A method for improving soil and increasing carbon sink of salt-alkali land castor planting and application

Abstract

This invention discloses a method and application for soil improvement and carbon sequestration enhancement in castor bean cultivation in saline-alkali land, relating to the field of agricultural planting technology. The method includes: planting castor beans in saline-alkali land using an agroforestry model; and spraying exogenous hormones during the seedling, budding, and flowering stages of the castor beans. The exogenous hormones are indoleacetic acid (IAA) or gibberellin, with the IAA and gibberellin spraying concentrations ranging from 50 μM to 200 μM. This method can lower the soil pH in coastal saline-alkali land, improve soil physicochemical properties, and increase soil organic carbon and organic matter content. Simultaneously, it can effectively enhance soil carbon storage, castor bean plant carbon sequestration, and total carbon accumulation. The method has low operating costs, stable improvement effects, and balances the needs of land use and ecological carbon sequestration improvement in saline-alkali land, providing a theoretical basis and technical support for enhancing soil carbon sequestration in coastal saline-alkali land.

Description

Technical Field

[0001] This invention relates to the field of agricultural planting technology, specifically to a method and application for soil improvement and carbon sequestration enhancement in castor bean cultivation in saline-alkali land. Background Technology

[0002] The coastal saline-alkali land has a loose soil structure, with sand accounting for over 80% of the total soil mass. It is characterized by low organic matter content, scarce nutrient reserves, and high soil mineralization. Due to the evaporation effect of the regional climate, soil salts can migrate upwards through capillary action and accumulate on the surface, easily forming salt frost and salt crust, which further exacerbates the damage caused by soil salinization.

[0003] Castor bean is a traditional and distinctive oilseed crop originating in Africa and now widely introduced to Asia, the Americas, Europe, and other regions. This crop has significant economic value; its seeds contain approximately 50% oil, and its processed products can be widely used in the production of various chemical materials such as surfactants, plasticizers, synthetic rubber, and foamed plastics. Existing research confirms that castor bean exhibits excellent environmental resilience, demonstrating strong tolerance to poor soil environments and combined heavy metal pollution in water and soil. Experimental studies have shown that, under pot cultivation conditions, castor bean has a certain enrichment and remediation effect on cadmium, lead, and zinc in heavy metal-contaminated soils, achieving in-situ passivation and absorption reduction of heavy metals. Simultaneously, castor bean can adapt to the adverse environment of coastal saline-alkali soils, and its cultivation can effectively improve the ecological environment of saline-alkali lands, possessing potential for application in saline-alkali land vegetation restoration and soil and water improvement. However, simple planting often suffers from long growth cycles, low carbon sequestration efficiency, and slow soil improvement. Especially in moderately to severely saline-alkali lands, plant growth is restricted, leading to reduced root exudates and difficulty in effectively lowering soil pH and increasing organic matter content.

[0004] To improve plant adaptability and growth rate in saline-alkali soils, existing technologies have attempted to use chemical fertilizers or single soil conditioners. However, chemical fertilizers tend to cause soil compaction, while traditional soil conditioners are often not targeted enough to simultaneously achieve the dual goals of rapidly reducing soil pH and significantly increasing soil carbon storage. Therefore, how to provide a method that can both promote castor bean growth and efficiently improve soil physicochemical properties and increase carbon sequestration is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] This invention addresses the shortcomings of existing technologies by providing a method and application for soil improvement and carbon sequestration enhancement through castor bean cultivation in saline-alkali land. By planting castor beans and applying specific concentrations of exogenous hormones, the physical and chemical properties of the soil are improved and the soil carbon sequestration is increased.

[0006] Therefore, in a first aspect, the present invention provides a method for soil improvement and carbon sequestration enhancement in castor bean cultivation on saline-alkali land, comprising: An agroforestry planting model was adopted, in which castor beans were planted in saline-alkali land, and exogenous hormones were sprayed during the seedling, budding and flowering stages of the castor beans. The exogenous hormones were indoleacetic acid or gibberellin, and the spraying concentration of indoleacetic acid was 50μM~200μM, and the spraying concentration of gibberellin was 50μM~200μM.

[0007] Furthermore, the castor bean variety is Zibo No. 5 or Zibo No. 8.

[0008] Furthermore, the spraying concentration of indoleacetic acid is 50 μM to 150 μM, and the spraying concentration of gibberellin is 50 μM to 150 μM.

[0009] Furthermore, before planting the castor beans, compound fertilizer is applied as base fertilizer, and the nitrogen application rate of the base fertilizer is 200 kg / ha to 300 kg / ha.

[0010] Furthermore, the compound fertilizer contains 10% to 20% nitrogen, 10% to 20% phosphorus, and 10% to 20% potassium.

[0011] Furthermore, the castor beans are planted using a hole-sowing method, with 2 to 3 seeds sown per hole and a plant spacing of 70cm to 90cm.

[0012] Furthermore, the organic matter content of the saline-alkali land is 17 g·kg⁻¹. -1 ~22g·kg -1 The total nitrogen content is 0.5 g·kg⁻¹. -1 ~1g·kg -1 The available phosphorus content is 1 mg·kg -1 ~2mg·kg -1 The available potassium content is 250 mg / kg. -1 ~300mg·kg -1 The salt content is 2 g / kg. -1 ~2.5g·kg -1 The soil pH value is 8.2~8.8.

[0013] Furthermore, the forest tree species in the agroforestry planting model include Zelkova and / or Ulmus pumila.

[0014] A second aspect of the invention provides the application of the method in the improvement of coastal saline-alkali soils and / or the enhancement of soil carbon sequestration.

[0015] Furthermore, the application includes at least one of the following: (1) Lower soil pH; (2) Increase soil organic carbon content; (3) Increase soil organic matter content; (4) Increase soil carbon storage, plant carbon sequestration or total carbon.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: This invention provides a method for soil improvement and carbon sequestration enhancement in castor bean cultivation on saline-alkali land. By precisely spraying specific concentrations of indoleacetic acid or gibberellin during the seedling, budding, and flowering stages of castor beans, it significantly reduces the soil pH in coastal saline-alkali land, improves soil physicochemical properties, and increases soil organic carbon and organic matter content. Simultaneously, it effectively enhances soil carbon storage, castor plant carbon sequestration, and total carbon accumulation. Employing an agroforestry integrated planting model, it exhibits strong adaptability to coastal saline-alkali land conditions, low operating costs, and stable improvement effects. It addresses both land use and ecological carbon sequestration improvement needs in saline-alkali land, providing a theoretical basis and technical support for enhancing soil carbon sequestration in coastal saline-alkali land. Detailed Implementation

[0017] To better understand the above technical solutions, the technical solutions of the embodiments of this application are described in detail below through specific examples. It should be understood that the embodiments of this application and the specific features in the embodiments are detailed descriptions of the technical solutions of the embodiments of this application, rather than limitations on the technical solutions of this application. Unless otherwise specified, the embodiments of this application and the technical features in the embodiments can be combined with each other. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply.

[0018] A first aspect of this invention provides a method for soil improvement and carbon sequestration enhancement in castor bean cultivation on saline-alkali land, comprising: An agroforestry planting model was adopted, in which castor beans were planted in saline-alkali land, and exogenous hormones were sprayed during the seedling, budding and flowering stages of the castor beans. The exogenous hormones were indoleacetic acid or gibberellin, with the spraying concentration of indoleacetic acid being 50μM~200μM and the spraying concentration of gibberellin being 50μM~200μM.

[0019] Specifically, based on the "low-promoting, high-inhibiting" concentration effect of exogenous hormones, this embodiment of the invention strictly limits the spraying concentration of indoleacetic acid (IAA) or gibberellin (GA3) to the range of 50 μM to 200 μM. Within this concentration range, exogenous hormones, as highly efficient signaling molecules, can effectively activate the stress resistance physiological pathways of castor beans: on the one hand, they upregulate the SOS ion balance pathway and the activity of antioxidant enzymes such as SOD, POD, and CAT in the plant, significantly reducing the accumulation of reactive oxygen species under salt and alkali stress and lowering the MDA content to protect cell membrane integrity; on the other hand, they induce the synthesis of osmotic regulatory substances such as proline and soluble sugars, enhancing the water retention and absorption capacity of the roots in a salt and alkali environment, thereby alleviating ion toxicity and oxidative damage, and ensuring that castor beans can still maintain a high chlorophyll content and photosynthetic efficiency under stress.

[0020] Multiple applications throughout the entire growth cycle significantly promoted root development and above-ground growth in castor beans, resulting in a substantial increase in plant biomass accumulation. This biomass enhancement laid the material foundation for soil improvement: the developed root system, increased litter, and root exudates significantly increased soil organic matter content, promoted soil aggregate formation, and improved permeability. Simultaneously, the metabolic activities of plant roots effectively reduced soil pH and salinity, optimized the rhizosphere microbial community structure, and enhanced soil buffering capacity.

[0021] The embodiments of the present invention not only solve the problem of low crop yield in saline-alkali land, but also significantly increase the total carbon content of the saline-alkali land ecosystem by increasing plant carbon storage and soil organic carbon density, thus achieving the dual goals of ecological restoration and carbon sequestration enhancement of saline-alkali land.

[0022] In some embodiments, the castor bean variety is Zibo No. 5 or Zibo No. 8.

[0023] Specifically, in the early stage of the experiment, 13 castor bean varieties (Zibi No. 5, Tongbi No. 17, Zibi No. 8, Zibi No. 11, Tongbi No. 101, V Zibi No. 9, Jinbi No. 6, Tongbi No. 16, Youbi No. 9, Youbi No. 6, Thai 302, Tongbi No. 7, and French hybrid) were planted in saline-alkali land. Through comparison of growth, physiology, and yield, Zibi No. 5 and Zibi No. 8 performed the best overall, with higher growth, physiological characteristics, and yield than other varieties, and stronger salt tolerance.

[0024] In some embodiments, the spraying concentration of indoleacetic acid is 50 μM to 150 μM, and the spraying concentration of gibberellin is 50 μM to 150 μM.

[0025] Specifically, within this concentration range, the hormone has the most significant effect in alleviating salt-alkali stress, while avoiding the risk of growth inhibition or cost waste that may result from high concentrations.

[0026] In some embodiments, compound fertilizer is applied as base fertilizer before planting the castor beans, with a nitrogen application rate of 200 kg / ha to 300 kg / ha.

[0027] Preferably, the nitrogen application rate of the base fertilizer is 250 kg / ha.

[0028] In some embodiments, the compound fertilizer contains 10% to 20% nitrogen, 10% to 20% phosphorus, and 10% to 20% potassium.

[0029] Specifically, this range ensures a balanced supply of nitrogen, phosphorus, and potassium nutrients. Phosphorus promotes root growth and energy metabolism, while potassium enhances the plant's resistance to lodging and salinity, thus synergistically improving soil fertility.

[0030] Preferably, the compound fertilizer contains 15% nitrogen, 15% phosphorus, and 15% potassium.

[0031] In some embodiments, castor beans are planted by hill sowing, with 2 to 3 seeds sown per hill and a plant spacing of 70 to 90 cm.

[0032] Specifically, hill planting ensures accurate spacing between plants and rows, which in turn allows for precise control of planting density.

[0033] Preferably, two seeds are sown per hole, with a plant spacing of 80cm.

[0034] In some embodiments, the organic matter content of the saline-alkali land is 17 g·kg⁻¹. -1 ~22g·kg -1 The total nitrogen content is 0.5 g·kg⁻¹. -1 ~1g·kg -1 The available phosphorus content is 1 mg·kg -1 ~2mg·kg -1 The available potassium content is 250 mg / kg. -1 ~300mg·kg -1 The salt content is 2 g / kg. -1 ~2.5g·kg -1 The soil pH value is 8.2~8.8.

[0035] In some embodiments, the forest tree species in agroforestry planting patterns include Zelkova and / or Ulmus pumila.

[0036] Specifically, the core advantage of the Zelkova serrata lies in the high degree of synergy between its growth cycle and the understory crops. Its autumn leaf fall frees up space for the sowing and growth of understory crops, and by the time new leaves sprout in spring, the crops are nearly ready for harvest. The natural shade provided by its canopy effectively improves the microclimate. Simultaneously, the loosening of soil and fertilization of understory crops benefits the Zelkova serrata's growth, creating a virtuous cycle. The advantages of the white elm lie in its strong resistance to salt and drought, its tall and upright trunk, and its smaller crown, which minimizes its impact on crop shading.

[0037] A second aspect of the present invention provides the application of the above-described method in the improvement of coastal saline-alkali land soil and / or the enhancement of soil carbon sequestration.

[0038] The above applications include at least one of the following: (1) Lower soil pH; (2) Increase soil organic carbon content; (3) Increase soil organic matter content; (4) Increase soil carbon storage, plant carbon sequestration or total carbon.

[0039] Example 1: A method for soil improvement and carbon sequestration enhancement in castor bean cultivation on saline-alkali land. Based on the typical habitat of coastal saline-alkali land in Jiangsu, this invention constructs a differentiated exogenous hormone spraying regulation technology system, explores the synergistic effect of different hormone types and application concentrations on the improvement of the physicochemical properties of saline-alkali soil, and clarifies the changing characteristics of soil carbon sequestration capacity under the hormone regulation mode.

[0040] 1. Experimental Design The topsoil layer of the experimental site contained 19.75 g·kg⁻¹ of organic matter. -1 Total nitrogen 0.72 g·kg -1 Available phosphorus 1.45 mg·kg -1 Available potassium 279 mg·kg -1 Salt content 2.18 g / kg -1 The soil pH value is 8.5.

[0041] The agroforestry planting model primarily includes two tree species: Zelkova serrata and Ulmus pumila. Based on standard wood measurement, the average Zelkova serrata tree height was 2.8m (±0.1m), diameter at breast height (DBH) was 4.3cm (±0.4cm), base diameter was 5.6cm (±0.6cm), north-south crown width was 1.1m (±0.3m), and tree spacing was 1m. The average Ulmus pumila tree height was 2.7m (±0.4m), DBH was 3.38cm (±0.8cm), base diameter was 5.2cm (±1.0cm), north-south crown width was 1.1m (±0.3m) (n=5, mean ± standard deviation), and tree spacing was 2m. All forest belts were configured in a north-south direction with a row spacing of 5m.

[0042] Two castor bean varieties, Zibo No. 5 (V1) and Zibo No. 8 (V3), were selected for the experiment. An agroforestry planting model was adopted. During the seedling, budding, and flowering stages of the castor beans, two hormones, indoleacetic acid (S1) and gibberellin (S2), were sprayed at the same concentration. Each hormone was applied at concentrations of 50 μM, 100 μM, 150 μM, and 200 μM. Sowing was carried out on April 16, 2024, and harvesting and yield measurement were conducted on October 15, 2024. Two seeds were sown per hill, with a plant spacing of 80 cm. Compound fertilizer was applied as basal fertilizer before sowing, with a nitrogen application rate of 250 kg / ha. The compound fertilizer used contained 15% nitrogen, phosphorus, and potassium. The spraying concentrations of S1 and S2 are shown in Table 1. Treatment groups I1, I2, I3, and I4 were treated with S1, and treatment groups G1, G2, G3, and G4 were treated with S2. CK was the blank control.

[0043] Table 1. Specific concentrations of the two hormones applied as sprays.

[0044] 2. Measurement Items Soil samples were randomly collected from each treatment before castor bean sowing, and 40 and 70 days after sowing. After collection, soil samples were air-dried and filtered through a 60-mesh sieve for determining soil pH and organic carbon. At castor bean harvest, 10 castor bean plants were randomly harvested from each plot, weighed immediately, blanched at 105℃ for 30 minutes, and then dried at 80℃ to constant weight. The dry weight was measured, and the samples were ground into powder using a mill and filtered through a 100-mesh sieve for determining plant organic carbon content. Soil pH was determined using a 5:1 soil-to-water ratio solution. Soil organic carbon and plant organic carbon were determined using the potassium dichromate external heating method.

[0045] 2.1 Soil Organic Matter Calculation OM = m1 × 1.724 × 1.08 / m × 10 3 ×100.

[0046] Where: OM—mass fraction of soil organic matter, % m1—Carbon content of the soil sample obtained from the standard curve, in mg; 1.724 — The coefficient for converting organic carbon to organic matter; 1.08 — Oxidation correction factor; m — soil sample mass, in grams.

[0047] 2.2 Soil carbon storage determination The organic carbon density estimation method was used. SOCi = Ci × Di × Ei × (1 - Gi) / 10.

[0048] Where: SOCi—soil organic carbon density of a single soil layer, kg·m³ -2 ; Ci – Soil organic carbon content, % Di — Soil bulk density, g·cm³ -3 ; Ei – Soil thickness, cm.

[0049] 2.3 Determination of castor bean carbon reserves Weigh 20 mg of dried castor bean plant sample and place it in a 50 ml test tube. Add 5 ml of 0.8 mol / L K2Cr2O7 solution and 5 ml of H2SO4, shake well, place in a 100℃ constant temperature incubator, cool in a cold water bath after 90 min, add ultrapure water to make up to 50 ml, shake well and let stand overnight. Take the supernatant and measure the color at a wavelength of 590 nm. Calculate the organic carbon content according to the standard curve.

[0050] Castor bean carbon reserves = castor bean biomass × castor bean carbon content.

[0051] Farmland carbon sequestration = castor bean carbon storage + soil carbon storage.

[0052] 3. Data Analysis Excel 2016 was used for data entry and calculation. Statistix 9 software was used for data analysis based on a single-factor randomization design. LSD was used for multiple comparisons. 0.05 The method was used. In the table below, different lowercase letters indicate that the differences between treatments are significant at the 0.05 level.

[0053] 4. Results Analysis 4.1 Effects of hormone spraying on castor bean on soil pH Table 2. Effects of hormone spraying on castor bean under agroforestry planting model on soil pH (Zibo No. 5)

[0054] As shown in Table 2, under the agroforestry planting model, different concentrations of indoleacetic acid (IAA) and gibberellin (GA3) treatments had a significant regulatory effect on the soil pH of Zima No. 5.

[0055] IAA treatments showed a concentration-dependent alkalinity-reducing effect, with medium to high concentrations (I2 and I3) significantly lowering soil pH and alleviating alkaline stress. GA3 treatments exhibited a "low-promoting, high-suppressing" characteristic, with low-concentration GA3 (G1) showing the best alkalinity-reducing effect. Soil pH was generally lower at 70 days post-sowing than at 40 days post-sowing, indicating that the regulatory effect of hormones on soil pH gradually increased as the growth stage progressed.

[0056] Table 3. Effects of hormone spraying on castor bean under agroforestry planting model on soil pH (Zibo No. 8)

[0057] As shown in Table 3, under the agroforestry planting model, different concentrations of indoleacetic acid (IAA) and gibberellin (GA3) treatments had a significant regulatory effect on the soil pH of Zima No. 8.

[0058] IAA treatments generally had a soil pH-lowering effect, with medium to high concentrations (I1, I2, and I3) significantly reducing soil pH. At 40 days post-sowing, the soil pH in the I3 treatment group was the lowest (8.09); at 70 days post-sowing, the soil pH in both the I2 and I4 treatment groups decreased to 6.99, significantly lower than the control, indicating that medium to high concentrations of IAA can effectively alleviate soil alkalinity stress.

[0059] 40 days after sowing, the soil pH was lowest under the G3 treatment (7.78), significantly lower than other GA3 treatments; 70 days after sowing, the soil pH was still lowest under the G3 treatment (6.78), indicating that medium concentration of GA3 (G3) is the appropriate concentration to reduce soil pH and can effectively alleviate soil alkalinity.

[0060] 4.2 Effects of hormone spraying on castor beans on soil organic carbon content Table 4. Effects of hormone spraying on castor bean under agroforestry planting model on soil organic carbon content (Zibo No. 5)

[0061] As shown in Table 4, under the agroforestry planting model, hormone spraying had a significant impact on the soil organic carbon content of Zibi No. 5.

[0062] The pre-sowing soil organic carbon content was 9.07 mg / g, and the organic carbon content at flowering time was higher in all treatments than the pre-sowing level. For the V1 variety, under IAA treatment, the organic carbon content at flowering time in treatments I1, I2, I3, and I4 were 11.05 mg / g, 11.30 mg / g, 10.29 mg / g, and 9.92 mg / g, respectively, representing increases of 21.83%, 24.59%, 13.45%, and 9.37% compared to pre-sowing levels, with the largest increases observed in treatments I1-I3. Under GA3 treatment, the organic carbon content at flowering time in treatments G1, G2, G3, and G4 were 11.09 mg / g, 10.29 mg / g, 9.68 mg / g, and 9.61 mg / g, respectively, representing increases of 22.27%, 13.45%, 6.73%, and 5.95% compared to pre-sowing levels, with the largest increases observed in treatments G1 and G2.

[0063] Table 5. Effects of hormone spraying on castor bean under agroforestry planting model on soil organic carbon content (Zibo No. 8)

[0064] As shown in Table 5, under the agroforestry planting model, different concentrations of indoleacetic acid (IAA) and gibberellin (GA3) treatments had a significant impact on the soil organic carbon content during the flowering period of Zibi No. 8.

[0065] The pre-sowing soil organic carbon content was 9.47 mg / g. For the V3 variety, under IAA treatment, the organic carbon content at flowering time under treatments I1, I2, I3, and I4 were 10.41 mg / g, 9.9 mg / g, 9.82 mg / g, and 9.58 mg / g, respectively, representing increases of 9.92%, 4.54%, 3.70%, and 1.17% compared to pre-sowing levels, with the largest increase observed under treatment I1. Under GA3 treatment, the organic carbon content at flowering time for varieties G1, G2, G3, and G4 were 9.82 mg / g, 9.73 mg / g, 10.15 mg / g, and 9.50 mg / g, respectively, representing increases of 3.70%, 2.75%, 7.18%, and 0.3% compared to pre-sowing levels.

[0066] 4.3 Effects of hormone spraying on castor beans on soil organic matter content Table 6. Effects of hormone spraying on castor bean under agroforestry planting model on soil organic matter content (Zibo No. 5)

[0067] As shown in Table 6, under the agroforestry planting model, different concentrations of indoleacetic acid (IAA) and gibberellin (GA3) treatments had a significant regulatory effect on the soil organic matter content of Zibi No. 5, and the soil organic matter content during the flowering period of each treatment group was higher than the pre-sowing level (1.71 mg / g).

[0068] Under IAA hormone treatment, the soil organic matter content of each IAA treatment increased compared with that before sowing, with an increase of 8.19%-21.05%. Among them, the I2 group had the highest organic matter content, at 2.07 mg / g, an increase of 21.05% compared with that before sowing; the I4 group had the lowest content, at 1.85 mg / g, an increase of 8.19% compared with that before sowing.

[0069] Under gibberellin treatment, the soil organic matter content in each GA treatment group also showed an increasing trend compared with that before sowing, with an increase of 4.68%-21.05%. The G1 group had the highest organic matter content, at 2.07 mg / g, an increase of 21.05% compared with that before sowing; the G4 group had the lowest content, at 1.79 mg / g, an increase of 4.68% compared with that before sowing.

[0070] Table 7. Effects of hormone spraying on castor bean under agroforestry planting model on soil organic matter content (Zibo No. 8)

[0071] As shown in Table 7, under the agroforestry planting model, different concentrations of indoleacetic acid (IAA) and gibberellin (GA3) treatments had a significant regulatory effect on the soil organic matter content of Zibi No. 8.

[0072] Under IAA hormone treatment, the soil organic matter content in all IAA treatment groups increased compared with that before sowing, with an increase of 1.14%-10.23%. Among them, the I1 group had the highest organic matter content, at 1.94 mg / g, an increase of 10.23% compared with that before sowing; the I2 and I4 treatments had the lowest content, at 1.78 mg / g.

[0073] Under gibberellin treatment, the soil organic matter content in each GA3 treatment group showed an increasing trend compared with that before sowing, with an increase of 1.14%-7.39%. The organic matter content in the G3 group was the highest, increasing by 7.39% compared with that before sowing; the content in the G4 treatment was the lowest, increasing by 1.14% compared with that before sowing.

[0074] 4.4 The impact of hormone spraying on soil carbon sequestration Table 8. The impact of hormone spraying on castor bean under an agroforestry planting model on soil carbon sequestration (Zibo No. 5)

[0075] As shown in Table 8, under the agroforestry planting model, different concentrations of IAA and GA3 treatments have a significant regulatory effect on the soil carbon sequestration capacity of Zibi No. 5, mainly reflected in three aspects: soil carbon storage, plant carbon sequestration, and total carbon accumulation.

[0076] Regarding soil carbon storage, compared with the control (CK), soil carbon storage was significantly increased under low-concentration IAA (I1) and low-concentration GA3 (G1) treatments, with increases of 17.97% and 18.47%, respectively. As the application concentrations of IAA and GA3 increased, soil carbon storage showed an overall decreasing trend. I3, I4, and G2 decreased to the same significant level, while G3 and G4 decreased significantly, but all were significantly higher than the control (CK).

[0077] Regarding plant carbon sequestration, the carbon sequestration of plants in the low-concentration IAA (I1) treatment group was significantly higher than that in the control group (CK), with an increase of 173.74%. As the hormone concentration increased, the carbon sequestration of plants in the IAA and GA3 treatments showed a gradual decreasing trend, with the decrease being more pronounced in the high-concentration treatments. The carbon sequestration of plants in the G3 and G4 treatments dropped to the lowest level among all treatments, but their carbon sequestration was still higher than that of the control group (CK).

[0078] Regarding the total carbon content of plants, the total carbon content of the I1 treatment was significantly higher than that of the CK, with an increase of 89.86%. Overall, the total carbon content decreased significantly with increasing IAA and GA3 concentrations, and there were significant differences among the treatments.

[0079] Table 9. The impact of hormone spraying on castor bean under an agroforestry planting model on soil carbon sequestration (Zibo No. 8)

[0080] As shown in Table 9, under the agroforestry planting model, different concentrations of IAA and GA3 treatments had a significant regulatory effect on the soil carbon sequestration capacity of Zibi No. 8. Regarding soil carbon storage, compared with the control (CK), all hormone treatments showed a significant increase in soil carbon storage to varying degrees, with an overall increase ranging from 4.27% to 19.44%. Among them, the I1 treatment showed the largest increase in soil carbon storage. With the increase of IAA and GA3 concentrations, the soil carbon storage generally showed a fluctuating downward trend. The soil carbon storage of I3 and G4 was at the lowest level among all treatments, but it was still significantly higher than that of the control (CK).

[0081] Regarding plant carbon sequestration, the carbon sequestration of plants treated with low and medium concentrations of IAA (I1, I2) was significantly higher than that of the control, with increases of 89.06% and 198.87%, respectively. The carbon sequestration of plants in all GA3 treatment groups was also higher than that of the control, while the carbon sequestration of plants treated with high concentrations of IAA (I4) was lower than that of the control and was the lowest among all treatments.

[0082] Regarding total carbon emissions, all hormone treatments showed significantly higher total carbon emissions than the control. The I2 treatment had the highest total carbon emissions, indicating that spraying with medium to low concentrations of hormones was more conducive to increasing total carbon emissions. As the hormone concentration further increased, the total carbon emissions showed a downward trend, with the G4 treatment reaching a relatively low level in the GA3 gradient.

[0083] 5. Conclusion 5.1 Effects of hormone spraying on castor beans on soil pH Soil pH is a crucial indicator for assessing the salinity and alkalinity of saline-alkali soils and the effectiveness of soil improvement, controlling the activity of soil microorganisms and related chemical reactions. This experiment shows that planting castor beans and spraying exogenous hormones can lower soil pH, bringing the soil pH closer to neutral. Low to medium concentrations of IAA and GA3 significantly reduced soil pH and alleviated saline-alkali stress, while high concentrations of hormones showed an inhibitory effect. For Zibo No. 5, medium concentrations of IAA and low concentrations of GA3 showed more pronounced alkalinity-lowering effects; Zibo No. 8 showed stronger pH regulation capabilities under medium concentrations of GA3. Exogenous hormones (IAA, GA3) achieve targeted regulation of soil pH by precisely controlling the physicochemical properties and biological community structure of the rhizosphere microenvironment. This process is the result of the synergistic effect of hormones, roots, and microorganisms, and its regulatory mechanism has clear specificity and systematicity. On the one hand, hormones, as key signaling molecules for plant growth and development, can activate related metabolic pathways within root cells, inducing the secretion of low-molecular-weight organic acids such as citric acid, malic acid, and oxalic acid. These organic acids can directly react with free alkaline ions in the soil (such as Na+). + Ca 2+ (etc.) undergo chelation reactions, reducing the OH- content in the soil solution. - The concentration of hormones directly reduces the pH of the rhizosphere soil, mitigating the adverse effects of salt and alkali stress. Previous studies have confirmed that exogenous IAA and GA3 can significantly upregulate the activity of key enzymes in root organic acid synthesis (such as citrate synthase and malate dehydrogenase), promoting the synthesis and secretion of organic acids. This induction effect exhibits a clear concentration dependence; appropriate hormone concentrations achieve optimal secretion effects, while high concentrations inhibit root metabolism and reduce organic acid secretion.

[0084] On the other hand, exogenous hormones can indirectly regulate soil pH by reshaping the structure and function of the rhizosphere microbial community. Hormones can act as "regulatory signals" for the rhizosphere microbial community, selectively promoting the proliferation and activity of functional microorganisms such as phosphate-solubilizing bacteria (e.g., Bacillus megatherium) and nitrogen-fixing bacteria (e.g., Azotobacter chroococcum), while inhibiting the growth of alkali-tolerant pathogens (e.g., Fusarium). Phosphate-solubilizing bacteria can secrete organic acids (e.g., gluconic acid and lactic acid) during metabolism, converting insoluble phosphates in the soil into soluble phosphorus that can be absorbed and utilized by plants, while releasing acidic substances to neutralize soil alkalinity and significantly reduce the pH of saline-alkali soils. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia nitrogen through biological nitrogen fixation, and the amino acids produced during their metabolism can be further decomposed to produce acidic secretions, synergistically improving the soil acid-base environment. Furthermore, these functional microorganisms can also indirectly promote the optimal regulation of soil pH by increasing soil enzyme activity (e.g., dehydrogenase and alkaline phosphatase) and accelerating soil material cycling.

[0085] 5.2 Effects of hormone spraying on castor beans on soil organic carbon content Exogenous hormones (IAA and GA3) exert a significant directional regulatory effect on soil organic carbon content by modulating the rhizosphere microenvironment, plant growth, and soil carbon cycling processes in castor bean plants. The intensity and direction of this regulation depend on the hormone type, application concentration, and characteristics of the castor bean variety, exhibiting clear systematicity and specificity. On one hand, hormones can promote castor bean plant growth and development, increase photosynthetic product accumulation and underground carbon input, thus providing a sufficient source of soil organic carbon. As core signaling molecules for plant growth regulation, appropriate concentrations of IAA and GA3 can significantly promote castor bean root elongation, lateral root germination, and aboveground biomass accumulation, enhancing photosynthetic efficiency and enabling more photosynthetic products (such as sucrose and starch) to be transported to the roots. These products are then input into the soil through root secretion and residue return, increasing soil organic carbon content.

[0086] Hormone application has significant ecological and agricultural implications for regulating soil organic carbon content in castor bean fields, serving as a key pathway to improve soil fertility in saline-alkali land and enhance the carbon sequestration function of farmland ecosystems. Increased soil organic carbon content can significantly improve soil physicochemical properties, enhance water and fertilizer retention capacity, optimize soil pore structure, create a suitable environment for castor bean root growth and nutrient absorption, and simultaneously promote soil microbial diversity, further improving soil ecosystem functions. For coastal saline-alkali lands, hormone-regulated soil organic carbon accumulation can effectively alleviate saline-alkali stress, reduce soil bulk density, improve soil aggregate stability, and accelerate the soil improvement process. Furthermore, as a core component of the carbon sequestration in farmland ecosystems, increased soil organic carbon content can significantly enhance the carbon sequestration potential of castor bean farmland, reduce atmospheric CO2 emissions, mitigate the greenhouse effect, and achieve synergistic development of "soil improvement - high crop yield - enhanced carbon sequestration." Different castor bean varieties showed significant differences in their response to hormone regulation. Zibo No. 5 exhibited superior organic carbon accumulation capacity under medium-concentration IAA and low-concentration GA3 treatments, while Zibo No. 8 showed a more significant increase in soil organic carbon content under low-concentration IAA and medium-concentration GA3 treatments. This varietal difference stems from the differences in the sensitivity of different castor bean genotypes to hormone signals, the distribution of photosynthetic products, and the composition of rhizosphere exudates. This provides a scientific basis for targeted optimization of hormone application programs and improvement of soil organic carbon content.

[0087] 5.3 Effects of hormone spraying on castor beans on soil organic matter content Hormone spraying has significant ecological and agricultural implications for regulating soil organic matter content in castor bean areas. It is a key approach to improving soil fertility in saline-alkali lands, accelerating ecological restoration, and strengthening farmland ecosystem functions. Soil organic matter, as a core indicator of soil fertility, significantly improves soil physicochemical properties, enhances water and fertilizer retention capacity, optimizes soil pore structure, reduces soil bulk density, and adsorbs alkaline ions, alleviating salt-alkali stress and creating a suitable soil environment for castor bean root growth and nutrient absorption. For coastal saline-alkali lands, hormone-regulated soil organic matter accumulation effectively improves soil structure, promotes soil aggregate formation, enhances soil permeability, accelerates desalination and dealkali removal processes, and drives continuous improvement in soil quality.

[0088] 5.4 The impact of hormone spraying on castor beans on soil carbon sequestration Exogenous hormones (indoleacetic acid IAA and gibberellin GA3) exert a significant directional regulatory effect on the carbon sequestration function of castor bean soil by systematically regulating the carbon fixation capacity of castor bean plants, soil carbon input and carbon fixation processes, and rhizosphere microenvironment evolution. The intensity and direction of this regulation mainly depend on the hormone type, application concentration, and characteristics of the castor bean variety, exhibiting clear systematicity, specificity, and concentration dependence. They are key regulatory factors influencing the carbon sequestration potential of farmland ecosystems. On the one hand, hormones can enhance the carbon fixation capacity of castor bean plants by promoting plant growth and development and improving photosynthetic efficiency, thereby increasing the total amount of soil carbon input and laying the core material basis for strengthening soil carbon sequestration function. As key signaling molecules regulating plant growth and photosynthetic metabolism, appropriate concentrations of IAA and GA3 can significantly promote robust root growth, lateral root germination, and lush aboveground stems and leaves in castor bean plants. They also upregulate the activity and expression levels of key photosynthetic enzymes (Rubisco enzymes and core proteins of the photosynthetic system II), improve the carbon fixation efficiency of photosynthesis, increase the net photosynthetic rate of plants, and promote the accumulation and distribution of photosynthetic products (organic carbon). A portion of photosynthetic products is used for plant growth, while the remainder is introduced into the soil through root exudation, litter return, and root residue decomposition, forming active and inert organic carbon in the soil, respectively, replenishing the soil carbon pool and increasing soil carbon sequestration capacity. Low to medium concentrations of hormones have the most significant promoting effect on plant carbon sequestration and soil carbon input, while high concentrations of hormones inhibit plant metabolism, leading to premature root aging, reducing photosynthetic efficiency and carbon input, and thus inhibiting soil carbon sequestration function.

[0089] In summary, under the agroforestry intercropping model, the application of exogenous hormones (IAA, GA3) has a significant comprehensive regulatory effect on the soil physicochemical properties such as pH, organic carbon, and organic matter, as well as the soil carbon sequestration function of castor bean (Zibo No. 5 and Zibo No. 8) in coastal saline-alkali land. This provides a scientific theoretical basis and technical support for the ecological restoration of coastal saline-alkali land, high-quality and high-yield cultivation of castor bean, and the enhancement of farmland carbon sequestration.

[0090] It will be readily understood by those skilled in the art that the above-described advantageous methods can be freely combined and superimposed without conflict. The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application. The above are merely preferred embodiments of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the protection scope of this application.

Claims

1. A method for soil improvement and carbon sequestration enhancement in castor bean cultivation on saline-alkali land, characterized in that, include: An agroforestry planting model was adopted, in which castor beans were planted in saline-alkali land, and exogenous hormones were sprayed during the seedling, budding and flowering stages of the castor beans. The exogenous hormone is indoleacetic acid or gibberellin, wherein the spray concentration of indoleacetic acid is 50 μM to 200 μM, and the spray concentration of gibberellin is 50 μM to 200 μM.

2. The method according to claim 1, characterized in that, The castor bean variety is Zibo No. 5 or Zibo No.

8.

3. The method according to claim 1, characterized in that, The spraying concentration of indoleacetic acid is 50 μM to 150 μM, and the spraying concentration of gibberellin is 50 μM to 150 μM.

4. The method according to claim 1, characterized in that, Before planting the castor beans, apply compound fertilizer as base fertilizer, with a nitrogen application rate of 200 kg / ha to 300 kg / ha.

5. The method according to claim 4, characterized in that, The compound fertilizer contains 10% to 20% nitrogen, 10% to 20% phosphorus, and 10% to 20% potassium.

6. The method according to claim 1, characterized in that, The castor beans are planted by sowing in holes, with 2 to 3 seeds sown per hole and a plant spacing of 70 to 90 cm.

7. The method according to claim 1, characterized in that, The organic matter content of the saline-alkali land is 17 g·kg⁻¹. -1 ~22g·kg -1 The total nitrogen content is 0.5 g·kg⁻¹. -1 ~1g·kg -1 The available phosphorus content is 1 mg·kg -1 ~2mg·kg -1 The available potassium content is 250 mg / kg. -1 ~300mg·kg -1 The salt content is 2 g / kg. -1 ~2.5g·kg -1 The soil pH value is 8.2~8.

8.

8. The method according to claim 1, characterized in that, The forest tree species in the agroforestry planting model include Zelkova and / or Ulmus pumila.

9. The application of the method according to any one of claims 1-8 in the improvement of coastal saline-alkali land soil and / or the enhancement of soil carbon sequestration.

10. The application according to claim 9, characterized in that, The application includes at least one of the following: (1) Lower soil pH; (2) Increase soil organic carbon content; (3) Increase soil organic matter content; (4) Increase soil carbon storage, plant carbon sequestration or total carbon.

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