Soil-Improving Effect of Sesbania–Sorghum Rotation in a Heavily Saline–Alkaline Coastal Region

Abstract

Planting salt-tolerant plants is an efficient method of biological improvement for saline–alkali land. However, few studies have examined the soil improvement effects of the rotation of the green manure plant sesbania and the grain crop sorghum. Thus, we planted sesbania in native soil on heavily saline–alkaline coastal land and subsequently planted sorghum after returning the sesbania straw to the soil. The effect of this sesbania–sorghum rotation on soil improvement was clarified by comparing indicators of soil quality before and after sesbania and sorghum were planted, such as the soil structure, water infiltration, soil salt content, and soil microbial changes. The results showed that the soil bulk density of the plow layer (0–40 cm) after crop harvest decreased by 9.63% compared with that of bare land, and the soil porosity increased by 5.67%. The cumulative infiltration, initial infiltration rate, and stable infiltration rate of saline soil were 3.6 times, 2.8 times, and 3.3 times higher than those of bare land, respectively. With the growth of sesbania and sorghum, the soil salt content in the plow layer of the cultivated land decreased by 37.73%, while that of bare land decreased by 9.1%. A further analysis of desalination showed that the total desalination amount in the plow layer was 15.58 t/ha, of which 5% was due to plant absorption, and the rest was from salt leaching. Moreover, sesbania–sorghum rotation increased the soil organic matter content in the plow layer from 69.1 t/ha to 73.8 t/ha. The quantities of some microorganisms that are mainly found in coastal saline soil decreased, while those of some common soil microorganisms increased, reflecting an improvement in the soil quality. The above results prove that sesbania–sorghum rotation had a significant effect on soil improvement and salt reduction, which is of great significance for the further utilization of saline–alkali land to enhance crop productivity.

1. Introduction

Soil salinization is a worldwide problem restricting agricultural production. China has approximately 100 million hectares of saline–alkali land, of which approximately 12 million hectares are available for agricultural use [1]. China is a country with a large population but limited cultivated land. Therefore, fully utilizing this saline–alkali land for agricultural production is of great significance to ensure food security [2]. At present, many practices for the improvement of saline–alkali land have been applied, mainly including physical, chemical, and biological measures. However, many saline–alkali land improvement measures are strongly dependent on the availability of fresh water [3]. For conditions where freshwater resources are lacking, biological improvement measures involving saline–alkali-tolerant plants have become an important choice for the improvement of heavily saline–alkali land.

Some halophytes or salt-tolerant plants can remove soil salt by absorbing salt and accumulating it in their tissues. A study by Sun et al. showed that Tamarix chinensis Lour. can absorb salt through its root system and accumulate salt in its leaves and twigs, so as to effectively reduce the soil salt content [4]. Yu et al. conducted a two-year field experiment rotating Sesbania cannabina and Triticosecale Wittm. on coastal saline–alkali land, and the results showed that the soil salt content was significantly decreased after the green manure plant sesbania was planted, thereby improving the yield of Triticosecale Wittm. [5]. Zhang et al. also found a similar soil-salt-decreasing effect after rotating triticale and sweet sorghum in saline–alkali soil [6]. Furthermore, the growth of salt-tolerant plants can improve the soil’s physical and chemical properties, such as by reducing the soil bulk density and increasing the soil porosity. These beneficial effects comprehensively enhance the soil quality and increase crop productivity [7].

Sesbania is usually regarded as a green manure plant because it provides the function of nitrogen fixation by its rhizobia [8]. It can grow in saline soil with a 0.3% salt content and in alkaline soil with a pH of 9.5. Thus, in practice, sesbania has been grown to improve the quality of saline–alkali soil and has shown good effects in alleviating soil salinization and improving soil fertility [9]. Some studies showed that planting sesbania on saline–alkali land can not only reduce the soil pH and soluble salt content but also significantly increase the soil organic matter content and improve the soil microbial population structure [9,10]. The soil microbial structure can reflect changes in the soil quality [11]. Due to the high salt content and poor permeability of coastal saline soil, which distinguish it from common soil, the microorganisms present in coastal saline soil usually exhibit salt tolerance or a predominance of anaerobic microorganisms, such as denitrifying bacteria in saline soil [12,13]. As plants grow, the soil permeability around their rhizospheres increases, and the quantities of some common soil microorganisms, such as nitrifying bacteria, increase. The activity of these bacteria promotes further improvements in the soil quality [12].

Sorghum is the fifth key crop among cereals and, like sesbania, displays good adaptability to various adversities, such as saline stress, drought, and barren soils [14,15]. Therefore, planting sorghum on saline–alkali land is of great significance for ensuring food security. In addition, with the growth of plants and the expansion of their root systems, soil structure qualities such as soil aggregates, the soil bulk density, and the soil porosity also improve, leading to increases in water infiltration and salt-leaching capacity [16,17]. Weather conditions and agricultural measures also play important roles in soil salt leaching and salt accumulation [1]. Reports by Guo et al. and Shaygan et al. showed that little rainfall and strong winds in spring could lead to significant salt return, while increased rainfall in summer or drip irrigation measures could significantly reduce the soil salinity in the plow layer [16,18]. Therefore, when studying the soil improvement and salt reduction effects of salt-tolerant plants, these important factors should also be considered.

There are about 0.7 million hectares of coastal saline–alkali land distributed in the Hebei province, China, where the soil is silty, has poor permeability and has a low content of organic matter [1]. Considering the salt tolerance and soil-improving effects of the green manure plant sesbania and the grain crop sorghum, we propose a sesbania–sorghum rotation mode, namely, planting sesbania during the drought and salt-return period of spring and planting sorghum in the rainy season of summer, so as to achieve high productivity on saline–alkali land. However, among prior studies of biological improvements for saline–alkali soil, researchers often focused on salt absorption by salt-tolerant plants as a means of removing soil salt during their growth, and less attention was paid to the improvement of soil physical properties via plant growth to promote salt leaching [2,16]. Therefore, in this study, we comprehensively analyze the soil-improving effect of the sesbania–sorghum rotation mode on coastal saline–alkali land.

2. Materials and Methods

2.1. Study Area Description

The experiment was conducted at the experimental base of the Institute of Coastal Agriculture, Hebei Academy of Agriculture and Forestry Sciences, Tangshan, Hebei Province (118°27′19″ E, 39°17′38″ N). The groundwater level was about 1.5 m, and the salinity of the groundwater was higher than 0.5%. The climate type belongs to the warm temperate-zone semi-humid continental monsoon. The historical annual average temperature is 12.1 °C, and the annual average precipitation for the past 10 years is 576 mm, mainly concentrated in July to August. However, the distribution of rainfall in 2023 was different from that in previous years, as shown in Figure 1 (rainfall data were obtained from the meteorological monitoring station at the experimental base). Under the influence of the climate conditions, the soil salinity showed significant overall seasonal variation characteristics, namely, salt accumulation in spring, autumn, and winter and salt leaching in summer.

2.2. Experimental Design

Saline soils were taken from the typical coastal saline–alkali area in the study locality. The soil bulk densities of the native soil at depths of 0–20 cm, 20–40 cm, 40–60 cm, 60–80 cm, and 80–100 cm were determined. Then, the saline soils were mixed mechanically to form soil samples. Four bottomless cement tanks with a waterproof layer on the wall were built; the length, width, and depth of each tank were 6 m, 3 m, and 1 m, respectively. The soil samples were layered and packed into the four tanks. The amount of soil samples used was calculated from the soil bulk density of the native soil in each 20 cm soil layer and from the weights and soil water contents of the soil samples. During this process, soil sensors were embedded in each soil layer to monitor the dynamic changes in the soil’s electrical conductivity. Finally, the soil samples in the tanks underwent a season of natural settlement from October 2022 to April 2023. The physical and chemical properties of the soil samples, such as the soil bulk density, aggregates, porosity, and infiltration characteristics, were then examined, as were the content of organic matter and the diversity of soil microorganisms. Among them, the soil bulk density, soil porosity, and aggregates were sampled in layers at depths of 0–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm; the contents of soil organic matter and soil microorganisms were sampled in layers at depths of 0–20 cm and 20–40 cm. Three tanks were set up to test the sesbania–sorghum rotation, while the other tank was kept in its original state as a control (bare land). The structure of the cement tanks and the experimental design are shown in Figure 2.

After one season of natural settlement, sesbania was sown on 18 April 2023. The variety “Zhongkejing 1” was provided by the Institute of Genetics and Developmental Biology, Chinese Academy of Sciences. The row spacing and plant spacing were 50 cm and 30 cm, respectively. Fresh water was provided via drip irrigation at a rate of 225 m³/ha to provide temporally suitable conditions for germination in the plow layer; after the seedlings emerged, 1 seedling was retained in each hole. The sesbania was harvested on 15 June 2023, and the biomass, ion contents, and total salt content in plant were measured. On 20th June, after the sesbania straw had been returned to the soil, sorghum was sown with a plant spacing of 30 cm and a row spacing of 50 cm, and 1 seedling was retained in each hole. The sorghum variety “Jiniang 2” was provided by Hebei Academy of Agriculture and Forestry Sciences. At the time of sorghum planting, fresh water was provided via drip irrigation at a rate of 225 m³/ha. An additional 225 m³/ha was applied via drip irrigation on the 25th of August because of drought (Figure 1). Throughout the entire growing season, no additional water or fertilizer was applied. Harvesting was carried out on the 15th of October, and the sorghum yield, biomass, ion contents, and total salt content in the plant body were determined. Simultaneously, the physical and chemical properties of the soil, as mentioned above, were measured. By comparing these indicators of soil quality before and after sesbania and sorghum planting, the effect of the sesbania–sorghum rotation on soil improvement was comprehensively analyzed.

2.3. Sample Parameters Determination

The soil cumulative infiltration, initial infiltration rate, and stable infiltration rate were recorded using a double-ring infiltrometer within 180 min [19]. The soil bulk density and porosity were determined using stainless steel rings after drying at 105 °C in an oven [20]; aggregates were measured using a Malvern laser particle size analyzer (Mastersizer 3000, Malvern Instruments Ltd., Worcestershire, UK) [21]. The soil water content was determined via the oven drying method (75 °C, 12 h). The soil electrical conductivity was measured in a soil extraction solution of water and dry soil (5:1 ratio) using a laboratory electrical conductivity meter (DDS-307A, Scientific Instrument Co., Ltd., Shanghai, China). The soil salt content was determined after the extraction solution was filtered and dried, and a linear equation relating the soil electrical conductivity and actual soil salt content was built to calculate the soil salt content from the soil electrical conductivity [22]. The total amount of desalination in the plow layer (0–40 cm) was calculated according to the soil bulk density and the decrease in the soil salt content before and after sesbania–sorghum planting. In addition, the salinity distribution around the rhizosphere was determined after sorghum harvesting. Soil samples within the soil profile at depths of 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm, and 60–80 cm and at distances to the sorghum roots of 5 cm, 10 cm, 15 cm, and 20 cm were taken, and their soil electrical conductivities were measured. The soil salt profile was simulated on the basis of the soil electrical conductivity via the contour method in Origin 8.0 software (OriginLab Corporation, Northampton, MA, USA).

The contents of soluble cations such as Ca²⁺, Mg²⁺, K⁺, and Na⁺ were quantified by means of ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry; CAP 6500, Thermo Fisher Scientific Inc., Waltham, MA, USA) [23,24]; the contents of anions HCO₃⁻, SO₄²⁻, and Cl⁻ were determined via a laboratory method, as described by Asad et al. [25]. To determine the total salt content in sesbania or sorghum, whole plants of sesbania or sorghum after harvest were dried, crushed, and mixed, and then the ion contents of Ca²⁺, Mg²⁺, K⁺, Na⁺, Cl⁻, HCO₃⁻, and SO₄²⁻ were measured according to the method above. The total amount of salt absorbed by the plants was estimated from the total content of these ions and the plant biomass.

The soil organic matter content was measured according to the Chinese National Standard NY/T 85-1988 [26]. The soil microbial diversity was analyzed by means of Illumina high-throughput sequencing and compared with the bacterial 16S ribosome database, fungal 18S ribosome database, etc., to investigate the changes in the soil microorganisms before and after sesbania–sorghum planting using online tools and default analysis methods (https://cloud.majorbio.com/page/tools/, accessed on 16 September 2024) [11].

2.4. Data Analysis

Data were randomly collected, and all results were subjected to analysis of variance (one-way ANOVA) with the SPSS 16.0 software package (SPSS Inc., Chicago, IL, USA). Mean separation was analyzed using Duncan’s Multiple Range Test (DMRT) at p ≤ 0.05 or 0.01. All figures were created using Origin 8.0 software (OriginLab Corporation, Northampton, MA, USA).

3. Results

3.1. Effects of Sesbania–Sorghum Growth on Soil Structure of Coastal Saline Soils

The soil in the experimental area was silty, with the main characteristics of a high soil bulk density, low porosity, and small soil particles. However, after the growth of sesbania and sorghum, the structure of the plow layer (0–40 cm) was significantly improved (Figure 3 and

Table 1. Soil particle size distribution after planting sesbania–sorghum.

Depth	0–20 (cm)		20–40 (cm)		40–60 (cm)		60–80 (cm)
	Before	After	Before	After	Before	After	Before	After
D(10) (μm)	2.93 ± 0.1	4.7 ± 0.5 **	2.86 ± 0.2	4.7 ± 0.5 **	2.61 ± 0.1	3.3 ± 0.7	2.67 ± 0.1	2.6 ± 0.1
D(50) (μm)	39.1 ± 1.3	48.1 ± 0.8 **	36.6 ± 1.2	38.1 ± 2.2	35.6 ± 1.3	35.3 ± 3.5	35.2 ± 0.8	35.6 ± 1.3
D(90) (μm)	101.4 ± 3.9	110 ± 5.2 *	96.6 ± 1.6	107.7 ± 1.2 *	98.2 ± 2.7	99.3 ± 1.4	98.4 ± 2.2	99.8 ± 1.1

* or ** in the column indicates the significant differences (p < 0.05 or p < 0.01) in each soil layer between before and after growing sesbania–sorghum. Data are shown as mean ± standard error (n = 9).

). The investigation results showed that the bulk density of the soil layers at 0–20 cm and 20–40 cm significantly decreased after the growth of sesbania and sorghum, from 1.43 g/cm³ and 1.5 g/cm³ to 1.25 g/cm³ and 1.40 g/cm³, respectively. Correspondingly, the soil porosity in the same two layers increased from 46% and 43.4% to 52.8% and 47.2%, respectively. This increase in soil porosity was related to variations in the soil particle size. After sesbania and sorghum were grown, the particle size of the topsoil (0–40 cm) significantly increased (Table 1), which enhanced the soil’s permeability. However, for the layer at 40–80 cm, the soil bulk density, soil porosity, and soil particle size did not show significant variation, indicating that the growth of sesbania–sorghum in the field had a significant improvement effect on the plow layer alone.

3.2. Effects of Sesbania–Sorghum Growth on Water Infiltration in Coastal Saline Soils

The results showed that during the same infiltration time (180 min), the cumulative infiltration, initial infiltration rate, and stable infiltration rate of coastal saline soil were all significantly enhanced after sesbania–sorghum growth; the final values of 17.5 mm, 0.14 mm/min, and 0.1 mm/min, respectively, represent increases of 3.6, 2.8, and 3.3 times compared with those before the plants were grown. These results indicate that the growth of the plants accelerated water infiltration, which plays an important role in salt leaching (Figure 4).

3.3. Effects of Sesbania–Sorghum Growth on Soil Salt in Coastal Saline Soils

Soil salinity changes are comprehensively affected by climate conditions, plant growth, and water management measures. The salt content in the plow layer (0–40 cm) varied significantly with the season, but overall, it exhibited a pattern of decrease. The salt content in the soil layers below 60 cm was relatively stable and remained at a high level (Figure 5A). In May, the salt contents in the soil layers at 0–20 cm and 20–40 cm increased by 15.8% and 37.3%, respectively, compared with those in April. In August, the salt contents increased in all soil layers except the 20–40 cm soil layer, with the largest increase of 14.2% observed in the 40–60 cm soil layer and the highest content of 11.45 g/kg observed in the 60–80 cm soil layer. In June, the salt contents in the 0–20 cm and 20–40 cm soil layers, compared with those in April, decreased by 37.3% and 48.3%, respectively. In September, the salt contents in the same soil layers decreased by 22.9% and 10.9%, respectively, compared with those in April (Figure 5A).

We also observed that the salinity around the crop rhizosphere changed in echelons (Figure 5B). The salt content was the lowest in the 0–40 cm soil layer within 5 cm of the roots, and it increased with increasing distance from the roots, forming a U-shaped low-salinity zone around the rhizosphere. The above phenomenon is related to the growth activities of plant roots, and it suggests that water-saving techniques with salt-control technology for saline–alkali land could be developed according to this U-shaped low-salinity zone.

3.4. Effects of Sesbania–Sorghum Growth on Desalting and Soil Quality

Based on the above experimental results, we found that the salt decrease and soil structure significantly varied within the plow layer (0–40 cm); thus, we analyzed the soil quality in the plow layer through the aspects of desalination and the soil organic matter content.

After the sesbania and sorghum harvests, the soil salt content in the plow layer decreased. The total soil desalination rate for the bare land was 9.1%, while that for the cultivated land was 3.15 times higher, at 37.73% (Figure 6). The total amount of salt removed from the cultivated land was 15.58 t/ha; the salt removed by plants was 0.78 t/ha, accounting for 5% of the total amount. Thus, the amount of eluted salt accounted for 95% of the total salt removed (

Table 2. Salt content in plant body after growing sesbania–sorghum.

	Sesbania (g/kg)	Sorghum (g/kg)	Total Removal (kg/ha) *
Ca2+	1.27	1.58	42.2
Mg2+	0.11	0.15	3.8
K+	0.03	0.02	0.8
Na+	7.30	14.22	281.1
Cl−	11.27	17.02	396.3
SO42−	1.08	2.98	48.3
HCO3−	0.14	0.13	4.3
In total	21.22	36.10	776.9

* Data of the total removal of soil salts in kg/ha is calculated by the iron content in plants and the total biomass of sesbania and sorghum, which are 23.67 t/ha and 7.61 t/ha, respectively. The total biomass of sorghum is excluded from its grain yield, and its grain yield is 4.3 t/ha.

). From this, combined with previous results, we could infer that the soil desalination was mainly due to improvements in the soil structure in the process of plant growth, including synergetic effects with agronomic measures such as straw return and drip irrigation [18,23].

The soil quality improvement was also reflected in the content of soil organic matter. With the growth of sesbania and sorghum, the content of soil organic matter in the 0–40 cm soil layer increased from 69.1 t/ha to 73.8 t/ha (Figure 6), indicating an increase in soil fertility that was beneficial for plant growth and microbial activity.

3.5. Effects of Sesbania–Sorghum Growth on Soil Microorganisms

Variations in soil microorganisms reflect soil quality to a certain extent. After sesbania and sorghum planting, the numbers of species of bacteria and fungi in the soil increased by 22.1% and 176%, respectively (Figure 7); however, the quantities of microorganisms that are normally dominant in coastal saline soil and common soil changed significantly before and after planting. For bacteria, after the sesbania and sorghum were harvested, the quantities of the bacterial groups Actinomarinales, PAUC43f_marine_benthic_group, BD2-11_terrestrial_group, Nitriliruptoraceae, and Gammaproteobacteria decreased by 19.4%, 28.3%, 27.6%, 26.9%, and 28.4%, respectively (Table S1). To the contrary, the quantities of the bacterial groups SBR1031, Bacillus, Thalassobacillus, and A4b increased by 119.9%, 15.7%, 18.8%, and 14.8%, respectively (Table S1). Similarly, for fungi, the quantities of the fungal groups Microascaceae, Penicillium, Alternaria, Ascomycota, and Cladosporium decreased by 14%, 30.6%, 17.7%, 27.8%, and 25.7%, respectively (Table S2), while the quantities of unclassified Fungi, Mycothermus, and Acaulium increased by 3.15, 24.81, and 1.71 times, respectively (Figure 7 and Table S2).

In addition, by comparing the correlations among soil microorganisms (Figure S1), it was found that the bacterial groups Actinomarinales, SBR1031, Nitriliruptoraceae, etc., and the fungal groups Microascaceae, Mycothermus, unclassified Fungi, etc., showed closer relationships with others. Because of this and the changes in their quantities before and after planting, these soil microorganisms not only reflect the exuberance of substance and energy metabolism in the soil but also are an important indicator reflecting soil quality [27].

4. Discussion

4.1. Soil Property Improvement Promotes Soil Water–Salt Transport and Desalination

Soil water–salt transport refers to the changes in soil moisture and salt contents over time and space under the influence of various natural and human factors. Its essence is that “salt comes and goes with water”. Studying the law of soil water–salt transport is the theoretical basis and core issue for improving soil and preventing soil secondary salinization [16,28].

Soil properties such as particle size, bulk density, and porosity affect the soil’s permeability, water preservation and storage capacity, water–salt transport law, and conversion rates and existence status of nutrients, as well as the growth and physiological activities of plant roots, all of which finally determine the degree of soil salinization [9,29]. An et al. reported a significant positive correlation between soil salinity and bulk density, as well as the proportion of sand in coastal saline soil, and a significant negative correlation between soil salinity and nutrient elements such as organic matter, the proportions of silt and clay, porosity, etc. [9]. The fundamental reason for this lies in the improvement of the soil structure in saline soil, which manifested as an increase in the soil porosity. Specifically, the amount of small aggregates in the soil decreased significantly after the growth of sesbania and sorghum; this was visually observed as the topsoil (0–10 cm), changing from powder to granular in form. Thus, the improvement in the soil structure led to a decrease in soil bulk density and an increase in porosity, thereby enhancing the capacity for water infiltration and salt leaching into the lower layer. Sun et al. and Basset et al. found similar results in the improvement of saline soil. They found that soil structure properties like the soil bulk density, aggregates, porosity, etc., significantly improved after biochar (size below 0.25 mm) was applied to coastal saline–alkali land, and the water infiltration rate increased by 10% [30,31].

In addition, when studying the distribution of soil salinity, we found a U-shaped low-salinity zone around the rhizosphere, which was similar to the result of our previous research on the effect of drip irrigation on salt leaching in coastal saline soil [1]. The difference was that the U-shaped zone formed after drip irrigation was directly related to soil water infiltration, while in this study, the soil porosity around the rhizosphere slowly increased during plant root growth, accelerating salt leaching around the rhizosphere into the lower layer [18]. However, regardless of the situation, the salt distribution could essentially be attributed to soil water–salt transport; this phenomenon could also inspire the development of water-saving and salt control technologies, which are of great significance for the improvement of saline–alkali land in areas lacking fresh water.

4.2. Environmental Factors and Agronomic Measures Affecting Salt Reduction in Soil

Changes in soil salinity are closely related to climatic conditions and agricultural measures. The atmospheric temperature, precipitation, evaporation, humidity, and wind speed all affect soil water–salt transport, among which rainfall and evaporation are the most important factors [18]. The current experimental area belongs to a semi-humid climate zone, with annual rainfall that is less than or equal to evaporation, and soil salinity showing obvious seasonal variations. In June, when there was more rainfall, the soil salt content was the lowest, while in May, August, and October, the decrease in rainfall corresponded to a dynamic upward trend in the soil salt content (Figure 1 and Figure 5). These changes are consistent with the current assertion that during high-temperature and dry seasons, evaporation is strong, and topsoil is prone to an increase in salt accumulation. With increased rainfall, the soil desalination rate also gradually increased [16,18,28].

Besides climatic factors, the greater desalination in cultivated soil than in bare land may also relate to soil tillage (e.g., straw return) and drip irrigation. From July onward, rainfall decreased, and the weather entered a period of sustained high temperatures, leading to an upward trend in the soil salt content in July and August. However, the salt content in the plow layer (0–40 cm) was lower than that in May, which was speculated to be related to straw return and supplementary drip irrigation. Wang et al. and Zhang et al. reported that straw return can significantly improve soil structures and permeability, which is favorable for salt leaching into lower layers and the prevention of salt accumulation. Meanwhile, the additional drip irrigation promoted further salt leaching, as evidenced by the accumulation of salt in the soil layer at 60–80 cm, reaching the highest level during the lowest rainfall period in August (Figure 5) [1,32].

4.3. Growth of Salt-Tolerant Plant Affecting Desalination and Soil Quality

Biological improvement measures include planting salt-tolerant plants and applying microbial fertilizers. Sesbania is often used as a green manure plant, and sorghum is a traditional grain crop. This study carried out the rotation of sesbania and sorghum on heavily saline–alkali coastal land, and we found that plant growth produced a significant effect on desalination, especially in June and September (Figure 5).

We speculated that this was related not only to rainfall but also to plant growth. Research has shown that the decomposition of plant abscission and root exudates during plant growth plays a significant role in improving soil structure [5]. Firstly, the growth of plant roots can increase soil porosity, thereby increasing the water infiltration rate and promoting desalination. Secondly, the organic acids secreted by the sesbania root system have a promoting effect on the release of Ca²⁺. At the same time, Ca²⁺ in soil is beneficial for promoting the formation of benign soil structures, inhibiting the adverse effects of Na⁺, and creating a favorable growth environment for plants, producing positive feedback [18,23]. When the above-ground biomass increases, it also reduces soil water evaporation and the rise in soil salt.

4.4. Soil Microorganisms Affecting Desalination and Soil Quality

After sesbania and sorghum were planted in this study, the quantities of common soil microorganisms increased significantly, while the quantities of microorganisms normally found in saline soils decreased significantly (Figure 7). Therefore, changes in soil microorganisms can, to some extent, reflect the quality of soil.

Among the microorganisms that decreased in quantity, bacteria from the groups Actinomarinales and Microtrichales are keystone heterotrophs for carbon mineralization and are mainly found in saline soil or marine environments [12,33]. Bacteria from the PAUC43f_marine_benthic_group are also among the most frequently detected in marine environments and are usually among the dominant bacterial genera in oily soil [34]. Similarly, the bacterial group Gammaproteobacteria, including Escherichia bacteria, is frequently present in polluted soil. The bacterial groups BD2-11_terrestrial_group and Nitriliruptoraceae are usually considered to have desulfurization and denitrification effects and are mainly found in wetlands or coastal saline soils [13]. The above-mentioned microorganisms are mainly dominant in coastal saline soil environments, which is ascribed to the typical characteristics of coastal saline soil, namely, being silty, having poor permeability, and having a high salt content [35]. In addition, the fungal groups Microascaceae, Ascomycota, Cladosporium, etc., showed a decrease in quantity. When the soil environment improves, some bacteria of genera such as SBR1031 become dominant microorganisms, inhibiting the above-mentioned microorganisms [27,36].

Our results also showed that the quantities of some microorganisms increased, such as bacteria of the genus SBR1031. These bacteria have an opposite function to the bacterial groups BD2-11_terrestrial_group and Nitriliruptoraceae: they can promote the transformation of ammonia nitrogen to nitrate nitrogen and jointly maintain the regional soil nutrient balance [12]. Additionally, the quantity of Mycothermus increased by 1.71 times after sesbania and sorghum planting. This fungal group is mainly dominant in the microbial community during the heating, high-temperature, and maturation stages of aerobic composting. The straw return performed in this study promoted the proliferation of this microorganism, accelerating the conversion of the straw to organic matter, increasing the contents of soil nutrients, improving the soil quality, and promoting plant growth. This is in agreement with research showing that beneficial associated bacteria, such as those from the genera A4b and Thalassobacillus, significantly increased in number around the roots of sesbania and sorghum [11,37,38].

Briefly, from the above, it can be inferred that due to improvements in the soil quality brought about by growing sesbania and sorghum, the quantities of some common soil microorganisms significantly increased, while some microorganisms associated with coastal saline soils decreased in number. These microorganism changes reflect, to some extent, the effect of sesbania–sorghum rotation in improving the soil’s quality and reducing its salt content [27,39].

5. Conclusions

Growing sesbania and sorghum is an effective and economical method for saline–alkali land improvement. Compared with bare land, the soil bulk density decreased by an average of 9.63%, the soil porosity increased by 5.67%, and the cumulative water infiltration increased by 3.6 times for soil in which sesbania and sorghum were grown, which promoted salt leaching into lower layers. The soil salt content in the plow layer decreased by 37.73%, and the total amount of salt removed was 15.58 t/ha. The improvements in the soil structure and the decrease in salinity ensured the normal growth of the sesbania and sorghum. The salt absorbed by the plants accounted for only 5% of the total desalination amount, which meant that the improved soil structure and enhanced salt-leaching ability were the main reasons for the soil desalination. The increased organic matter in the plow layer was beneficial to the soil microbial activity. Some common soil microorganisms significantly increased in quantity, while some microorganisms that are normally found in coastal saline soils decreased in quantity. These changes reflect the improvement in the soil quality. Our research proved that sesbania–sorghum rotation has a significant soil-improving effect. However, this study was based on a one-year test; considering weather conditions, it still needs to be further tested.