Lignin-based dom structure remodeling agent for reducing cadmium accumulation in rice brown rice and application method thereof

Abstract

This invention belongs to the field of agricultural environmental remediation and safe agricultural production technology, specifically relating to a lignin-based DOM structure remodeling agent for reducing cadmium accumulation in brown rice and its application method. The lignin-based DOM structure remodeling agent of this invention is based on dealkalized lignin, compounded with nitrogen, phosphorus, and potassium nutrients. It has high organic carbon content, high cation exchange capacity, and extremely low Cd background. After application to the soil, it will not introduce new cadmium pollution sources, ensuring good environmental safety. By remodeling the DOM structure and stabilizing soil Cd, it can significantly reduce the Cd content in brown rice and promote Cd accumulation in the rice roots, effectively controlling the translocation of Cd to brown rice and achieving safe rice production in cadmium-contaminated paddy fields. Furthermore, the method of this invention is simple to operate, highly compatible with conventional paddy field flooding management systems, requires no additional equipment investment, and is particularly suitable for the remediation of acidic or weakly acidic cadmium-contaminated paddy soils in southern regions and for safe rice production.

Description

Technical Field

[0001] This invention belongs to the field of agricultural environmental remediation and safe agricultural production technology, specifically relating to a lignin-based DOM structure remodeling agent for reducing cadmium accumulation in brown rice and its application method. Background Technology

[0002] Cadmium (Cd) is one of the most biotoxic heavy metal pollutants in farmland soils. In the acidic or slightly acidic paddy soil areas of southern my country, cadmium pollution in paddy soils is a prominent problem due to human activities such as industrial development, wastewater irrigation, and fertilizer application. Rice (… Oryza sativa L. has a strong capacity to accumulate Cd in soil. After being absorbed by the roots, Cd can be transported to the grains (brown rice) through the stems and leaves, and then enter the human body through dietary intake, posing a potential threat to human health. Therefore, achieving safe rice production in cadmium-contaminated paddy fields has become an urgent technical problem to be solved in the field of agricultural environment.

[0003] Currently, the main technical means for remediating cadmium pollution in paddy fields and reducing cadmium in brown rice include: applying alkaline materials such as lime to increase soil pH, applying silicate or phosphate passivating materials, using water management, planting low-cadmium-accumulating varieties, and soil replacement. However, all of these technologies have certain limitations. Although alkaline materials such as lime can increase soil pH and reduce Cd bioavailability in the short term, long-term application may affect soil nutrient balance, and their passivation effect is easily affected by environmental conditions; single passivating agents have limited adsorption selectivity for Cd and pose a risk of desorption under acidic conditions; water management is constrained by water sources and climate conditions; the selection cycle for low-accumulation varieties is long and their regional adaptability is limited; soil replacement is a large-scale and costly project, and it significantly disturbs the physical structure and microecological environment of the original topsoil. In addition, most existing remediation technologies focus on single factors such as Cd adsorption-desorption balance or soil pH regulation, and there is no technical solution that systematically reduces the bioavailability of Cd in paddy fields and controls Cd translocation to brown rice from the perspective of targeted regulation of soil dissolved organic matter (DOM) structure.

[0004] Soil dissolved organic matter (DOM) is the most active component of soil organic matter, playing a crucial regulatory role in the migration, transformation, and bioavailability of heavy metals. DOM's influence on Cd is dual: it can fix Cd through complexation, reducing its bioavailability; it can also act as a migration carrier, promoting Cd transport. The actual impact of DOM on Cd bioavailability depends on its content and structural characteristics, with aromaticity, hydrophobicity, degree of humification, and molecular weight being key factors determining DOM's ability to complex and fix Cd.

[0005] Lignin is a naturally abundant aromatic polymer with a three-dimensional network structure, rich polar functional groups (such as phenolic hydroxyl, carboxyl, and methoxy groups), and high cation exchange capacity, exhibiting potential for complexing and adsorbing heavy metal ions. Previous studies have attempted to use lignin as a soil conditioner or heavy metal adsorbent. However, existing lignin-based materials have the following shortcomings in paddy field soil remediation applications: their formulations are often monotonous, lacking proper compatibility with nutrients such as nitrogen, phosphorus, and potassium, making it difficult to simultaneously meet the growth needs of rice; material properties have not been specifically designed for the acidic cadmium-contaminated paddy field-rice system in southern China, and key indicators such as organic carbon content, cation exchange capacity, pH, and heavy metal background values ​​lack systematic optimization; application techniques are often crude and lack synergy with paddy field water management systems, making it difficult to fully utilize lignin's function in reshaping the soil DOM structure. Currently, there is a lack of technical solutions that utilize lignin-based materials as DOM structure-directed regulators to reduce Cd bioavailability and control Cd translocation to rice grains by enhancing the humification and aromaticity of paddy field soil DOM.

[0006] Therefore, developing a lignin-based DOM remodeling agent that can efficiently reshape the DOM structure of paddy soil, enhance the humification and aroma of DOM, has a suitable cation exchange capacity and low Cd background, and establishing a corresponding field application method is of great significance for the safe production of rice in acidic cadmium-contaminated paddy soil. Summary of the Invention

[0007] The purpose of this invention is to address existing problems by providing a lignin-based DOM structure remodeling agent for reducing cadmium accumulation in brown rice and its application method.

[0008] This invention is achieved through the following technical solution: The first objective of this invention is to provide a lignin-based DOM structure remodeling agent for reducing cadmium accumulation in brown rice, comprising, by weight percentage, 80%~98% dealkalized lignin, 0.3%~1.5% nitrogen source, 0.03%~0.3% phosphorus source, and 0.05%~0.5% potassium source; The reshaping agent has an organic carbon content of 150~300 g·kg. -1 The cation exchange capacity is 30~70 cmol·kg -1 The pH value is 4.5–6.5, and the Cd content is less than 0.1 mg·kg⁻¹. -1 ; The remodeling agent, when applied to the soil, can increase the UV absorbance of dissolved organic matter (DOM). 254 and UV 260 SUVA UV absorbance 254 and SUVA 260This increases the humification index (HIX) and decreases the E2 / E3 and E3 / E4 ratios.

[0009] The second objective of this invention is to provide the application of the lignin-based DOM structure remodeling agent in reducing cadmium accumulation in brown rice.

[0010] Furthermore, the dealkalized lignin is derived from one or more of rice straw, rice husk, wheat straw, corn straw, sugarcane bagasse, or papermaking black liquor, preferably alkali-extracted lignin from rice straw.

[0011] A third objective of this invention is to provide a method for reducing cadmium accumulation in brown rice using the aforementioned lignin-based DOM structure remodeling agent, comprising the following steps: (1) Apply the lignin-based DOM structure remodeling agent to cadmium-contaminated paddy soil at an application rate of 0.5% to 2.0% of the soil dry weight; (2) Mix the remodeling agent thoroughly with the paddy soil and react for 14 to 35 days to improve the aroma, hydrophobicity and humification of the soil DOM; (3) Plant rice in the treated paddy soil and carry out flooding management: After transplanting rice, maintain the water level of the paddy field at 1-5 cm. Stop watering 7-21 days before the rice matures and is harvested and allow the soil to dry naturally. The method enables UV radiation from the soil DOM. 254 UV 260 SUVA 254 and SUVA 260 Increased levels of HIX and decreased E2 / E3 and E3 / E4 ratios promote the conversion of exchangeable Cd in the soil to organically bound, iron-manganese oxide-bound, and / or residual forms, thereby reducing the content of available Cd in the soil and decreasing Cd accumulation in brown rice.

[0012] Furthermore, the improvement of soil DOM humification is manifested by: the humification index (HIX) increasing to above 6, and / or the relative contribution rate of humus-like fluorescent components C2 and / or C3 increasing, and the relative contribution rate of protein-humus-like mixed component C1 decreasing.

[0013] Furthermore, the flood management is as follows: maintain the water level on the field surface at 2 cm; and stop replenishing water 14 days before the rice matures and is harvested.

[0014] Furthermore, the cadmium-contaminated paddy field soil is southern acidic or weakly acidic paddy soil with a pH of 4.5–6.5 and a total Cd content of 0.6–5.0 mg·kg⁻¹. -1 Preferably, the dosage is 1.0~3.0 mg·kg. -1 .

[0015] Furthermore, the method reduces the Cd content in brown rice by more than 33%, increases the Cd content in rice roots, and / or reduces the Cd translocation coefficient from roots to brown rice by more than 80%.

[0016] Furthermore, the method also increases the content of amorphous iron and / or organic matter in the soil.

[0017] Furthermore, the nitrogen, phosphorus, and potassium nutrients applied by the method are calculated based on soil dry weight as follows: N 30~120 mg·kg -1 P10~60 mg·kg -1 K 20~100 mg·kg -1 .

[0018] Furthermore, the nutrients applied by the method are: N 60 mg·kg -1 P 20 mg·kg -1 K 50 mg·kg -1 .

[0019] The fourth objective of this invention is to provide the application of the method in the remediation of cadmium-contaminated paddy field soil.

[0020] The present invention has the following advantages over the prior art: 1. The lignin-based DOM structure remodeling agent of this invention is based on dealkalized lignin and compounded with nitrogen, phosphorus and potassium nutrients. It has high organic carbon content, high cation exchange capacity and extremely low Cd background. After being applied to the soil, it will not introduce new cadmium pollution sources and has good environmental safety.

[0021] 2. The remodeling agent of this invention can directionally remodel the soil DOM structure and significantly improve the UV absorbance of the DOM. 254 and UV 260 SUVA UV absorbance 254 and SUVA 260 It also increases the humification index (HIX) and reduces the E2 / E3 and E3 / E4 ratios, causing DOM to transform towards higher aromaticity, higher hydrophobicity, higher humification degree and relatively high molecular weight, thereby enhancing the DOM's ability to complex and fix Cd.

[0022] 3. This invention can promote the transformation of highly active exchangeable Cd in soil into relatively stable forms such as organically bound, iron-manganese oxide bound, and / or residual forms, significantly reducing the content of available Cd in the soil, thereby reducing the migration and bioavailability of Cd in the soil-rice system.

[0023] 4. This invention can significantly reduce the Cd content in brown rice by reshaping the DOM structure and stabilizing soil Cd, and promote the accumulation of more Cd in the rice roots, effectively controlling the translocation of Cd to brown rice and achieving safe production of rice in cadmium-contaminated paddy fields.

[0024] 5. The method of the present invention is simple to operate, has good compatibility with conventional paddy field flooding management systems, requires no additional equipment investment, and is particularly suitable for the remediation of acidic or weakly acidic cadmium-contaminated paddy soil in the south and for safe rice production. Attached Figure Description

[0025] Figure 1 The effects of lignin on the absorption, translocation, and accumulation of Cd in rice; Figure 2 The effects of lignin on Cd content, availability, and chemical speciation in soil; Figure 3 The effects of lignin on the content of dissolved organic matter (DOM) in soil and its ultraviolet-visible (UV-Vis) spectral characteristics; Figure 4 The fluorescent components of soil dissolved organic matter (DOM) were analyzed based on parallel factor analysis (PARAFAC). Figure 5 The effect of lignin on the fluorescence spectral characteristic index of soil dissolved organic matter (DOM) and the relative contribution rate of each component; Figure 6 To analyze the correlation between soil physicochemical and DOM characteristics and Cd migration in the soil-rice system. Detailed Implementation

[0026] To further explain the present invention, the following specific embodiments are described.

[0027] Example 1: Materials and Methods 1.1 Test Materials Soil samples were collected from the 0-20 cm soil layer of a rice-Chinese cabbage rotation field near Guangzhou City, Guangdong Province (23.395141°N, 113.805797°E). The area has an average annual precipitation of 2040 mm and an average annual temperature of 22.1℃. The rice-Chinese cabbage rotation has been practiced continuously for over 20 years. After collection, the soil was air-dried, mixed, and sieved through a 2 mm nylon sieve for later use. The physicochemical properties of the soil were: pH 5.47, organic carbon 12.82 g·kg⁻¹. -1 Cation exchange capacity (CEC) 4.07 cmol·kg -1 Total nitrogen 0.66 g·kg -1 Total phosphorus 0.69 g·kg -1 Total potassium 24.90 g·kg-1 Alkaline nitrogen uptake: 61.62 mg·kg -1 Available phosphorus 105.64 mg·kg -1 Available potassium 189.65 mg·kg -1 Total Cd 0.18 mg·kg -1 The concentration is lower than the risk screening value for Cd in the "Soil Environmental Quality Standard for Agricultural Land Soil Pollution Risk Control (Trial)" (GB 15618-2018) (0.3 mg·kg for pH ≤ 5.5). -1 ).

[0028] The tested lignin was purchased from Shanghai Maclean Biochemical Technology Co., Ltd., and its basic properties are as follows: pH 5.21, organic carbon 204.95 g·kg⁻¹. -1 Cation exchange capacity 45.71 cmol·kg -1 Total nitrogen 0.66%, total phosphorus 0.08%, total potassium 0.14%, total Cd 0.047 mg·kg⁻¹ -1 .

[0029] 1.2 Pot Experiment Design The experiment was conducted from April 7 to July 22, 2024, at the experimental base of the Institute of Agricultural Resources and Environment, Guangdong Academy of Agricultural Sciences (22.774049° N, 113.445430° E, Guangzhou). Each polyethylene pot (30 cm in diameter and 25 cm in height) was accurately filled with 5 kg of test soil. Four treatments were set up in the experiment: (1) control (no lignin and Cd added), (2) +1% lignin (w / w), (3) +2 mg·kg -1 Cd (added as CdCl2 solution), (4) + 1% lignin + 2 mg·kg -1 Cd was applied in triplicate for each treatment. After each treatment was completed, the pots were left to stand for 30 days to allow the exogenously added Cd to age. The rice variety used was "Meixiangzhan 2," a predominantly cultivated indica rice variety in the soil collection area. Rice seedlings were raised in seedling trays and transplanted when they reached the three-leaf stage, with three seedlings per pot. The ambient temperature during the rice growth period was 14℃~38℃. Following the guidelines for scientific fertilization of major crops in Guangdong Province, nitrogen was applied at a rate of 60 mg / kg. -1 Phosphorus 20 mg·kg -1 Potassium 50 mg·kg -1The application rate should be adjusted so that the base fertilizer is evenly mixed into the soil of each pot one day before transplanting to meet the growth needs of rice seedlings. During the rice growth period, pest and disease control should be carried out according to local planting practices. From the time the seedlings are transplanted, they should be weighed regularly every day and deionized water should be added to maintain a water layer of about 2 cm in the pot. Watering should be stopped 14 days before the rice is harvested and the soil in the pot is allowed to dry naturally to promote rice maturity and subsequent sampling.

[0030] After the rice was fully mature, destructive sampling was performed independently on each treatment pot. For soil sampling, after removing surface debris, a uniformly mixed non-rhizosphere soil sample was collected from the pot, air-dried, and used for subsequent physicochemical property and DOM analysis. For plant sampling, the entire rice plant was uprooted and divided into three parts: roots, stems and leaves, and brown rice. Root samples were first rinsed with tap water to remove adhering soil, then eluted with sodium dithionite-sodium citrate-sodium bicarbonate (DCB) extract to remove the iron film on the root surface, and then thoroughly rinsed with deionized water. Stems, leaves, and brown rice were directly washed with tap water and deionized water. After washing, all plant samples were blanched at 105℃ for 30 min, then dried at 60℃ to constant weight, ground, and passed through a 100-mesh sieve for later use.

[0031] Example 2: Indicator Measurement and Analysis 2.1 Determination of Cd content in soil and rice The physicochemical properties of the tested soils were determined according to the method of Bao Shidan (Bao Shidan. Soil Agrochemical Analysis [M]. 3rd ed. Beijing: China Agriculture Press, 2000.). For soil samples collected during the experiment, the Cd content in the soil was determined by nitric acid-perchloric acid digestion, and the available Cd content in the soil was determined by diethylenetriaminepentaacetic acid (DTPA) extraction. For rice plant samples (including stems, leaves, roots, and brown rice) collected during the experiment, the Cd content in different plant organs was determined by perchloric acid-sulfuric acid-nitric acid digestion. All samples that underwent digestion or extraction were uniformly diluted to volume with 1% hydrochloric acid, filtered through a 0.45 μm filter membrane, and the Cd content in the filtrate was determined using inductively coupled plasma mass spectrometry (ICP-MS 7800, Agilent Technologies, USA). During the determination process, standard reference materials (soil GBW 074568 and plant GBW 10048) provided by the National Center for Standard Materials Research were used for quality control. The recovery rates of Cd in soil and plant standard samples were 93% and 97%, respectively.

[0032] 2.2 Cd speciation in soil The modified Tessier sequential extraction method was used to determine the Cd speciation in soil (Tessier A, Campbell PGC, Bisson M. Sequential extraction procedure for the speciation of particulatetrace metals[J]. Analytical Chemistry, 1979, 51(7): 844-851. and Yin H, Tan N, Liu C, et al. The associations of heavy metals with crystalline iron oxides in the polluted soils around the mining areas in Guangdong Province, China[J]. Chemosphere, 2016, 161: 181-189.). The improved method mainly includes the following two points: First, the amorphous iron-manganese oxide bound states are extracted together, called easily reducible iron-manganese oxide bound states, while the crystalline iron-manganese oxide bound states are extracted separately; Second, given that the tested soil is acidic, the carbonate bound states extracted with sodium acetate / acetic acid are omitted. During the extraction process, the soil sample is mixed with the corresponding extractant and shaken. The extracts of each speciation are subjected to 4000 rpm. g The supernatant was collected after centrifugation for 20 min. The analyzed Cd speciation included water-soluble, exchangeable, amorphous iron-manganese oxide-bound, crystalline iron-manganese oxide-bound, organically bound, and residual forms. Detailed procedures were performed according to the method of Li et al. (Li Q, Chang J, Li L, et al. Soil amendmentsalter cadmium distribution and bacterial community structure in paddy soils[J]. Science of the Total Environment, 2024, 924: 171399.) (Table 1). The Cd concentration in each extract was determined using inductively coupled plasma mass spectrometry (ICP-MS 7800, Agilent Technologies, USA). The relative error between the sum of the contents of each Cd speciation and the total Cd content was controlled within 10%.

[0033] Table 1. Methods for continuous extraction of Cd speciation from soil

[0034] 2.3 DOM Sample Preparation and Analysis DOM was extracted from the soil using the water-soil shaking method. 3.0 g of air-dried soil was weighed and placed in a 100 mL centrifuge tube. 60 mL of ultrapure water (Millipore, resistivity 18.25 MΩ∙cm) was added at a water-soil ratio of 20:1. The mixture was then centrifuged at 200 r·min under light and at room temperature. -1 Oscillate for 16 hours, then at 4000 g Centrifuge for 30 min, filter the supernatant through a 0.45 μm aqueous filter membrane to obtain the DOM solution, and store at 4℃ in the dark. Before fluorescence spectroscopy, dilute the DOM sample with ultrapure water to UV concentration. 254 < 0.3, to eliminate the internal filtration effect.

[0035] The UV-Vis absorption spectra of soil DOM were determined using a double-beam UV-Vis spectrophotometer (UV-2450, Shimadzu, Japan). A 10 mm path length quartz cuvette was used, with ultrapure water as a blank control. The scanning range was 200–800 nm, and the scan step was 1 nm. The UV absorbance at 254 nm (UV...) was... 254 UV is used to characterize the degree of humification and aromaticity of DOM; a higher value indicates a higher degree of humification and a stronger aromaticity. 254 The ratio of SUVA to soluble organic carbon (DOC) concentration is defined as SUVA. 254 This allows for further quantification of the aromatic characteristics of the DOM; a higher value indicates a higher proportion of aromatic structures in the DOM. Ultraviolet absorbance (UV) at 260 nm... 260 ) is used to characterize the hydrophobicity level of DOM; a higher value indicates a higher content of hydrophobic substances; UV 260 The ratio of DOC concentration to SUVA is defined as... 260 This can further reflect the hydrophobicity of DOM. The absorbance ratio E2 / E3 (the ratio of absorbance at 254 nm to 365 nm) can be used to estimate the relative molecular weight of DOM, and the two are negatively correlated; another ratio, E3 / E4 (the ratio of absorbance at 300 nm to 400 nm), is used to distinguish the types of humic DOM. When E3 / E4 > 3.5, humic DOM is dominated by fulvic acid, and vice versa. Spectral slope ratio S R Defined as the ratio of the slope of the absorbance exponential function curve in the wavelength range of 275–295 nm to that in the wavelength range of 350–400 nm (S R = S 275~295 / S 350~400 ), which also has the function of characterizing the source of DOM and its relative molecular weight: its value is negatively correlated with the relative molecular weight, S R > 1 indicates that DOM is mainly biologically derived, S R <1 indicates that the source is mainly terrestrial.

[0036] The three-dimensional fluorescence spectra of soil DOM were determined using a three-dimensional fluorescence spectrometer (F-7000, Hitachi, Japan). The light source was a 450 W xenon arc lamp, and the scan rate was 30,000 nm·min. -1 The scanning bandwidth for both emission wavelength (Em) and excitation wavelength (Ex) is 5 nm. The Em scanning range is 250–550 nm, and the Ex scanning range is 200–450 nm. The fluorescence index (FI) is the ratio of fluorescence intensity at emission wavelengths of 450 nm and 500 nm at an excitation wavelength of 370 nm. It is mainly used to indicate the origin of DOM and the contribution of aromatic structures to the fluorescence signal. FI > 1.9 indicates that DOM mainly originates from biological metabolism and release, with prominent autogenic characteristics; FI < 1.4 indicates that DOM mainly originates from plants and soil organic matter. The humification index (HIX) is the ratio of the integrated area of ​​emission wavelengths of 435–480 nm and 300–345 nm at an excitation wavelength of 254 nm. It can be used to characterize the degree and origin of DOM humification. HIX > 6 indicates a high degree of DOM humification and a large terrestrial contribution; 4–6 indicates high humification and weak autogenic characteristics; HIX < 4 indicates a weak degree of humification and predominantly autogenic origin. The biogenicity index (BIX) is the ratio of fluorescence intensity at emission wavelengths of 380 nm and 430 nm at an excitation wavelength of 310 nm. It reflects the autogenic contribution rate of DOM and can be used to assess the degree of biological activity of the environment in which the DOM is located. A BIX > 1 indicates obvious biogenicity, 0.8~1 indicates high autogenicity, 0.7~0.8 indicates medium autogenicity, and BIX < 0.7 indicates low autogenicity.

[0037] 2.4 Data Processing and Analysis The rice Cd translocation coefficient (TF) is the ratio of Cd content in brown rice to Cd content in roots or stems and leaves. The bioaccumulation factor (BCF) is the ratio of Cd content in roots or stems and leaves to Cd content in soil. SPSS 25.0 was used for statistical analysis and the construction of a multiple stepwise regression model. Differences among different treatments were compared using one-way ANOVA and Tukey's HSD post-hoc test. P < 0.05 indicates a significant difference. Parallel factor analysis (PARAFAC) was performed on the fluorescence spectra using the DOMFluor toolbox in MATLAB R2021a. Before modeling, the data underwent Raman normalization and outlier removal. The model reliability was then verified using a split-half test and residual analysis. The analyzed fluorescence components were uploaded to the OpenFluor online database for comparison to identify the types and characteristics of DOM substances. Microsoft Excel 2021 and Origin 2021 Pro software were used to organize and plot the UV-Vis absorption and three-dimensional fluorescence spectral data.

[0038] Example 3: Results Analysis 3.1 Changes in soil physicochemical properties The changes in soil physicochemical properties under different treatments are shown in Table 2. Compared with the control, lignin application alone significantly increased soil pH, organic carbon content, and CEC. Specifically, soil pH increased from 5.34 to 5.74. P < 0.05), organic carbon content and CEC increased significantly by 24% and 17%, respectively. Regarding available nutrients, lignin application increased available potassium content from 192.35 mg·kg⁻¹. -1 Significantly increased to 237.84 mg·kg -1 However, the contents of available nitrogen and phosphorus in the soil decreased significantly, by 6% and 18%, respectively. Compared with the +Cd treatment, the +lignin +Cd treatment significantly increased soil pH, organic carbon content, CEC, and available potassium content. Specifically, soil pH increased from 5.51 to 5.78, and organic carbon content increased from 11.88 g·kg⁻¹. -1 Increased to 15.32 g·kg -1 CEC is 3.93 cmol·kg -1 Increased to 4.78 cmol·kg -1 The available potassium content is 71.84 mg·kg⁻¹ -1 Increased to 202.24 mg·kg -1 Meanwhile, the soil available nitrogen content in the +lignin +Cd treatment decreased significantly compared to the +Cd treatment, with a decrease of 6%, while the available phosphorus content did not change significantly. Overall, the application of lignin (+lignin)... vs. Control treatment and +lignin +Cd vs. The +Cd treatment significantly increased soil pH, organic carbon content, CEC, and available potassium content, but the effects on available nitrogen and available phosphorus varied, indicating that lignin can improve soil acidity and organic carbon levels while altering the characteristics of available soil nutrients.

[0039] Table 2. Changes in soil physicochemical properties under different treatments

[0040] 3.2 Cd distribution in rice Lignin treatment significantly altered the distribution pattern of Cd in various organs of rice. Figure 1 Compared to the control, the +lignin treatment reduced the Cd content of brown rice from 0.34 mg·kg⁻¹. -1 Reduced to 0.05 mg·kg -1 The reduction was 85%; compared with the +Cd treatment, the +lignin +Cd treatment reduced the Cd content of brown rice from 2.32 mg·kg⁻¹. -1Reduced to 1.55 mg·kg -1 The decrease reached 33% ( Figure 1 A). Unlike the decrease in Cd content in brown rice, the Cd content in the roots significantly increased under lignin treatment. Compared to the control, the +lignin treatment increased the Cd content in the roots by 117%; compared to the +Cd treatment, the +lignin +Cd treatment reduced the Cd content in the roots from 15.78 mg·kg⁻¹. -1 Increased to 60.68 mg·kg -1 The increase reached 285% ( Figure 1 B). Regarding the Cd content in stems and leaves, the Cd content in the control and the + lignin treatment was 0.31 mg·kg⁻¹. -1 and 0.16 mg·kg -1 The Cd content in stems and leaves was at a low level; compared with the +Cd treatment, the Cd content in stems and leaves after the +lignin +Cd treatment reached 7.38 mg·kg⁻¹. -1 The increase was 30% ( Figure 1 C). Translocation coefficient results showed that lignin significantly reduced the translocation capacity of Cd to brown rice. Compared with the control, the +lignin treatment significantly reduced the translocation coefficients of Cd from roots and from stems and leaves by 99% and 71%, respectively; compared with the +Cd treatment, the +lignin +Cd treatment significantly reduced the Cd translocation coefficient from roots to brown rice (from 0.148 to 0.026) and the Cd translocation coefficient from stems and leaves to brown rice (from 0.411 to 0.210). Figure 1 D). The bioaccumulation coefficient results further indicate that lignin treatment promotes Cd accumulation in the roots and reduces its translocation to brown rice. Figure 1 E). Compared with the control, the +lignin treatment significantly increased the bioaccumulation coefficient of Cd in rice roots (by 160%), while significantly decreasing the bioaccumulation coefficient of Cd in stems and leaves (by 36%). Compared with the +Cd treatment, the +lignin +Cd treatment significantly increased the bioaccumulation coefficients of Cd in roots and stems and leaves, by 298% and 34%, respectively. In summary, lignin treatment altered the distribution pattern of Cd in rice, causing Cd to accumulate more in roots and in inedible parts, and significantly inhibiting its translocation to brown rice.

[0041] Figure 1 (AC) Cd content in brown rice, roots, and stems / leaves; (D) Cd translocation coefficient in rice (TF). 根-糙米 TF 茎叶-糙米 (E) Cd bioaccumulation coefficient (BCF) in rice roots and stems 根 BCF 茎叶 Different letters on the bar chart indicate significant differences between treatments. P < 0.05).

[0042] 3.3 Distribution of Cd speciation in soil Depend on Figure 2 It can be seen that lignin significantly altered the chemical speciation of Cd. Compared with the +Cd treatment, the +lignin +Cd treatment significantly reduced the soil DTPA-Cd content by 16%. Figure 2 (C) indicates that lignin application reduced the bioavailability of Cd in the soil. In the control soil, Cd mainly existed in the exchangeable form, accounting for 42%, followed by amorphous iron-manganese oxide-bound and residual forms, accounting for 19% and 16%, respectively. After adding lignin, the proportion of exchangeable Cd in the soil significantly decreased from 42% to 35%; at the same time, the proportions of amorphous iron-manganese oxide-bound and residual forms increased to 22% and 19%, respectively. This indicates that even in a low Cd background, lignin can promote the transformation of soil Cd from a more active form to a relatively stable form. The input of exogenous Cd led to a significant change in the Cd speciation in the soil. In the +Cd treatment, the proportion of exchangeable Cd significantly increased to over 75%, becoming the main form of Cd, while the proportions of organically bound and residual forms were extremely low (only 4% and 1%, respectively). Compared with the +Cd treatment, the +lignin +Cd treatment significantly decreased the proportion of exchangeable Cd from 75% to 70%, while significantly increasing the proportion of organically bound Cd from 4% to 7%. In summary, lignin application (+lignin) vs. Control treatment and +lignin +Cd vs. Both +Cd treatment significantly reduced the proportion of highly bioavailable exchangeable Cd in the soil and promoted the transformation of Cd into relatively stable forms such as crystalline iron-manganese oxide bound, organic bound, and residual forms, thereby reducing the mobility and bioavailability of Cd in the soil.

[0043] Figure 2 (A) Total Cd content in soil; (B) Distribution characteristics of different chemical forms of Cd in soil (including residual form, organically bound form, crystalline iron-manganese oxide bound form, amorphous iron-manganese oxide bound form, exchangeable form, and water-soluble Cd content less than 0.01 mg·kg⁻¹). -1 (C) Soil available Cd (DTPA-Cd) content. Different lowercase letters on the bar chart indicate significant differences between treatments. P < 0.05).

[0044] 3.4 Characteristics of UV-Vis absorption spectral parameters of soil DOM Figure 3 A shows the changes in soil DOC content under different treatments. The results indicate that the addition of lignin (+lignin)... vs. Control treatment and +lignin +Cd vs.+Cd treatment significantly increased soil DOC content. Compared with the control, the +lignin treatment increased DOC content by 3.0 times; compared with the +Cd treatment, the +lignin +Cd treatment increased DOC content by 4.1 times. To further analyze the structural characteristics of DOM, ultraviolet-visible absorption spectroscopy analysis was performed. Figure 3 B). Overall, the trends in the spectral curves of each treatment were similar, but there were significant differences in absorption intensity, showing an overall pattern of "+lignin > control > +lignin +Cd > +Cd". To clarify the regulatory role of lignin on DOM structure, the different treatments were divided into two groups for comparative analysis. Compared with the control, the application of lignin alone significantly altered the UV-Vis spectral characteristics of DOM. The +lignin treatment significantly increased the UV intensity of DOM. 254 UV 260 SUVA 254 and SUVA 260 The values ​​indicate that lignin application enhanced the aromaticity, hydrophobicity, and humification of the DOM. Simultaneously, the +lignin treatment significantly reduced the E2 / E3 and E3 / E4 values ​​(…). Figure 3 The presence of lignin (HI) indicates that the application of lignin significantly increased the relative molecular weight of DOM and the proportion of humic acid components in DOM. Under Cd stress, the regulatory effect of lignin on DOM structure remained significant. Compared with +Cd treatment, +lignin +Cd treatment also significantly improved the UV index, which characterizes aromaticity and hydrophobicity. 254 UV 260 SUVA 254 and SUVA 260 The values ​​increased by 15.4 times, 15.3 times, 2.1 times, and 2.1 times, respectively; at the same time, the E2 / E3 and E3 / E4 values ​​decreased significantly, by 40% and 51%, respectively. Furthermore, compared with the +Cd treatment, the +lignin +Cd treatment significantly increased the S content of the soil DOM. R The value is high, but its average value is still less than 1. Figure 3 (G) indicates that the DOM under this treatment still exhibits predominantly terrestrial input characteristics. Comprehensive comparative analysis of the two groups reveals that lignin application significantly increased soil DOC content and altered the UV-Vis spectral characteristics of the DOM, leading to a transformation of the DOM towards higher aromaticity, higher hydrophobicity, higher humification, and relatively higher molecular weight. These results demonstrate that exogenous lignin not only increased the quantity of DOM but also optimized its structural composition.

[0045] Figure 3 (A) Soil soluble organic carbon (DOC) content; (B) UV-Vis absorption spectrum of DOM; (CD) UV characteristic parameters. 254 and UV 260 (EF) Specific UV Absorbance SUVA 254and SUVA 260 (G) Spectral slope ratio (S) R (HI) Absorbance ratios E2 / E3 and E3 / E4. Different lowercase letters on the box plot indicate significant differences between treatments. P <0.05). Abs represents absorbance; UV... 254 and UV 260 The ultraviolet absorbance at wavelengths of 254 nm and 260 nm are respectively; SUVA 254 and SUVA 260 UV 254 and UV 260 The ratio of the concentration of soluble organic carbon (DOC) to the concentration of S; R S is the ratio of the spectral slopes of the absorbance exponential function curves for wavelengths of 275–295 nm and 350–400 nm, i.e., S R = S 275~295 / S 350~400 E2 / E3 is the ratio of ultraviolet absorbance at 254 nm and 365 nm; E3 / E4 is the ratio of ultraviolet absorbance at 300 nm and 400 nm.

[0046] 3.5 Three-dimensional fluorescence spectral characteristics of soil DOM Depend on Figure 4 It was found that different treatments significantly affected the fluorescence characteristics of soil DOM. Parallel factor analysis identified three main fluorescent components (C1-C3). Component 1 (C1) was a mixture of protein-like and humic-like substances, with fluorescence peaks located at Ex / Em = 225 / 400 nm (corresponding to tryptophan-like substances) and 280 / 400 nm (corresponding to humic acid-like substances), respectively. Component 2 (C2) was identified as a humic-like component, with fluorescence peaks located at Ex / Em = 245 / 410 nm and 335 / 410 nm. Component 3 (C3) also belonged to the humic-like component, with fluorescence peaks located at Ex / Em = 270 / 470 nm and 365 / 470 nm, exhibiting spectral characteristics of large-molecule aromatic compounds.

[0047] Figure 4 In the middle: The left side of the spectrum is the three-dimensional excitation-emission matrix (EEM) fluorescence contour plot of the three fluorescent components (C1, C2 and C3) identified by the model; the right side of the spectrum is the excitation (Ex, solid line) and emission (Em, dashed line) spectral loading of each component.

[0048] Further calculations were performed on the fluorescence index (FI), biogenic index (BIX), humification index (HIX), and the relative contribution rate of each fluorescent component based on three-dimensional fluorescence spectroscopy. Figure 5To clarify the regulatory effect of lignin on the fluorescence characteristics of DOM, different treatments were divided into two groups for comparison. First, compared with the control, lignin application alone significantly altered the fluorescence index and component distribution of soil DOM. Regarding the fluorescence index, the +lignin treatment significantly increased both BIX and HIX, with BIX increasing to 0.86, indicating that lignin application increased the contribution of autogenic DOM and promoted the enhancement of DOM humification. In terms of the relative contribution rate of components, the +lignin treatment significantly reduced the proportion of component C1 from 42% in the control to 36%; simultaneously, the relative contribution rates of humus-like components C2 and C3 increased, with component C2 increasing to 35%. This suggests that lignin application promotes the transformation of DOM components from a protein-humus-like mixed component to a humus-like component.

[0049] Secondly, under Cd stress, compared with the +Cd treatment, the +lignin +Cd treatment significantly increased the DOM's FI by 14%, and the HIX also significantly increased, reaching the highest level among all treatments. Regarding component contribution rates, the C1 component accounted for 43% in the +Cd treatment, while the +lignin +Cd treatment significantly reduced it to 36%; conversely, the C3 component accounted for the highest proportion in the +lignin +Cd treatment, at 30%. This indicates that under Cd stress, lignin can still reduce the relative contribution rate of the C1 component and increase the proportion of humic components, especially the C3 component. In summary, the addition of lignin (+lignin)... vs. Control treatment and +lignin +Cd vs. +Cd treatment significantly altered the three-dimensional fluorescence characteristics of DOM, mainly by reducing the relative contribution rate of the C1 component, increasing the contribution rates of humic components C2 and C3, and enhancing the HIX of DOM. These results indicate that lignin can promote the transformation of soil DOM from a protein-humic mixture to a more humified and aromatic humic component.

[0050] Figure 5 In the figure: (A) Fluorescence index (FI); (B) Biogenicity index (BIX); (C) Humicity index (HIX); (D) Fluorescence intensity contribution rate of the fluorescent components (C1~C3) extracted by parallel factor analysis. Different lowercase letters in the figure indicate significant differences between treatments. P < 0.05).

[0051] 3.6 Correlation Analysis To clarify the relationship between soil physicochemical properties and DOM characteristic parameters and Cd migration and enrichment in the soil-rice system, this study conducted Mantel test and Pearson correlation analysis. Figure 6 A). Mantel test results showed that the degree of association between Cd accumulation in different parts of rice and environmental factors varied. Specifically, Cd in brown rice was associated with DOC and UV.254 UV 260 Cd was significantly correlated with DOC, HIX, and C2, with highly significant correlations observed with DOC, HIX, and C2. Stem and leaf Cd was mainly significantly correlated with DOC and C2; DTPA-Cd was only highly significantly correlated with DOC. Furthermore, this study selected five DOM parameters significantly associated with Cd migration and accumulation (DOC, HIX, C2, UV). 254 UV 260 Redundancy analysis (RDA) was performed on DTPA-Cd and Cd content in various parts of rice to reveal its multidimensional spatial response relationship. Figure 6 B). The results showed that the first two axes of RDA explained 98.90% of the total variance, indicating that the selected core variables could well explain the variation characteristics of Cd migration and accumulation in the soil-rice system. The angle between DOC and brown rice Cd and DTPA-Cd was very small, and both extended along the positive half-axis of RDA1, indicating that DOC was strongly positively correlated with soil Cd availability and brown rice Cd accumulation. Conversely, UV, which characterizes the aromaticity and hydrophobicity of DOM... 254 and UV 260 The direction points to the third quadrant and forms a significant obtuse angle with the Cd content indices of each part, indicating that the enhanced aromaticity and hydrophobicity of the DOM may be related to the reduced Cd migration and bioavailability.

[0052] Figure 6 (A) Mantel test and Pearson correlation network heatmap of environmental factors with soil DTPA-Cd and Cd content in various parts of rice; (B) Redundancy analysis of core DOM parameters with soil DTPA-Cd and Cd content in various parts of rice. The line width in the figure represents the magnitude of Mantel's r value, and the line color represents the significance level (Mantel's r-value). p (The red arrows represent response variables, and the blue arrows represent explanatory variables.)

[0053] To further identify key environmental factors (DOC, HIX, C2, UV) 254 UV 260 To study the effects of Cd in brown rice, Cd in stems and leaves, and DTPA-Cd on the Cd content of brown rice, this study optimized and screened the model using multiple stepwise linear regression. Redundant variables with insignificant partial correlations (C2, UV) were removed. 254 UV 260 The final fitted regression models all reached a highly significant level. P< 0.001 (Table 3). The results show that DOC was a common predictor retained in all three regression models, and its regression coefficients were all positive. In the stem-leaf Cd model, only DOC was retained, indicating that stem-leaf Cd content is mainly positively correlated with DOC. For the brown rice Cd and DTPA-Cd models, in addition to DOC, HIX was also retained as a significant predictor. DOC and HIX together explained 98.6% and 98.1% of the variation in brown rice Cd and DTPA-Cd, respectively, indicating that DOM content and its humification degree are important factors affecting soil Cd bioavailability and brown rice Cd accumulation. DOC showed a positive effect, while HIX showed a negative effect. These results indicate that DOM has a dual regulatory effect on Cd migration and brown rice Cd accumulation: increased DOC may increase Cd migration potential, while increased DOM humification degree may decrease Cd bioavailability.

[0054] Table 3. Multiple stepwise linear regression analysis of Cd content in brown rice, stem and leaf Cd, and DTPA-Cd.

[0055] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lignin-based DOM structure remodeling agent for reducing cadmium accumulation in brown rice, characterized in that, By weight percentage, it includes 80%–98% alkali-degraded lignin, 0.3%–1.5% nitrogen source, 0.03%–0.3% phosphorus source, and 0.05%–0.5% potassium source; The reshaping agent has an organic carbon content of 150~300 g·kg. -1 The cation exchange capacity is 30~70 cmol·kg -1 The pH value is 4.5–6.5, and the Cd content is less than 0.1 mg·kg⁻¹. -1 ; The reshaping agent, when applied to the soil, can increase the UV absorbance of dissolved organic matter (DOM) in the soil. 254 and UV 260 SUVA UV absorbance 254 and SUVA 260 This increases HIX and decreases the E2 / E3 and E3 / E4 ratios.

2. The application of the lignin-based DOM structure remodeling agent according to claim 1 in reducing cadmium accumulation in brown rice.

3. The lignin-based DOM structure remodeling agent according to claim 1, characterized in that, The dealkalized lignin is derived from one or more of rice straw, rice husk, wheat straw, corn straw, sugarcane bagasse, or papermaking black liquor.

4. A method for reducing cadmium accumulation in brown rice using the lignin-based DOM structure remodeling agent according to any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Apply the lignin-based DOM structure remodeling agent to cadmium-contaminated paddy soil at an application rate of 0.5% to 2.0% of the soil dry weight; (2) Mix the remodeling agent thoroughly with the paddy soil and react for 14 to 35 days to improve the aroma, hydrophobicity and humification of the soil DOM; (3) Plant rice in the treated paddy soil and carry out flooding management: After transplanting rice, maintain the water level of the paddy field at 1-5 cm. Stop watering 7-21 days before the rice matures and is harvested and allow the soil to dry naturally. The method enables UV radiation from the soil DOM. 254 UV 260 SUVA 254 and SUVA 260 Increased levels of HIX and decreased E2 / E3 and E3 / E4 ratios promote the conversion of exchangeable Cd in the soil to organically bound, iron-manganese oxide-bound, and / or residual forms, thereby reducing the content of available Cd in the soil and decreasing Cd accumulation in brown rice.

5. The method according to claim 4, characterized in that, The improvement in soil DOM humification is manifested by: HIX increasing to above 6, and / or the relative contribution rate of humus-like fluorescent components C2 and / or C3 increasing, and the relative contribution rate of protein-humus-like mixed component C1 decreasing.

6. The method according to claim 4, characterized in that, The flood management measures are as follows: maintain the water level on the field surface at 2 cm; and stop replenishing water 14 days before the rice is harvested.

7. The method according to claim 4, characterized in that, The cadmium-contaminated paddy field soil was southern acidic or weakly acidic paddy soil with a pH of 4.5–6.5 and a total Cd content of 0.6–5.0 mg·kg⁻¹. -1 .

8. The method according to claim 4, characterized in that, The method reduces the Cd content in brown rice by more than 33%, increases the Cd content in rice roots, and / or reduces the Cd translocation coefficient from roots to brown rice by more than 80%.

9. The method according to claim 4, characterized in that, The nitrogen, phosphorus, and potassium nutrients applied by the method are calculated based on soil dry weight as follows: N 30~120 mg·kg -1 P 10~60 mg·kg -1 K 20~100 mg·kg -1 .

10. The application of the method according to any one of claims 4 to 9 in the remediation of cadmium-contaminated paddy field soil.

Images