Currently, scholars at home and abroad have proposed a series of improvement measures for saline-alkali soil, including water conservancy, physics, chemistry, and biology. Physical improvement is usually achieved by reducing soil salinity, mainly based on the principles of water and salt movement. Basic methods include soil leaching, which involves the dissolution of salts in saline-alkali soils through freshwater irrigation. The resulting salt components are then transported to deeper soil layers through infiltration or drained away through drainage measures. These methods can also be classified as water conservancy engineering restoration technologies (Kumawat et al., 2022). The chemical improvement method for saline-alkali soil is mainly achieved by adding chemical reagents that react with salt-alkali ions (mainly Na⁺) in the soil. This reduces the content of soil salt-alkali components and improves the physical and chemical properties and soil structure. Commonly used chemical reagents include gypsum/phosphogypsum, humic acid, superphosphate, peat, and vinegar residue, etc. The biological improvement method (mainly plants and microorganisms) can essentially improve the physical and chemical properties of soil, while increasing soil fertility. It is a technology that integrates economic, environmental, and ecological effects. Among them, soil microorganisms are the most dynamic component of soil, affecting soil energy flow and material circulation through metabolic activities. Many salt-tolerant and alkali-tolerant microorganisms can not only reduce soil salinity, but also have the functions of nitrogen fixation, increasing potassium and phosphate dissolution. Meanwhile, these microorganisms secrete active substances, including plant hormones, iron carriers, antioxidants, and extracellular polysaccharides, which can activate the antioxidant enzyme system in plants and promote their growth. Compared to conventional physical and chemical improvement or remediation methods, in agricultural soil improvement, the choice of technology needs to be very cautious, which also leads to many physical and chemical improvement methods being rejected due to cost, environmental protection, etc. In current research, microbial remediation of saline-alkali soil has occupied a major position in most countries. Therefore, the introduction of salt-tolerant functional microorganisms can bolster the remediation of saline-alkali soils and augment plant resistance to salt stress.

PSMs are a type of soil functional microorganism that was first discovered in the roots of farmland crops (Dey et al., 2021). It showed that in the community of salt-tolerant microorganisms, most species of PSMs are considered to be key plant growth-promoting microorganisms, which have the ability to solubilize P and K, and produce various metabolites that promote plant growth under saline conditions (such as plant growth hormones, iron carriers, ACC deaminase, and antagonists of plant pathogens) (Su et al., 2023). The most beneficial function of PSMs is to convert non-biologically available sources of P (both organic and inorganic) in soil into bio-available forms: 1. solubilization of inorganic P salts through cell-produced biological functions such as protonation, acidification, chelation, etc. 2. mineralization of organic P salts through enzyme activities (such as phosphatase, phosphonate hydrolase, phytases, and C-P lyase) (Ahemad, 2015; Hu and Chen, 2023). However, we must recognize that high salt concentration environments can lead to cell dehydration, shrinkage, and loss of activity (Rath and Rousk, 2015). Therefore, in order to improve the effect of microorganism's application in saline-alkali soil, it is necessary to screen out and select highly tolerant PSMs in saline-alkali soil during the process, and to use other technologies to maintain the activity of PSMs and their effect.

The application of gypsum in improving saline-alkali soil has been widely recognized, and the key lies in effectively reducing soil salt content through the displacement of Ca²⁺ and Na⁺ (Basak et al., 2022). Non-renewable natural gypsum minerals pose challenges for large-scale agricultural use. Meanwhile, due to industrial growth, government subsidies have decreased (especially in developing countries), and the quality of gypsum raw materials has decreased, leading to high costs of chemical remediation and further impeding the repair of saline-alkali soils (Qadir and Oster, 2004).

With the rapid development of industry, the production of gypsum and its similar by-products (such as PG and desulfurization gypsum) is gradually increasing, and the main component of these by-products is CaSO₄ · 2H₂O. PG is a solid waste from the wet phosphoric acid process, and its treatment is difficult. It is usually only stored or discarded in a centralized manner. The environmental hazards of PG mainly come from the release of P and heavy metals that are enriched in the soil or water body after large-scale accumulation, leading to high local concentrations and ecological damage. However, with the progress of process technology and the implementation of relevant policies, the content of polluting elements in PG produced in industrial production has been significantly reduced. For example, the weight percentage of heavy metals such as Pb, Cd and Cr in the PG used in the study of Chen et al. (2021) was ≤0.002 wt%. Additionally, countries such as Brazil, Spain and Lebanon have widely used PG as a soil amendment (Abril et al., 2009; Kassir et al., 2011; Costa et al., 2021).

Besides reducing salinity by replacing Na⁺, PG can also improve soil pore structure, increase water permeability, reduce soil redox potential (reduce negative values), and reduce CH₄ emissions in saline-alkali soils (Khatun et al., 2021). Armstrong (1989) compared the solubility, exchangeable sodium replacement, and clay dispersion inhibition of three calcium amendments (PG, rock gypsum, saturated gypsum solution), and the results showed that PG was the most effective in saline-alkali soil improvement. Additionally, the total P content in PG is about 2.5–7.5%, and its reuse as a P fertilizer can reduce the damage to the environment and natural resources caused by P mining. However, under alkaline soil conditions, the application of PG can lead to the precipitation of Ca–P, thereby limiting the utilization efficiency of P in PG.

It is gratifying that the utilization of PSMs to solubilize insoluble P minerals and enhance P release has been proven to be feasible. The secretion of multiple organic acids makes PSMs more suitable for working with mineral soil adsorbents than other types of plant growth-promoting bacteria. For instance, the PSMs (Burkholderia sp. strain PH10) can completely dissolve apatite crystals within 22 h (Fontaine et al., 2016). The phosphate-solubilizing B. megaterium (TBRC 1396) can solubilize up to 835.45 ± 11.76 mg/L phosphorus from struvite in 14 days (Jokkaew et al., 2022).

Multiple studies have confirmed that oxalic acid, citric acid, acetic acid, lactic acid, gluconic acid, and malic acid, which are organic acids secreted by PSMs, are the main means for dissolving inorganic insoluble phosphorus sources because these organic acids have strong acidity. Tian et al. (2022) found that Aspergillus niger (ANG) released up to 1,103 mg/L P from PG in 7 days. In addition, the rich P, Ca and organic matter in PG have been confirmed to be useful as fertilizers to promote plant growth. For example, adding 30 g/kg PG and a mixed microbial agent containing phosphate-solubilizing bacteria Bacillus megaterium var. phosphaticum and Pseudomonas fluorescens to the soil can increase the available phosphorus in the soil by ~80%, and increase the dry weight of corn (mg per plant) from 398 to 624 (Al-Enazy et al., 2017). Meanwhile, the application of PG (9 t ha⁻¹) and bacteria (A. lipoferum + B. circulance) can significantly improve the physiological status, antioxidant enzyme activity, microbial activity, nutrient absorption, and productivity of maize plants under saline-sodic soil conditions (Khalifa et al., 2021). It is worth noting that the combination of microorganisms and PG can also reduce potentially harmful elements in PG, such as Fe, Al, F, etc., through their own cells and secretions (such as, extracellular polymers) (Jalali et al., 2016; Chen et al., 2021). Therefore, the combined application of PSMs and PG for saline-alkali soil remediation is feasible and effective, as the combination of the two not only leverages their respective advantages, but also leads to mutual enhancement (Figure 1).

The improvement of saline-alkali soil with PSMs and PG, and the promotion of plant growth in saline-alkali soil, is a potentially efficient and economic measure in the future. However, when trying to implement both technologies simultaneously in practical engineering applications, we should consider the following aspects. Firstly, attention should be paid to microbial diversity, and the beneficial microbial communities should be protected and utilized to achieve comprehensive soil improvement and ecological restoration. Secondly, the utilization of waste PG should be optimized (including raw material composition, usage amount, application frequency, application process, etc.) to reduce its negative impact on soil and the environment, while exploring its potential phosphorus resource value. Thirdly, long-term effect evaluation should be emphasized, and the impact of improvement measures on soil physical and chemical properties, microbial community structure, crop growth, and other aspects should be continuously monitored to ensure the stability and sustainability of the improvement measures. Fourth, emphasis should be placed on assessing environmental risks, particularly the damage and residual effects of radioactive elements on plants, to ensure that the improvement measures do not have negative impacts on the environment and human health. Finally, attention should be paid to technological innovation and application, exploring new composite improvement technologies and methods, and improving the utilization rate and improvement effect of PSMs and PG resources, thus making greater contributions to agricultural sustainable development and ecological environmental protection.

LL: Data curation, Formal analysis, Writing – original draft. SY: Data curation, Methodology, Writing – original draft. XH: Data curation, Methodology, Writing – original draft. ZL: Methodology, Writing – review & editing. HC: Writing – original draft, Writing – review & editing.