the 41 evidence-base studies, two reviewers independently extracted the following data: technology cluster; study design and setting (field, greenhouse, or laboratory); replication and controls; treatment duration; and quantitative performance metrics, including salt removal efficiency (in % or absolute units), crop yield response (in % relative to control), cost estimates (in USD per hectare or per unit), and reported maturity level. To enable cross-technology comparison, the extracted metrics were standardized as follows: (i) salt removal efficiency was recorded directly from studies reporting pre- and post-treatment soil salinity; (ii) crop yield increase was calculated as the percentage difference between treated and control groups; (iii) cost estimates were converted to USD ha⁻¹ based on values reported in each study; and (iv) technology maturity was classified into three levels—low (proof-of-concept, ≤2 studies), medium (3–10 studies), and high (>10 studies or established practice). Because the primary studies differ substantially in endpoints, soil conditions, and experimental designs, no meta-analytic pooling was performed. Ranges in Tables 1–3 indicate the spread of individual study values rather than weighted estimates. Accordingly, the cross-technology comparisons presented in Table 3 are qualitative syntheses of these ranges, not statistical comparisons.

Study-quality assessment. Given the heterogeneity of study designs, a conventional risk-of-bias instrument (e.g., Cochrane RoB) was not applicable. Instead, each evidence-base study was assessed against a structured framework covering: (i) experimental design (randomized, controlled, or uncontrolled); (ii) replication (number of replicates/plots); (iii) setting (field, greenhouse, or laboratory); (iv) treatment duration; and (v) reporting completeness (quantitative data with defined units). Studies were rated as High, Medium, or Low by both reviewers, and cluster-level ratings are summarized in Table 3. These ratings were used to qualify the evidence, not to exclude studies.

Two reviewers (Z.S.L. and L.K.Y) independently screened titles and abstracts, and subsequently assessed full-text articles. Disagreements were resolved by discussion with a third reviewer (P.K.). Data were extracted independently by two reviewers using a standardized form, including technology type, salt removal efficiency, crop yield, cost, and application stage. Inter-reviewer agreement was not formally quantified (e.g., via Cohen’s κ), which we acknowledge as a limitation. Disagreements at both the screening and data extraction stages were resolved by discussion with a third reviewer (P.K.).

This review was not registered in any systematic review registry.

Systematic evidence base (n = 41): Gene-based breeding and MAS: 14 studies; halophyte phytoremediation: 7 studies; microbial remediation: 6 studies; rotations, intercropping and straw: 13 studies; and solar-driven desalination: 1 study. Background/supporting references: 41 studies.

3. Biological Approaches to Saline–Alkali Land Remediation

Biological remediation harnesses plants, microorganisms, and their ecological interactions to elevate crop salt tolerance, reduce soil salinity, and improve rhizosphere conditions. The underlying mechanisms function at two complementary levels: enhancing the crop’s intrinsic salt–alkali tolerance through genetic improvement, and employing plant root activity and microbial metabolism to ameliorate soil properties while exporting salt via biomass harvest [5,18]. Throughout this review, we distinguish three related but mechanistically different outcomes: (i) plant stress mitigation—improving the ability of crops to tolerate salinity without necessarily altering soil properties (e.g., breeding, PGPR-mediated tolerance); (ii) soil amelioration—changes in soil physicochemical properties such as pH, ESP, structure, or organic matter content; and (iii) salt removal—net export of salts from the soil system through biomass harvest, leaching, or physical desalination. This distinction is applied consistently in Section 3 and Section 4.

3.1. Breeding Salt–Alkali-Tolerant Crop Varieties: Crop Adaptation to Saline Conditions

Enhancing crop tolerance to saline–alkali stress is a central strategy for making salinized soils productive. The physiological and molecular basis of salt tolerance, including osmotic stress tolerance, Na+ exclusion, and tissue tolerance, has been comprehensively reviewed [7,9]. Transgenic technology, CRISPR/Cas-mediated gene editing, and marker-assisted selection (MAS) have all substantially accelerated the development of tolerant varieties [19,20]. Notably, ‘Zhongke-fa 5’, ‘Hai-dao 86’, and several salt-tolerant japonica rice lines perform well in soils with salt contents of 3–6 g·kg⁻¹, yielding 6.0–9.0 t·ha⁻¹ (values reported across more than twelve independent field trials in eastern China [21,22]). Because salt-tolerant cultivars improve the agricultural utilization of salt-affected land but do not, by themselves, remove salts or reduce sodicity, they are classified here as crop-adaptation technologies rather than soil-reclamation technologies.

3.1.1. Genetic Engineering and Gene Editing

Transgenic approaches have been the most extensively explored route.

Table 1. Representative genes used to improve salt–alkali tolerance in major crops, grouped by functional category.

Functional Category	Gene	Source Species	Target Crop	Effect on Salt Tolerance	Key Reference
Transcription factor	OsSTAP1	Rice	Rice	Not quantified	[23]
Stress-associated protein	OsASR6	Rice	Rice	Not quantified	[24]
Antioxidant/ascorbate	SS3	Rice	Rice	Not quantified	[25]
Ion transporter (Na+/K+)	OsHKT2 (KO)	Rice	Rice	Not quantified	[26]
Ion transporter (Na+/K+)	HKT (CRISPR)	Wheat	Wheat	Not quantified	[28]
Vacuolar Na+/H+ antiporter	AtNHX1	Arabidopsis	Cotton	25% increase	[27]
Vacuolar H+-pyrophosphatase	TsVP	T. halophila	Cotton	Co-expressed with AtNHX1	[27]
G protein γ subunit (alkali)	AT1 (KO)	Sorghum	Rice/sorghum	20.1–27.8% yield gain	[29]

Note: KO = loss-of-function knockout; CRISPR = gene editing via CRISPR-Cas9.

summarizes key target genes grouped by functional category. Overexpression of transcription factors such as OsSTAP1, the stress-associated protein OsASR6, and the ascorbate-regulating enzyme SS3 all significantly improve salt tolerance in rice [23,24,25]. Knockout of the Na+ transporter OsHKT2 reduces shoot Na+ accumulation and enhances salt tolerance [26]. Co-expression of the Arabidopsis vacuolar Na+/H+ antiporter gene AtNHX1 together with the Thellungiella halophila vacuolar H+-pyrophosphatase gene TsVP raised cotton salt tolerance by 25% relative to wild-type controls [27]. In wheat, CRISPR-Cas9 editing of HKT family ion transporters strengthens root Na+ exclusion [28]. A landmark advance came when Zhang et al. [29] cloned the G protein γ-subunit gene AT1 from sorghum through genome-wide association analysis. AT1 is highly conserved across the Poaceae (grass family), and its loss-of-function mutants boosted grain yields by 20.1% in sorghum and 22.4–27.8% in rice on sodic saline–alkali fields, providing a powerful breeding target.

3.1.2. Marker-Assisted Selection

MAS employs molecular markers tightly linked to quantitative trait loci (QTLs) to accelerate selection. Thomson et al. mapped the major salt-tolerance QTL Saltol to rice chromosome 1 and developed the co-segregating markers RM341 and RM493 [30]; introgressing Saltol into the elite variety ‘Pusa Basmati 1121’ markedly improved seedling-stage salt tolerance [31]. In wheat, genome-wide SNP scans have identified multiple loci associated with salt tolerance at germination and seedling stages [32], and MAS has since been deployed for cotton [33] and other staple crops [19].

3.1.3. Specialty Salt-Tolerant Crops and Salt-Induced Quality Enhancement

Barley (Hordeum vulgare) outperforms wheat and maize in sodium tolerance and is grown as both a food and forage crop on saline–alkali soils of northwestern China [34]. Cotton, sunflower, sweet sorghum, and sugar beet likewise display robust salt tolerance [35,36]. Importantly, moderate salt stress can elevate the quality of certain horticultural commodities: tomato fruit from saline–alkali fields accumulates more soluble sugars, organic acids, vitamin C, and lycopene under the control of the SlSnRK2.6-SlZHD8 signaling cascade [10,37]. Salt-tolerant asparagus (Asparagus officinalis) and quinoa (Chenopodium quinoa) are being promoted in coastal saline areas [17,38].

3.2. Halophyte-Based Ecological Restoration

Phytoremediation with halophytes offers unique advantages for saline soil rehabilitation [39,40]. China’s halophyte flora exceeds 400 species [40,41]. The halo-xerophyte Suaeda salsa, a typical salt-accumulating species, produces 18.8–28.5 t·ha⁻¹ of above-ground dry matter on heavily saline land (field trials in Xinjiang, n = 6 plots) and removes > 600 kg·ha⁻¹ of salt in a single harvest; after three consecutive years of cultivation, soil salinity and alkalinity fall by 83–91% [42]. Salicornia bigelovii, a halophyte species native to North America and the Caribbean, has been introduced experimentally in China for seawater-irrigated cultivation, producing 10–20 t·ha⁻¹ of biomass with seed oil and protein contents of 30% and 35%, respectively [43]. Its potential for saline–alkali land remediation in China remains under investigation. The ‘Zhongke-jing’ Sesbania cannabina lines developed by Cao’s group at the Chinese Academy of Sciences thrive in sodic soils of pH > 9 in Northeast China; when incorporated as green manure, they lower soil pH by 0.4 units and boost organic matter by more than 30% [44]. Salt-secreting species such as Spartina alterniflora and Sesuvium portulacastrum stabilize coastal sediments and improve soil structure [45,46,47]. Experimental seawater-based cultivation of Salicornia and related Sarcocornia species has also been demonstrated in other arid coastal regions, such as Israel [48]. It should be noted that reported declines in soil salinity following halophyte cultivation can reflect net salt export through harvested biomass, downward leaching, or redistribution of salts within the soil profile. Only net export constitutes permanent salt removal, and salinity reductions reported in individual studies should be interpreted with this distinction in mind.

3.3. Microbial Remediation of Saline–Alkali Soils

Under salt stress, roots actively recruit beneficial rhizosphere microorganisms by secreting organic acids, amino acids, and sugars, establishing a plant–microbe synergism that bolsters stress resistance [49,50].

3.3.1. Functional Microbial Resources and Mechanisms of Action

Plant growth-promoting rhizobacteria (PGPR) are among the most widely studied beneficial microorganisms in saline–alkali soils. Bacillus subtilis secretes indole-3-acetic acid (IAA), fixes N₂, solubilizes P, and releases K, collectively improving rice salt tolerance under both greenhouse and field conditions [51]. Pseudomonas species mitigate salt stress through exopolysaccharide secretion, activation of antioxidant enzymes, and osmotic adjustment [52]. Arbuscular mycorrhizal fungi (AMF) reduce Na⁺ accumulation in maize by modifying ion transporter expression in roots [53]. Aspergillus niger and Penicillium oxalicum mobilize sparingly soluble nutrients [54]. The photosynthetic bacterium Rhodopseudomonas palustris produces extracellular polymeric substances (EPSs) that chelate salt ions and, upon degradation, enrich soil organic matter [55]. Halotolerant bacteria and archaea remain underexplored but represent a promising resource pool [56]. The effectiveness of microbial inoculation should therefore be interpreted within the broader plant–soil–microbiome system, because microbial contributions to plant nutrient acquisition and stress tolerance are strongly conditioned by soil properties, plant genotype, environmental conditions, and rhizosphere interactions [57].

3.3.2. Multitrophic Microbial Strategies

Microorganisms improve saline soils through both direct mechanisms (EPS-mediated aggregation, extracellular enzyme-driven nutrient cycling, and organic-acid-mediated pH reduction) and indirect mechanisms, namely phytohormone synthesis, ACC deaminase activity, and induced systemic resistance [58]. Composite microbial consortia, such as those combining Bacillus amyloliquefaciens with Stenotrophomonas, consistently outperform single-strain inoculants in field trials, with reported yield improvements of 15–35% over uninoculated controls depending on crop and soil type [59]. These gains are, however, highly context-dependent: the same consortia show coefficients of variation of 30–60% across field trials, and improved crop tolerance should not be equated with demonstrated soil salt reduction (see the introductory paragraph of Section 3). Moreover, soil microbial responses to agricultural interventions may involve changes not only in community diversity but also in functional metabolic potential, reinforcing the need to evaluate microbial remediation strategies beyond short-term inoculation responses [60]. Similarly, co-applying PGPR with organic fertilizer synergistically raises soil organic matter, available potassium, and enzyme activities while reshaping the rhizosphere microbial community toward a more beneficial structure [59].

3.4. The Case for Integrated Biological Strategies

Each biological approach carries inherent limitations when deployed alone. Gene-based breeding draws on a finite pool of characterized tolerance genes and faces biosafety hurdles and regulatory barriers in many countries. Halophyte remediation demands three to five years to achieve meaningful salt reduction, generating minimal economic return during that window. Microbial inoculants suffer from environmental sensitivity, leading to inconsistent field performance with coefficients of variation of 30–60% across trials [18]. Three primary sources of variability can be identified. First, competitive exclusion by indigenous microbiota: introduced PGPR strains often face intense competition from well-adapted native microbial communities for rhizosphere niches and nutrient resources, which results in poor colonization and rapid population decline [18]. Second, environmental stress exceeding survival thresholds: soil moisture fluctuations, temperature extremes, and osmotic stress in saline–alkali soils can sharply reduce the viability of inoculated strains before they establish functional populations [50]. Third, inappropriate carrier formulation and delivery methods: the choice of carrier material (e.g., peat, biochar, alginate beads) and application technique (seed coating, soil drench, or foliar spray) significantly affects the survival and dispersal of inoculants. Standardized protocols for carrier selection tailored to specific soil types and cropping systems remain largely absent [59]. Addressing these sources of inconsistency will require systematic optimization of strain selection, formulation, and application timing under field-relevant conditions. These shortcomings underscore the need for combinatorial strategies. For instance, salt-tolerant crop varieties can be rotated with halophyte pre-treatment, while microbial inoculants can be applied during both the halophyte and crop phases. Such integration maximizes synergies and operationalizes the “remediation–production” cycle. Realizing this vision, however, will require systematic optimization of timing, spatial arrangement, and management practices across diverse saline–alkali land types. We emphasize that some of the proposed combinations (e.g., halophyte pre-treatment followed by salt-tolerant crops) are supported by field evidence, whereas others (e.g., solar desalination coupled with PGPR inoculation) remain conceptual hypotheses awaiting empirical testing. These two categories should not be assigned equal evidentiary weight.

4. Agronomic and Physical Salt-Reduction Technologies

Physical and agronomic technologies suppress capillary rise of salts and remove salinity without chemical additives, making them particularly suitable for arid and semi-arid saline areas where freshwater is scarce. Principal approaches include rotation and intercropping systems, integrated crop–livestock operations, straw mulching, incorporation and the emerging solar-driven interfacial evaporation desalination [1,12,58].

4.1. Rotations, Intercropping, and Crop–Livestock Integration

4.1.1. Halophyte–Crop Rotations and Intercropping

Rotating or intercropping halophytes with conventional crops can optimize soil micro-ecology, balance nutrient cycles, and inhibit surface salt accumulation. Documented patterns include Suaeda salsa–rice, Tamarix–cotton, Phragmites–maize, and barley–mixed grain sequences [15]. Cotton–Suaeda salsa intercropping reduces soil salinity and bulk density relative to cotton monoculture [61,62], with the halophyte removing up to 3839 kg·ha⁻¹ of salt each year [63]. Wheat–maize–alfalfa intercropping lowers salinity in the 0–40 cm layer while raising whole-system productivity [64]. Reported reductions in topsoil salinity under these systems can arise from several distinct mechanisms—net salt removal through halophyte biomass harvest, downward leaching by irrigation or rainfall, altered capillary transport, dilution, or plant uptake. These mechanisms have substantially different implications for long-term reclamation, yet few studies to date partition them explicitly.

Mushroom–vegetable rotations represent an emerging variant. Fungal mycelia secrete organic acids that neutralize soil alkalinity, and the spent substrate functions as a carbon-rich amendment. The Volvariella volvacea–tomato rotation promoted in Shandong Province boosted subsequent tomato yield by more than 10% [65]. Stropharia rugosoannulata–tomato rotation has also been explored: S. rugosoannulata, a straw-rotting basidiomycete, grows on lignocellulosic substrate without direct soil contact, permitting cultivation on saline–alkali land. Preliminary data indicate that substrate incorporation elevates soil organic matter and porosity, although the underlying mechanisms and long-term field effects remain to be systematically examined (authors’ unpublished data).

4.1.2. Integrated Crop–Livestock Systems

The “forage for salt reduction–livestock for income–manure for fertility” model has yielded tangible results. In a demonstration project in Dongying, Shandong Province, salt-tolerant forages (sweet sorghum, triticale) were grown to feed sheep, and the composted manure was returned to the fields. After treatment, soil salinity dropped from 11 to 2.6%, and soil fertility improved markedly [66].

Beyond China, similar integrated crop–livestock systems have demonstrated success in other saline regions. In Australia, the salt-tolerant grass Distichlis spicata has been integrated with sheep grazing in the Western Australian wheatbelt, reducing water table salinity while providing supplementary forage during dry seasons [67,68,69,70]. In the United States, salt-tolerant grass–beef cattle integration has been implemented in the San Joaquin Valley, demonstrating that forage production on saline land can be economically viable when coupled with livestock operations [69,71]. A 34-year long-term field experiment carried out at Ludhiana’s Punjab Agricultural University in India’s Indo-Gangetic Plain illustrated remarkable improvements in soil health, nutrient cycling, and rice–wheat system productivity under integrated nutrient management. This strategy applied farmyard manure (FYM) together with 50% of the standard NPK fertilizer rate. FYM was superior to green manure and wheat straw incorporation in elevating soil organic carbon, stimulating microbial activity, and increasing plant-available nutrients. These results confirm FYM as the most efficient organic amendment in this long-term trial for sodic soil amelioration and long-term yield preservation [72]. Collectively, these international cases demonstrate that the integrated crop–livestock approach is transferable across diverse agro-ecological and socio-economic contexts, though region-specific optimization of species selection, stocking rates, and manure management remains necessary.

4.2. Straw Mulching and Incorporation

Straw amendment is a low-cost, widely adopted practice that integrates physical and biological processes. Surface mulching reduces evaporative water loss by 30–50% and blocks capillary rise, lowering topsoil salt content by 30–50% [73]. Deep burial of straw as a 5 cm thick interlayer fractures compacted horizons and increases soil water content by 8.2–11.0%; the organic acids released during decomposition further neutralize soil alkalinity [74]. Straw return combined with drip irrigation is regarded as a preferred long-term strategy for arid and semi-arid saline areas, as demonstrated by long-term trials in North China [75,76]; analogous drip-irrigation-based salinity management has likewise proven effective in other arid regions, including the western United States [77,78]. Risks of incomplete decomposition—especially in cold or dry climates—and localized salt re-accumulation call for careful management, including C:N ratio adjustment and complementary tillage.

4.3. Solar-Driven Straw-Based Interfacial Evaporation Desalination

Dong et al. recently reported a non-contact solar interfacial evaporation device with potential as a low-cost physical desalination approach [14]. The device consists of an ordinary drinking straw wrapped with hydrophilic filter paper and capped with black agricultural film. Capillary action lifts saline pore water upward; solar energy drives evaporation, and salt crystallizes on the outer straw surface—achieving desalination with zero freshwater consumption and zero liquid discharge. As illustrated in Figure 3, the mechanism is entirely passive. As this technology cluster currently comprises a single proof-of-concept study (Table 3), the performance metrics and cost estimates reported below should be regarded as preliminary.

Field trials on heavily saline coastal soil in Qingdao, eastern China (single-site, three replicate plots per treatment), showed that three months of continuous treatment reduced soil salinity from 11.04 to 0.95 mg·g⁻¹, a 91.4% removal rate approaching that of traditional freshwater leaching [14]; these results, while promising, derive from a single site with three replicate plots per treatment and should be treated as proof-of-concept evidence. The material cost is approximately USD 0.004 per straw unit, and each unit can be reused more than eighteen times. At a deployment density of 10,000 units ha⁻¹, the material cost is approximately USD 40 ha⁻¹, excluding labor and periodic maintenance—substantially lower than the USD 1000–3000 ha⁻¹ for subsurface drainage or the annual USD 100–500 ha⁻¹ for chemical amendments reported in Chinese studies [5,13]. However, these cost comparisons are preliminary; a full life-cycle assessment incorporating manufacturing, deployment, recovery, and disposal has not been conducted.

Several limitations must be acknowledged. First, the long-term effects of salt crystallization on soil properties remain unknown. These include pore continuity, aggregate stability, and subsurface microbial communities. This is a critical knowledge gap, because crystallized salt may redissolve and re-enter the soil during rainfall events. Second, the optimal spatial density and geometric arrangement of straw units for different soil textures have not been systematically determined. Third, performance under variable climatic conditions (cloud cover, temperature extremes, wind) is unknown. Fourth, integration with field-scale farming operations such as tillage, irrigation, and harvesting remains to be prototyped. Fifth, the risk of cyclic salt dissolution and recrystallization under alternating wet–dry conditions has not been evaluated. This is particularly important during rainfall or irrigation events. If crystallized salt re-enters the soil solution during wet seasons, the net desalination efficiency over annual cycles could be substantially lower than the 91.4% removal rate reported from a single three-month trial. Long-term monitoring of salt mass balance under field-relevant precipitation and irrigation regimes is urgently needed. Sixth, the proposed deployment density of 10,000 units ha⁻¹ (approximately one straw unit per square meter) poses significant challenges for mechanized farming operations. Coordination with routine tillage, seeding, irrigation, and harvesting activities has not been addressed; the physical presence of straw units may obstruct machinery, interfere with drip irrigation lines, and complicate field access during the growing season. Engineering solutions—such as seasonal deployment and retrieval cycles, integration with raised-bed planting systems, or alternative unit geometries compatible with standard row spacing—remain to be developed. Despite these caveats, the technology warrants further investigation given its potential for ultra-low-cost, passive, zero-discharge desalination, particularly in the context of saline–alkali land management in China and similar regions worldwide. Conclusive statements on scalability, long-term soil effects, crop safety, salt disposal, energy balance, and maintenance await multi-site validation.

Saline pore water rises through the straw core via capillary action; solar radiation absorbed by the black agricultural film drives water evaporation while salt crystallizes on the outer straw surface. Performance summary: 91.4% salt removal in three months, ~USD 0.004 per unit, >18 reuses. Key uncertainties: single-site trial, soil structural impacts not assessed, field-scale deployment protocols pending [14].

Table 2. Comparative summary of major rotation, intercropping, and integrated crop–livestock systems for saline–alkali land improvement.

System Type	Pattern/Species	Salt Reduction	Yield Effect	Applicable Region	Source
Halophyte–crop rotation	Suaeda salsa → rice	83–91% (3 yr)	Stable rice yield	Coastal saline soils	[15,42]
Halophyte–crop rotation	Sesbania → rice	pH ↓ 0.4, OM ↑ 30%	Rice yield ↑ 22–28%	Sodic soils, NE China	[44]
Intercropping	Cotton + Suaeda salsa	Salt & BD ↓	Cotton yield maintained	Arid NW China	[61,63]
Intercropping	Wheat/maize + alfalfa	0–40 cm salt ↓	System yield ↑	Huang-Huai-Hai	[64]
Mushroom–vegetable	Volvariella → tomato	Alkalinity ↓	Tomato ↑ >10%	Shandong	[65]
Crop–livestock	Forage → sheep → manure	Salinity 11 → 2.6%	Meat production +	Yellow River Delta	[66]
Solar desalination	Straw evaporation	91.4% (3 months)	Wheat +7.8 Mt·yr−1	Universal (est.)	[14]

Note: BD = bulk density; OM = organic matter; est. = global extrapolation estimate.

summarizes the rotation, intercropping and crop-livestock systems in this section.

5. Toward Bio-Physical Synergy: A Conceptual Framework

A central premise emerging from this review is that biological and physical approaches, when combined thoughtfully, can produce synergistic effects that neither achieves alone. The conceptual basis for such synergy draws on ecological niche theory and functional complementarity: biological methods (genetic tolerance, microbial facilitation) address the plant’s ability to withstand stress, while physical methods (desalination, structural amendment) reduce the stress itself. In principle, coupling the two can shorten the remediation timeline, increase system resilience, and diversify economic outputs. We present this coupling framework explicitly as a conceptual research framework rather than an established remediation strategy. Although the individual components are evidence-based, the integrated deployment model has not been validated as a whole. Biological restoration should also consider the recovery of soil ecological functioning, since severe land degradation can substantially restructure microbial communities associated with biologically active soil surfaces [79].

Potential positive synergies include: (i) solar straw desalination reducing soil salinity below the threshold that constrains microbial inoculant survival (<6 g·kg⁻¹), thereby enabling PGPR establishment on previously inhospitable soils; (ii) straw amendment simultaneously providing physical salt suppression and a carbon substrate for halotolerant decomposers that further reduce pH; and (iii) mushroom–vegetable rotations combining fungal acid-secretion with crop production in a single cycle. Conversely, antagonistic interactions must also be considered. For instance, rapid salt removal by solar straw could temporarily alter the osmotic balance that salt-accumulating halophytes depend on, and heavy straw mulching may reduce soil temperature and delay microbial activity in cool climates. Systematic, multi-factorial field experiments that manipulate both biological and physical variables are urgently needed to map these interaction surfaces and identify optimal coupling regimes for each saline–alkali land type. We regard such multi-factorial field validation as the single most important prerequisite for the practical deployment of the integrated framework.

6. Critical Analysis of Current Challenges and Limitations

A quantitative comparison of the key performance metrics, cost profiles, and maturity levels of all five technology clusters is presented in

Table 3. Quantitative comparison of five technology clusters for saline–alkali land remediation.

Technology	Salt Reduction/Yield Effect	Time Scale	Cost (USD ha−1)	Applicable Soil Type	Maturity	Application Stage	Key Limitations	Supporting Studies (n)	Evidence Quality	Field Validation Level
Gene-based breeding (CRISPR, MAS)	Yield gain 20–28% (sodic soils)	2–5 years	High R&D input (>106 per trait)	Mild–moderate saline–alkali	Medium–high	Establishment → Production	Biosafety regulation; limited multi-stress data	14	Medium (mixed greenhouse/field, replicated)	Multiple field trials (rice, cotton, wheat)
Halophyte phytoremediation	Salt removal 83–91% (3 growing seasons)	3–5 years	Low (<100)	All saline–alkali types	High	Establishment	3–5 yr yield gap; biomass disposal needed	7	Medium–High (multi-site field trials)	Field-validated (regional trials, China)
Microbial inoculation	Salt tolerance↑ 15–35%	1 growing season	50–200	Mild–moderate saline–alkali	Medium	Establishment → Production	Field inconsistency; soil-dependent	6	Low–Medium (high field variability, CV 30–60%)	Limited; inconsistent field performance
Straw amendment (mulch + burial)	Topsoil salinity ↓ 30–50%	1–2 years	20–50	Arid/semi-arid regions	High	Establishment → Production	Slow decomposition in cold climates	13	Medium–High (incl. long-term trials)	Field-validated (incl. 34-yr trial, India)
Solar straw evaporation	91.4% (3 months)	3 months	~40 (materials only)	All types (not yet validated)	Low (proof-of-concept)	Rapid desalination	Single-site trial; long-term effects unknown	1	Low (single-site proof-of-concept)	Proof-of-concept only (1 site, 3 plots)

Note: Cost estimates are compiled from the reviewed literature and should be considered approximate. R&D costs for breeding are excluded from per-hectare estimates. Solar straw cost is for materials only (USD 0.004 per unit × 10,000 units ha⁻¹), excluding labor and maintenance. Quantitative values are reported exactly as given in the primary studies: ranges indicate the spread of values across studies, whereas single values (e.g., 91.4% salt removal for solar straw desalination) derive from individual studies and are not pooled estimates. See Section 2 for the data-standardization and study-quality framework.

.

Despite the considerable progress cataloged above, the practical deployment of saline–alkali land improvement technologies at meaningful scale confronts several interrelated obstacles. As summarized in Table 3, the five technology clusters exhibit markedly different cost profiles, time scales, and maturity levels, underscoring the need for context-specific deployment strategies.

First, most salt-tolerant crop varieties have been developed and evaluated under single-stress conditions (salinity or alkalinity in isolation) and perform substantially less well under the combined stresses that prevail in the field. The molecular architecture of multi-stress tolerance (encompassing Na⁺/K⁺ homeostasis, pH regulation, and reactive oxygen species (ROS) scavenging) remains incompletely characterized [80]. Moreover, cultivation protocols for specialty salt-tolerant crops (e.g., quinoa, asparagus) have not been optimized, and the mechanisms governing quality traits under field salinity are treated only in fragmented case studies, impeding the design of integrated remediation–production–quality programs.

Second, the scalability of ecological improvement models is constrained by economic realities. Halophyte phytoremediation typically requires three to five years to achieve substantial desalinization, during which the land yields minimal economic return. This temporal mismatch sharply limits farmer adoption in smallholder-dominated agricultural landscapes [81]. Biological restoration should also consider recovery of soil ecological functioning, since severe land degradation can substantially restructure microbial communities associated with biologically active soil surfaces [79]. Rotation and straw return practices lack region-specific standardization: optimal incorporation depth, rate, and timing differ markedly between coastal saline soils (high clay, fast decomposition) and sodic soils (high exchangeable sodium, slow decomposition). Incomplete straw decomposition can immobilize nitrogen and favor pathogenic fungi, and localized salt re-accumulation has been documented under certain management regimes. Mushroom–vegetable rotations, though promising, remain at the pilot-demonstration stage with few replicated, multi-site field validations [82].

Third, the solar straw desalination technology, despite its impressive preliminary metrics, requires validation on at least four fronts: (i) long-term physical and chemical consequences of salt crystallization for soil pore architecture, hydraulic conductivity, and microbial habitat; (ii) optimal spatial density and arrangement across soil textures; (iii) performance under variable climatic conditions; and (iv) integration with field-scale operations. As noted in Section 4.3, the cost comparison with conventional methods is based on limited data and should be interpreted cautiously.

Fourth, and most critically from a systems perspective, an integrative framework for deploying biological and physical technologies in concert does not yet exist. The “soil improvement → crop production → quality enhancement” value chain has not been quantitatively modeled at farm or regional scales [82]. These gaps define the research agenda moving forward.

7. Research Outlook and Proposed Roadmap

Building on the critical analysis in Section 6, we propose a structured roadmap organized around three strategic priorities, with explicit timelines and policy recommendations. Figure 1 provides a visual synthesis.

Priority 1: Discovery and safe deployment of salt–alkali tolerance genes (target: 3–5 years). The AT1 and GS3 genes [29] are promising targets for CRISPR/Cas-mediated genome editing across cereal crops. We recommend (i) systematic functional characterization of AT1 orthologs in wheat, maize, and millet to evaluate conservation of alkali tolerance mechanisms; (ii) development of transgene-free edited lines via ribonucleoprotein (RNP)-based CRISPR delivery to circumvent regulatory restrictions; and (iii) pyramiding AT1 with established salt-tolerance QTLs (Saltol, Nax1/2) through combined MAS and genomic selection to produce varieties with robust, multi-stress resistance. Table 1 provides a template for organizing target genes by functional category, which should be expanded as new candidates emerge.

Priority 2: Development and long-term testing of integrated bio-physical remediation systems (target: 5–10 years). We envision a phased coupling model: (a) a rapid desalination phase (0–3 months) employing solar straw technology to reduce soil salinity below 3 g·kg⁻¹—the approximate tolerance threshold of most salt-tolerant crop varieties; (b) a biological establishment phase (3–12 months) featuring halophyte planting or salt-tolerant crop cultivation supported by microbial inoculants; and (c) a sustainable production phase (>12 months) that integrates rotation cycles with periodic straw amendment and reduced-rate solar desalination as needed. We recommend that this model be tested in multi-site, randomized complete-block trials (>5 years duration) at a minimum of three representative locations: a coastal site (e.g., Qingdao, Shandong), a sodic site (Songnen Plain, Jilin), and an irrigated alluvial site (Hetao District, Inner Mongolia). Monitoring should include soil physicochemical properties, microbial community dynamics (via 16S/ITS amplicon sequencing), crop yield and quality, and full economic cost–benefit accounting. The conceptual basis for synergy outlined in Section 5 should be tested explicitly: for instance, a 2 × 2 factorial design (solar desalination: with/without × halophyte: with/without) would reveal interaction effects.

Priority 3: Technology demonstration, standardization, and policy translation (target: 5–15 years). Region-specific technical protocols should be co-developed with farmers and extension agencies through participatory research. Standardized demonstration bases in the three aforementioned regions should validate performance under real-world conditions and serve as training hubs. Critically, to accelerate adoption, enabling policy mechanisms must be established. These include: (i) integrating saline–alkali land remediation into carbon credit markets. For instance, soil organic carbon sequestration under halophyte–crop rotations could generate tradable credits at an estimated 0.5–2.0 t CO₂-eq·ha⁻¹·yr⁻¹ [44,59,66]; (ii) providing targeted subsidies for early adopters of integrated bio-physical systems, analogous to payment for ecosystem services (PES) programs; (iii) establishing public–private partnerships to scale up solar straw manufacturing and distribution; and (iv) developing salt crystal collection and disposal protocols, including mechanical harvesting of crystallized salt from straw surfaces, periodic replacement of saturated straw units, and assessment of collected salt as a potential industrial feedstock (e.g., for deicing or chemical production), to prevent recontamination and enable circular utilization of the removed salts. Economic analyses comparing the cost–benefit profiles of individual versus bundled technologies across the three target landscapes are urgently needed to inform both farmer decision-making and national policy frameworks.

8. Conclusions

Soil salinization threatens global agricultural sustainability. This review evaluated five technology clusters for saline–alkali land reclamation: gene-based breeding, halophyte phytoremediation, microbial inoculation, agronomic rotations with straw amendment, and solar-driven interfacial evaporation. Biological approaches have achieved notable progress—CRISPR-edited varieties carrying AT1 show reported yield gains of 20–28% on sodic soils, halophytes achieve reported salt removals of 83–91% over three years, and microbial consortia improve salt tolerance by a reported 15–35%. Physical technologies offer complementary advantages: straw amendment reduces topsoil salinity by 30–50%. Solar straw desalination achieved 91.4% salt removal in a single proof-of-concept field trial at ~USD 0.004 per unit. Critically, no single technology can address this multidimensional challenge alone. An integrated bio-physical coupling model—combining rapid physical desalination with sustained biological remediation—offers a promising research direction that requires systematic field validation. Key knowledge gaps include the long-term edaphic effects of solar desalination, field-scale reliability of microbial inoculants, and absence of regionally validated protocols. The proposed roadmap, organized around three strategic priorities with 3–15 year timelines, provides a framework for accelerating translation from research to practice through interdisciplinary collaboration and enabling policy mechanisms.

This review has several limitations: (i) only English and Chinese literature were included; (ii) the review was not prospectively registered, and inter-reviewer agreement was not formally quantified; (iii) no formal risk-of-bias assessment was conducted due to heterogeneous study designs; instead, a structured study-quality framework (experimental design, replication, setting, duration, and reporting completeness) was applied as described in Section 2; and (iv) the quantitative comparisons in Table 3 integrate evidence of heterogeneous quality, so the values should be interpreted as indicative ranges rather than pooled effect estimates.