Method for safely utilizing rice field having combined mercury / cadmium pollution

Abstract

Disclosed is a method for safely utilizing a rice field having combined mercury / cadmium pollution, belonging to the technical field of rice planting. The method comprises: plowing the soil of a rice field based on the soil pollution characteristics of the rice field; conditioning the plowed soil; screening a low-accumulation rice variety and planting same in the conditioned rice field; managing the water content of the rice field during the rice planting process; managing the fertilization of the rice field during the rice planting process; performing leaf surface control from the 8th to 12th day of the rice tillering period to the beginning of the jointing period, the booting period and the filling period; harvesting the rice to obtain rice. The present invention uses multiple linked effective measures to reduce mercury / cadmium enrichment of rice, thereby realizing safe production of rice in a rice field having combined mercury / cadmium pollution.

Description

A safe utilization method for rice fields contaminated by mercury / cadmium complex Technical Field

[0001] The invention relates to the technical field of rice planting, and in particular to a method for safely utilizing rice fields contaminated by mercury / cadmium complex. Background Art

[0002] my country's rice paddies face a serious problem of combined mercury and cadmium pollution. A national soil pollution survey shows that cadmium and mercury soil concentrations exceed standards at 7.0% and 1.6% of sites, respectively, with a combined excess rate of 19.4%. Mercury is the only highly concentrated and toxic global persistent pollutant that can participate in a complete cycle within an ecosystem, posing a significant threat. Rice also faces a significant risk of exceeding standards for mercury and cadmium. The prevention and control of combined mercury and cadmium contamination in rice paddy soils and their safe utilization have been incorporated into the Chinese government's overall food security goals. Currently, a variety of technologies are available for the safe utilization of combined mercury and cadmium contaminated soils, including physical remediation, chemical remediation, and phytoremediation.

[0003] Physical remediation technology mainly uses physical means to remove pollutants from the soil. Common technologies include physical separation remediation technology, soil steam extraction remediation technology, fixed / stabilized soil remediation technology, vitrification remediation technology, thermal desorption remediation technology, and electrokinetic remediation technology. For mercury-contaminated soil, thermal desorption / desorption technology is often used for remediation, taking advantage of the volatility of mercury. For heavily cadmium-contaminated soil, removing and replacing contaminated topsoil has been used to remediate some contaminated rice fields. For lightly cadmium-contaminated soil, soil turnover and dilution are the most widely used options for remediation of paddy soil pollution. In addition, electrokinetic remediation technology is an emerging and effective remediation technology for mercury-cadmium contaminated soil. Inert electrodes are implanted on both sides of the contaminated soil and a voltage is applied to establish an electric field gradient of appropriate strength. Under the action of the electric field, mercury and cadmium in the soil are concentrated at both ends of the electrodes through electromigration, electroosmosis or electrophoresis for centralized treatment, thereby effectively reducing the concentration of mercury and cadmium pollutants in the soil.

[0004] However, the choice of physical remediation solution is specific to the soil environment and has the disadvantages of being environmentally damaging, costly, time-consuming, and reducing crop yields after treatment. The thermal desorption / desorption method used to remediate mercury pollution will destroy soil organic matter due to excessively high temperatures, accelerating its decomposition and loss. For land used for agricultural use, soil that has undergone high-temperature thermal desorption remediation has lost its agricultural value. Maintaining an acceptable level of soil quality is also very important for soil that will be used for agricultural purposes in the future. Thermal desorption technology has high energy consumption costs, but is likely to have little effect on the remediation of soil cadmium pollution. Electric remediation technology requires electrical energy to maintain the electric field, which has high energy consumption, and the cost of equipment and operation is usually relatively high, especially when applied to large-scale contaminated sites. In addition, the operation requirements are also high. Incorrect electric field design or operation may lead to the random migration of pollutants, posing an ecological safety hazard to beneficial soil animals.

[0005] Chemical remediation technology involves using chemical remediation agents added to the soil to react with pollutants, degrading them and removing or reducing their toxicity. Currently, the main chemical remediation methods include chemical leaching, chemical fixation, chemical oxidation remediation, chemical reduction and reductive dechlorination remediation, solvent extraction, and soil property improvement remediation. Chemical leaching involves injecting chemical / biochemical solvents into the contaminated soil layer. The liquid containing the pollutants is then extracted from the soil for separation and treatment. This technology effectively extracts mobile or exchangeable cadmium from contaminated soil. Chemical fixation involves adding chemical reagents or chemical materials (such as biochar, activated carbon, phosphates, and iron-manganese oxides) to contaminated soil to immobilize heavy metals in the soil through complexation, precipitation, and adsorption reactions, thereby reducing their mobility and bioavailability. Although soil leaching technology can reduce the content of soluble and exchangeable mercury and cadmium in heavily contaminated soil, some beneficial essential elements in the soil will also be washed out. Chemical reagents can easily damage the basic physical and chemical properties of farmland soil, which may cause the repaired soil to lose its agricultural value. However, soil leaching technology is costly, requires a large amount of space, and the disturbance of farmland soil also needs to be weighed. Chemical fixation technology requires regular monitoring of the soil to ensure that the fixed material is still effective. Maintenance and monitoring costs are high. Highly mobile soils or soils with complex chemical properties may not be suitable. The introduction of new chemicals may cause unexpected problems and have potential side effects. For large-scale soil contaminated sites, chemical fixation will be uneconomical or impractical due to the need for large amounts of fixatives and maintenance.

[0006] Phytoremediation technology utilizes plants to degrade, extract, and fix pollutants in the soil. After plant roots absorb heavy metals, they are transported from the roots to the aboveground parts of the plant through relevant tissue structures. During this absorption and transport process, heavy metals interact with various parts of the plant. Specific plants can use these interactions to remove or fix heavy metals in the soil, thereby remediating the soil. Phytoremediation technologies for contaminated soil primarily include phytostabilization and phytoremoval. Phytostabilization is phytoremoval, while phytoremoval includes phytoextraction, phytovolatilization, and phytodegradation. Phytoremediation primarily targets the root zone of plants, reducing the bioavailability and mobility of heavy metals in the soil through absorption and accumulation by plant roots or through interactions between plant rhizosphere material and heavy metals. However, phytoremediation does not reduce the levels of mercury and cadmium in the soil; it simply changes their forms. For paddy field soil, planting hyperaccumulator plants to repair the soil is less feasible. On the one hand, because rice is the staple food of local residents, it is difficult to get local people to change their diet structure to plant hyperaccumulator plants that are inedible and have low economic value in the short term. On the other hand, the restoration cycle of this technology is long and no plants that co-hyperaccumulate mercury and cadmium have been found so far.

[0007] In summary, existing methods for preventing and controlling mercury / cadmium compound-contaminated soils are unable to achieve safe utilization of mercury / cadmium compound-contaminated soils due to the aforementioned deficiencies. Furthermore, most of the existing inorganic, organic, and microbial passivators that can effectively passivate mercury and cadmium are still in the potted plant trial stage and have not been promoted in the field. Most soil passivators are generally expensive and have the potential to cause secondary pollution, which limits their large-scale application. The application of single control / remediation technologies in actual rice fields is not very effective, and the remediation efforts are limited.

[0008] Therefore, there is an urgent need to develop a safe utilization method for mercury / cadmium compound-contaminated rice fields, which can produce rice that meets national safety standards in mercury / cadmium compound-contaminated rice fields and ensure food security. Summary of the Invention

[0009] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for the safe utilization of rice fields contaminated by mercury / cadmium compounds, which can reduce the mercury / cadmium enrichment effect of rice through multiple links, thereby achieving safe rice production in rice fields contaminated by mercury / cadmium compounds.

[0010] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0011] A method for safely utilizing rice fields contaminated by mercury / cadmium complexes comprises the following steps:

[0012] According to the soil pollution characteristics of the paddy fields, the paddy fields are plowed;

[0013] Conditioning the soil after plowing;

[0014] Select low-accumulation rice varieties and plant them in conditioned rice fields;

[0015] Managing water in rice fields during rice cultivation;

[0016] Fertilization management of rice fields during rice cultivation;

[0017] Carry out leaf control from the 8th to 12th day after the rice tillering stage to the beginning of the jointing stage, the booting stage and the grain filling stage;

[0018] Harvest rice and get rice.

[0019] In specific implementation, the tillage of the paddy field soil according to the soil pollution characteristics of the paddy field includes:

[0020] For paddy fields where the mercury concentration in the top 15 cm of soil is less than 1.0 mg / kg, the soil should be plowed to a depth of 30 cm;

[0021] For the rest of the paddy fields, shallow tillage was carried out, with the soil plowed to a depth of 15 cm.

[0022] In specific implementation, the conditioning of the plowed soil includes, for acidic soil, using alkaline soil passivator for soil conditioning; for alkaline or neutral soil, using lime or calcium magnesium phosphate for soil conditioning.

[0023] The alkaline soil passivator is preferably iron-based biochar.

[0024] Iron-based biochar was used to condition soils with mercury concentrations of 0.6-2.0 mg / kg and cadmium concentrations of 0.10-0.20 mg / kg. Cadmium is a highly mobile heavy metal, with a high transfer rate from soil to plant stems and rice (i.e., a high enrichment coefficient). Although cadmium concentrations in soil did not exceed national soil environmental quality standards, cadmium levels in rice were excessive. Therefore, even if cadmium concentrations in soil are below the control standard, the risk of contamination remains significant. Iron-based biochar, with its large surface area and abundant functional groups, can adsorb large amounts of mercury and cadmium, reducing their availability and thereby simultaneously reducing the combined contamination caused by these elements. Furthermore, iron-based biochar can rapidly increase soil pH, reaching a pH of 12 or above (with a maximum limit of 14), making it particularly effective in improving acidic soils.

[0025] For soils with mercury and / or cadmium concentrations exceeding the above ranges, in addition to applying iron-based biochar for soil conditioning, it is necessary to maintain a 3-5 cm water layer on the field surface between the heading and maturity stages of the rice, and to begin draining the field after the rice reaches maturity. Foliar fertilizers containing sodium selenite and Metarhizium anisopliae should be applied simultaneously during the rice's tillering, booting, heading, and grain filling stages. Iron-based biochar, with its inherent pH of 12-14, can raise the pH of paddy soil, thereby increasing the amount of exchangeable cations in the soil and converting cadmium from an acid-extractable form to a more stable residual form. This reduces cadmium absorption by rice and lowers the cadmium accumulation coefficient, thereby reducing cadmium uptake by the aboveground rice. Furthermore, by applying more foliar fertilizers throughout the rice's growth period, the risk of heavy metal mercury and cadmium contamination can be further reduced.

[0026] Preferably, the screening of low accumulation rice varieties includes:

[0027] Select rice varieties based on the main rice varieties and planting habits in the area;

[0028] Randomly select the experimental area, divide it into plots of equal area, and plant the selected rice varieties in each plot;

[0029] The heavy metal concentrations of rice harvested after planting in each plot were analyzed, and rice varieties with a rice enrichment coefficient less than 1 and a rice yield reduction of no more than 10% compared with the rice yield in conventional unpolluted rice fields were selected.

[0030] Preferably, the water management of the paddy field during the rice planting process includes:

[0031] For soils with mercury concentrations of 0.6-2.0 mg / kg, alternate wet-drying cycles are implemented during the rice growing season, and paddy fields are drained during the rice maturation period.

[0032] For the remaining soils, paddy fields were drained only during the rice maturity period.

[0033] Among them, the alternating wet-dry water management is specifically as follows: shallow water irrigation during rice transplanting, controlling the water layer height of the rice field to 2-5 cm during rice tillering, drying the field after the rice seedlings have grown, and deep water irrigation during the rice heading stage, controlling the water layer height to 5-10 cm.

[0034] Drainage management involves laying out drainage ditches at the lowest point of the farmland or on the side closest to the drainage canal. The length and width of the drainage ditch are determined based on the farmland area and drainage needs. The ditch is excavated manually, maintaining a slope to ensure smooth water flow. Weeds and debris are regularly cleared from the ditch to ensure it remains unobstructed. Drainage operation involves manually opening the sluice gates to allow water from the paddy field to flow into the nearby canal. The gates are closed when the water level in the paddy field reaches the required level.

[0035] Because the soil in the rice field is in a reducing state when it is irrigated, reducing bacteria can easily reduce sulfate into sulfide or sulfur ions, forming stable cadmium sulfide precipitates with cadmium ions; after flooding, crystalline iron oxide not only adsorbs cadmium, but also promotes the fixation of cadmium by soil organic matter, converting the exchangeable cadmium into an organically bound state, thereby reducing the absorption of cadmium by the aboveground parts of rice.

[0036] Preferably, the soil fertilization management during the rice planting process includes:

[0037] The application amount of fertilizer and the ratio of nitrogen, phosphorus and potassium are determined according to the requirements of rice soil testing formula and fertilizer quota technology.

[0038] Preferably, the foliar control from the 8th to 12th day after the tillering stage to the beginning of the jointing stage, the booting stage and the filling stage of the rice includes:

[0039] From the 10th day after tillering to the beginning of jointing, heading and grain filling stages, spray foliar fertilizer on rice 2-3 times, with an interval of 7-10 days between each application.

[0040] Preferably, the foliage fertilizer includes sodium selenite, Metarhizium anisopliae, Daleweiwang or Dongjie amino acid.

[0041] Preferably, the spraying concentration of sodium selenite is 5 mg / L, and the dosage is 0.4 g / mu; the dosage of Metarhizium anisopliae is 80 ml / mu; the dosage of Dialevitra is 800 ml / mu; and the dosage of Dongjie amino acid foliar fertilizer is 200 ml / mu.

[0042] Compared with the prior art, the present invention has the following advantages:

[0043] The method for safe utilization of rice fields with combined mercury / cadmium pollution provided by the present invention reduces the rice / soil enrichment coefficient by adopting a multi-process integrated technology of low-accumulation rice varieties-controlling soil pH-agronomic management-selenium-mercury antagonism / microbial combined enhancement, thereby reducing the accumulation of mercury and cadmium elements in rice, so that the produced rice meets the national safety production standards and at the same time increases the rice yield. Among them, selenium and mercury have similar chemical properties, and there is a competitive relationship in the chemical reaction, which can produce antagonism. Therefore, the accumulation of mercury in rice is reduced by the antagonism between selenium and mercury; or by reducing the activity of mercury by Metarhizium anisopliae. By alternately managing the wet and dry irrigation of rice fields and controlling the soil pH, the bioavailability of cadmium is reduced, thereby reducing the accumulation of cadmium in rice. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG1 is a graph showing the total mercury and total cadmium contents in soil after rice harvest in an embodiment of the present invention, wherein different lowercase letters indicate significant differences (P < 0.05).

[0045] FIG2 is a graph showing the total mercury and total cadmium contents in rice according to an embodiment of the present invention, wherein different lowercase letters indicate significant differences (P < 0.05).

[0046] FIG3 shows the enrichment coefficients of total mercury and total cadmium in different rice varieties according to the present invention. In the figure, enrichment coefficient = total mercury / cadmium content in rice ÷ total mercury / cadmium content in soil. Different lowercase letters indicate significant differences (P < 0.05). DETAILED DESCRIPTION

[0047] The embodiment of the present invention discloses a method for safely utilizing rice fields contaminated by mercury / cadmium complex, comprising the following steps:

[0048] According to the soil pollution characteristics of the paddy fields, the paddy fields are plowed;

[0049] Conditioning the soil after plowing;

[0050] Select low-accumulation rice varieties and plant them in conditioned rice fields;

[0051] Managing water in rice fields during rice cultivation;

[0052] Fertilization management of rice fields during rice cultivation;

[0053] Carry out leaf control from the 8th to 12th day after the rice tillering stage to the beginning of the jointing stage, the booting stage and the grain filling stage;

[0054] Harvest rice and get rice.

[0055] The method for safely utilizing rice fields contaminated with mercury and cadmium, disclosed in the embodiments of this invention, is primarily targeted at pristine soil with a total mercury concentration ranging from 0.60 to 2.00 mg / kg and a total cadmium concentration ranging from 0.10 to 0.20 mg / kg. The method provided by this invention enables safe rice production in such pristine soil. The resulting rice meets the national food safety standard (GB 2762-2022) and increases rice yield.

[0056] During implementation, different tillage measures should be adopted based on soil contamination characteristics to loosen the soil and break up the plow layer, promoting rice root growth. For areas where mercury concentrations in the top 15 cm of soil are less than 1.0 mg / kg and where deep plowing is suitable for safe utilization, it is recommended to use a high-powered crawler tractor-tillager for deep plowing of the tillage layer, with the plow's penetration depth maintained at a stable 30 cm. During deep plowing, the soil should be moist. If the field surface is waterlogged, the water should be removed two days before plowing to keep the soil moist. The plowing route should be designed based on the size and shape of the field, preferably a circular route. Maintain a steady tractor movement during plowing. For other areas, shallow tillage should be performed to a depth of 15 cm. Deep plowing involves significant soil movement, resulting in high energy consumption and operational costs, as well as high tractor power requirements. Furthermore, deep plowing creates surface furrows that are difficult to level, impacting seeding quality. Therefore, shallow plowing to a depth of 15 cm is recommended for areas unsuitable for deep plowing. In particular, soil with a higher pollution level in the bottom layer than in the surface layer should not be deep plowed, as this will bring pollutants from the bottom layer to the surface of the soil, increasing the risk of excessive pollutants in rice.

[0057] When conditioning soil, for acidic soils, choose an alkaline soil passivator containing calcium, magnesium, and silicon (preferably one with a pH greater than 9). The application rate of soil passivator should be determined based on the soil's physical and chemical properties, the degree of contamination, and the rice variety being planted. For soils with mild to moderate mercury contamination, the recommended application rate for iron-based biochar is 56.25 kg / mu. Sodium selenite should be applied at a rate of 192.02 g / mu. Dissolve in an appropriate amount of water and evenly pour or spray the applied solution over the plot. Ammonium molybdate should be applied at a rate of 104.17 g / mu. Apply the solution once before the experiment and supplement with sodium selenite and ammonium molybdate (at the same pre-experimental doses) during the grain filling period. Application should be carried out mechanically using a tractor or rotary tiller with a hopper attached 7-10 days before rice planting and transplanting. After application, thoroughly level the field to ensure the soil passivator is thoroughly mixed with the soil.

[0058] The selection of low-accumulation rice varieties should be based on the prevailing local rice varieties and planting practices. Varieties with relatively low heavy metal accumulation characteristics that have passed provincial variety approval or registration and are suitable for widespread cultivation should be selected. By combining the selection of low-accumulation varieties with high-yield cultivation techniques and measures, the impact of mercury and cadmium in the soil on agricultural product yield and quality can be reduced. Through small-plot trials, field verification, and demonstration area promotion, rice varieties will be selected based on an enrichment coefficient less than 1 and a yield reduction of no more than 10% compared to rice in conventional, unpolluted rice fields. These selected rice varieties will then be planted throughout the region.

[0059] For soils with mercury concentrations of 0.6-2.0 mg / kg, alternate wet-dry cycles should be implemented during the rice growing season, and paddy fields should be drained during the rice maturation period. For other soils, drainage should be applied only during the rice maturation period. For paddy fields contaminated with cadmium, where low-accumulation rice varieties were previously planted but soil deactivators were not applied, based on previously assessed risks of agricultural products exceeding standards in contaminated soil, rice-growing areas with mild cadmium contamination, where water sources are sufficient, water quality meets the "Agricultural Irrigation Water Quality Standard" (GB 5084-2021), and drainage is readily available, should maintain a 3-5 cm surface water layer during the rice filling period and not begin drainage until after maturity. During the rice filling period, foliar fertilizers such as silicon and zinc should be sprayed once at 500 ml / mu, diluted with water according to the product instructions. This will reduce cadmium accumulation in rice. A fertilizer quota system should be implemented during rice cultivation. Fertilizer application rates and nitrogen, phosphorus, and potassium ratios are determined based on soil testing and fertilizer quota requirements for rice. The total amount of fertilizer is equivalent to 14 kg of pure nitrogen per mu (approximately 1400 lb) and the total amount of fertilizer is equivalent to 23 kg of pure nitrogen per mu (approximately 100 lb). This saves over 10% of fertilizer compared to traditional fertilization. Alkaline fertilizers should be preferred. Calcium magnesium phosphate fertilizers are recommended for phosphate fertilizers, along with organic fertilizers. The recommended rate of organic fertilizer is 200-500 kg per mu (approximately 200-500 kg per mu) depending on soil fertility. Acidic or physiologically acidic fertilizers and chlorine-containing fertilizers that can activate heavy metals should be avoided. Instead, qualified nitrate nitrogen fertilizers, calcium magnesium phosphate fertilizers, potassium sulfate fertilizers, potassium silicate fertilizers, selenium fertilizers, silicon fertilizers, zinc fertilizers, or compound fertilizers containing these ingredients should be used. Furthermore, effective water management should be implemented during critical growth stages of crops to reduce heavy metal accumulation.

[0060] During rice cultivation, foliar control of mercury levels is also necessary. During key mercury control periods, such as from peak tillering to jointing, booting, and grain filling, spray foliar fertilizers containing selenium two to three times around 4 p.m. on clear days with consistent sunny weather for the next few days. Leave 7-10 days between each application. Refer to the foliar fertilizer instructions for specific application rates. For cultivated soils with mild to moderate mercury contamination, it is recommended to apply sodium selenite at a concentration of 5 mg / L and a dosage of 0.4 g / mu; Metarhizium anisopliae (a microbial pesticide) at a dosage of 80 ml / mu; or Dialeviwang at a dosage of 800 ml / mu; or Dongjie amino acid foliar fertilizer at a dosage of 200 ml / mu.

[0061] After the rice harvest, contaminated crops must be safely disposed of. Crop straw from contaminated farmland should not be directly returned to the fields for fertilizer. Instead, it should be collected and removed from the fields after the crops are harvested, leaving no stubble. In conjunction with local industrial development, it can be used as biomass fuel or other resource-based disposal options.

[0062] If heavy metal levels in rice produced from contaminated farmland exceed the limits for contaminants in food (GB 2762-2022), the rice should be removed from the food market and agricultural products exceeding the limits should not be used as food ingredients. Rice that does not meet the standards should be stored in designated warehouses and sold in a targeted manner. Example

[0063] The soil type in the experimental area for screening rice varieties with low heavy metal accumulation is yellow-spotted purple mud fields with a clayey texture. Testing revealed that the soil was generally acidic (range, 4.89-6.68, average, 5.33). Soil organic matter content was moderate (average, 2.1%), and relatively low in nitrogen and phosphorus (total nitrogen, 0.15%, available phosphorus, 6.6 mg / kg). Potassium was abundant (available potassium, 312 mg / kg). Total mercury content was 1.84 ± 0.57 mg / kg, and total cadmium content was 0.15 ± 0.02. The soil pollution risk screening values ​​for agricultural land specified in the "Standard for Soil Pollution Risk Control in Agricultural Land (Trial)" (GB15618-2018) were used as the basis (pH ≤ 5.5, paddy field standard, Cd standard of 0.3 mg / kg, and Hg standard of 0.5 mg / kg).

[0064] The experiment used a completely randomized block design, with 75 plots divided into three blocks, each containing 25 plots of 5 m × 5 m. Twenty-five rice varieties were planted in each of the 25 plots within each block: Yongyou 17, Yongyou 1540, Yongyou 7860, Jiafengyou 2, Zhejingyou 1578, Zhongzheyou 8, Yongyou 15, Yongyou 12, Zheyou 18, Zhejing 100, Nanjing 46, Zhenuo 106, Zhongjia 8, Xiushui 519, Xiushui 14, Xiushui 121, Jiahe 247, Jia67, Ningruanxiang 85, Ning68, Ning77, Ning75, Ningnuo 69, Ning84, and Ning88.

[0065] After rice harvest, total mercury and total cadmium levels in the soil are shown in Figure 1. Different lowercase letters indicate significant differences (P < 0.05). Although rice's absorption and accumulation of mercury reduces soil total mercury levels, background soil total mercury levels are still relatively high. Even after safe utilization during rice cultivation, soil total mercury levels remained above 0.5 mg / kg (average 1.84 mg / kg), exceeding the standard, as shown in Figure 1(a). Therefore, long-term, stable, and safe utilization is necessary. In contrast, as shown in Figure 1(b), soil total cadmium levels were all below 0.3 mg / kg, remaining within the standard.

[0066] The total mercury and total cadmium contents in rice are shown in Figure 2. Different lowercase letters indicate significant differences (P < 0.05). Figure 2 shows significant differences in the total mercury and total cadmium contents of rice. As shown in Figure 2a, Ning 75, Yongyou 17, Ningruanxiang 85, Yongyou 1540, and Zhejingyou 1578 rice varieties have relatively low mercury contents (below 0.005 mg / kg). Ning 68, Ning 84, Ning 77, Nanjing 46, and Ningnuo 69 rice varieties have moderate mercury contents (below 0.007 mg / kg). Xiushui 14, Xiushui 519, Ning 88, and Zhongjia 8 rice varieties have relatively high mercury contents (above 0.008 mg / kg). Overall, the mercury content in rice of these rice varieties did not exceed the limit standard for total mercury (0.02 mg / kg) in the "National Food Safety Standard Limit of Contaminants in Food" (GB 2762-2022).

[0067] As shown in Figure 2b, the cadmium accumulation levels in rice grains of different rice varieties are within the standard, with the exception of a few rice varieties (such as Zheyou 18 and Zhejingyou 1578) where small amounts of cadmium exceeded the standard. Overall, indica-japonica hybrid rice varieties have a much higher cadmium accumulation capacity than conventional rice varieties.

[0068] Figure 3 shows the accumulation coefficients of total mercury and total cadmium in different rice varieties. In the figure, accumulation coefficient = total mercury / cadmium content in rice grains / total mercury / cadmium content in soil. Different lowercase letters indicate significant differences (P < 0.05). As shown in Figure 3a, the order of mercury accumulation among different rice varieties is as follows: Ning 75 < Yongyou 15 < Yongyou 17 < Zhenuo 106 < Ning 77 < Ning 68 < Ningruanxiang 85 < Zhejing You 15 < 78 < Jiafeng You 2 < Zhongzhe You 8 < Yongyou 15 < Zheyou 18 < Yongyou 78 < Ning 88 < Xiushui 121 < Ningnuo 69 < Jia 67 < Nanjing 46 < Ning 84 < Yongyou 12 < Zhejing 100 < Xiushui 519 < Jiahe 247 < Zhongjia 8 < Xiushui 14. Therefore, for areas with lightly to moderately mercury-contaminated cultivated soil, we recommend giving priority to rice varieties with low accumulation of heavy metal mercury, such as Ning 75, Yongyou 17, Ningruanxiang 85, and Yongyou 1540. These varieties can significantly reduce the risk of excessive mercury in rice and ensure the safe production of rice.

[0069] As shown in Figure 3b, the order of cadmium accumulation is Ningnuo 69 < Jia 67 < Ning 88 < Zhongjia 8 < Jiahe 247 < Zhejing 100 < Zhejing 106 < Xiushui 14 < Xiushui 519 < Zhejing 106 < Yongyou 17 < Ning 68 < Xiushui 121 < Ningruanxiang 85 < Yongyou 15 < Nanjing 46 < Ning 75 < Ning 84 < Ning 77 < Yongyou 1540 < Zhongzheyou 8 < Jiafengyou 2 < Yongyou 12 < Zheyou 18 < Yongyou 7860 < Zhejing You 1578. Among these, indica-japonica hybrid rice varieties with relatively high cadmium accumulation include Zhejing You 1578, Yongyou 7860, Zheyou 18, Yongyou 12, and Jiafengyou 2.

[0070] A comparative analysis of yields among different rice varieties is shown in Table 1. Table 1 shows that yields among different rice varieties are relatively similar, but significant differences exist. The order of rice yield, from highest to lowest, is: Zheyou 18 > Yongyou 1540 > Zhejing You 1578 > Yongyou 12 > Nanjing 46 > Zhejing 100 > Ning 75 > Jiahe 247 > Jia67 > Yongyou 7860 > Xiushui 519 > Xiushui 14 > Xiushui 121 > Ning 88 > Ning 68 > Jiafeng You 2 > Ning 84 > Ningruanxiang 85 > Ningnuo 69 > Yongyou 15 > Yongyou 17 > Zhongzhe You 8 > Ning 77 > Zhongjia 8 > Zhenuo 106. Zheyou 18 had the highest yield, 3174.84 kg / hectare higher than Zhenuo 106 (Table 1). Overall, the yield of indica-japonica hybrid rice is higher than that of conventional japonica rice.

[0071] Table 1 Yield and significant differences of different rice varieties

[0072] Rice variety yield (kg / hectare) Standard deviation Significance label Variety type Zhe Nuo 1067534.62750.16g Conventional japonica rice Zhongjia 8 7789.611271.34fg Conventional japonica rice Ning 778219.59339.06efg Conventional japonica rice Zhongzheyou 8 8304.58995.43efg Indica hybrid rice Yongyou 178319.58743.99efg Indica-japonica hybrid rice Yongyou 158384.581612.12defg Indica-japonica Hybrid rice Ningnuo 698654.57432.64defg Conventional japonica rice Ningruanxiang 858734.56210.70cdefg Conventional japonica rice Ning 848744.56532.63cdefg Conventional japonica rice Ning 688784.56482.86bcdefg Conventional japonica rice Jiafengyou 2 8784.561269.57bcdefg Indica-japonica hybrid rice Ning 888949.55834.63bcdefg Conventional japonica rice Xiushui 1219104. 541065.58bcdefg conventional japonica rice Xiushui 149154.54322.51abcdef conventional japonica rice Xiushui 5199159.54181.44abcdef conventional japonica rice Yongyou 78609169.5462.45abcdef indica-japonica hybrid rice Jia 679214.54173.20abcdef conventional japonica rice Jiahe 2479269.54913.48abcdef conventional japonica rice Ning 759414.53789. 33abcdeConventional japonica rice Zhejiang Jing 1009444.53816.78abcdeConventional japonica rice Nan Jing 469554.52657.92abcdeConventional japonica rice Yong You 129934.501332.35abcdIndica-japonica hybrid rice Zhejiang Jing You 157810279.49720.17abcIndica-japonica hybrid rice Yong You 154010359.48383.68abIndica-japonica hybrid rice Zhejiang You 1810709.461106.19aIndica-japonica hybrid rice

[0073] Note: Different lowercase letters indicate significant differences (P < 0.05).

[0074] In summary, considering rice yield, Zhejingyou 1578, Nanjing 46, and Ning 84 are recommended for soils with mild to moderate mercury contamination; Ning 88, Xiushui 519, Xiushui 14, and Nanjing 46 are recommended for soils with mild to moderate cadmium contamination. Nanjing 46, Ning 88, and Ning 84 are recommended for soils with combined mercury and cadmium contamination.

[0075] It can be seen that the method for safe utilization of mercury / cadmium complex-contaminated rice fields provided by the present invention reduces the rice / soil enrichment coefficient by adopting a multi-process integrated technology of low-accumulation rice varieties - regulating soil pH - agronomic management - selenium-mercury antagonism / microbial combined enhancement, thereby reducing the accumulation of mercury and cadmium elements in rice, making the produced rice meet the national safety production standards, and at the same time increasing rice yield, which is worthy of promotion and use.

[0076] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for safely utilizing rice fields contaminated by mercury / cadmium, characterized in that: The method comprises the following steps: plowing the soil of the paddy field according to the soil pollution characteristics of the paddy field; Conditioning the soil after tillage; Select low-accumulation rice varieties and plant them in conditioned rice fields; Water management in rice fields during rice cultivation; Fertilization management of rice fields during rice cultivation; Carry out leaf control from the 8th to 12th day after the rice tillering stage to the beginning of the jointing stage, the booting stage and the grain filling stage; Harvest rice and get rice; The tillage of the paddy field soil according to the soil pollution characteristics of the paddy field includes: for paddy fields where the mercury concentration in the top 15 cm of the soil is less than 1.0 mg / kg, the tillage depth is 30 cm; For the rest of the rice fields, shallow tillage was carried out, with the soil plowed to a depth of 15 cm; The conditioning of the tilled soil includes, for acidic soil, using an alkaline soil passivator to condition the soil; For alkaline or neutral soils, use lime or calcium magnesium phosphate fertilizers for soil conditioning; The alkaline soil passivator is iron-based biochar; The screening of low-accumulation rice varieties includes selecting rice varieties according to the main rice varieties and planting habits in the local area; Randomly select the test area, divide it into small plots of equal area, and plant the selected rice varieties in each small plot; The heavy metal concentration of rice harvested after planting in each plot was analyzed, and rice varieties with a rice enrichment coefficient less than 1 and a rice yield reduction of no more than 10% compared with the rice yield of conventional unpolluted rice fields were selected; The foliar control from the 8th to 12th day of the tillering stage to the beginning of the jointing stage, the booting stage and the filling stage of the rice includes spraying foliar fertilizer on the rice 2-3 times from the 8th to 12th day of the tillering stage to the beginning of the jointing stage, the booting stage and the filling stage, with an interval of 7-10 days between each application; the foliar fertilizer includes sodium selenite, Metarhizium anisopliae, Dalvevan or Dongjie amino acid; The spraying concentration of the sodium selenite is 5 mg / L, and the dosage is 0.4 g / mu; the dosage of the green muscardine fungus is 80 ml / mu; the dosage of Daleweiwang is 800 ml / mu; and the dosage of Dongjie amino acid is 200 ml / mu.

2. The method for safe utilization of rice fields contaminated by mercury / cadmium complex according to claim 1, characterized in that: The water management of the paddy field during the rice planting process includes: for the soil with a mercury concentration of 0.6-2.0 mg / kg, alternately managing the water between wet and dry during the rice growth period, and draining the paddy field during the rice maturity period; For the rest of the soils, paddy fields were drained only during the rice maturity period.

3. The method for safe utilization of rice fields contaminated by mercury / cadmium complex according to claim 1, characterized in that: The soil fertilization management during the rice planting process includes determining the application amount of fertilizer and the nitrogen, phosphorus and potassium ratio according to the rice soil testing formula and the requirements of fertilizer quota technology.

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