Agricultural hydrogel preparation method and application

Abstract

The application discloses a kind of agricultural hydrogel preparation method and application.The preparation method of agricultural hydrogel CPAUH provided in the application comprises the following steps: acrylamide and potassium acrylate are dissolved in water, then functional components and carboxylated cellulose nanofiber are added, and then crosslinking agent, initiator and catalyst are added, and the composite hydrogel with interpenetrating polymer network is formed by stirring under normal temperature and pressure.The preparation strategy provided in the application not only includes the advantages of mild synthesis conditions and energy saving of normal temperature hydrogel, but also has the advantages of systematicity of material design, accuracy of function integration and practicability for practical application.The obtained agricultural hydrogel has multiple functions of improving nutrient utilization efficiency, water retention and saline-alkali resistance, which provides a feasible strategy for relieving drought and abiotic stress and promoting efficient and sustainable agricultural practice.

Description

Technical Field

[0001] This invention belongs to the field of hydrogel preparation and agricultural fertilizer technology, specifically relating to a method for preparing agricultural hydrogels and their applications. Background Technology

[0002] Hydrogels are a class of polymer materials with a three-dimensional network structure. As a highly absorbent and water-retaining material, hydrogels are widely used in various fields, such as agricultural and environmental materials, and medical applications. However, there are certain limitations in their preparation processes and functionalities.

[0003] In the field of agricultural and environmental materials, the traditional preparation process of hydrogels has long relied on external heating to drive gelation, polymerization, or cross-linking reactions. This commonality has become a standard practice documented in patents and academic literature. For example, the synthesis of humic acid-based hydrogels requires a rapid high-temperature reaction at 60-120°C for 10-20 minutes to drive free radical polymerization. For heat-sensitive biomass such as hemicellulose, existing processes still require heating to dissolve at 60°C first, followed by cross-linking in a 70°C oven for 2 hours to complete the shaping. In starch-based systems, the starch / dPET-based hydrogel studied by Gerling E et al. requires gelatinization at 85°C and drying at 40°C to obtain the gel product. The aforementioned processes collectively demonstrate that traditional hydrogel preparation methods heavily rely on continuous and precise external heating in key steps such as material activation, polymerization initiation, network crosslinking, and final drying. This not only leads to high energy consumption, complex equipment, and long process cycles, but may also cause degradation or inactivation of heat-sensitive bioactive components (such as natural polymer functional groups, microorganisms, enzymes, and plant extracts), limiting the application potential of hydrogels in the agricultural field. Therefore, developing a synthesis method capable of completing gelation and crosslinking at room temperature (e.g., 25°C) and atmospheric pressure is crucial for addressing the problems of high production energy consumption, complex processes, and limited bio / chemical compatibility of hydrogels used in existing agricultural and environmental materials.

[0004] In recent years, synthetic strategies conducted at room temperature or under mild conditions have become an important research direction due to their simplicity, mild conditions, and effective protection of heat-sensitive bioactivity. For example, hydrogels based on natural polysaccharides, such as sodium alginate, can be prepared through two main approaches: chemical cross-linking via EDC / NHS, where the carboxyl groups react with the amino groups of adipic acid hydrazide at room temperature, followed by washing and freeze-drying to form a porous network. The resulting product exhibits good biocompatibility and biodegradability. Another common approach is ionic cross-linking, such as immersing an SA solution in a CaCl2 solution to cross-link and form a gel at room temperature. Adjusting the concentrations of both solutions allows for systematic study of the relationship between cross-linking density and performance. Composite hydrogels based on natural polymers, such as chitosan, are often combined with other components to expand their functionality. For instance, cross-linking lignin with glutaraldehyde at room temperature can form a three-dimensional network, which, after freeze-drying, yields a porous aerogel suitable for adsorption and environmental protection materials. Simultaneously, various mild processes have been developed for the preparation of artificially synthesized polymers and composite hydrogels. PAM-CMC-SLS@Fe 3+ Hydrogels utilize Fe 3+ The mediated self-redox system synergistically with free radical polymerization enables rapid crosslinking at room temperature, making it suitable for flexible sensing. By varying the amount of SiO2 nanoparticles added and optimizing the dispersion process, a series of PAM-based composite gels can be prepared for mechanical enhancement or functionalization. Although the above methods can synthesize hydrogels under ambient temperature and pressure, the process is complex and energy-intensive. Furthermore, the resulting hydrogels exhibit poor compatibility with various heat-sensitive agricultural active ingredients and poor stability under salt stress, failing to simultaneously achieve both good mechanical properties and functional sustained-release effects.

[0005] In addition, hydrogel-based slow-release fertilizers have attracted increasing attention due to their potential to improve nutrient use efficiency, save water, and promote crop growth. However, most studies focus on nutrient loading and water retention capacity, while research on the regulatory role of these hydrogels under salt stress conditions remains limited. CN118724660A discloses an active composite microalgae bio-fertilizer that can improve crop salt stress resistance, utilizing modified biochar. Hydrogel systems encapsulate active composite microalgae, improving fertilizer utilization and promoting microalgae attachment and growth. The resulting microalgae bio-fertilizer significantly enhances crop salt stress resistance and further increases crop yield. However, this method requires the hydrogel to be used in combination with modified biochar to maintain the stability of the hydrogel's structure and function under salt stress conditions, which increases the complexity of preparation and raw material costs. Meanwhile, in the pursuit of high-performance hydrogel carriers, the incorporation of biomass waste and nanomaterials shows great potential. CN120647452A discloses a hydrogel fertilizer and its preparation method. This hydrogel is formed by cross-linking sodium carboxymethyl cellulose, dialdehyde starch, and gelatin to create a three-dimensional network structure. After loading fertilizers such as urea, it can achieve slow nutrient release (25-day release rate ≥95%), and has high water absorption (35-45 times) and biodegradability (7-day soil degradation rate ≥60%). This product is suitable for crop cultivation, significantly improving fertilizer utilization and reducing environmental pollution. However, the structural and functional stability of hydrogels is challenged under salt stress conditions.

[0006] In summary, developing composite hydrogels that combine simple and mild preparation processes with multiple functions is of great significance for building a resource-saving and sustainable agricultural system, but it also presents significant technical challenges. Summary of the Invention

[0007] This invention provides an agricultural hydrogel with a semi-interpenetrating network structure prepared under ambient temperature and pressure conditions. The ambient temperature and pressure synthesis strategy proposed in this invention aims to fundamentally eliminate the dependence on heat-driven processes in the preparation of hydrogels in the agricultural and environmental materials fields. While simplifying the process and reducing energy consumption, it also provides the possibility for the safe and efficient compounding of hydrogels with various heat-sensitive agricultural active ingredients. The agricultural hydrogel obtained by this invention possesses multiple benefits, including improved nutrient utilization efficiency, water retention, and salt and alkali resistance.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] In a first aspect, the present invention provides a method for preparing agricultural hydrogel, comprising the following steps: adding acrylamide and potassium acrylate to water to dissolve, then adding functional components, carboxylated cellulose nanofibers, crosslinking agents, and initiators, mixing and adding catalysts, and stirring at room temperature and normal pressure to form hydrogel.

[0010] The mass ratio of acrylamide, potassium acrylate, and carboxylated cellulose nanofibers is 1:(0.1-1.0):(0.1-1.0). Specifically, it can be 1:(0.2-0.3):(0.1-0.2).

[0011] Based on the mass of acrylamide, the crosslinking agent is N,N'-methylenebisacrylamide, and its dosage range is 0.1-1.0%; specifically, it can be 0.5-0.8%. The initiator is ammonium persulfate, and its dosage range is 1.0-5.0%; specifically, it can be 2-3%.

[0012] The catalyst is N,N,N',N'-tetramethylethylenediamine, and its dosage range is 0.1-1.0%; specifically, it can be 0.3-0.6%.

[0013] The functional component accounts for 5.0-20.0% of the total mass of the hydrogel fertilizer. Specifically, it can be 13-16%. The functional component includes urea and humic acid; the mass ratio of urea to humic acid is 1:1.

[0014] In one specific embodiment of the present invention, the agricultural hydrogel is prepared according to the following operation: Dissolve 60-80 g of acrylamide (AM) and 10-20 g of potassium acrylate (KAA) in deionized water. Add 6-20 g of urea, 6-20 g of humic acid, and 5-15 g of carboxylated cellulose nanofibers (CNF) and mix thoroughly. Then add 0.1-1.0 g of N,N'-methylenebisacrylamide (MBA) as a crosslinking agent and 1.0-5.0 g of ammonium persulfate (APS) as an initiator. After mixing thoroughly, slowly add 0.1-1.0 mL of the catalyst N,N,N',N'-tetramethylethylenediamine (TMED) and stir at room temperature and atmospheric pressure to form an interpenetrating polymer network hydrogel loaded with urea and humic acid, denoted as CPAUH hydrogel.

[0015] Secondly, the present invention also provides agricultural hydrogels obtained by the preparation method described above.

[0016] The agricultural hydrogel has an interpenetrating polymer network structure.

[0017] Thirdly, the present invention provides a method for simultaneously improving the salt stress resistance of crops and improving soil by applying the aforementioned agricultural hydrogel during the crop planting process.

[0018] The agricultural hydrogel is applied by mixing it with soil, and the application rate ranges from 0.01% to 2.0% of the soil weight. Specifically, it can be 0.5% or 1.5%.

[0019] The crops mentioned are grains and vegetables.

[0020] The method for synthesizing agricultural hydrogels (CPAUH) provided by this invention exhibits the following significant advantages: In terms of synthesis methods, the CPAUH construction pathway provided by this invention is more systematic and complete. Existing methods often focus on single crosslinking mechanisms or simple component composites, while CPAUH employs a clear three-level construction strategy of "basic polymerization—structural reinforcement—functional loading." It uses room-temperature free radical copolymerization as the basic network, followed by the sequential introduction of cellulose nanofibers (CNF) for mechanical reinforcement, and precise loading of urea or humic acid to achieve functionalization. This design makes the synthesis process easy to control and repeat, and enables precise step-by-step control of the material's structure and properties, surpassing the relatively simple one-step or two-component composite strategies in most literature.

[0021] In terms of functional integration, the CPAUH provided by this invention achieves a clear unity of "structure-performance-application". While other existing gels cover various functional directions (such as adsorption, sensing, and sustained release), CPAUH synergistically solves the two key issues of mechanical strength and functional sustained release within the same system. The introduction of CNF effectively enhances the mechanical stability of the hydrogel, while the loading of urea / humic acid directly endows it with the function of nutrient controlled release as a smart agricultural material. This design, which integrates the reinforcing phase and functional components into one system, makes the material more practical and reliable for practical applications (such as agricultural sustained release). CPAUH also has significant advantages in terms of process and sustainability. The entire process is carried out at room temperature and pressure, with clear steps and simple operation, possessing good potential for large-scale production. At the same time, it uses some bio-based raw materials (such as CNF) and incorporates the concept of agricultural nutrient resource utilization, which is highly consistent with the emphasized green and environmentally friendly research direction, demonstrating a more systematic "green design" approach.

[0022] In summary, the CPAUH preparation strategy provided by this invention not only encompasses the advantages of mild room-temperature hydrogel synthesis conditions and energy saving, but also excels in the systematic nature of material design, the precision of functional integration, and the practicality for real-world applications. Attached Figure Description

[0023] Figure 1 The Fourier transform infrared (FTIR) spectra and thermogravimetric analysis (TGA) curves of the hydrogels are shown; where (a) is the Fourier transform infrared (FTIR) spectrum of CPA, CPAU and CPAUH hydrogels, and (b) is the thermogravimetric analysis (TGA) curve.

[0024] Figure 2 Scanning electron microscope images of CPA, CPAU, and CPAUH hydrogels.

[0025] Figure 3 XPS wide-scan spectra, C 1s spectra, and O 1s spectra of CPA (a, b, c) and CPAUH (d, e, f).

[0026] Figure 4 The mechanical property test results of the hydrogels are shown below; (a) is a digital photograph of the sample used for mechanical property testing, (b) is the water contact angle of the sample, (c) is the tensile stress-strain curve, and (d) is the tensile strength and fracture strain value of CCA, CPA, CPAU and CPAUH hydrogels.

[0027] Figure 5 The swelling behavior of CCA, CPA, CPAU and CPAUH hydrogels is shown; where (a) is the equilibrium swelling ratio (g / g) in distilled water, (b) is the effect of pH on the swelling ratio, and (c) is the effect of different salt solutions on the swelling ratio.

[0028] Figure 6 The results show the water retention curve and swelling recovery ability test results of CPAUH hydrogel; where (a) is the water retention curve of CPAUH hydrogel and (b) is the swelling recovery ability of CPAUH hydrogel in three cycles.

[0029] Figure 7 The cumulative release curves of (a) urea and (b) humic acid in CPAUH hydrogel are shown.

[0030] Figure 8 These are parameters for plant growth and nutrient absorption; where (a) is plant height, (b) is chlorophyll content (SPAD value), (c) is fresh weight and dry weight, (d) is total nitrogen, phosphorus and potassium content, (e) is nitrogen absorption, and (f) is phosphorus absorption.

[0031] Figure 9 Digital photographs of wheat plant phenotypes.

[0032] Figure 10 The soil physicochemical properties are: (a) electrical conductivity (EC), (b) ammonium nitrogen (NH4+). + (-N), (c) is nitrate nitrogen (NO3) - -N), (d) represents total nitrogen.

[0033] Figure 11 This is a schematic diagram of the preparation process of the CPAUH hydrogel of the present invention. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0035] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0036] Unless otherwise specified, all reagents, materials, instruments, etc. used in the following examples are commercially available.

[0037] The materials used in the following embodiments are sourced from: Acrylamide (AM, 99%), potassium acrylate (KAA, 95%), urea, humic acid (HA ≥ 90%), carboxylated cellulose nanofibers (CNF, diameter: 4-10 nm, length: ~200 nm), N,N'-methylenebisacrylamide (MBA), ammonium persulfate (APS), and N,N,N',N'-tetramethylethylenediamine (TMED) were all purchased from Maclean Biotech Ltd. (Shanghai, China).

[0038] Sodium chloride, sodium sulfate, sodium carbonate, and sodium bicarbonate were all purchased from Sinopharm Chemical Reagent Co., Ltd.

[0039] All reagents were of analytical grade.

[0040] Phosphate-buffered saline (PBS) was purchased from Pinggen Technology Co., Ltd.

[0041] Example 1: Preparation of agricultural hydrogel CPAUH This embodiment provides a method for preparing agricultural hydrogel CPAUH, the specific steps of which are as follows: 72 g of acrylamide (AM) and 14.4 g of potassium acrylate (KAA) were dissolved in deionized water. Then, 8 g of urea, 8 g of humic acid, and 8 g of carboxylated cellulose nanofibers (CNF) were added. Subsequently, 0.48 g of N,N'-methylenebisacrylamide (MBA) was added as a crosslinking agent, and 1.6 g of ammonium persulfate (APS) was added as an initiator. After thorough mixing, 0.4 mL of the catalyst N,N,N',N'-tetramethylethylenediamine (TMED) was slowly added. The mixture was stirred at room temperature and atmospheric pressure to form an interpenetrating polymer network hydrogel loaded with urea and humic acid, denoted as CPAUH hydrogel (CNF / p(AM-co-KAA)).

[0042] Comparative Example 1 The difference from Example 1 is that it lacks the materials urea, humic acid, and carboxylated cellulose nanofibers (CNF). The resulting hydrogel is designated as CCA hydrogel.

[0043] Comparative Example 2 The difference from Example 1 is that urea and humic acid are missing. The resulting hydrogel is designated as CPA hydrogel.

[0044] Comparative Example 3 The difference from Example 1 is the absence of humic acid. The resulting hydrogel is designated as CPAU hydrogel.

[0045] The structure and properties of the hydrogels obtained in the above embodiments and comparative examples were systematically characterized.

[0046] 1. Expansion rate The swelling rate of the hydrogel was determined by gravimetric method.

[0047] Test method: The hydrogel was dried in an oven at 60℃ to constant weight to obtain a dried hydrogel (W). d Then, immerse the sample in distilled water, PBS buffer (0.01 mol / L, pH 3, 5, 7, 9, 10), or salt solution (0.01 mol / L NaCl, Na2SO4, Na2CO3, NaHCO3). Remove the sample at intervals (0.5, 1, 2, 4, 8, 12, and 24 hours), blot off surface moisture with absorbent paper, and weigh (W). s Each experiment was repeated three times.

[0048] The equilibrium expansion rate is calculated using formula (1): Q eq = (W s - W d ) / W d ; In formula (1), Q eq W represents the swelling index. s W represents the weight of a sample over a certain time interval. d This indicates the dry weight of the sample.

[0049] 2. Moisture retention capacity and reswelling capacity To assess moisture retention capacity, fully swollen hydrogels were air-dried at room temperature, and their weight was recorded periodically. Moisture retention rate (R0) i ,%) is calculated using formula (2): R i =(M i / M0) × 100; In formula (2), M0 and M i These represent the initial weight and the weight as a function of time, respectively.

[0050] For the reswelling capacity analysis, the dried hydrogel was re-immersed in distilled water until equilibrium was reached, and then dried again to constant weight. This swelling-drying cycle was repeated three times, and the swelling rate of each cycle was determined according to the above method to assess the structural toughness.

[0051] 3. Nutrient release behavior Nutrient release kinetics were studied by incubating 0.5 g of dried hydrogel in 100 mL of distilled water at 25 °C. The solution was replaced with fresh distilled water at predetermined time intervals. The extracted samples were diluted to 100 mL for quantitative analysis. Urea concentration was determined spectrophotometrically using dimethylaminobenzaldehyde at 430 nm. Humic acid concentration was determined using a UV-Vis spectrophotometer at 465 nm.

[0052] 4. Pot experiment under salt stress conditions The pot experiment was conducted in a wheat greenhouse at the Beijing Academy of Agricultural and Forestry Sciences. Soil was placed in pots measuring 52 × 35 × 15 cm. The initial physicochemical properties of the soil were as follows: pH 5.76, electrical conductivity (EC) 5.68 mS / cm, total nitrogen 1.70%, available phosphorus 561.79 mg / kg, and available potassium 1076.00 mg / kg. Four treatments (CK, P0, P1, and P2) were set up, with each treatment replicated three times. Each pot contained 10 kg of air-dried soil. Hydrogel was applied at 0% (CK, P0), 0.5% (P1), and 1.5% (P2) of soil weight. The total nitrogen content of each treatment was equalized by supplementing with urea. 2000 mL of 200 mM salt solution was added to P0, P1, and P2; CK was not treated. One day after salinization, the soil EC values ​​for P0, P1, and P2 were 11.32, 12.13, and 12.06 mS / cm, respectively. During wheat growth, P1 and P2 received only 70% of the water used by CK and P0.

[0053] The salt solution (200 mM) was prepared by dissolving 11.688 g NaCl, 28.408 g Na₂SO₄, 21.200 g Na₂CO₃, and 16.802 g NaHCO₃ in distilled water. The solution was then transferred to a 1000 mL volumetric flask and diluted to volume with distilled water. The pH and conductivity (EC) of this solution were 9.63 and 107.17 mS / cm, respectively.

[0054] 5. Characterization Before freeze-drying, immerse the freshly prepared hydrogel in deionized water for 1 hour.

[0055] The microstructure of the freeze-dried samples was observed using a scanning electron microscope (SEM, Hitachi S4800). Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific Nicolet iS20) was used at wavenumbers of 400–4000 cm⁻¹. -1 Record the spectrum within the range.

[0056] The surface elemental composition was analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha), and the thermal stability was evaluated using thermogravimetric analysis (TG, Netzsch STA449C).

[0057] During the wheat seedling stage, plant height was measured, and leaf chlorophyll content was determined using a SPAD instrument. Soil volumetric water content was measured using a TDR350 probe. At harvest, fresh and dry weights were recorded. Nutrient content was determined after digestion of plant tissues: total nitrogen (Kjeldahl method), total phosphorus (UV spectrophotometry), and total potassium (flame photometry). After harvest, soil samples were air-dried, sieved, and the following parameters were determined: total nitrogen (Kjeldahl method using sulfuric acid digestion), available phosphorus (sodium bicarbonate extraction-molybdenum antimony colorimetric method), available potassium (ammonium acetate extraction-flame photometry), soil organic matter (potassium dichromate titration method), pH (soil-water ratio 2.5:1), and electrical conductivity (EC, soil-water ratio 5:1).

[0058] Data Analysis: Data were processed using Excel 2019, SPSS 22.0, and Origin 2024 software. One-way ANOVA combined with multiple comparison tests was used to compare differences between different treatments. The statistical significance level was set at [value missing]. P <0.05.

[0059] The characterization results are as follows: 1. Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TG) Figure 1 (a) shows the FTIR spectra of CPA, CPAU, and CPAUH hydrogels. At approximately 3120 cm⁻¹ -1 and 3450 cm -1 The broad and strong absorption bands that appear at the point correspond to the binding stretching vibrations of the -OH and -NH- groups, confirming the presence of hydroxyl groups from cellulose nanofibers (CNF) and amino groups (-NH2) from acrylamide (AM).

[0060] The further broadening of these peaks in CPAUH indicates the formation of intermolecular hydrogen bonds after the incorporation of urea and humic acid. 2932 cm⁻¹ -1 The absorption band at 1672 cm⁻¹ is attributed to the stretching vibrations of -CH₂- in the CNF / p(AM-co-KAA) hydrogel polymer backbone. -1 The peak at 1614 cm⁻¹ originates from the C=O stretching vibration of the carboxyl group, while the peak at approximately 1614 cm⁻¹ originates from this vibration. -1 The peak at 1452 cm⁻¹ is related to the p=π conjugation between the lone pair electrons of nitrogen in the amide group and the carbonyl group. Furthermore, the peak at 1452 cm⁻¹... -1 and 1408 cm -1 The peak at that point corresponds to the symmetric stretching vibration of the carboxylate anion.

[0061] Figure 1(b) shows the TG curves of the CPA, CPAU, and CPAUH hydrogels. The TGA curves reveal different thermal decomposition behaviors of the three samples under a nitrogen atmosphere. An initial mass loss of approximately 10% occurs between 50 °C and 200 °C, mainly attributed to the evaporation of residual water trapped in the hydrogel network. Significant mass loss is observed in the 300–500 °C range, corresponding to the decomposition of the polymer backbone. At 800 °C, the residual weights of CPA and CPAU are approximately 20% and 22%, respectively. The CPAU hydrogel exhibits slightly enhanced thermal stability, retaining approximately 30% of its weight at 800 °C, indicating that the incorporation of urea does not significantly alter the thermodynamic behavior of the underlying CPA matrix. In contrast, CPAUH exhibits the highest thermal stability, attributed to the presence of humic acid, whose aromatic structure confers additional resistance to thermal degradation and enhances the overall thermal stability of the composite hydrogel.

[0062] 2. Morphological observation The surface morphology of CPA, CPAU, and CPAUH gels after immersion for 30 minutes was observed using scanning electron microscopy.

[0063] like Figure 2 As shown, the structural characteristics of hydrogels vary depending on their composition. CPA and CPAU exhibit similar porous structures, while CPAUH displays a unique network structure.

[0064] like Figure 2 As shown in (a), the surface pore size of the CPA hydrogel ranges from 6 to 12 μm. Figure 2 As shown in (b), CPAU has a denser and more compact pore structure with pore sizes between 7 and 18 μm. Figure 2 As shown in (c), the CPAUH hydrogel exhibits an interconnected network with pore sizes of approximately 7–14 μm. The presence of these interconnected porous networks provides ample internal space for the loading of urea and humic acid, thereby facilitating their diffusion and controlled release in aquatic and soil environments.

[0065] 3. XPS Analysis The surface chemical composition of CPA and CPAUH hydrogels was determined using XPS.

[0066] The broad-scan spectrum showed three main peaks at 285 eV (C 1s), 400 eV (N 1s), and 532 eV (O 1s), corresponding to carbon, nitrogen, and oxygen, respectively. Figure 3 (a) and 3 (d)). Compared with CCA, CPAUH hydrogels exhibited higher nitrogen content (17.05%) and oxygen content (24.33%) due to the successful introduction of urea and humic acid.

[0067] The high-resolution C 1s spectrum of CPAUH can be decomposed into four binding components: 284.80 eV (CC / CH), 287.87 eV (CN / CO), 292.59 eV (C=O), and 295.43 eV (OC=O). Figure 3 (b) Since the main chain structure of CPAUH is similar to that of CPA, their C 1s binding environments are basically the same. O 1s spectroscopy confirms the simultaneous presence of C=O (532.3 eV) and CO (533.5 eV) functional groups in the hydrogel matrix.

[0068] Notably, the CO component area of ​​CPAUH increased compared to CCA, which is attributed to the introduction of oxygen-rich humic acid. These results collectively confirm that urea and humic acid have been successfully integrated into the CNF / p(AM-co-KAA) hydrogel network.

[0069] 4. Mechanical strength Dumbbell-shaped samples were used. Figure 4 (a) The hydrophilicity and mechanical properties of the hydrogels were evaluated. CPAUH had a contact angle of approximately 62°, higher than CCA, CPA, and CPAU, indicating relatively strong surface hydrophobicity. This phenomenon may originate from the aromatic rings present in the humic acid molecules ( Figure 4 (b)).

[0070] To systematically evaluate the effects of the semi-interpenetrating network structure (semi-IPN) and nutrient load on mechanical properties, tensile tests were conducted. Figure 4 (c) The introduction of CNF significantly improved the tensile properties of P(AM-co-KAA) hydrogels, increasing the tensile stress from 105 kPa to 152 kPa and the elongation at break from 295% to 384% (CCA vs. CPA). This improvement is due to the introduction of CNF, which forms a semi-interpenetrating network structure through hydrogen bonding and physical entanglement, enhancing the polymer network and surpassing the effect of traditional chemical crosslinking. CNF can simultaneously serve as a gel scaffold and reinforcing phase, forming a stable three-dimensional network through physical entanglement, significantly improving mechanical properties. After loading urea, the tensile stress further increased from 152 kPa to 212 kPa, indicating that the hydrogen bonds between urea, CNF, and the P(AM-co-KAA) network contribute to the formation of a denser crosslinked structure. The surface carboxyl groups form stable hydrogen bonds and electrostatic interactions, enhancing the gel network and improving mechanical integrity. For CPAUH loaded with both urea and humic acid, a slight decrease in tensile strength was observed. This may be because the high loading level interfered with the formation of chemical crosslinks. Nevertheless, the results confirm that CNF-based semi-interpenetrating networks significantly improve the mechanical properties of P(AM-co-KAA) hydrogels while maintaining flexibility and functional load-bearing capacity.

[0071] 5. Water absorption capacity (expansion capacity) The water absorption and swelling behavior of the hydrogels were studied to evaluate their responsiveness to different environments. For example... Figure 5 As shown in (a), CPAUH reached its maximum expansion ratio of 121.65 g / g after 24 hours in distilled water. During the initial 0–12 hours, the expansion ratio increased rapidly, then gradually slowed and eventually stabilized. This kinetic behavior indicates a strong water absorption capacity in the early stages, followed by saturation as the network structure reaches its expansion limit. The significantly high initial expansion rate is likely attributed to the rapid formation of extensive hydrogen bonds between water molecules and the amide groups (-CONH2) in the CPAUH molecular chains, as well as the rapid unfolding of the polymer chains after water absorption, creating space for water permeation and storage. The equilibrium state reached after 12 hours reflects the balance between the elastic contractile force of the fully swollen crosslinked network and the permeation driving force, thus limiting further expansion of the network.

[0072] Figure 5 (b) The water absorption properties of the four materials under different pH conditions were compared. CPAUH exhibited the best swelling behavior in a neutral environment (pH 7), with a swelling ratio of 40.10 g / g, while its swelling capacity decreased significantly under acidic and alkaline conditions. This phenomenon indicates that CPAUH has significant pH sensitivity. In acidic media, a large amount of H+... + Ionized protonated carboxylates (-COO) - The formation of the -COOH group by the Na group enhances hydrogen bonding and electrostatic repulsion, leading to network structure compression and reduced water absorption capacity. Under alkaline conditions, Na... + Ion shielding -COO - The negative charge on the groups weakens the electrostatic repulsion between polymer chains, while the increased ionic strength reduces the osmotic pressure gradient. These combined effects inhibit gel swelling. Therefore, CPAUH's significant pH responsiveness indicates its tunable water absorption capacity in different acidic and alkaline environments, supporting its potential application in soils or crop rhizospheres under specific pH conditions.

[0073] Figure 5 (c) illustrates the swelling behavior of four hydrogels in various sodium salt solutions (NaCl, Na₂SO₄, Na₂CO₃, NaHCO₃). All materials exhibited significantly lower water absorption capacity in salt solutions compared to distilled water, with Na₂SO₄ and Na₂CO₃ showing significantly lower swelling ratios than NaCl and NaHCO₃. The overall decrease in swelling capacity under saline conditions is primarily attributed to cations (such as Na₂SO₄, Na₂CO₃, NaCl, NaHCO₃, NaCl ... + The shielding effect of this on anionic carboxylate groups in the hydrogel network reduces electrostatic repulsion and limits network expansion. It is noteworthy that this effect is particularly pronounced in hydrogels containing divalent anions (such as SO42-). 2- CO32- In a salt solution of ), the higher Na content per mole of salt + Concentration exacerbates the charge shielding effect, resulting in a greater reduction in water absorption than in monovalent salt systems. These results indicate that, in practical agricultural applications, the type and ionic strength of soil salts play a crucial role in determining the water retention capacity of hydrogels and serve as key criteria for assessing their suitability.

[0074] 6. Moisture retention and reswelling properties Figure 6 (a) shows the moisture loss curve of CPAUH at 25°C. The sample completed the dehydration process within 48 hours, exhibiting a relatively constant dehydration rate during this period. This stable and linear dehydration trend indicates that the moisture evaporation process is mainly controlled by environmental conditions (such as temperature and humidity) rather than by the inherent diffusion resistance of the material. This behavior suggests a relatively uniform pore size distribution within the hydrogel network, allowing moisture to diffuse out at a consistent rate. This controlled dehydration is significant for practical agricultural applications because it enables stable moisture release in alternating wet and dry environments, thus providing crops with a more sustained water supply.

[0075] The reusability of CPAUH was evaluated through three consecutive desorption-absorption cycles, and the results are as follows: Figure 6 As shown in (b), the swelling ratio gradually decreased with increasing cycle number, but remained at a relatively high level of approximately 118 g / g after the third cycle. This gradual decrease in swelling ratio is primarily attributed to mechanical fatigue of the polymer network during repeated swelling and shrinkage. In each cycle, the crosslinked chains stretch and shrink, potentially leading to irreversible breakdown of weaker physical crosslinks (such as hydrogen bonds and chain entanglements) or breakage of chemical crosslinks, resulting in slight network degradation and permanent loss of water absorption capacity. Nevertheless, CPAUH retained approximately 97% of its initial swelling ratio (121.65 g / g) after three cycles, demonstrating the excellent mechanical toughness and structural stability of its crosslinked network. This result confirms the excellent reliability and long-term application potential of CPAUH as a reusable moisture management material.

[0076] 7. Urea and humic acid release performance To evaluate the environmental application potential of the prepared gel fertilizer, its nutrient release kinetics were systematically studied under hydroponic conditions at 25°C.

[0077] Figure 7The cumulative release curves of urea and humic acid over 15 days are shown. Both nutrients exhibit a typical biphasic release pattern. In the first 10 days, approximately 66.91% of urea and 92.45% of humic acid are released. This sequential release pattern, with the total cumulative release of urea below 80%, indicates its sustained release capacity, suggesting that nutrient supply can be synchronized with crop nitrogen demand. In contrast, humic acid exhibits a gradual and stable release, with only an additional 1% cumulative release between days 10 and 15. Although the release rate in this stage is significantly slower than in the initial stage, this sustained release contributes to long-term soil improvement. The combination of rapid early release and sustained later release is beneficial for initial soil conditioning and long-term nutrient supply.

[0078] The initial rapid release is attributed to readily soluble nutrients physically adsorbed on the gel surface or in the macropores. These nutrients dissolve rapidly upon contact with water and diffuse out via short pathways. This characteristic offers agronomical advantages, as it provides immediate and necessary starting nutrients for seed germination and early seedling growth, ensuring nutritional support during critical initial stages. After 10 days, the release rate significantly slows and eventually stabilizes, primarily due to two factors: first, the rapid depletion of nutrients on the gel surface and in easily accessible areas leads to longer diffusion paths and increased resistance, reducing the concentration gradient driving diffusion; second, the release mechanism gradually shifts from an initial dissolution-dominated mode to a sustained release mode controlled by the swelling and degradation behavior of the gel network. Over time, the gel carrier may undergo slight microbial degradation or erosion, forming new diffusion pathways that collectively regulate the later, slower nutrient release. This biphasic release pattern, characterized by rapid initial release followed by stable release, closely aligns with crop nutrient requirements—high demand in the early growth stages and greater stability in later stages. This indicates the significant potential of CPAUH gel fertilizers in controlling nutrient release rates and improving fertilizer utilization efficiency.

[0079] 8. The effect of hydrogels on wheat growth under salt stress. To investigate the salt tolerance, water retention, and slow-release fertilizer capacity of CPAUH hydrogel, different fertilization treatments were designed to elucidate its growth-promoting effect on wheat and its intrinsic mechanism of improving soil physicochemical properties.

[0080] Plant height data indicate that CPAUH can alleviate salt stress ( Figure 8 (a) Figure 9On day 21, high-dose CPAUH (P2) promoted plant growth to 12.5 cm, significantly exceeding the control treatment (CK, 7.9 cm), indicating its early physiological regulatory role. By day 35, P2 exhibited supercompensatory growth (22.8 cm), significantly higher than the CK treatment (19.3 cm) and the severely inhibited P0 treatment (11.8 cm). This established a stable growth gradient (P2>CK>P1>P0), confirming its dose-dependent synergistic effect.

[0081] From a mechanistic perspective, CPAUH mainly works through three pathways: physically improving soil structure and water retention capacity; chemically adsorbing Na through ion exchange. + This reduces ion toxicity and physiologically enables sustained nutrient release. The supercompensatory growth of P2 treatment likely stems from this combined stress relief and rhizosphere optimization, activating compensatory growth through resource redistribution and hormonal regulation. Therefore, sufficient application doses are required for CPAUH to effectively alleviate salt stress.

[0082] The dynamic changes in chlorophyll content reflect both salt stress damage and the restorative effect of CPAUH. Figure 8 (b) Figure 9 On day 32, the chlorophyll content of the P0 treatment (15.09) was significantly lower than that of the CK treatment (17.64), indicating that its chloroplasts were severely damaged; while the P2 treatment (22.23) showed the protective effect of CPAUH. This protection is achieved through three levels: (1) cellular level: maintaining ion homeostasis to reduce membrane lipid peroxidation; (2) metabolic level: promoting chlorophyll biosynthesis through continuous nutrient release; (3) systemic level: possibly enhancing reactive oxygen species scavenging capacity by regulating antioxidant enzyme activity. The dynamic changes further revealed the plant's active adaptation strategy. By day 43, the plants treated with CPAUH shifted from passive damage relief to active adaptation. By day 63, there was no significant difference between the P2 treatment (37.03) and the CK treatment (33.81), indicating that CPAUH can not only protect against salt damage, but also activate the plant's intrinsic self-repair ability. Maintaining a higher chlorophyll content improved light capture efficiency, which may explain the higher plant height of the P2-treated plants. The consistently high chlorophyll content in the P2 treatment at all stages was consistent with its superior growth performance, confirming the principle that photosynthetic performance determines growth outcomes. This provides physiological evidence for the finding that "sufficient CPAUH can both alleviate stress and activate endogenous adaptation mechanisms in saline environments."

[0083] Biomass data confirmed the alleviating effect of CPAUH under salt stress. Figure 8 (c) Figure 9Salt stress severely inhibited plant growth: the fresh weight and dry weight of the P0 treatment (0.19 g and 0.05 g) were only 22.9% and 27.8% of those of the CK treatment (0.83 g and 0.18 g), respectively, indicating that water absorption and photosynthetic assimilation accumulation were inhibited. Application of CPAUH reversed this inhibition. The fresh weight and dry weight of the P2 treatment reached 1.57 g and 0.36 g, respectively, which were 189.2% and 200.0% of those of the CK treatment, significantly exceeding the control treatment. The higher fresh weight of the P2 treatment suggests that CPAUH may have alleviated osmotic stress and enhanced water absorption and retention capacity by promoting root development and improving soil structure. This improvement in water balance is consistent with the continuous increase in plant height, confirming the role of CPAUH in maintaining water balance. In terms of dry matter, the dry weight of the P2 treatment was twice that of the CK treatment, indicating that CPAUH alleviated photosynthetic inhibition and promoted carbon assimilation and distribution. This improvement may stem from: (1) stable water supply and reduced ion toxicity maintaining photosynthesis; (2) continuous nutrient release supporting photosynthetic function; and (3) reduced osmotic regulation energy consumption allowing more photosynthetic assimilates to be allocated to growth. Notably, the P2 treatment outperformed the CK treatment in both biomass parameters, indicating that optimized soil improvement not only counteracts stress effects but also stimulates plant growth potential. The mechanism may lie in CPAUH improving the rhizosphere environment, thereby regulating endogenous hormones and optimizing the utilization of photosynthetic assimilates. In conclusion, this study confirms that high-dose CPAUH can effectively promote biomass accumulation in saline soils, providing a theoretical and applied basis for its remediation application.

[0084] The total nitrogen (N), phosphorus (P), and potassium (K) content and absorption dynamics indicate that CPAUH can enhance nutrient uptake under salt stress. Figure 8 (d)-(f)). Salt stress reduced nutrient uptake; for example, the total nitrogen content in the P0 treatment (4.08%) was lower than that in the CK treatment (5.49%), which may be due to Na+. + This is due to induced inhibition of transport proteins and decreased reductase activity. In contrast, the P2 treatment not only alleviated this limitation, but its total nitrogen content (6.04%) and nitrogen uptake even exceeded those of the CK treatment, showing a significant nutrient-promoting effect. This improvement reflects the multiple regulatory effects of CPAUH. In terms of nitrogen metabolism, the "nitrogen hyperaccumulation" observed in the P2 treatment may be due to: (1) Na + Adsorption reduced ion toxicity and increased transport protein activity; (2) continuous nitrogen release ensured supply; (3) rhizosphere optimization promoted root development and increased absorption area. For phosphorus, CPUA increased its content and absorption, indicating that it can provide phosphorus source and enhance bioavailability. Possible pathways include improving the rhizosphere environment and activating soil-fixed phosphorus. Potassium dynamics were particularly significant: the performance of P2 treatment confirmed that CPAUH can effectively alleviate Na+ ion toxicity.+ Caused K + The absorption of competitive inhibition helps maintain ion balance and osmotic regulation. From a nutritional perspective, the synergistic promotion of nitrogen, phosphorus, and potassium absorption in the P2 treatment helps to restore metabolic balance, which is consistent with the plant's growth advantage in plant height and biomass, further highlighting the role of CPAUH in maintaining nutrient homeostasis under salt stress.

[0085] Soil electrical conductivity (EC), a key indicator of soil salinity, reveals the significant ameliorative effect of CPAUH. Figure 10 (a) Compared to the control treatment (CK), the EC value of the salt stress treatment (P0) was significantly higher, exceeding the CK treatment by 3.02 mS / cm. CPAUH application showed a dose-dependent effect: the EC values ​​of the P1 treatment were similar to those of the P0 treatment, while the EC value of the P2 treatment was significantly lower, decreasing by 4.38 mS / cm compared to the P0 treatment. The significant reduction in EC value of the P2 treatment stemmed from its multi-scale remediation mechanism. The hydrogel's network structure improved soil porosity and promoted salt leaching; simultaneously, its functional groups directly reduced the ion concentration in the soil solution through ion exchange. This physical... Chemical synergy effectively optimized the rhizosphere microenvironment. The EC value of the P2 treatment was closely related to its excellent plant height and biomass performance, confirming that improving the rhizosphere through materials science is an effective strategy for alleviating salt stress. This "materials-structure-function" synergistic mechanism provides important prospects for sustainable soil management.

[0086] like Figure 10 As shown in (b)-(d), the regulatory effects of different treatments on soil nitrogen forms reveal the function of CPAUH in regulating nitrogen cycling. The ammonium nitrogen (NH4+) under salt stress treatment (P0) was significantly reduced. + The ammonium nitrogen (NO3-) content was not significantly different from the control (CK) treatment, while the application of CPAUH dose-dependently reduced the ammonium nitrogen content (P1 and P2 treatments reduced it by 535.99 and 935.68 mg / kg, respectively, compared to the CK treatment). Conversely, the nitrate nitrogen (NO3-) content in the P0 and P1 treatments was significantly lower than that in the CK treatment. - The nitrogen (N-N) content in the P2 treatment was significantly higher than that in the CK treatment (increasing by 1011.33 and 1512.35 mg / kg, respectively), but the nitrate nitrogen content in the P2 treatment was significantly lower. The total nitrogen content was also the lowest in the P2 treatment (0.73%), forming a clear "low-residue" nitrogen pattern, indicating its systemic regulatory role in nitrogen transformation. The simultaneous reduction of all nitrogen forms under the P2 treatment reflects the optimization of nitrogen flux in the soil-plant system. This effect likely stems from the improvement of the rhizosphere environment by CPAUH, which simultaneously promotes nitrification and plant nitrogen uptake, thereby driving the efficient transfer of nitrogen from the soil to the plant. This synergistic effect of "promoting transformation and enhancing uptake" shortens the retention time of soil nitrogen and reduces potential losses.

[0087] From an agroecological perspective, the nitrogen allocation pattern of the P2 treatment is of significant value: maintaining low soil nitrogen residues while meeting crop needs implies improved fertilizer use efficiency and reduced non-point source pollution risk. Therefore, from a nitrogen cycle perspective, high-dose CPAUH can effectively optimize nutrient management under salt stress.

[0088] Comparative Example 4 The difference from Example 1 is that conventional carboxymethyl cellulose (CMC) is used instead of carboxylated cellulose nanofibers (CNF).

[0089] Comparative Example 5 The difference from Example 1 is that alginate is used instead of carboxylated cellulose nanofibers (CNF).

[0090] The hydrogel fertilizers obtained in Comparative Example 4 and Comparative Example 5 were tested according to the above test method, and the results showed that: (1) The tensile strength and toughness of the hydrogel obtained in Comparative Example 4 were 25 kPa and 418 kJ / m, respectively. 3 The hydrogel obtained in Comparative Example 5 had a tensile strength of 42 kPa and a toughness of 422 kJ / m. 3 The hydrogel obtained in Example 1 has a tensile strength of 178 kPa and a toughness of 490 kJ / m. 3 This demonstrates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate possesses excellent mechanical properties.

[0091] (2) The water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Comparative Example 4 were 106.38 g / g and 67.64 g / g, respectively; the water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Comparative Example 5 were 148.52 g / g and 88.21 g / g, respectively; and the water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Example 1 were 121.65 g / g and 118 g / g, respectively. This indicates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate has excellent synergistic and comprehensive regulatory ability in water retention and swelling, which is beneficial for the preservation of nutrients and water.

[0092] (3) The cumulative urea release rate of the hydrogel obtained in Comparative Example 4 was 80.63% and the humic acid release rate was 78.62% within 15 days, with a decrease in conductivity of 20.67%; the cumulative urea release rate of the hydrogel obtained in Comparative Example 5 was 75.32% and the humic acid release rate was 74.39% within 15 days, with a decrease in conductivity of 28.65%; the cumulative urea release rate of the hydrogel obtained in Example 1 was 66.91% and the humic acid release rate was 92.45% within 15 days, with a decrease in conductivity of 39.16%. This indicates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate has a good sustained-release effect.

[0093] Comparative Example 6 The difference from Example 1 is that hydroxyethyl acrylate (HEMA) is used instead of acrylamide.

[0094] Comparative Example 7 The difference from Example 1 is that sodium acrylate is used instead of potassium acrylate.

[0095] The hydrogel fertilizers obtained in Comparative Example 6 and Comparative Example 7 were tested according to the above test method, and the results showed that: (1) The tensile strength and toughness of the hydrogel obtained in Comparative Example 6 were 152 kPa and 488 kJ / m, respectively. 3 The hydrogel obtained in Comparative Example 7 had a tensile strength of 155 kPa and a toughness of 465 kJ / m. 3 The hydrogel obtained in Example 1 has a tensile strength of 178 kPa and a toughness of 490 kJ / m. 3 This demonstrates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate possesses excellent mechanical properties.

[0096] (2) The water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Comparative Example 6 were 110.81 g / g and 106.15 g / g, respectively; the water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Comparative Example 7 were 116.48 g / g and 109.65 g / g, respectively; and the water absorption properties and water absorption capacity after three adsorption-desorption cycles of the hydrogel obtained in Example 1 were 121.65 g / g and 118 g / g, respectively. This indicates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate has excellent water retention and swelling capacity, which is beneficial for the preservation of nutrients and moisture.

[0097] (3) The cumulative urea release rate of the hydrogel obtained in Comparative Example 6 within 15 days was 60.94%, the humic acid release rate was 84.37%, and the conductivity decreased by 32.57%; the cumulative urea release rate of the hydrogel obtained in Comparative Example 7 within 15 days was 64.54%, the humic acid release rate was 87.84%, and the conductivity decreased by 31.72%; the cumulative urea release rate of the hydrogel obtained in Example 1 within 15 days was 66.91%, the humic acid release rate was 92.45%, and the conductivity decreased by 39.16%. This indicates that the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate has a good sustained-release effect.

[0098] In summary, the composite hydrogel based on carboxylated cellulose nanofibers, acrylamide, and potassium acrylate of this invention has good mechanical properties, water retention, and sustained-release effect.

[0099] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing agricultural hydrogel, characterized in that, The process includes the following steps: acrylamide and potassium acrylate are dissolved in water, followed by the addition of functional components, carboxylated cellulose nanofibers, crosslinking agents, and initiators. After mixing, a catalyst is added, and the mixture is stirred at room temperature and normal pressure to form a hydrogel.

2. The preparation method according to claim 1, characterized in that, The mass ratio of acrylamide, potassium acrylate, and carboxylated cellulose nanofibers is 1:(0.1-1.0):(0.1-1.0).

3. The preparation method according to claim 2, characterized in that, Based on the mass of acrylamide, the crosslinking agent is N,N'-methylenebisacrylamide, and its dosage range is 0.1-1.0%. The initiator is ammonium persulfate, and its dosage range is 1.0-5.0%. The catalyst is N,N,N',N'-tetramethylethylenediamine, and its dosage range is 0.1-1.0%.

4. The preparation method according to claim 2, characterized in that, The functional component accounts for 5.0-20.0% of the total mass of the hydrogel fertilizer.

5. The preparation method according to claim 2, characterized in that, The functional components include: urea and humic acid; the mass ratio of urea to humic acid is 1:

1.

6. The agricultural hydrogel obtained by the preparation method according to any one of claims 1-5.

7. The agricultural hydrogel according to claim 6, characterized in that, The agricultural hydrogel has an interpenetrating polymer network structure.

8. A method for simultaneously improving crop salt stress resistance and soil improvement, characterized in that, During crop cultivation, the agricultural hydrogel described in claim 6 or 7 is applied.

9. The method according to claim 8, characterized in that, The agricultural hydrogel is applied by mixing it with soil, and the application amount ranges from 0.01% to 2.0% of the soil weight.

10. The method according to claim 8, characterized in that, The crops mentioned are grains and vegetables.

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