Method for treating rare earth tailing area by exogenous nutrient combined with guge

Abstract

The application discloses a method for treating rare earth tailing area by exogenous nutrients and Yugu, which comprises the following steps: firstly, applying base fertilizer to the soil of the rare earth tailing area to be treated; 7 days later, directly sowing Yugu seeds in the soil; when the plants grow to the tillering stage, applying the first topdressing; 3 days after mowing, applying the second topdressing; the base fertilizer, the first topdressing and the second topdressing adopt biogas residue or biogas slurry; the biogas slurry is directly applied or applied after pretreatment; the pretreatment method of the biogas slurry comprises the following steps: adding montmorillonite powder into the biogas slurry and stirring uniformly, standing, then adding saccharide compounds and stirring uniformly. The application of specific amount of exogenous nutrients biogas slurry and biogas residue, the control of suitable physicochemical properties and the pretreatment of the biogas slurry can all obviously promote the growth of Yugu, relieve soil acidification, improve soil nutrients and nutrient transmission capacity, reduce the heavy metal level of the rare earth tailing area soil, and the treatment effect of the tailing area is obvious.

Description

Technical Field

[0001] This invention belongs to the field of ecological restoration technology, specifically relating to a method for treating rare earth tailings areas using exogenous nutrients in conjunction with grain remediation. Background Technology

[0002] Rare earth tailings are an important strategic mineral resource in southern my country. However, long-term over-exploitation and the extensive use of acidic leaching agents have led to numerous environmental pollution problems, including soil acidification, fertility degradation, and heavy metal contamination. This makes the restoration and reconstruction of rare earth mining ecosystems more unique and complex than other types of mining areas. Currently, phytoremediation, with vegetation restoration as its goal, is the main method for ecological restoration in mining areas. However, research shows that soil restoration often lags behind vegetation restoration and requires external nutrient support; otherwise, restored vegetation will decline due to insufficient nutrients. Furthermore, conventional remediation plants suffer from low biomass, slow growth, lack of post-harvest utilization value, and poor economic benefits, severely hindering the ecological restoration process in mining areas. Therefore, in recent years, enterprises around rare earth tailings areas have introduced biogas projects that centrally treat livestock and poultry manure. These projects centrally utilize biogas slurry or residue for energy plant cultivation. Leveraging the advantages of energy plants—such as tolerance to poor soil conditions, strong resilience, and high biomass—this approach not only restores the soil but also expands post-harvest utilization pathways, providing economic support for sustainable restoration.

[0003] In terms of energy content and biomass yield, energy plants refer to those plants that utilize light energy efficiently, can synthesize components similar to petroleum or diesel, and are rich in oils, carbohydrates, or hydrogen. Terrestrial types mainly include herbaceous and woody plants, while aquatic plants are primarily algae. Different types of energy plants exhibit different growth patterns and thus have varying effects on rare earth tailings remediation.

[0004] Yugu (scientific name: Pennisetumamer *Caragana korshinskii* (L.) Leeke. is an annual herbaceous plant belonging to the Poaceae family and the *Caragana* genus. *Caragana korshinskii* is characterized by its wide adaptability and strong resistance to adverse conditions, making it a dual-purpose forage crop that yields both high-quality grain and high-quality feed.

[0005] The growth and biomass of energy plants are closely related to the type and amount of fertilizer applied. However, the correlation between fertilizer application and plant uptake is not entirely positive. Taking nitrogen application as an example, when the nitrogen level reaches the crop's absorption threshold, the plant's nitrogen uptake tends to be saturated. Excessive nitrogen application not only leads to a decrease in nitrogen utilization efficiency but also delays the growth period and reduces crop yield. Currently, in southern my country, most energy plants used for biogas production through anaerobic fermentation are from the Poaceae family. Related studies have shown that nitrogen fixation in Poaceae plants largely relies on non-symbiotic nitrogen fixation, and the nitrogen required for their growth and development depends on the absorption of nitrogen from the soil by the plant roots. Rational application of nitrogen fertilizer can significantly promote plant growth (e.g., agronomic traits such as tiller number, plant height, and biomass) and improve energy quality.

[0006] Biogas slurry and biogas residue are the liquid and solid components obtained after solid-liquid separation of anaerobic fermentation residues. They are rich in organic matter, nitrogen, phosphorus, potassium, and other nutrients, providing essential nutrients for plants. However, the physicochemical properties of biogas slurry and residue, and even different batches of biogas slurry or residue, vary, affecting plant nutrient absorption. Therefore, when using biogas slurry or residue as an exogenous nutrient source, the application rate and physicochemical properties must be considered to ensure effective treatment. However, current technologies rarely take these factors into account, resulting in less than satisfactory treatment outcomes.

[0007] In addition, the nutrient concentration in biogas slurry is low, and a large amount of biogas slurry needs to be applied to ensure the optimal amount of fertilizer. This results in a lot of water being carried away when the biogas slurry is applied to the soil, which affects the original soil structure, easily causes nutrient loss, and affects the soil environment for plant growth.

[0008] In summary, how to provide a method for the combined treatment of rare earth tailings areas with exogenous nutrients and by rationally applying exogenous nutrients to improve the treatment effect of rare earth tailings areas is an urgent problem to be solved. Summary of the Invention

[0009] The first objective of this invention is to study the effects of different application rates of biogas slurry or biogas residue on the growth of rice and the treatment effect of rare earth tailings areas, using biogas slurry or biogas residue as exogenous nutrients and rice as an energy plant, so as to determine the range of biogas slurry or biogas residue application rates.

[0010] The second objective of this invention is to study the effects of different pH values ​​of biogas slurry or biogas residue on the growth of rice and the treatment effect of rare earth tailings areas, thereby determining the pH range of biogas slurry or biogas residue.

[0011] The third objective of this invention is to pretreat biogas slurry to reduce the impact of high water content in biogas slurry on soil structure.

[0012] The technical solution of the present invention is as follows:

[0013] A method for treating rare earth tailings areas using exogenous nutrients in conjunction with *Yugu* (a type of millet) involves planting *Yugu* in the rare earth tailings areas, specifically including the following steps:

[0014] First, apply base fertilizer to the soil in the rare earth tailings area to be treated. Seven days later, sow the seeds of the rare earth cod directly into the soil. When the plants reach the tillering stage, apply the first top dressing. When the best-growing plants reach a height of 1.2 m, harvest them. Apply the second top dressing three days after harvesting. The application rates of the base fertilizer, the first top dressing, and the second top dressing are the same, with a pure nitrogen content of 800-1100 kg / hm². 2 The fertilizers used for the base fertilizer, the first topdressing, and the second topdressing are biogas residue or biogas slurry.

[0015] Preferably, the application rates of the basal fertilizer, the first topdressing, and the second topdressing are the same, with a pure nitrogen content of 960 kg / hm². 2 .

[0016] Preferably, the biogas slurry has the following physicochemical properties: pH 7.90-8.46, organic matter (OM) content 0.34-0.41%, total nitrogen (TN) content 0.26-0.36%, P2O5 content 0.06-0.21%, and K2O content 0.17-0.31%.

[0017] Preferably, the physicochemical properties of the biogas residue are as follows: moisture content 64.69-69.78%, pH 8.02-8.43, organic matter (OM) content 28.80-37.27%, total nitrogen (TN) content 2.95-4.02%, P2O5 content 24.78-36.82%, and K2O content 0.78-1.19%.

[0018] Preferably, the planting density of the rice is 1-30 plants / m². 2 .

[0019] Preferably, the rare earth tailings area is an ion-type rare earth tailings area.

[0020] The biogas slurry can be applied directly or after pretreatment. The pretreatment method for the biogas slurry includes the following steps: adding montmorillonite powder to the biogas slurry and stirring evenly, letting it stand for 10-20 minutes, and then adding sugar compounds and stirring evenly. The amount of montmorillonite powder and sugar compounds added is 4-7% and 3-6% of the mass of the biogas slurry, respectively.

[0021] Preferably, the carbohydrate compound is trigalacturonic acid.

[0022] The beneficial effects of this invention are:

[0023] 1. Using the specific application amount of exogenous nutrients biogas slurry and biogas residue of this invention can significantly promote the growth of rice, alleviate soil acidification, enhance soil nutrients and nutrient transfer capacity, and reduce the heavy metal level in the soil of rare earth tailings areas, with obvious treatment effects.

[0024] 2. This invention selects biogas slurry and biogas residue with suitable physicochemical properties, and by controlling their pH, it can effectively improve the treatment effect of tailings areas.

[0025] 3. Due to the low nutrient concentration in biogas slurry, a large amount of biogas slurry needs to be applied to ensure the optimal fertilization amount. This results in a large amount of water being carried away when the biogas slurry is applied to the soil, affecting the original fertilizer retention structure of the soil and easily causing nutrient loss. Therefore, this invention pre-treats the biogas slurry. First, montmorillonite powder with water-absorbing properties is added to concentrate the biogas slurry. Then, sugar compounds are added to thicken the solution. After this pre-treatment, the biogas slurry applied to the soil can improve the fertilizer retention effect, improve plant growth, and enhance the tailings treatment effect.

[0026] The saccharide compound of the present invention is trigalacturonic acid. The unique structure of this substance can encapsulate the organic matter in the biogas slurry, making it easier for plants to absorb, thereby improving the fertilization absorption effect and plant growth. In addition, trigalacturonic acid can also adsorb heavy metals in the soil, thereby improving the removal effect of heavy metals in tailings areas. Attached Figure Description

[0027] Figure 1 The growth diagrams of the energy plant *Erythrina variegata* under different experimental treatments in 2019 are shown.

[0028] Figure 2 The growth diagrams of the energy plant *Erythrina variegata* under different experimental treatments in 2020 are shown.

[0029] Figure 3 This is a growth chart of the energy plant *Erythrina variegata* under different experimental treatments in 2021, according to the present invention. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Unless otherwise specified, the experimental methods and materials used in the following embodiments are conventional methods and commonly available materials. Where specific techniques or conditions are not specified in the embodiments, they should be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Example 1

[0032] This embodiment provides a method for treating rare earth tailings areas using exogenous nutrients in conjunction with *Gynostemma pentaphyllum*, wherein *Gynostemma pentaphyllum* is planted in the rare earth tailings area at a planting density of 20 plants / m². 2 Specifically, it includes the following steps:

[0033] First, apply base fertilizer to the soil in the rare earth tailings area to be treated. Seven days later, sow the seeds of Yugu (a type of grain) directly into the soil. When the plants reach the tillering stage, apply the first top dressing. When the best-growing plants reach a height of 1.2 m, they are harvested. Apply the second top dressing three days after harvesting. The application rates of the base fertilizer, the first top dressing, and the second top dressing are the same, with a pure nitrogen content of 960 kg / hm². 2 The fertilizer used for the base fertilizer, the first topdressing, and the second topdressing is biogas slurry. The treatment period is three years (2019-2021).

[0034] The physicochemical properties of the biogas slurry are as follows:

[0035] Example 2

[0036] This embodiment provides a method for treating rare earth tailings areas using exogenous nutrients in conjunction with *Gynostemma pentaphyllum*, wherein *Gynostemma pentaphyllum* is planted in the rare earth tailings area at a planting density of 20 plants / m². 2 Specifically, the following steps are taken: First, apply base fertilizer to the soil in the rare earth tailings area to be treated. Seven days later, sow the seeds of the rare earth grain directly into the soil. When the plants reach the tillering stage, apply the first top dressing. When the best-growing plants reach a height of 1.2 m, they are harvested. A second top dressing is applied three days after harvesting. The application rates of the base fertilizer, the first top dressing, and the second top dressing are the same, with a pure nitrogen content of 960 kg / hm². 2 The fertilizer used for the base fertilizer, the first topdressing, and the second topdressing is biogas residue. The treatment period is three years (2019-2021).

[0037] The physicochemical properties of the biogas residue are as follows:

[0038] Example 3

[0039] This embodiment provides a method for the combined treatment of rare earth tailings areas using exogenous nutrients and grain remediation. The application rates of the base fertilizer, the first topdressing, and the second topdressing are the same, with a pure nitrogen content of 800 kg / hm². 2 The fertilizer used was biogas slurry, and the rest was the same as in Example 1. Example 4

[0040] This embodiment provides a method for the combined treatment of rare earth tailings areas using exogenous nutrients and grain remediation. The application rates of the base fertilizer, the first topdressing, and the second topdressing are the same, with a pure nitrogen content of 1100 kg / hm².2 The fertilizer used was biogas slurry, and the rest was the same as in Example 1. Example 5

[0041] This embodiment provides a method for the combined treatment of rare earth tailings areas using exogenous nutrients and grain remediation. The application rates of the base fertilizer, the first topdressing, and the second topdressing are the same, with a pure nitrogen content of 800 kg / hm². 2 The fertilizer used was biogas residue, and the rest was the same as in Example 2. Example 6

[0042] This embodiment provides a method for the combined treatment of rare earth tailings areas using exogenous nutrients and grain remediation. The application rates of the base fertilizer, the first topdressing, and the second topdressing are the same, with a pure nitrogen content of 1100 kg / hm². 2 The fertilizer used was biogas residue, and the rest was the same as in Example 2. Example 7

[0043] Based on Example 1, this example provides a method for treating rare earth tailings areas using exogenous nutrients in combination with gluten. The biogas slurry used is pretreated before application. The pretreatment method is as follows: add montmorillonite powder to the biogas slurry and stir evenly, let it stand for 15 minutes, and then add a sugar compound (trigalacturonic acid) and stir evenly. The amount of montmorillonite powder and sugar compound added is 5.5% and 4.5% of the biogas slurry mass, respectively.

[0044] The rest is the same as in Example 1. Example 8

[0045] This embodiment provides a method for treating rare earth tailings areas using exogenous nutrients in combination with grain. The biogas slurry used is pretreated before application. The pretreatment method is as follows: add montmorillonite powder to the biogas slurry and stir evenly, let it stand for 10 minutes, and then add a sugar compound (trigalacturonic acid) and stir evenly. The amount of montmorillonite powder and sugar compound added is 4% and 3% of the mass of the biogas slurry, respectively.

[0046] The rest is the same as in Example 1. Example 9

[0047] This embodiment provides a method for treating rare earth tailings areas using exogenous nutrients in combination with grain. The biogas slurry used is pretreated before application. The pretreatment method is as follows: add montmorillonite powder to the biogas slurry and stir evenly, let it stand for 20 minutes, and then add a sugar compound (trigalacturonic acid) and stir evenly. The amount of montmorillonite powder and sugar compound added is 7% and 6% of the mass of the biogas slurry, respectively.

[0048] The rest is the same as in Example 1.

[0049] Comparative Examples 1-2 (Changes in Energy Plants):

[0050] The difference between Comparative Example 1 and Example 1 is that the oyakodon was replaced with hybrid pampas grass, while the rest remained the same.

[0051] The difference between Comparative Example 2 and Example 2 is that the oyakoyaki was replaced with hybrid pampas grass, while the rest remained the same.

[0052] Comparative Examples 3-10 (with variations in the amount of biogas slurry or biogas residue applied):

[0053] The difference between Comparative Examples 3-6 and Example 1 is that the amount of biogas slurry applied each time is changed, while the rest remains the same.

[0054] The difference between Comparative Examples 7-10 and Example 2 is that the amount of biogas residue applied each time is changed, while the rest remains the same.

[0055] Specifically as follows:

[0056]

[0057] Comparative Examples 11 and 12 (changes in biogas slurry pH):

[0058] The difference between Comparative Example 11 and Example 1 is that the pH of the biogas slurry applied each year is adjusted to 7.00 using acid (sulfuric acid).

[0059] The difference between Comparative Example 12 and Example 1 is that the pH value of the biogas slurry applied each year is adjusted to 8.60 using alkali (sodium hydroxide).

[0060] Comparative Examples 13 and 14 (pH changes in biogas residue):

[0061] The difference between Comparative Example 13 and Example 2 is that the pH value of the biogas residue applied each year is adjusted to 7.80 using acid (sulfuric acid).

[0062] The difference between Comparative Example 14 and Example 2 is that the pH value of the biogas residue applied each year is adjusted to 8.60 using alkali (sodium hydroxide).

[0063] Comparative Examples 15 and 16 (with changes in biogas slurry pretreatment methods):

[0064] Comparative Example 15

[0065] The difference between this comparative example and Example 7 is that no sugar compounds are added in the biogas slurry pretreatment method.

[0066] Comparative Example 16

[0067] The difference between this comparative example and Example 7 is that the carbohydrate compound is glucose.

[0068] Pot experiment

[0069] I. Experimental Materials

[0070] The energy plant used in the pot experiment (hybrid Napier grass) Pennisetum hybrid PH), Miya ( Pennisetumamcrieanum The biogas project (PA) and exogenous nutrients (Biogas slurry (BS) and biogas residues (BR)) were all provided by Ganzhou Ruiyuan Biotechnology Co., Ltd. The company has built a super-large biogas project (total fermentation volume of 20,000 m³) in Lingbei Town, Dingnan County, Ganzhou City, Jiangxi Province. 3 In 2017, the research team, in collaboration with the company, established a sustainable remediation zone for energy plants in the abandoned mining area surrounding the biogas project. Hybrid Napier grass and wild rice were selected from the varietal resource nursery within the sustainable remediation zone. Biogas slurry and biogas residue were taken from the biogas project's fermentation tank. Every year on April 10th, the fermentation output from the tank was separated into solid and liquid components, then packaged into plastic containers and transported to the laboratory for storage at 4°C. Testing showed that the heavy metal content in the biogas slurry and biogas residue did not exceed the limits specified in the agricultural industry standard "Biogas Fertilizer" (NY / T 2596-2022) (see Table 2 below).

[0071] Primary soil (PS) from rare earth tailings: Soil samples for the pot experiment were taken from an abandoned rare earth tailings remediation area in Dingnan County, Ganzhou City, Jiangxi Province (115.054917°E, 24.979109°N). This area was originally a rare earth tailings dump, with sandy red soil and poor nutrients. The soil physicochemical properties are shown in Table 1. ① The soil pH of the rare earth tailings site is acidic; ② The EC value, representing soil electrical conductivity, is at a low level, indicating poor nutrient transport capacity; ③ The soil nutrient levels are low, with OM, TN, and AN at the extremely deficient level (level 6), and AP and AK at the moderate levels (level 4 and level 3, respectively); ④ The content of the five heavy metals in the soil samples is lower than the risk screening value (pH≤5.5) in the "Soil Environmental Quality Standard for Agricultural Land Soil Pollution Risk Control" (GB 15618-2018), indicating that the heavy metal content in the soil is low (see Table 2).

[0072] Table 1. Soil physicochemical properties of rare earth tailings sites

[0073]

[0074] Note: 1 refers to the soil nutrient and pH value classification standard, which is based on the "Survey on pH Value and Nutrient Status of Cultivated Soil in Jiangxi Province"; 2 refers to the soil EC value classification standard, which is based on the "Relationship between Substrate EC Value and Crop Growth and Comparison of Measurement Methods"; pH refers to acidity and alkalinity, EC refers to electrical conductivity, OM refers to organic matter, AN refers to alkaline available nitrogen, AP refers to available phosphorus, AK refers to available potassium, and PS refers to the soil sample of this invention.

[0075] Table 2. Heavy metal content in biogas slurry, biogas residue, and soil samples.

[0076]

[0077] Note 1: Agricultural industry standard "Biogas Fertilizer" (NY / T 2596-2022);

[0078] Note 2: National Standard "Soil Environmental Quality Standard for Risk Control of Soil Pollution in Agricultural Land" (GB 15618-2018).

[0079] II. Test Methods

[0080] The outdoor pot experiment was conducted at the experimental base of Jiangxi Academy of Agricultural Sciences from 2019 to 2021. The energy plant selected was *Paspalum notatum* (hereinafter referred to as PA), and the control plant was hybrid *Pennisetum affine* (hereinafter referred to as PH). A PACK (no fertilizer, planted with *Paspalum notatum*) was set up, and the plants were treated according to the methods of Examples 1, 2, 7 and Comparative Examples 1-17, respectively.

[0081] Potted plant operation procedure: ① Base fertilizer: Mix biogas slurry and biogas residue with 8 kg of soil, mix well, and let stand for 7 days in pots; ② Planting: Soak the seeds of *Gnaphalium affine* in 10% hydrogen peroxide solution for 15 minutes, rinse 5 times with sterile water, and soak for 4 hours. Sow 10 seeds directly. After the seedlings have grown 3-4 leaves, thin them out, leaving the 3 best-growing plants; transplant 3 hybrid *Pennisetum alopecuroides* seedlings into pots. After the seedlings have grown 2 leaves, thin them out, leaving only 1 plant; ③ First top dressing: When the plants reach the tillering stage, apply the first top dressing according to the application ratio; ④ Harvesting and second top dressing: When the plants of the best-growing experimental treatment groups reach a height of 1.5 m (hybrid *Pennisetum alopecuroides*) and 1.2 m (*Gnaphalium affine*), respectively, they are harvested, and the second top dressing is applied 3 days after harvesting.

[0082] III. Measurement Method

[0083] (1) Measurement of plant growth and biomass

[0084] Chlorophyll content was measured before plant harvesting on a sunny day. A SPAD-502 chlorophyll meter was used to measure chlorophyll in 5 healthy, mature leaves, with each leaf measured three times. The average value was taken as the result. After each harvest, the number of tillers, plant height, and above-ground fresh weight were measured. The root system was removed after the second harvest, cleaned of dirt and sand, washed, blanched, and dried to constant weight; the dry weight of the root system was then measured.

[0085] (2) Determination of physicochemical properties of soil samples

[0086] The physicochemical properties of soil samples were determined using the methods described in the third edition of "Soil Agrochemical Analysis" by Bao Shidan. Specifically: ① pH value: measured using a PHS-3C pH meter after extraction at a water-to-soil ratio of 2.5:1; ② Electrical conductivity (EC) value: measured using a Thermo Orion Star A conductivity meter after extraction at a water-to-soil ratio of 5:1; ③ Soil OM content: determined using the potassium dichromate volumetric method with external heating; ④ Soil TN: determined using the semi-micro Kjeldahl method; ⑤ Soil AN: determined using the alkaline diffusion method; ⑥ Soil AP: determined using the ammonium fluoride-hydrochloric acid method suitable for acidic soils; ⑦ Soil AK: determined using the flame photometric method (ammonium acetate extraction).

[0087] (3) Determination of soil heavy metal content

[0088] Weigh 0.1 g of soil sample into a microwave digestion tube, add 0.5 mL of water to moisten, then add 1.5 mL of HCl and 4.5 mL of HNO3 (both of analytical grade). Shake well, cap, and let stand overnight. Pre-digest at 100 °C for 30 min in a graphite digester, cool, and then digest in a microwave digester for 30 min (190 °C). Remove acid to approximately 0.5 mL in a graphite digester, and then bring the volume to 50 mL with ultrapure water. Soil composition analysis standard material GBW07425 was used for quality control.

[0089] IV. Test Results

[0090] 1. The effect of different application rates of biogas slurry or biogas residue on the effectiveness of Yugu in treating rare earth tailings areas.

[0091] (1) Growth and biomass of the rice paddies

[0092] See the growth status of Miyako from 2019 to 2021. Figure 1-3 The specific growth data are shown in Table 3-8 below.

[0093] A. Number of tillers in Mitsuya

[0094] Tillering in grasses occurs on relatively swollen and nutrient-rich tillering nodes, and is usually related to external conditions such as water, fertilizer, light, temperature, and agricultural practices. As long as the conditions are suitable, the more tillers a plant produces, the more it indirectly reflects the plant's growth status and is also one of the important factors determining yield.

[0095] Table 3. Number of tillers per plant (2019-2021)

[0096]

[0097] B. Plant height of Mitsuya

[0098] Plant height, as a morphological indicator reflecting the growth vigor of grasses, is closely related to their yield.

[0099] Table 4. Plant height of Miko (2019-2021) (cm / plant)

[0100]

[0101] C. Spandex content (SPAD)

[0102] SPAD value, as an indicator of the relative chlorophyll content in leaves, is commonly used to reflect leaf photosynthetic efficiency, nitrogen nutrition, and plant yield levels.

[0103] Table 5. Chlorophyll content (SPAD) of Yugu vineyards, 2019-2021

[0104]

[0105] D. Fresh weight on the ground of the imperial valley

[0106] Yugu is a high-biomass C4 plant. The level of its biomass determines the plant's potential for remediation by accumulating soil pollutants and is also the material basis for its high-value post-harvest utilization.

[0107] Table 6 Fresh weight of above-ground parts of Yugu rice (g / pot) from 2019 to 2021

[0108]

[0109] E. Dry weight of Oya root

[0110] The root system is an important part of a plant's absorption of soil nutrients and its influence on the soil system. Therefore, root dry weight is often used as a key indicator to reflect plant growth.

[0111] Table 7. Dry weight of Yugu roots (g / plant) from 2019 to 2021

[0112]

[0113] F. Miya Negatsu

[0114] Table 8. Root length of Yugu (cm / plant) from 2019 to 2021

[0115]

[0116] The growth and biomass data of the *Prunus cerasifera* obtained from Tables 3-8 above show that, under different exogenous nutrient application conditions (biogas slurry / biogas residue), the growth of *Prunus cerasifera* was superior to that of the control group PACK (no fertilizer, planted *Prunus cerasifera*), indicating that both biogas slurry and biogas residue application can promote the growth of *Prunus cerasifera*. Furthermore, regardless of whether it was the biogas slurry group or the biogas residue group, the number of tillers, plant height, chlorophyll content (SPAD), aboveground fresh weight, root dry weight, and root length of *Prunus cerasifera* of this invention were all at the fourth nitrogen application level (PABS-4 / PABR-4, 960 kg / hm²). 2 The fact that the amount of fertilizer applied in this invention reaches its maximum indicates that the amount of fertilizer applied must be controlled within a suitable range in order to effectively improve the growth of the grain.

[0117] (2) Influence of soil physicochemical properties

[0118] Three years after planting the rice, its soil physicochemical properties were tested in 2021, and the results are shown in Table 9 below.

[0119] Table 9. Soil physicochemical properties for planting Yugu (a type of grain).

[0120]

[0121] As shown in Table 9, compared with the control group PACK, the physical and chemical properties of the tailings soil were significantly improved after 3 years of application of biogas slurry or biogas residue.

[0122] Soil pH: The improvement effect on soil acidification in the tailings area was significant. In both the biogas slurry and biogas residue groups, soil pH gradually increased with increasing fertilizer application, reaching the 4th and 5th nitrogen application levels (PABS-4 / PABR-4, 960 kg / hm²). 2 The upward trend was not obvious after the fourth nitrogen application level. In particular, the soil pH in the biogas residue group rapidly increased to the neutral range under the fourth nitrogen application level. Considering the cost and the adverse effects of excessive fertilization, the fourth nitrogen application level was considered to be more appropriate.

[0123] Soil EC, OM (organic matter), TN (total nitrogen), and AN (alkaline nitrogen): Compared with the control group PACK, after 3 years of planting millet with biogas slurry or biogas residue, the soil EC (nutrient transport capacity), OM, TN, and AN in the tailings area were significantly improved, with biogas residue showing a more significant improvement. Specifically, for both the biogas slurry and biogas residue groups, all of the above nutrient indicators were highest at the fourth nitrogen application level (PABS-4 / PABR-4, 960 kg / hm²). 2 The fact that the amount of fertilizer applied in this invention reaches its maximum indicates that the amount of fertilizer applied in this invention is suitable for this nitrogen application level range.

[0124] Soil available phosphorus (AP) and available potassium (AK): Compared with the control group (PACK), the application of biogas slurry or biogas residue for planting rice significantly improved the available phosphorus (AP) and available potassium (AK) in the tailings area soil. Since excessive phosphorus and potassium can cause soil salinization, soil acidification, and water pollution, excessive phosphorus can lead to crop chlorosis and yellowing, and excessive potassium can cause crop lodging, based on the soil nutrient and pH grading standards in Table 1, and considering the cost of fertilization and the adverse effects of excessive fertilization, the fourth nitrogen application level is considered more suitable.

[0125] 2. Effects of different biogas slurries or biogas residues on the growth of rice.

[0126] The average annual plant height and above-ground fresh weight of *Erythrina variegata* were obtained by applying different biogas slurries or biogas residues as fertilizer. The results are shown in Table 10 below.

[0127] Table 10. Plant height and above-ground fresh weight of Okoji (a type of grain)

[0128]

[0129] As shown in Table 10, the plant height and above-ground fresh weight of the *Erythrina* plants in Examples 1 and 2 of this invention were higher than those in Comparative Examples 11-14 (where the pH of the biogas slurry and biogas residue was changed), indicating that controlling the biogas slurry or biogas residue within a suitable pH range in this invention can promote the growth of *Erythrina*. Compared with Example 1, Example 7, which pretreated the biogas slurry before application, further increased the plant height and biomass of *Erythrina*. Furthermore, the growth of *Erythrina* plants in Comparative Examples 15 and 16 (where the biogas slurry pretreatment method was changed) was lower than that in Example 7, indicating that pretreatment of the biogas slurry according to the method of this invention can effectively promote the growth of *Erythrina*.

[0130] 3. The impact of different energy plants and different biogas slurry or biogas residue on soil improvement

[0131] Using different energy plants and different biogas slurry or biogas residue, the soil physicochemical properties were tested in 2021 after 3 years of treatment. The results are shown in Table 11 below.

[0132] Table 11 Soil physicochemical properties of plants grown with *Gnaphalium affine* and hybrid *Pennisetum affine*

[0133]

[0134] As shown in Table 11, after 3 years of treatment, the soil physicochemical properties of the plants grown in Examples 1, 2, and 7 of this invention were better than those of the soil grown in Comparative Examples 1 and 2, which were grown in Hybrid Napier grass. They were also better than those of Comparative Examples 11-14 (where the pH of the biogas slurry and biogas residue was changed) and Comparative Examples 15 and 16 (where the biogas slurry pretreatment method was changed). This indicates that the soil physicochemical properties improved by planting the plants in this invention were better than those of Hybrid Napier grass. Controlling the biogas slurry or biogas residue within a suitable pH range and pretreating the biogas slurry also resulted in better soil improvement.

[0135] 4. The impact of biogas slurry or biogas residue on the improvement of heavy metals in soil used for planting millet.

[0136] After three years of treatment using the methods of Examples 1, 2, and 7 and Comparative Examples 15 and 16, the heavy metal content of the soil was tested in 2021, and the results are shown in Table 12 below.

[0137] Table 12 Heavy metal content in soil where Yugu is planted (mg / kg)

[0138]

[0139] As shown in Table 12, compared with the original tailings soil, the planting of PACK in this invention can significantly reduce the Cr and Pb content in the soil. Application of biogas slurry and biogas residue (Examples 1 and 2) can further reduce the Cr and Pb content, and has a slight reducing effect on Cd and Hg content. Example 7 (pretreatment of biogas slurry) showed a significant reduction effect on the content of all five heavy metals, which was superior to Examples 1 and 2, indicating that pretreatment of biogas slurry can effectively improve the removal of heavy metals from the soil.

[0140] Compared with Comparative Examples 15 and 16, the method of Example 7 of the present invention for pretreatment of biogas slurry can effectively improve the removal of heavy metals from soil.

Claims

1. A method for treating rare earth tailings areas using exogenous nutrients combined with grain remediation, characterized in that: In rare earth tailings areas, millet is planted and external nutrients are applied, namely biogas slurry, which is pretreated before application. The pretreatment method for biogas slurry includes the following steps: adding montmorillonite powder to the biogas slurry and stirring evenly, letting it stand for 10-20 minutes, and then adding a sugar compound and stirring evenly. The amount of montmorillonite powder and sugar compound added is 4-7% and 3-6% of the biogas slurry mass, respectively. The carbohydrate compound is trigalacturonic acid; The specific steps for planting Yugu (a type of wild rice) in rare earth tailings areas include: First, apply base fertilizer to the soil in the rare earth tailings area to be treated. Seven days later, sow the seeds of the rare earth cod directly into the soil. When the plants reach the tillering stage, apply the first top dressing. When the best-growing plants reach a height of 1.2 m, harvest them. Apply the second top dressing three days after harvesting. The application rates of the base fertilizer, the first top dressing, and the second top dressing are the same, with a pure nitrogen content of 800-1100 kg / hm². 2 The fertilizer used for the base fertilizer, the first topdressing and the second topdressing is pretreated biogas slurry; The physicochemical properties of the biogas slurry are as follows: pH 7.90-8.46, organic matter (OM) content 0.34-0.41%, total nitrogen (TN) content 0.26-0.36%, P2O5 content 0.06-0.21%, and K2O content 0.17-0.31%.

2. The method according to claim 1, characterized in that: The planting density of the rice is 1-30 plants / m². 2 .

3. The method according to claim 1, characterized in that: The rare earth tailings area is an ion-adsorption rare earth tailings area.

4. The method according to claim 1, characterized in that: The application rates of the base fertilizer, the first topdressing, and the second topdressing were all the same, with a pure nitrogen content of 960 kg / hm². 2 .

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