Coastal saline-alkali land green manure combined desalting and fertilizing method based on multistage screening

Abstract

The invention belongs to the technical field of saline-alkali soil improvement and agricultural biology, and particularly relates to a coastal saline-alkali soil green manure combined desalting and fertilizing method based on multistage screening. According to the method, sand culture and water culture are combined, different NaCl concentration gradients are set, the salt tolerance of green manure crops is evaluated, and green manure varieties suitable for different salinization degrees are screened out; planting the screened green manure varieties in different areas according to the salinization degree of the saline-alkali soil; after the green manure grows to a proper period, turning and returning to the field to realize soil desalination and fertilization. According to the method, multi-dimensional agronomic characters such as the emergence rate, the root length, the plant height and the fresh weight are comprehensively evaluated through a multi-stage screening method combining sand culture primary screening and water culture secondary screening, green manure varieties suitable for mild, moderate and severe salinized soil are scientifically screened out, the defects that in the prior art, green manure selection is blind, and a targeted evaluation system is lacked are overcome, and the method is suitable for large-scale popularization and application. And accurate variety support is provided for biological improvement of the coastal saline-alkali land.

Description

Technical Field

[0001] This invention belongs to the field of saline-alkali land improvement and agricultural biotechnology, and more specifically, it relates to a method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening. Background Technology

[0002] Currently, coastal saline-alkali land serves as an important reserve of arable land, and its improvement and utilization are of great significance to ensuring food security. Existing saline-alkali land improvement technologies mainly include physical, chemical, and biological methods. Among them, biological improvement, which promotes desalination and fertilization by planting salt-tolerant plants (green manure), has attracted much attention due to its environmental friendliness and strong sustainability.

[0003] However, the existing technological system still has significant shortcomings: on the one hand, relevant research is mostly concentrated in inland or other types of saline-alkali land, and focuses on the specific salt composition of coastal areas (with Na+ as the main component). + Cl - There is a lack of systematic research on the salt tolerance (mainly salt tolerance) and climate conditions, and the technology is not targeted enough. On the other hand, there is a shortage of green manure resources suitable for this region, and there is a lack of a scientific and efficient salt tolerance evaluation and screening system, which leads to blind selection of green manure varieties.

[0004] In addition, existing technologies mostly adopt single green manure planting or microbial inoculant application, lacking a systematic technical solution that combines salt-tolerant green manure screening, zoned and appropriate planting, and synergistic application of specific microbial inoculants. This fails to fully leverage the synergistic effect of the "microorganism-plant" symbiotic system, resulting in problems such as unstable effects, long cycles, and low efficiency in the improvement process, making it difficult to meet the needs of rapid and large-scale governance. Summary of the Invention

[0005] To address the problems of existing technologies, such as the lack of a systematic green manure screening system for the specific environment of coastal saline-alkali land and low improvement efficiency, this invention provides a multi-level screening method for combined green manure desalination and fertilization of coastal saline-alkali land. This method solves the problems of the existing technologies and achieves scientific screening of green manure resources and rapid desalination and fertilization of saline-alkali land.

[0006] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of this invention is to provide a method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening, the steps of which include: Under different NaCl concentration gradients, the salt tolerance of green manure crops was evaluated by combining sand culture primary screening and hydroponic secondary screening, and green manure varieties suitable for different degrees of salinization were screened out. Green manure varieties selected for planting in zones based on the degree of salinization of saline-alkali land; When the green manure has grown to a suitable age, it should be plowed into the field to achieve soil desalination and fertilization.

[0007] This invention, through a technical process that combines a multi-level screening system with field verification, selects green manure varieties suitable for different degrees of salinization, solving the problem of blind selection of green manure in existing technologies. Through targeted planting and returning to the field, it significantly improves the desalination efficiency and fertilization effect of coastal saline-alkali land.

[0008] Furthermore, the initial screening of sand culture includes: filling 3.5 kg of dry sand into a 2.5 L capacity bucket (16 cm upper diameter, 13 cm lower diameter, 17.5 cm height, no holes at the bottom), designing 5 replicates for each NaCl concentration, planting 3 green manure plants in each bucket, and using an aqueous solution of the corresponding NaCl concentration as daily irrigation water when the sand surface shows whiteness. In addition, the sand of each treatment is soaked in the aqueous solution of the corresponding NaCl concentration before the experiment, and the germination rate, root length, plant height and fresh weight of the green manure are observed and recorded.

[0009] Furthermore, the hydroponic re-screening includes: a light intensity of 15000 LUX, a CO2 concentration of 800 ppm, a daytime temperature controlled at 20-25℃, a nighttime temperature controlled at 15-18℃, a daytime light duration of 14 h, a nighttime light duration of 10 h, a daytime humidity controlled at 60%-70%, a nighttime humidity controlled at 70%-85%, hydroponics is carried out in the corresponding NaCl concentration, and the germination rate, root length, plant height, and fresh weight of the green manure are observed and recorded.

[0010] Furthermore, the different NaCl concentration gradients include five gradients: 0wt%, 0.2wt%, 0.3wt%, 0.5wt%, and 0.8wt%.

[0011] This invention, by setting a reasonable concentration gradient, can accurately evaluate the growth performance of green manure crops under different salt stresses, ensuring the accuracy of the screening results.

[0012] Furthermore, the salt tolerance evaluation includes germination rate, root length, plant height, and fresh weight.

[0013] Furthermore, the selection of green manure varieties suitable for different degrees of salinization includes: Summer green manure crops suitable for severely saline-alkali land include sesbania and sorghum; winter green manure crops suitable for severely saline-alkali land include ryegrass, sweet clover, alfalfa, tall grass, white clover, and red clover. Summer green manure crops suitable for mild to moderate saline-alkali land include Sudan grass, sunflower, Indian cowpea, Laba bean, and Indigofera tinctoria; winter green manure crops suitable for mild to moderate saline-alkali land include radish, rapeseed, and vetch. Suitable summer green manure crops for slightly saline-alkali land include black beans, bamboo beans, and black mung beans; suitable winter green manure crops for slightly saline-alkali land include milkvetch, crimson clover, horsehair fern, and gnaphalium affine.

[0014] The summer and winter green manure varieties screened by this invention, suitable for severely saline-alkali land, have extremely strong salt tolerance and can grow normally in severely saline-alkali land, providing suitable species resources for the biological improvement of severely saline-alkali land. This invention has screened out summer and winter green manure varieties suitable for mild to moderate saline-alkali land and mild saline-alkali land, providing a diverse range of green manure options that are conducive to flexible configuration according to specific planting seasons and crop rotation systems.

[0015] In this invention, the classification criteria for severely saline-alkali land, moderately saline-alkali land, and slightly saline-alkali land are based on salt concentration (degree of salinization) as follows: Slightly saline-alkali land: 0.15wt%-0.3wt% (excluding); Slightly to moderately saline-alkali land: 0.3wt%-0.5wt% (excluding); Severely saline-alkali land: ≥0.5wt%.

[0016] This invention maximizes the desalination and fertilization potential of green manure by precisely matching specific green manure varieties with soils of corresponding salinity levels.

[0017] Optionally, the green manure varieties selected for planting can be planted as a single variety or a mixture of multiple varieties.

[0018] By setting planting methods, this invention provides a flexible planting model. Mixed sowing can take advantage of the complementary effects between species to further improve biomass accumulation and soil cover.

[0019] Furthermore, the process of planting the selected green manure varieties also includes a step of mixing the arbuscular mycorrhizal fungal fertilizer with the green manure seeds at a mass ratio of (1-10):10.

[0020] This invention introduces arbuscular mycorrhizal fungal fertilizer, which utilizes the synergistic effect of microorganisms and plants to further promote the growth of green manure, enhance its stress resistance, and significantly improve soil microbial activity.

[0021] Optionally, the preparation steps of the arbuscular mycorrhizal fungal fertilizer include: The powders of *Bombyx mori*, *Bombyx umbellatus*, *Bombyx mori*, and *Bombyx mori* were mixed evenly in a mass ratio of 1:3:2:1 to obtain arbuscular mycorrhizal fungal fertilizer.

[0022] This invention constructs a highly efficient mycorrhizal symbiotic system by combining specific strains and mixing them in specific proportions, which can significantly improve the colonization ability and improvement effect of green manure in saline-alkali environments.

[0023] Furthermore, the method of turning over and returning to the field is: turning over and pressing the green manure into the soil when the green manure has grown to its full flowering period or before harvesting the intercropped crops.

[0024] This invention selects the time when green manure biomass is at its maximum or when it is combined with key agricultural operations to return it to the field, ensuring efficient input of organic materials and rapid improvement of soil physical and chemical properties.

[0025] The present invention discloses the following technical effects: This invention establishes a systematic green manure screening system for the specific environment of coastal saline-alkali land. Through a multi-stage screening method combining sand culture for initial screening and hydroponic screening, five NaCl concentration gradients (0wt%, 0.2wt%, 0.3wt%, 0.5wt%, and 0.8wt%) are set up. A comprehensive evaluation of multiple agronomic traits, such as germination rate, root length, plant height, and fresh weight, is conducted to scientifically screen green manure varieties suitable for mild, moderate, and severe salinization soils. This overcomes the shortcomings of existing technologies, such as blind selection of green manure and lack of a targeted evaluation system, providing precise variety support for the biological improvement of coastal saline-alkali land.

[0026] This invention achieves rapid desalination and significant fertilization effects. It can significantly reduce soil EC value, significantly reduce soil pH value, and significantly increase soil organic matter content within 90 days, thereby significantly improving soil physicochemical properties and enhancing soil water and fertilizer retention capacity.

[0027] The synergistic application of arbuscular mycorrhizal fungal fertilizer and green manure in the method of this invention produces a significant synergistic effect. Specifically, in terms of yield increase, the synergistic application of green manure and arbuscular mycorrhizal fungal fertilizer significantly increases the yield of cash crops compared to green manure alone. Regarding microbial activity, green manure alone increases the number of soil microorganisms, while the synergistic application further significantly increases this number. In terms of soil fertility improvement, the synergistic application significantly increases the organic matter content compared to green manure alone. This synergistic effect far exceeds the simple sum of the effects of applying the two technologies individually, demonstrating the unique advantages of the microbial-plant symbiotic system in the improvement of saline-alkali land.

[0028] This invention significantly improves the rhizosphere microenvironment through the synergistic effect of green manure root exudates and arbuscular mycorrhizal fungi, promotes the formation of soil aggregates, and enhances soil aeration and water permeability; it greatly enhances soil microbial activity and diversity, and accelerates the cycling and transformation of soil nutrients; through the synergistic effect of root organic acid secretion and microbial metabolites, it promotes the release of insoluble nutrients and salt leaching in the soil, thus achieving overall improvement of the soil ecosystem at the microscopic level.

[0029] The method of this invention does not require complex engineering measures and can achieve rapid improvement of saline-alkali land through biological measures. It is suitable for promotion and application in Fujian and similar coastal saline-alkali areas, and has good economic and ecological benefits. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 In Example 3, tomatoes were intercropped with ryegrass in severely saline-alkali land. Detailed Implementation

[0031] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0032] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0033] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0034] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0035] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0036] Unless otherwise specified, all raw materials and reagents involved in the specific embodiments of this invention are commercially available products. Specifically, the *Bombyx mori* powder, *Bombyx umbellatus* powder, *Bombyx mori* powder, and *Bombyx mori* powder used in the arbuscular mycorrhizal fungal fertilizer of this invention are all commercially available products, and their fungal content is not less than 10%. 8 cfu / g Unless otherwise specified, room temperature and ambient temperature in the specific embodiments of this invention refer to 20-30℃.

[0037] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0038] The sand culture screening method involved in this invention includes: filling 3.5 kg of dry sand into a 2.5 L capacity bucket (top diameter 16 cm, bottom diameter 13 cm, height 17.5 cm, no holes at the bottom), designing 5 replicates for each NaCl concentration, planting 3 green manure plants in each bucket, and using an aqueous solution of the corresponding NaCl concentration as daily irrigation water when the sand surface shows whiteness. Before the experiment, the sand of each treatment is soaked in an aqueous solution of the corresponding NaCl concentration, and the germination rate, root length, plant height and fresh weight of the green manure are observed and recorded.

[0039] The hydroponic re-screening involved in this invention includes: a light intensity of 15000 LUX, a CO2 concentration of 800 ppm, a daytime temperature controlled at 20-25℃, a nighttime temperature controlled at 15-18℃, a daytime light duration of 14 h, a nighttime light duration of 10 h, a daytime humidity controlled at 60%-70%, a nighttime humidity controlled at 70%-85%, hydroponics is carried out in the corresponding NaCl concentration, and the germination rate, root length, plant height and fresh weight of green manure are observed and recorded.

[0040] In this invention, the salt tolerance evaluation comprehensively assesses the salt tolerance of green manure crops by considering multiple agronomic traits, thereby improving the reliability of variety selection. The evaluation criteria are shown in Table 1. Table 1 Evaluation Indicators and Measurement Methods The average salt tolerance index of the four indicators is taken to obtain the comprehensive salt tolerance index. Then, the salt tolerance levels are classified as shown in Table 2. Table 2. Classification of Comprehensive Salt Tolerance Levels The selection criteria for varieties (corresponding to different degrees of salinization) are shown in Table 3.

[0041] Table 3 Comprehensive evaluation process: Sand culture preliminary screening: Sand culture experiments were conducted at five NaCl gradients of 0wt%, 0.2wt%, 0.3wt%, 0.5wt%, and 0.8wt%, and various indicators were measured. The salt tolerance index was calculated, and varieties that met the salt tolerance requirements were preliminarily screened.

[0042] Hydroponic re-screening: The varieties that passed the initial screening were evaluated again under hydroponic conditions to verify their salt tolerance stability.

[0043] Based on comprehensive judgment, only varieties that meet the corresponding salt tolerance requirements in both sand culture and hydroponics can be identified as green manure varieties suitable for that degree of salinity.

[0044] In the sand culture primary screening and hydroponic secondary screening experiments, each plant was used as a control at a 0% concentration, and the salt tolerance index was calculated.

[0045] For each indicator, the salt tolerance index is calculated using the following formula: Salt tolerance index = (measured value under a certain salt concentration treatment) / (measured value under the control (0% NaCl) treatment) 100%.

[0046] In some specific implementations, this invention provides a method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening, the steps of which include: S1. Under different NaCl concentration gradients (0wt%, 0.2wt%, 0.3wt%, 0.5wt%, and 0.8wt%), the salt tolerance of green manure crops was evaluated (seedling rate, root length, plant height, and fresh weight) by combining sand culture primary screening and hydroponic secondary screening, and green manure varieties suitable for different degrees of salinization were screened out. Suitable summer green manure crops for severely saline-alkali land include sesbania and sorghum; suitable winter green manure crops include ryegrass, sweet clover, alfalfa, tall grass, white clover, and red clover. Suitable summer green manure crops for mild to moderate saline-alkali land include Sudan grass, sunflower, Indian cowpea, Laba bean, and Indigofera tinctoria; suitable winter green manure crops include radish, rapeseed, and vetch. Suitable summer green manure crops for slightly saline-alkali soils include black beans, bamboo beans, and black mung beans; suitable winter green manure crops include milkvetch, crimson clover, horsehair fern, and gnaphalium affine.

[0047] The classification criteria for severely saline-alkali land, moderately saline-alkali land, and mildly saline-alkali land are based on salt concentration (degree of salinization): mildly saline-alkali land: 0.15wt%-0.3wt% (excluding); moderately saline-alkali land: 0.3wt%-0.5wt% (excluding); severely saline-alkali land: >0.5wt%.

[0048] In this step, establishing a multi-level screening system in the laboratory is the foundation of the entire method. This invention abandons traditional single evaluation methods and innovatively adopts a combination of sand culture and hydroponics. Hydroponics allows for precise control of the salt concentration in the nutrient solution, facilitating the observation of the morphological responses of green manure crop roots under pure salt stress; while sand culture more closely resembles the actual physical environment of field soil, simulating the mechanical resistance and nutrient absorption of roots in the substrate. The two methods complement and verify each other, thus constructing a scientific and rigorous salt tolerance evaluation standard. By setting a series of representative NaCl concentration gradients, various habitats from non-saline soil to heavily saline soil are simulated, comprehensively examining the survival ability and growth performance of green manure crops under different salt stress intensities, thereby screening out a green manure variety library truly suitable for coastal saline-alkali environments. This step effectively overcomes the shortcomings of previous direct introduction or experience-based planting, such as low survival rates and poor growth, providing a solid species resource guarantee for subsequent field applications.

[0049] S2. Based on the degree of salinization of saline-alkali land, select green manure varieties for planting in different zones; Before planting, mix the arbuscular mycorrhizal fungal fertilizer and green manure seeds evenly at a mass ratio of (1-10):10. The preparation steps of the arbuscular mycorrhizal fungal fertilizer include: Mix the powders of *Bombyx mori*, *Bombyx umbellatus*, *Bombyx latae*, and *Bombyx mori* in a mass ratio of 1:3:2:1 to obtain arbuscular mycorrhizal fungal fertilizer. Green manure varieties can be planted as a single variety or a mixture of multiple varieties.

[0050] This step embodies the planting logic of "adapting to local conditions and precise matching." The salinity distribution in coastal saline-alkali lands often exhibits significant spatial heterogeneity; different plots, and even different areas within the same plot, may have vastly different salinity levels. This invention emphasizes precisely matching selected green manure varieties with actual field salinity based on preliminary soil salinity surveys. For example, varieties with high salt tolerance thresholds are placed in areas with high salinity, while varieties with relatively low salt tolerance thresholds but high biomass accumulation are placed in areas with low salinity. This zonal matching model ensures that green manure crops can germinate, grow, and form substantial ground cover in soils with corresponding salinity levels, thereby maximizing their biological improvement potential and avoiding resource waste or ineffective improvement caused by a "one-size-fits-all" planting approach.

[0051] S3. When the green manure reaches its full flowering period or before the harvest of intercropped crops, turn the green manure into the soil to achieve soil desalination and fertilization.

[0052] This step is a key agronomic operation for achieving the goal of "combined desalination and fertilization". The mechanism of "combined desalination and fertilization" described in this invention is mainly achieved through the dual effects of green manure roots improving soil physical structure (desalination) and increasing organic matter input (fertilization). Specifically, during the growth of green manure, its extensive root system penetrates and divides the soil, effectively breaking up the soil compaction layer, increasing soil porosity, and improving soil aggregate structure. This optimization of physical structure significantly improves soil permeability and water retention capacity, allowing water to more smoothly leach salt from the soil surface during rainfall or irrigation, thereby achieving the effect of physical desalination. Simultaneously, when the green manure reaches its peak flowering period or a suitable stage with a large biomass, its above-ground parts are turned into the soil. After being turned in, the green manure plants decompose under the action of soil microorganisms, releasing a large amount of organic matter, nitrogen, phosphorus, potassium, and other nutrients, as well as various active substances. This not only directly improves the soil fertility level but also further promotes the formation of soil aggregate structure and enhances the soil's water and fertilizer retention capacity. Through the synergistic effect of the above-mentioned "root physical improvement" and "plant chemical fertilization", this invention achieves a comprehensive improvement in the physical and chemical properties of coastal saline-alkali soil, achieving the dual purpose of rapid desalination and continuous fertilization.

[0053] In step S1, the NaCl concentration gradient is not arbitrarily selected, but rather scientifically mapped based on the actual salinization grading standards of coastal saline-alkali land. Specifically, 0% serves as the control group, representing a non-saline soil environment, used to observe the normal growth potential of green manure crops; the 0.2%-0.3% concentration range corresponds to slightly to moderately saline soil, the most common salinity range in coastal farmland of Fujian; 0.5% concentration corresponds to severely saline soil, used to screen varieties with strong salt tolerance potential; and 0.8% concentration is close to the salinity threshold of coastal saline soil, used for salt tolerance verification under extreme conditions. Through these five gradient settings, this invention constructs an evaluation network covering the entire range from non-saline soil to severely saline soil, ensuring that the selected varieties can accurately adapt to field plots with different degrees of salinization, avoiding the variety suitability bias caused by overly narrow evaluation ranges in traditional methods.

[0054] The salt tolerance evaluation in step S1 includes germination rate, root length, plant height, and fresh weight. This invention adopts a comprehensive consideration of multiple dimensions rather than a single indicator. The specific measurement methods are as follows: Germination rate is used to evaluate the germination ability of seeds under salt stress and is the primary indicator determining whether seedlings will survive; root length is a key indicator for evaluating salt tolerance because in saline-alkali environments, roots are the first to come into contact with soil salt, and the development of the root system directly determines the plant's ability to absorb water and nutrients. Longer roots often mean that the plant has stronger penetration and salt-avoidance capabilities; plant height and fresh weight reflect the overall biomass accumulation level of the plant under salt stress. Through dual verification using sand culture and hydroponics, the changes in the above agronomic traits of green manure crops under different NaCl concentration gradients were observed, and their salt tolerance index was calculated, thus obtaining a scientific conclusion on salt tolerance evaluation.

[0055] Based on the above evaluation system, this invention has screened a library of green manure varieties suitable for different degrees of salinization. Varieties suitable for severely saline-alkali land maintain high germination rates and biomass accumulation even at high salt concentrations of 0.5%-0.8%, demonstrating extremely strong salt tolerance potential. For example, *Senecio scandens* and *Sorghum sulphureus* have well-developed taproot systems that can penetrate the hard salt crust layer and utilize deep soil moisture; ryegrass has a dense fibrous root system that can quickly form surface cover and effectively inhibit soil salinization. These characteristics make them the preferred pioneer plants for the biological improvement of severely saline-alkali land.

[0056] The selected green manure varieties suitable for mild to moderate saline-alkali soils and slightly saline-alkali soils grow vigorously at a salt concentration of 0.2%-0.3%, and have a large biomass accumulation and high potential for nutrient return to the soil. For example, vetch and radish not only have moderate salt tolerance, but also have strong nitrogen fixation or nutrient enrichment capabilities, making them suitable for planting in cultivated soils with relatively low salinity, thus achieving an organic combination of soil fertility improvement and crop rotation.

[0057] Through the above screening process, this invention establishes a complete "variety-salt tolerance level" correspondence table, providing solid variety resource support for the "local adaptation and precise matching" in the subsequent step S2. This variety library construction method based on a multi-level screening system overcomes the blindness of previous experience-based introductions and ensures the survival rate and improvement effect of green manure crops in actual field applications.

[0058] The core of step S2 in this invention is establishing a precise mapping relationship between soil salinity content and the salt tolerance threshold of green manure varieties. This is a key step in achieving "tailored and precise improvement based on local conditions." Planting selected green manure varieties according to the degree of salinization in saline-alkali land is based on a deep coupling between the biological characteristics of green manure crops and the needs of saline-alkali land improvement. For slightly saline-alkali land (salt content 0.15%-0.3%), soil salt stress is relatively small, and the focus of improvement is on rapidly enhancing soil fertility. For example, *Indigofera tinctoria*, as a leguminous green manure, has moderate salt tolerance and strong nitrogen-fixing ability. Its roots can form root nodules in symbiosis with rhizobia, converting atmospheric nitrogen into nitrogen that can be used by plants, thereby significantly improving soil fertility while improving soil structure. For moderately saline-alkali land (salt content 0.3%-0.5%), soil salinity already inhibits the growth of common crops. It is necessary to select varieties with a certain salt tolerance and large biomass, such as vetch (hairy vetch). Vetch not only has moderate salt tolerance, but its lush foliage allows it to inject a large amount of organic matter into the soil after being turned over. Furthermore, its creeping vines effectively cover the surface, reducing soil moisture evaporation and inhibiting salt return. For severely saline-alkali land (salt content 0.5%-0.7%), the high salinity and poor soil structure make improvement the most difficult. Pioneer plants with high salt tolerance thresholds must be selected, such as ryegrass. Ryegrass has a well-developed fibrous root system that can penetrate deep into soil pores, breaking up the compacted layer through root penetration, improving soil permeability, and promoting rainwater leaching of salt. At the same time, ryegrass has strong salt tolerance, maintaining a high biomass accumulation even in high-salt environments, ensuring a sufficient supply of improvement materials.

[0059] In terms of planting methods, this invention provides flexible operational space, where green manure varieties can be planted as a single variety or a mixture of multiple varieties. Single-variety planting facilitates field management and unified operation, and is suitable for areas with regular plots and uniform salinity distribution. However, in plots with uneven salinity distribution or diverse improvement needs, the mixed planting mode exhibits unique advantages. For example, in the summer improvement of severely saline-alkali land, sesbania and sorghum can be mixed and sown. Sesbania is a deep-rooted legume with a well-developed taproot, which can utilize deep soil moisture and increase soil nitrogen through nitrogen fixation; sorghum is a grass with a well-developed fibrous root system, mainly distributed in shallow soil, and grows rapidly, quickly forming surface cover. The mixed sowing of the two constructs a three-dimensional root structure of "deep and shallow combination, legume and grass combination": spatially, deep roots and shallow roots occupy different soil layers, reducing interspecific competition and expanding the depth range of soil improvement; functionally, the nitrogen fixation of legumes compensates for the large nitrogen demand of grass, while the high biomass of grass provides a rich carbon source for the soil. This complementary ecological niche advantage means that the total biomass of mixed-sown populations is often higher than that of monocultures, thereby significantly increasing the amount of organic matter input per unit area and accelerating the improvement process of saline-alkali land.

[0060] Furthermore, in agricultural production practices, green manure planting is often combined with cash crops. The following examples and comparative models further demonstrate the application scenarios of intercropping green manure with cash crops. Sowing green manure between tomato planting beds utilizes the space between rows during the green manure's growth period, improving land utilization. On the other hand, the root exudates and decomposition products of the green manure after being turned over improve the rhizosphere soil environment, promoting tomato root growth. Especially before or after tomato harvest, timely turning over of the green manure not only solves the problem of green manure disposal but also achieves on-site fertilization, creating favorable soil conditions for the next crop. This intercropping model organically combines "improvement" and "production," significantly enhancing the comprehensive benefits of agricultural utilization in saline-alkali land.

[0061] In saline-alkali environments, soil microbial communities are often structurally simple and have low activity, which limits the colonization and nutrient absorption capacity of green manure crop roots. This invention introduces screened arbuscular mycorrhizal fungi into the fertilizer, allowing them to form a dominant microbial community around the green manure roots. Mycorrhizal fungi can form a symbiotic relationship with green manure roots; their extensive hyphal network not only greatly expands the root's absorption surface area, helping plants more effectively absorb water and mineral nutrients such as phosphorus under salt stress, but also secretes specific proteins such as glomerulosin, which act as an important "binder" for soil aggregate structure. Therefore, the addition of the fertilizer effectively equips the green manure with a "biological support system," significantly enhancing its survival ability and improvement potential in harsh environments. Furthermore, the fertilizer's formula is not a random combination but is designed based on the functional complementarity of different fungal species. Specifically, *Bombyx mori* and *Pterocarya stenoptera*, common soil fungi, possess strong resistance to adverse conditions and the ability to decompose organic matter, thus improving the rhizosphere microenvironment. *Gymnospermum latifolium* and *Gymnospermum moses*, typical arbuscular mycorrhizal fungi, are the main forces in forming mycorrhizal symbionts, directly infecting plant roots and establishing symbiotic relationships. *Pterocarya stenoptera* has the highest proportion, aiming to rapidly occupy rhizosphere niches and suppress soil-borne pathogens through its rapid reproduction. *Gymnospermum moses* and *Gymnospermum latifolium* together account for 3 parts, ensuring sufficient effective infection potential. *Bombyx mori* accounts for 1 part, playing a supporting regulatory role. This specific quality ratio achieves a balance between "resistance and colonization" and "functional symbiosis," maximizing the survival rate and functional expression of the microbial fertilizer in saline-alkali environments.

[0062] Example 1 A multi-stage screening-based method for combining green manure with desalination and fertilization in coastal saline-alkali land includes the following steps: S1. Using a combination of sand culture and hydroponics, different NaCl concentration gradients (0%, 0.2%, 0.3%, 0.5% and 0.8%) were set up to evaluate the salt tolerance of green manure crops (seedling rate, root length, plant height and fresh weight), and to screen out green manure varieties suitable for different degrees of salinization. Suitable summer green manure crops for severely saline-alkali land include sesbania and sorghum; suitable winter green manure crops include ryegrass, sweet clover, alfalfa, tall grass, white clover, and red clover.

[0063] Suitable summer green manure crops for mild to moderate saline-alkali land include Sudan grass, sunflower, Indian cowpea, Laba bean, and Indigofera tinctoria; suitable winter green manure crops include radish, rapeseed, and vetch.

[0064] Suitable summer green manure crops for slightly saline-alkali soils include black beans, bamboo beans, and black mung beans; suitable winter green manure crops include milkvetch, crimson clover, horsehair fern, and gnaphalium affine.

[0065] The classification criteria for severely saline-alkali land, moderately saline-alkali land, and mildly saline-alkali land are based on salt concentration (degree of salinization): mildly saline-alkali land: 0.15%-0.3% (excluding); moderately saline-alkali land: 0.3%-0.5% (excluding); severely saline-alkali land: >0.5%.

[0066] S2. Based on the degree of salinization of saline-alkali land, select green manure varieties for planting in different zones; Before planting, mix arbuscular mycorrhizal fungal fertilizer with green manure seeds (vetch) at a mass ratio of 1:1. Intercropping vetch and tomatoes is carried out in slightly to moderately saline-alkali soil (salinity level of 0.3%). Tomatoes are prepared in raised beds, and after transplanting, vetch is sown between the beds. The preparation steps of the arbuscular mycorrhizal fungal fertilizer include: Mix the powders of *Bombyx mori*, *Bombyx umbellatus*, *Bombyx latae*, and *Bombyx mori* in a mass ratio of 1:3:2:1 to obtain arbuscular mycorrhizal fungal fertilizer. S3. Before harvesting tomatoes, turn green manure into the soil to desalinate and enrich the soil. Field management measures are routine.

[0067] Comparative Example 1 Compared with Example 1, the difference is that in step S2, the green manure seeds were not combined with arbuscular mycorrhizal fungal fertilizer, that is, only green manure seeds were sown.

[0068] A comparative experiment was conducted using the schemes of Example 1 and Comparative Example 1. Three plots were planted in both Example 1 and Comparative Example 1 to study soil microbial activity (Table 4) and economic crop yield (Table 5).

[0069] Table 4 Table 5 As can be seen from the data in Tables 4 and 5, after green manure was turned into the field, compared with the control group 1 which did not use arbuscular mycorrhizal fungal fertilizer, the EC content decreased, soil fertility increased, the number of microorganisms increased significantly, and the tomato yield increased by 48.8%.

[0070] Example 2 A multi-stage screening-based method for combining green manure with desalination and fertilization in coastal saline-alkali land includes the following steps: S1, Same as Example 1.

[0071] S2. Plant the selected green manure varieties according to the degree of salinization of the saline-alkali land; In severely saline-alkali land (salinization level of 0.5%), ryegrass and tomatoes were intercropped. Tomatoes were planted in raised beds, and after transplanting, ryegrass was sown between the beds. S3. Before harvesting tomatoes, turn green manure into the soil to desalinate and enrich the soil. Field management measures are routine.

[0072] Comparative Example 2 The difference from Example 2 is that ryegrass was not planted in step S2.

[0073] A comparative experiment was conducted using the schemes of Example 2 and Comparative Example 2. Three plots were planted in both Example 2 and Comparative Example 2 to study soil microbial activity (Table 6) and economic crop yield (Table 7).

[0074] Table 6 Table 7 As can be seen from the data in Tables 6 and 7, compared with Comparative Example 2, Example 2 showed a decrease in EC content, an increase in soil fertility, an increase in the number of microorganisms, and a 30.2% increase in tomato yield.

[0075] Example 3 A multi-stage screening-based method for combining green manure with desalination and fertilization in coastal saline-alkali land includes the following steps: S1, Same as Example 1.

[0076] S2. Plant the selected green manure varieties according to the degree of salinization of the saline-alkali land; Before planting, arbuscular mycorrhizal fungal fertilizer and green manure seeds (ryegrass) are mixed at a mass ratio of 1:1. Ryegrass and tomatoes are intercropped in severely saline-alkali land (salinization level 0.5%). Tomatoes are planted in raised beds, and after transplanting, ryegrass is sown between the beds. The preparation steps of the arbuscular mycorrhizal fungal fertilizer include: Mix the powders of *Bombyx mori*, *Bombyx umbellatus*, *Bombyx latae*, and *Bombyx mori* in a mass ratio of 1:3:2:1 to obtain arbuscular mycorrhizal fungal fertilizer. S3. Before harvesting tomatoes, turn green manure into the soil to desalinate and enrich the soil. Field management measures are routine.

[0077] Comparative Example 3 Compared with Example 3, the difference is that in step S2, the green manure seeds were not combined with arbuscular mycorrhizal fungal fertilizer, that is, only green manure seeds were sown.

[0078] A comparative experiment was conducted using the schemes of Example 3 and Comparative Example 3. Three plots were planted in both Example 3 and Comparative Example 3 to study soil microbial activity (Table 8) and economic crop yield (Table 9).

[0079] Table 8 Table 9 As can be seen from the data in Tables 8 and 9, compared with Comparative Example 3, Example 3 showed a decrease in EC content, an increase in soil fertility, an increase in the number of microorganisms, and a 45.8% increase in tomato yield.

[0080] As can be seen from the data in Tables 4-9, the synergistic effect of arbuscular mycorrhizal fungal fertilizer and green manure returning to the field has a significant effect on improving the fertility of saline-alkali land. After the combined use, the EC content decreased, soil fertility increased, the number of microorganisms increased significantly, and the tomato yield increased significantly.

[0081] Test case Field verification of desalination effect: The selected green manure (Indigofera tinctoria, Vigna arvense, and Lorrae ryegrass) were planted in lightly, moderately, and severely saline-alkali lands respectively. Soil salinity, chloride content, pH value, EC value, organic matter and other indicators were measured regularly to evaluate their desalination and fertilization effects. The unplanted land was used as the control group (CK). The effects of lightly saline-alkali land are shown in Table 10, moderately saline-alkali land in Table 11, and severely saline-alkali land in Table 12.

[0082] Table 10 Table 11 Table 12 Figure 1 In Example 3, tomatoes were intercropped with ryegrass in severely saline-alkali land.

[0083] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0084] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening, characterized by the following steps: include: Under different NaCl concentration gradients, the salt tolerance of green manure crops was evaluated by combining sand culture primary screening and hydroponic secondary screening, and green manure varieties suitable for different degrees of salinization were screened out. Green manure varieties selected for planting in zones based on the degree of salinization of saline-alkali land; When the green manure has grown to a suitable age, it should be plowed into the field to achieve soil desalination and fertilization. The different NaCl concentration gradients include five gradients: 0 wt%, 0.2 wt%, 0.3 wt%, 0.5 wt%, and 0.8 wt%. The salt tolerance evaluation includes germination rate, root length, plant height, and fresh weight; The degree of salinization includes: slightly saline-alkali land: 0.15wt%-0.3wt%; slightly to moderately saline-alkali land: 0.3wt%-0.5wt%; severely saline-alkali land: ≥0.5wt%; wherein, slightly saline-alkali land contains no more than 0.3wt%, and slightly to moderately saline-alkali land contains no more than 0.5wt%. The selected green manure varieties suitable for different degrees of salinization include: summer green manure crops suitable for severely saline-alkali land include sesbania and sorghum; winter green manure crops suitable for severely saline-alkali land include ryegrass, sweet clover, alfalfa, tall grass, white clover, and red clover; summer green manure crops suitable for mild to moderate saline-alkali land include Sudan grass, sunflower, cowpea, larvae, and indigofera; winter green manure crops suitable for mild to moderate saline-alkali land include radish, rapeseed, and vetch; summer green manure crops suitable for mild saline-alkali land include black bean, bamboo bean, and black mung bean; and winter green manure crops suitable for mild saline-alkali land include milkvetch, crimson clover, datura, and gnaphalium.

2. The method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening as described in claim 1, characterized in that, The initial screening of sand culture includes: 3.5 kg of dry sand is placed in a 2.5 L capacity bucket, 5 replicates are designed for each NaCl concentration, 3 green manure plants are planted in each bucket, and when the sand surface shows white, the corresponding NaCl concentration aqueous solution is used as daily irrigation water. Before the experiment, the sand of each treatment is soaked in the corresponding NaCl concentration aqueous solution. The germination rate, root length, plant height and fresh weight of the green manure are observed and recorded.

3. The method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening as described in claim 1, characterized in that, The hydroponic re-screening process includes: a light intensity of 15000 LUX, a CO2 concentration of 800 ppm, a daytime temperature controlled at 20-25℃, a nighttime temperature controlled at 15-18℃, a daytime light duration of 14 h, a nighttime light duration of 10 h, a daytime humidity controlled at 60%-70%, a nighttime humidity controlled at 70%-85%, and hydroponics conducted at the corresponding NaCl concentration. The germination rate, root length, plant height, and fresh weight of the green manure are observed and recorded.

4. The method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening as described in claim 1, characterized in that, The process of planting the selected green manure varieties also includes a step of mixing the arbuscular mycorrhizal fungal fertilizer with the green manure seeds at a mass ratio of (1-10):

10.

5. The method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening as described in claim 4, characterized in that, The preparation steps of the arbuscular mycorrhizal fungal fertilizer include: The powders of *Bombyx mori*, *Bombyx umbellatus*, *Bombyx mori*, and *Bombyx mori* were mixed evenly in a mass ratio of 1:3:2:1 to obtain arbuscular mycorrhizal fungal fertilizer.

6. The method for combined desalination and fertilization of coastal saline-alkali land using green manure based on multi-level screening as described in claim 1, characterized in that, The process of turning over and returning the green manure to the field involves turning the green manure into the soil before it reaches its full flowering period or before harvesting the intercropped crops.

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