Preparation method and application of special biochar-based nano-selenium for saline-alkali soil improvement

Abstract

The present application relates to the technical field of fertilizer, and more particularly to a preparation method of special biochar-based nano selenium for saline-alkali soil improvement and application thereof. The present application pyrolyzes rice straw under inert atmosphere to obtain rice straw biochar; mixes the rice straw biochar and a selenium source solution to load, and obtains selenium-loaded biochar; mixes the selenium-loaded biochar and lemon juice extract, adjusts pH to 9-11, and reacts under light-proof condition to obtain biochar-based nano selenium. The biochar-based nano selenium can significantly improve the physicochemical properties of soil, reduce the pH, conductivity and alkalization degree of soil, and improve the cation exchange capacity, thus providing a good rhizosphere environment for crop growth; and exhibits extremely high selenium bioavailability, can efficiently promote the transportation and accumulation of selenium element to crop fruits, makes the selenium content of fruits reach the standard of selenium-rich food, and realizes the synergistic improvement of yield and quality.

Description

Technical Field

[0001] This invention relates to the field of fertilizer technology, and in particular to a method for preparing biochar-based nano-selenium for saline-alkali soil improvement and its application. Background Technology

[0002] In agricultural production, although traditional selenium fertilizers can increase the selenium content of crops, they are primarily composed of inorganic selenium (SeO3). 2- SeO4 2- Selenium is primarily in its inorganic form, but it faces challenges such as low bioavailability (<10%), easy leaching and pollution, long-term excessive application leading to soil selenium enrichment, and disruption of microbial communities. Its extraction is also accompanied by energy consumption and ecological disturbance. However, research on nano-selenium has provided crucial support for selenium fertilizer innovation, and multiple comparative experiments have confirmed its significant advantages over traditional inorganic selenium. Existing technologies have shown that at the same concentration of 0.16 mg / L, nano-selenium and SeO4... 2- Both can promote the absorption of mineral elements such as N, P, and K, as well as selenium, by pomegranates, but nano-selenium has a superior biological effect. Furthermore, existing experiments have found that 50-100 mg / kg of nano-selenium can significantly increase the fresh weight of tobacco and promote root growth, while the same concentration of SeO4... 2- However, it exhibited an inhibitory effect. Meanwhile, the nano-selenium fertilizer developed by Li Fei et al., after foliar spraying or soil basal application, resulted in a selenium content of 250-5000 μg / kg in the finished tea, 2-50 times higher than traditional inorganic selenium fertilizers, with an organic selenium content exceeding 90%, far surpassing the conversion efficiency of traditional fertilizers (below 50%). In summary, at the same concentration, nano-selenium has a higher bio-promoting effect and lower physiological toxicity than inorganic selenium, and soil application tests showed lower selenium residue in the soil, confirming its synergistic and load-reducing characteristics, highlighting its superior environmental friendliness and agricultural application potential.

[0003] While nano-selenium fertilizers possess significant application advantages, their large-scale promotion in agricultural settings is limited by prominent stability issues. Nano-selenium, with its inherent large specific surface area and high surface energy, is prone to aggregation and inactivation upon entering the soil environment, or to migration and loss through water leaching. This leads to a significant reduction in its bioavailability in the soil-plant system, becoming the core bottleneck restricting its widespread application. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing biochar-based nano-selenium for saline-alkali soil improvement and its application, so as to solve the problems existing in the prior art.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: One of the technical solutions of this invention provides a method for preparing biochar-based selenium nanoparticles, comprising the following steps: (1) Rice straw was pyrolyzed under an inert atmosphere to obtain rice straw biochar; (2) Mix rice straw biochar and selenium source solution, load them, and obtain selenium-loaded biochar; (3) After mixing selenium-loaded biochar and lemon juice extract, the pH was adjusted to 9-11 and the reaction was carried out under light-protected conditions to obtain biochar-based nano-selenium.

[0006] The second technical solution of the present invention provides biochar-based selenium nanoparticles prepared by the above-mentioned preparation method.

[0007] The third technical solution of this invention provides the application of the above-mentioned biochar-based nano-selenium in the improvement of saline-alkali soil.

[0008] Compared with the prior art, the present invention has the following beneficial effects: This invention prepares a carbon-based nano-selenium fertilizer (Nano-Se@RS) using rice straw biochar (RS) as a carrier. Selenium is uniformly loaded onto the biochar surface primarily in the form of crystalline zero-valent nano-selenium, without disrupting the porous structure of the carrier. This process combines slow-release fertilizer retention with the high bioactivity of nano-selenium. Nano-Se@RS significantly improves soil physicochemical properties, reducing soil pH, conductivity, and alkalinity, and increasing cation exchange capacity, providing a favorable rhizosphere environment for crop growth. Simultaneously, this material significantly enhances crop photosynthetic performance, providing sufficient material and energy for both vegetative and reproductive growth. Nano-Se@RS treatment significantly promotes crop growth and development, resulting in a significant increase in crop yield under salt-alkali stress. Furthermore, Nano-Se@RS exhibits extremely high selenium bioavailability, efficiently promoting the translocation and accumulation of selenium in crop fruits, enabling the fruit selenium content to meet selenium-enriched food standards, thus achieving a synergistic improvement in yield and quality. Attached Figure Description

[0009] Figure 1 The Langmuir and Freundlich isotherm adsorption model curves of sodium selenite on rice straw biochar are shown. Figure 2 SEM microstructure images and EDS elemental energy maps of RS and Nano-Se@RS are shown, where (a) is RS and (b) is Nano-Se@RS. Figure 3 XRD patterns and UV analysis plots of RS and Nano-Se@RS are shown, where (a) is the XRD pattern and (b) is the UV analysis plot; Figure 4 The effects of different treatment groups on soil salinization and alkalization parameters are shown, where (a) is pH, (b) is electrical conductivity, (c) is cation exchange capacity, and (d) is alkalinity. Figure 5 The effects of different treatments on the photosynthetic indices of cherry tomatoes are shown in the figure. (a) represents the net photosynthetic rate of leaves, (b) represents the transpiration rate of leaves, (c) represents the stomatal conductance of leaves, and (d) represents the intercellular CO2 concentration. Figure 6 The effects of different treatments on the plant height and fruit biomass of cherry tomatoes are shown in the figure. (a) represents the plant height of the tomato plant, and (b) represents the weight of the underground part. Figure 7 The effects of different treatments on the selenium content of cherry tomatoes. Detailed Implementation

[0010] 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.

[0011] 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. Every smaller range between any stated value or intermediate value within a stated range, and 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.

[0012] 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.

[0013] 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 obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0014] 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.

[0015] All raw materials used in this invention can be obtained commercially or prepared using existing technologies.

[0016] The room temperature mentioned in this invention is calculated as 18~30℃.

[0017] This invention provides a method for preparing biochar-based selenium nanoparticles, comprising the following steps: (1) Rice straw was pyrolyzed under an inert atmosphere to obtain rice straw biochar; (2) Mix rice straw biochar and selenium source solution, load them, and obtain selenium-loaded biochar; (3) After mixing selenium-loaded biochar and lemon juice extract, the pH was adjusted to 9-11 and the reaction was carried out under light-protected conditions to obtain biochar-based nano-selenium.

[0018] Step (1) of this invention involves crushing, sieving, and drying rice straw, then pyrolyzing it under an inert atmosphere. After pyrolysis, heating is stopped, nitrogen is introduced, and the mixture is allowed to cool naturally to room temperature. The product is then collected to obtain rice straw biochar.

[0019] In this invention, the particle size sieved is 20-40 mesh.

[0020] In this invention, the drying temperature is 105°C.

[0021] In this invention, the flow rate of the inert atmosphere is 0.8~1.2L / min, for example, it can be 0.8L / min, 0.9L / min, 1.1L / min or 1.2L / min; the heating rate of the pyrolysis is 5~15℃ / min, for example, it can be 5℃ / min, 7℃ / min, 10℃ / min, 12℃ / min or 15℃ / min; the pyrolysis temperature is 500~600℃, for example, it can be 500, 520, 550, 580 or 600; and the pyrolysis time is 1~3h, for example, it can be 1h, 2h or 3h.

[0022] In a preferred embodiment of the present invention, the pyrolysis in step (1) is carried out in a tubular muffle furnace, the model of which is KSL-1400X and the manufacturer is Hefei Kejing Materials Technology Co., Ltd.

[0023] In this invention, biochar, with its well-developed pore structure, large specific surface area, and abundant surface-active functional groups, can not only achieve slow-release regulation of nutrients but also effectively improve soil physicochemical properties and increase crop yield. Its characteristics are highly compatible with the need for improved stability of nano-selenium, providing an ideal carrier choice for solving the application challenges of nano-selenium fertilizers. More importantly, biochar, with aromatic hydrocarbons as its core carbon skeleton and densely distributed active functional groups such as carboxyl groups (-COOH), hydroxyl groups (-OH), and quinone groups (=OO=) on its surface, and naturally embedded with heteroatoms such as nitrogen, phosphorus, and silicon, endows it with extremely strong interfacial adsorption and loading capacity. This provides an irreplaceable structural foundation and functional advantages for constructing stable nano-selenium loading systems, making it an ideal nano-selenium carrier material. This technical approach is highly consistent with the international development concept of "nano-agriculture" and provides an important solution for interdisciplinary innovation in the field of selenium fertilizers.

[0024] Step (2) of this invention is to first prepare a sodium selenite solution, add the sodium selenite solution to the biochar, mix the biochar and the solution thoroughly, place the above-mentioned system containing the test in a constant temperature shaker, shake and adsorb, achieve full contact between solid and liquid, and obtain selenium-loaded biochar.

[0025] In this invention, the selenium source solution includes a sodium selenite solution; the concentration of the selenium source solution is 20~180 mg / L, for example, it can be 20 mg / L, 40 mg / L, 60 mg / L, 80 mg / L, 100 mg / L, 120 mg / L, 140 mg / L, 160 mg / L or 180 mg / L, etc.

[0026] In a preferred embodiment of the present invention, sodium selenite (Na2SeO3 content ≥99%) was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0027] In this invention, the ratio of rice straw biochar to selenium source solution is 0.5~1.5 mg:1 mL, for example, it can be 0.5 mg:1 mL, 0.7 mg:1 mL, 1 mg:1 mL, 1.2 mg:1 mL or 1.5 mg:1 mL, etc.

[0028] In this invention, the temperature of the load is room temperature, and the time is 12 to 36 hours, for example, 12 hours, 18 hours, 24 hours or 36 hours.

[0029] In a preferred embodiment of the present invention, the load is applied under oscillation conditions, and the oscillation speed is 150 rpm.

[0030] Step (3) of this invention involves mixing selenium-loaded biochar and lemon juice extract, then adjusting the pH to 9-11 and maintaining it stable, reacting under constant temperature and light protection, and finally freeze-drying under nitrogen protection to obtain biochar-based nano-selenium.

[0031] In this invention, the method for preparing the lemon juice extract includes the following steps: crushing lemons to extract juice, collecting the juice and centrifuging it, and the supernatant obtained by centrifugation is the lemon juice extract.

[0032] In a preferred embodiment of the present invention, the lemons are fresh and undamaged, and are rinsed with running water to remove impurities, and then washed three times with ultrapure water.

[0033] In this invention, the centrifugation speed is 8000 r / min and the time is 10 min.

[0034] In a preferred embodiment of the present invention, the centrifuge used for centrifugation is model TG16-WS.

[0035] In this invention, the ratio of selenium-supported biochar to lemon juice extract is 0.5~1.5g:1mL, for example, it can be 0.5g:1mL, 0.7g:1mL, 1g:1mL, 1.2g:1mL or 1.5g:1mL, etc.

[0036] In this invention, the reaction temperature is room temperature and the time is 24~48h, for example, it can be 24h, 36h or 48h.

[0037] In a preferred embodiment of the present invention, the reaction is carried out under oscillation conditions, and the oscillation speed is 150 rpm.

[0038] In this invention, the freeze-drying temperature is -50°C, the pressure is 10Pa, and the time is 12~18h, for example, 12, 14, 16 or 18h.

[0039] The present invention also provides biochar-based selenium nanoparticles prepared by the above-described preparation method.

[0040] This invention also provides the application of the above-mentioned biochar-based nano-selenium in the improvement of saline-alkali soil.

[0041] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0042] Example 1 (1) Take rice straw, crush it and pass it through a 20-40 mesh sieve, and dry it in an oven at 105℃ until constant weight; weigh 100 g of dried straw and put it into a 500 mL crucible and compact it. Place the crucible in a tube muffle furnace (KSL-1400X, Hefei Kejing Materials Technology Co., Ltd.), continuously introduce nitrogen gas at a flow rate of 1 L / min, and seal the system; heat up to 550℃ at a rate of 10℃ / min and pyrolyze at a constant temperature for 2 h; stop heating, keep nitrogen gas flowing and cool naturally to room temperature, collect the product, seal and store it to obtain rice straw biochar, denoted as RS.

[0043] (2) Weigh 50 mg RS and place it in a 50 mL centrifuge tube. Add 50 mL of sodium selenite solution with a concentration of 120 mg / L to mix RS with sodium selenite solution. Shake and adsorb at 25 °C and 150 rpm for 24 h to achieve full contact between solid and liquid and obtain selenium-loaded biochar, denoted as RS-Se.

[0044] (3) Select fresh, undamaged lemons, rinse them with running water to remove impurities, then wash them three times with ultrapure water. After drying them with clean gauze, cut them into pieces and squeeze out the juice. Transfer the collected juice to a 50 mL centrifuge tube and place it in a TG16-WS centrifuge. Centrifuge at 8000 r / min for 10 min. Take the supernatant, which is the lemon juice extract. Seal it in a clean beaker for later use.

[0045] (4) Accurately weigh 50.00 g of selenium-supported biochar (RS-Se), add it to a 250 mL Erlenmeyer flask, add lemon juice extract at a liquid-to-solid ratio of 1:1 (g:mL), then slowly add ammonia water to adjust the pH of the system to 9.0 and maintain stability. React at 25℃ and 150 rpm in the dark for 48 h. After the reaction is completed, freeze-dry under vacuum at -50℃ and 10 Pa for 10 h under N2 protection to obtain the Nano-Se@RS product.

[0046] Test Example 1 Adsorption test: First, prepare sodium selenite solutions with concentration gradients of 0~180 mg / L, setting no less than 5 concentration gradients (n≥5). Accurately weigh 50.00 mg RS into a 50 mL centrifuge tube, and add 50 mL of sodium selenite solution of the corresponding concentration to ensure thorough mixing of biochar and solution. Place the centrifuge tube containing the test system in a constant temperature shaker and shake for 24 h at 25 ℃ and 150 rpm to achieve full solid-liquid contact. After adsorption, centrifuge at 4000 rpm for 15 min, take the supernatant and filter it through a 0.22 μm aqueous filter membrane. Use inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher) to determine the selenium concentration in the filtrate. Calculate the amount of selenium adsorbed by RS based on the difference in selenium concentration in the solution before and after adsorption. The calculation formula is shown in equation (1). Two isothermal adsorption models, Langmuir model (2) and Freundlich model (3), were selected to fit the experimental data. The adsorption isotherm data were fitted using the Langmuir isotherm adsorption model. Figure 1 The results showed that RS had the maximum adsorption capacity (q) for sodium selenite. max The concentration reached 9.18 mg / g, with a correlation coefficient R. 2 =0.986, indicating that RS has a good adsorption capacity for selenium. The selenium loaded with biochar is named RS-Se.

[0047] = (1) Where: q e The adsorption capacity of the adsorbent for Se, in mg / g; CO and C e ν represents the initial and adsorption equilibrium concentrations of Se, in mg / L; V represents the solution volume, in L; and m represents the mass of biochar.

[0048] (2) (3) In the formula: q max The maximum adsorption capacity is expressed in mg / g; K is the Freundlich adsorption coefficient, which is related to the adsorption surface area and temperature; n is a temperature-dependent constant, where n>1.

[0049] Test Example 2 To verify the synthesis and product characteristics of nano-selenium, the microstructure and crystal structure of Nano-Se@RS were characterized using a high-resolution transmission electron microscope (JEOL JEM2100F) and an X-ray diffractometer (XRD, SmartLab 9 kW). In bright-field mode, the phase transition process was monitored using scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDS) elemental mapping. Meanwhile, the characteristic absorption peak of nano-selenium was detected at a wavelength of 280 nm using a UV-Vis spectrophotometer (UV-1800, Shimadzu). The multi-dimensional characterization results jointly confirmed that nano-selenium was successfully synthesized.

[0050] Comparative analysis using SEM-EDS characterization revealed that ( Figure 2 Nano-Se@RS samples prepared using lemon extract as a reducing agent exhibited distinct loading characteristics in both microstructure and elemental composition. The original RS support possessed a typical porous framework structure with uniform pore distribution. Under high magnification, the pore wall surface was smooth and flat, with no obvious particle adhesion. Its EDS spectrum only detected C, O, N, and a small amount of Si elements, without the characteristic signal of selenium, indicating that the support itself did not contain selenium components, providing clean porous sites for loading nano-selenium. After Se(IV) was reduced by lemon extract and then loaded onto the nano-Se@RS sample, the porous framework structure of the support was preserved. Simultaneously, a large number of uniformly distributed nanoscale particles appeared on the pore surface and inside the pores. High-magnification images further confirmed that these particles were uniform in size, without obvious agglomeration, and well-dispersed. EDS results showed that, in addition to the background elements of the support, the sample exhibited significant characteristic peaks of selenium at ~1.4 keV and ~10.9 keV, directly proving that Se(IV) was successfully reduced to zero-valent selenium by lemon extract and introduced into the material system, corresponding one-to-one with the nanoparticles observed in SEM. The results indicate that the lemon extract reduction method can effectively achieve uniform loading of nano-selenium on the RS biochar support, and the loading process did not damage the porous structure of the support, providing a good structural basis for subsequent functional applications.

[0051] XRD and UV-Vis characterization were used to further verify the formation of nano-selenium in Nano-Se@RS. Figure 3 In the XRD pattern of (a), the diffraction signals of the loaded sample (Nano-Se@RS) at 2θ ≈ 23° and 43°~45° are significantly enhanced compared to the original RS support. Comparison with the standard card (JCPDS No. 06-0362) corresponds to the (100), (102), and (111) crystal planes of nano-selenium, respectively. Due to the masking effect of the amorphous broad peaks of the support, the characteristic peaks of crystalline selenium do not exhibit typical sharp morphology, but the enhancement of signal intensity and positional matching prove that Se... 0Se was introduced into the system in a crystalline form; in contrast, amorphous selenium, due to its disordered atomic arrangement, only exhibits broad and diffuse diffraction scattering peaks. Therefore, XRD results indicate that Se loaded in Nano-Se@RS... 0 It possesses a crystalline structure, and the nano-selenium size, calculated using the Scherrer formula, is approximately 15–20 nm. Meanwhile, Figure 3 In the UV-Vis spectrum of (b), characteristic absorption bands of nano-selenium were observed at ~295 nm and ~500 nm. These characteristic absorption bands are unique optical properties of nanoscale elemental selenium particles, and their formation is closely related to the quantum size effect of nanocrystalline selenium. Amorphous selenium typically does not possess such characteristic absorption peaks, further confirming the presence of nano-selenium on the surface of biochar. In summary, the lemon extract reduction method successfully reduced Se... 4+ Restore to Se 0 It is stably loaded onto the surface of biochar in the form of crystalline nano-selenium.

[0052] Test Example 3 Application Experiment (1) Experimental design The tested soil (0–20 cm) was saline-alkali soil, collected from the Yellow River Delta Coastal Wetland Ecological Experimental Station of the Chinese Academy of Sciences in Kenli County, Shandong Province. The soil type was sandy loam, and the parent material was river alluvium and marine deposits. The salt composition was mainly chloride.

[0053] Soil pH was measured using a soil-to-water ratio of 1:2.5 with an extract, and soil electrical conductivity (EC) was measured using a GLP22 pH meter. Cation exchange capacity (CEC) was determined using the sodium acetate-flame photometry method, and soil alkalinity (ESP) was calculated as the ratio of exchangeable sodium content to cation exchange capacity. The measured soil pH was 8.69, electrical conductivity was 0.336 dS / m, CEC was 7.5 cmol / kg, available nitrogen content was 25.5 mg / kg, available phosphorus content was 3.86 mg / kg, organic matter content was 2.59 g / kg, available potassium was 26.5 mg / kg, and total selenium was 0.09 mg / kg.

[0054] Select plump cherry tomato seeds (Shandong Academy of Agricultural Sciences), first soak them in a 1% sodium hypochlorite solution for 10 minutes for disinfection, then rinse them three times with deionized water and drain them for later use.

[0055] Seedlings were cultivated in a light-controlled incubator with controlled day and night environmental parameters: temperature set at 25–32 °C (day) and 18–20 °C (night), relative humidity maintained at 70–80%, and appropriate light intensity (e.g., 500 μmol·m⁻¹). -2 ·s -1Provide a light cycle of 12-14 h / d and avoid strong light stress. Transplant the seedlings once they have grown to a uniform size.

[0056] The experiment adopted a randomized block design, with each pot filled with 5 kg of air-dried soil and a single plant planted in each pot (15 cm × 15 cm × 20 cm). A total of 5 treatment groups were set up, with 4 replicates for each treatment group to ensure the reliability and repeatability of the experimental data.

[0057] The treatment groups were set up as follows: ① Control (CK): No treatment was added; ② RS group: 0.06% biochar was applied (RS, as a percentage of dry soil mass); ③ Se group: 27.54 mg sodium selenite (Se) was applied; ④ RS-Se group: 0.06% selenium-adsorbed biochar was applied (RS-Se, as a percentage of dry soil mass); ⑤ Nano-Se@RS group: 0.06% selenium-loaded biochar was applied (Nano-Se@RS, as a percentage of dry soil mass).

[0058] Using a steel tape measure with an accuracy of 0.1 cm, the vertical distance from the base of the stem to the apical meristem was measured and recorded as the plant height in cm. Fruits were harvested in batches after full maturity, and the yield was tallied. At 50 days of age, during the stable light intensity period from 9:00 AM to 11:00 AM, photosynthetic indices of the upper and middle functional leaves were measured using a WALZ GFS-3000 portable photosynthesis system (Germany). These indices included net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO2 concentration (Ci). 25 mg of dried cherry tomato sample was accurately weighed and placed in a digestion vessel. A mixture of nitric acid and ultrapure water (4:3 volume ratio) was added, and the digestion was performed using a CEM Mars6 microwave digester with programmed temperature ramping. After the digestion solution was cooled to room temperature, it was filtered through a 0.22 μm PES filter membrane and then diluted to 50 mL with ultrapure water with a resistivity ≥18.2 MΩ·cm. Finally, the selenium content in the sample was detected by ICP-MS.

[0059] Plotting was performed using Origin 9.0, and one-way ANOVA and the least significant difference (LSD) method were used with SPSS 21.0 software to compare differences between different treatments.

[0060] (2) Test results The effects of different soil improvement treatments on soil salinity and alkalinity characteristics, such as Figure 4 As shown, compared with the CK group, RS, RS-Se, and Nano-Se@RS treatments can enhance the protonation of oxygen-containing functional groups on the biochar surface and improve the free Na+ content. +The adsorption and exchange of these substances reduced soil pH to varying degrees. The Nano-Se@RS group showed the largest reduction due to the optimized surface charge distribution of biochar further enhanced by the nano-selenium loading, while Se treatment had no significant effect on pH. Regarding electrical conductivity, biochar-based treatments reduced soluble salt content by adsorbing and immobilizing soluble salt ions and improving soil structure to promote salt leaching. Se treatment, due to the introduction of additional salts from exogenous sodium selenite, slightly increased electrical conductivity, with the Nano-Se@RS group showing the best salt reduction effect. In terms of CEC, the large specific surface area and abundant surface charge sites of biochar significantly enhanced the cation adsorption capacity of soil colloids. The CEC increases for RS, RS-Se, and Nano-Se@RS treatments reached 102.7%, 120.0%, and 137.3%, respectively. The nano-selenium modification in the Nano-Se@RS group further increased surface active sites and strengthened cation retention capacity, while Se treatment showed no significant improvement. Regarding alkalinity, biochar and its selenium-modified products can adsorb and exchange sodium... + Promotes leaching and removal, reducing Na+ in soil colloids. + Saturation was measured, with the Nano-Se@RS group showing a 41.3% reduction in ESP, while Se treatment had no significant effect on alkalinity. Overall, biochar-based amendments (especially Nano-Se@RS) synergistically reduced soil pH, electrical conductivity, and alkalinity, while significantly increasing cation exchange capacity, effectively improving the salinity and nutrient retention properties of coastal saline-alkali soils.

[0061] The effects of different treatments on photosynthetic gas exchange parameters of cherry tomato leaves, such as Figure 5 As shown, each treatment improved the photosynthetic performance of the plants to varying degrees, and the overall performance was Nano-Se@RS>RS-Se>Se ≈ RS>CK, with the Nano-Se@RS treatment showing the most significant improvement. Figure 5 The results of the net photosynthetic rate of leaves in the middle (a) group showed that the net photosynthetic rate of the CK group was 15.7 μmol CO2. 2 / (m 2 •s), RS and Se treatments showed slight improvements, while RS-Se and Nano-Se@RS treatments showed more significant improvements, with the Nano-Se@RS treatment group achieving a net photosynthetic rate of 21.7 μmol CO2. 2 / (m 2The photosynthetic rate (·s) was approximately 38.2% higher than that of the control group. The mechanism of this phenomenon mainly includes two aspects: First, the Nano-Se@RS treatment significantly improved the physicochemical properties of saline-alkali soil, reduced soil pH and alkalinity, alleviated the damage of salt and alkali stress to chloroplast structure, maintained the integrity of thylakoid membrane and the stability of photosynthetic electron transport chain, and ensured the energy conversion efficiency of the light reaction stage; Second, as a trace element, nano-selenium can participate in regulating the activity of key photosynthetic enzymes (such as Rubisco), promote the carbon fixation process of the Calvin cycle, and thus improve the net photosynthetic rate. Figure 5 (b) Leaf transpiration rate and Figure 5 The trend of stomatal conductance in the middle (c) group was consistent with that of the net photosynthetic rate. The transpiration rate and stomatal conductance of the Nano-Se@RS treatment group were significantly higher than those of other treatments, reaching 9.9 mmol H2O / (m²). 2 ·s) and 187 mmol H2O / (m 2 The levels of Na+ and Na+ in the control group were increased by approximately 38.6% and 76.4% respectively compared to the control group. The mechanism of action is as follows: biochar-based materials (RS, RS-Se, Nano-Se@RS) adsorb free Na+ in the soil. + It reduces soil solution osmotic pressure and alleviates stomatal closure induced by salt and alkali stress. At the same time, nano-selenium can reduce stomatal restriction under stress and promote stomatal opening by regulating the synthesis and signal transduction of abscisic acid (ABA) in leaves, thereby increasing leaf transpiration rate and stomatal conductance and providing sufficient channels for photosynthetic gas exchange. Figure 5 The intercellular CO2 concentration in the middle (d) cells showed the opposite trend: the intercellular CO2 concentration was highest in the CK group (287 μmol CO2 / (m)). 2 The concentration of CO2 / (m³) gradually decreased with increasing treatment effectiveness, reaching 170 μmol CO2 / (m³) in the Nano-Se@RS treatment group. 2The concentration of CO2 in the nano-selenium (·s) was approximately 40.7% lower than that in the control group. Based on the changes in stomatal conductance, this process can be identified as a non-stomatal-limited photosynthetic enhancement effect: on the one hand, Nano-Se@RS treatment alleviated salt-alkali stress, protected the structural and functional integrity of chloroplasts, improved the carboxylation efficiency of mesophyll cells for CO2, enhanced the catalytic activity of key photosynthetic enzymes such as Rubisco, and accelerated the fixation and consumption rate of intercellular CO2; on the other hand, nano-selenium can promote photosynthetic electron transfer, reduce the accumulation of reactive oxygen species (ROS), reduce their oxidative damage to the photosynthetic system, ensure the synergistic progress of the light and dark reactions, further enhance the leaf's ability to utilize intercellular CO2, and ultimately lead to a significant decrease in intercellular CO2 concentration with the increase of net photosynthetic rate. In summary, the Nano-Se@RS treatment, through the synergistic effects of soil improvement, ion regulation, enzyme activity enhancement, and oxidative damage mitigation, not only improved the stress effect of the saline-alkali environment on the plants but also strengthened the intrinsic function of the leaf photosynthetic system, significantly enhancing the photosynthetic performance of cherry tomatoes and laying a good physiological foundation for plant growth, development, and yield formation.

[0062] Different soil improvement treatments had significant regulatory effects on the growth and yield of cherry tomatoes under saline-alkali stress. The overall effect of each treatment showed a gradient characteristic of Nano-Se@RS>RS-Se>Se>RS>CK, which was highly consistent with the pattern of soil salinity improvement and photosynthetic performance enhancement. Figure 6 The dynamic changes in plant height showed that the plant height of cherry tomatoes in each treatment exhibited a typical "S-shaped" growth pattern with the growth process. Growth was slow in the early stage (1-3 weeks), entered a rapid growth period in the middle stage (3-7 weeks), and slowed down in the later stage (7-9 weeks). Furthermore, the differences between treatments gradually increased with the progression of the growth period. At harvest time (9 weeks), the plant height of the CK group was 34.29 cm, while the Nano-Se@RS treatment reached 55.96 cm, a significant increase of 63.2% compared to the CK, demonstrating excellent growth-promoting effects. Data on underground biomass and fruit yield per plant at harvest further validated this trend. The underground biomass weight and fruit weight per plant in the CK group were 1.09 g and 192.30 g, respectively, while the Nano-Se@RS treatment reached 1.65 g and 292.62 g, respectively, representing increases of 51.4% and 52.2% compared to the CK, achieving a dual improvement in cherry tomato growth and yield under salt-alkali stress. Its underlying mechanism is mainly manifested in three aspects: First, Nano-Se@RS reduces Na+ by lowering soil pH, electrical conductivity, and alkalinity. +First, it provides a stable rhizosphere environment for plant growth by mitigating ion toxicity and osmotic stress on the roots. Second, it significantly enhances the net photosynthetic rate, stomatal conductance, and transpiration rate of leaves, reduces intercellular CO2 concentration, and strengthens photosynthetic carbon assimilation capacity, providing a sufficient material basis for biomass accumulation. Third, nano-selenium can regulate the synthesis and signal transduction of endogenous hormones, promote cell elongation and division, and enhance the activity of antioxidant enzymes to alleviate reactive oxygen species damage and maintain normal physiological metabolism in plants. In summary, carbon-based nano-selenium fertilizer (Nano-Se@RS) effectively alleviates the inhibitory effect of salt-alkali stress on cherry tomato growth through a synergistic regulatory mechanism of "soil-root-photosynthesis," significantly improves plant biomass accumulation and fruit yield, and provides a feasible technical approach for the green quality improvement and efficiency enhancement of cherry tomato cultivation in saline-alkali land.

[0063] Different treatments significantly affected the selenium content of cherry tomato fruits under salt-alkali stress, as shown in the results. Figure 7 As shown in the figure, the selenium content of the fruits in the CK and RS groups was 0.045 μg / g and 0.049 μg / g, respectively, which was at the natural background level. The selenium content of the fruits in the Se, RS-Se, and Nano-Se@RS treatment groups was significantly increased, reaching 0.298 μg / g, 0.326 μg / g, and 0.478 μg / g, respectively, representing increases of 562.2%, 624.4%, and 962.2% compared to the CK group, showing a gradient characteristic of Nano-Se@RS>RS-Se>Se>RS ≈ CK. Among them, the Nano-Se@RS treatment had the highest selenium content, which was within the suitable range for selenium-enriched fruits and vegetables in my country (0.10~0.50 μg / g), and did not exceed the safety threshold. This indicates that carbon-based nano-selenium fertilizer can effectively promote the transport and accumulation of selenium in fruits, achieving selenium enrichment and quality improvement in cherry tomatoes.

[0064] From the perspective of its mechanism of action, the effect of Nano-Se@RS on increasing the selenium content of fruits mainly stems from two aspects: Firstly, the porous structure and surface charge characteristics of the biochar carrier can effectively immobilize exogenous selenium, reduce selenium leaching loss, and slowly release nano-selenium, thereby improving the bioavailability of selenium. Secondly, nano-selenium can promote the absorption of selenium from the soil to the roots by regulating the expression of root selenium transport proteins, which is then transported through the xylem to the aboveground parts and finally accumulated in the fruit. In addition, the relief of salt and alkali stress improves the overall physiological metabolic activity of the plant, enhances the plant's ability to absorb and transport selenium, and further promotes the accumulation of selenium in the fruit. In summary, the carbon-based nano-selenium fertilizer (Nano-Se@RS) can alleviate salt and alkali stress, increase cherry tomato yield, and significantly increase the selenium content of the fruit, providing an effective way for the selenium-enriched cultivation of cherry tomatoes in saline-alkali land.

[0065] In summary, this invention successfully prepared a carbon-based nano-selenium fertilizer (Nano-Se@RS) using rice straw biochar (RS) as a carrier. Characterization by SEM-EDS, XRD, and UV-Vis confirmed that selenium is uniformly loaded onto the biochar surface primarily in the form of crystalline zero-valent nano-selenium, without disrupting the porous structure of the carrier. This process combines slow-release fertilizer retention with the high bioactivity of nano-selenium. Pot experiments in saline-alkali soil showed that Nano-Se@RS significantly improved soil physicochemical properties, reduced soil pH, electrical conductivity, and alkalinity, and increased cation exchange capacity, providing a favorable rhizosphere environment for cherry tomato growth. Simultaneously, this material significantly enhanced the photosynthetic performance of cherry tomato by increasing net photosynthetic rate, stomatal conductance, and transpiration rate, reducing intercellular CO2 concentration, and strengthening the plant's carbon fixation capacity, providing sufficient material and energy for both vegetative and reproductive growth. Compared with the control (CK) and traditional selenium fertilizer treatments, the Nano-Se@RS treatment significantly promoted the growth and development of cherry tomatoes, exhibiting optimal levels in plant height, underground biomass, and fruit yield per plant, achieving a significant increase in cherry tomato yield under salt-alkali stress. Simultaneously, Nano-Se@RS demonstrated extremely high selenium bioavailability, efficiently promoting the translocation and accumulation of selenium in cherry tomato fruits, ensuring that the fruit selenium content meets selenium-enriched food standards, significantly superior to single selenium fertilizer treatments, achieving a synergistic improvement in both yield and quality.

[0066] The carbon-based nano-selenium fertilizer developed in this invention integrates the salinity improvement and slow-release functions of biochar and the high bioactivity of nano-selenium. It shows good application potential in the quality improvement and efficiency enhancement of cherry tomatoes in saline-alkali land and can provide technical reference for the development of high-efficiency selenium fertilizer and the production of selenium-rich agricultural products.

[0067] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing biochar-based selenium nanoparticles, characterized in that, Includes the following steps: (1) Rice straw was pyrolyzed under an inert atmosphere to obtain rice straw biochar; (2) Mix rice straw biochar and selenium source solution, load them, and obtain selenium-loaded biochar; (3) After mixing selenium-loaded biochar and lemon juice extract, the pH was adjusted to 9-11 and the reaction was carried out under light-protected conditions to obtain biochar-based nano-selenium.

2. The preparation method according to claim 1, characterized in that, The flow rate of the inert atmosphere is 0.8~1.2L / min; the heating rate of the pyrolysis is 5~15℃ / min, the pyrolysis temperature is 500~600℃, and the pyrolysis time is 1~3h.

3. The preparation method according to claim 1, characterized in that, The selenium source solution includes a sodium selenite solution; the concentration of the selenium source solution is 20~180 mg / L.

4. The preparation method according to claim 1, characterized in that, The ratio of rice straw biochar to selenium source solution is 0.5~1.5 mg: 1 mL.

5. The preparation method according to claim 1, characterized in that, The load was kept at room temperature for 12 to 36 hours.

6. The preparation method according to claim 1, characterized in that, The ratio of selenium-loaded biochar to lemon juice extract is 0.5~1.5g:1mL.

7. The preparation method according to claim 1, characterized in that, The reaction was carried out at room temperature for 24–48 hours.

8. Biochar-based selenium nanoparticles prepared by the preparation method according to any one of claims 1 to 7.

9. The application of the biochar-based nano-selenium as described in claim 8 in the improvement of saline-alkali soil.

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