A method for reducing mercury content in brown rice from lightly to moderately mercury-contaminated paddy fields.

Abstract

A method for reducing the mercury content in brown rice from lightly to moderately mercury-contaminated paddy fields includes the following steps: 30-40 days after transplanting, intermittently drying the field at the end of the tillering stage, ensuring a total of 10 days of dryness regardless of rainfall. The degree of drying is determined by cracks in the paddy field, with rice leaves extending upwards in a sword-like shape and feeling prickly to the touch. After the waxy ripening stage, the field is left to dry naturally without irrigation. Water control is maintained throughout the growing season: moist irrigation is applied from the heading stage to the end of the grain-filling stage, with each irrigation water level not exceeding 2 cm. This field-drying method is not only an environmentally friendly agricultural practice but also demonstrates a significant synergistic effect in ensuring food security and improving rice quality, providing a new approach and effective method for solving heavy metal pollution problems in rice production.

Description

Technical Field

[0001] This invention belongs to the field of heavy metal pollution prevention and control technology for crops, specifically involving a method for reducing the mercury content in brown rice from lightly to moderately mercury-polluted paddy fields. Background Technology

[0002] Mercury (Hg) is one of the most toxic heavy metal pollutants globally, widely distributed in various environmental media, with soil being the world's largest reservoir of mercury. Globally, soil mercury pollution is more severe in the northern half of the world than in the southern half. Mercury in soil can be classified into elemental mercury, inorganic compound mercury, and organic compound mercury. Under anaerobic conditions, inorganic mercury is easily converted into neurotoxic methylmercury (MeHg) by some sulfur-reducing bacteria, some iron-reducing bacteria, and methanogens. Numerous studies have shown that methylmercury in rice grains mainly originates from the soil, and that soil methylmercury is primarily formed through in-situ methylation of inorganic mercury. Rice cultivation inevitably requires irrigation, which alters the soil's redox potential, affecting the formation of manganese / iron compounds and their adsorption of mercury. The prolonged wetland state of irrigated rice paddies enhances the activity of anaerobic microorganisms and increases methane production, creating an active site for methylmercury. Therefore, compared to other crops, rice absorbs and accumulates more mercury, posing a significant risk of exceeding safe levels.

[0003] Current research indicates that proper soil moisture management is a crucial way to reduce the accumulation of total mercury in crop grains in mercury-polluted areas. However, paddy field irrigation is generally required for rice cultivation. This is not only to ensure the water needs of rice throughout its growth cycle but also to create a suitable ecological environment for rice growth and development. Under flooded conditions, the soil is in a reduced state, organic matter decomposes more slowly and accumulates more easily, which is beneficial for soil fertility retention and supply. Therefore, using inappropriate paddy field management methods for rice water control may lead to a decrease in rice yield. Summary of the Invention

[0004] Technical Problem Solved: To address the issue that continuous flooding throughout the rice growing season exacerbates mercury pollution in paddy fields, leading to mercury absorption and accumulation in rice and ultimately resulting in excessive mercury content in brown rice. This invention provides a method for reducing mercury content in brown rice from lightly to moderately mercury-contaminated paddy fields. This method effectively reduces mercury content in brown rice without significantly impacting the yield of this rice variety.

[0005] Technical solution: A method for reducing the mercury content in brown rice from lightly to moderately mercury-polluted paddy fields by drying the fields, the steps of which include: 30-40 days after transplanting, intermittently drying the fields at the end of the tillering stage, ensuring a total of 10 days of dryness regardless of rainfall, with the degree of drying determined by cracks in the paddy field, rice leaves extending upwards in a sword shape, and feeling prickly to the touch; drying the fields after the waxy ripening stage, stopping irrigation, and allowing the paddy fields to dry naturally; water control during the growth period: moist irrigation is implemented from the heading stage to the end of the grain filling stage, with each irrigation water layer not exceeding 2 cm.

[0006] Preferably, the above-mentioned lightly to moderately mercury-contaminated paddy fields have a soil mercury concentration of 0.6 to 1.6 mg / kg.

[0007] Preferably, the field is left to dry after the waxy ripening period, and drainage is carried out when it rains.

[0008] Preferably, water is controlled and drainage is carried out in a timely manner during the above-mentioned growth period to prevent the paddy field from maintaining a high water level for a long time after rainfall, that is, the water layer does not exceed 2 cm.

[0009] Preferably, the above-mentioned rice varieties are Suxiangjing (Sujing) 1176, Chunyou 927, Ningxiangjing 9, Changxiangjing 1813, Xiushui 121 or Nanjing 46.

[0010] Preferably, the above-mentioned field-keeping method is applicable to the southern Jiangsu region.

[0011] Preferably, the above-mentioned rice is treated with herbicide at least once after transplanting.

[0012] Beneficial Effects: The paddy field drying method described in this invention is specifically designed for paddy fields in Jiangsu and Zhejiang provinces, showcasing an innovative agricultural management strategy aimed at achieving a dual improvement in yield and quality, demonstrating significant synergistic effects. The core of this method lies in the scientific regulation of paddy field water management, specifically implementing appropriate drying measures at specific growth stages. This not only ensures the normal growth and development of rice and stable yield, but more importantly, this measure effectively promotes the natural degradation and migration of harmful elements in the soil, such as mercury, significantly reducing the mercury content in brown rice, thereby improving the safety and health quality of the rice. Specifically, during the drying process, moderate soil drying is conducive to enhancing the activity of soil microorganisms. These microorganisms can transform or fix some mercury elements during their metabolic activities, reducing the chance of it being absorbed by the rice roots. Simultaneously, the improvement in soil physicochemical properties also promotes the transformation of mercury forms, reducing its bioavailability. Furthermore, the field-drying process promotes deeper and stronger rice root systems, enhances the plant's efficient absorption and utilization of nutrients, and indirectly improves the rice's resistance to adverse environmental conditions, including its ability to combat heavy metal pollution, thus ensuring the simultaneous improvement of rice yield and quality. In summary, the field-drying method proposed in this invention is not only an environmentally friendly agricultural practice, but also demonstrates a significant synergistic effect in ensuring food security and improving rice quality, providing a new approach and effective method for solving the problem of heavy metal pollution in rice production. Attached Figure Description

[0013] Figure 1 This is a comparison chart of the mercury content in brown rice of Suxiangjing (Sujing) 1176 under different field conditions during the field investigation in Example 1.

[0014] Figure 2 This is a comparison chart of the mercury content in brown rice of Nanjing 3908 under different field storage methods in Example 1.

[0015] Figure 3 This is a comparison chart of mercury content and BCF (bioaccumulation factor) in Changxiangjing 1813 brown rice under different field management methods in Example 2.

[0016] Figure 4 This is a comparison chart of the yield of Changxiangjing 1813 under different field resting methods in Example 2.

[0017] Figure 5 This is a comparison chart of the mercury content in brown rice of five different rice varieties under different paddy drying methods in Example 2.

[0018] Figure 6 This is a comparison chart showing the yields of five different rice varieties under different paddy field management methods in Example 2.

[0019] Figure 7This is a comparison chart of mercury content and cumulative amount in Xiushui 121 plants under different water layer heights in Example 3.

[0020] Figure 8 This is a comparison chart of mercury content in Nanjing 46 brown rice under four different rice storage methods in Example 4.

[0021] Figure 9 This is a comparison chart of the yield of Nanjing 46 under four different field resting methods in Example 4.

[0022] Figure 1 In the bar chart, the asterisks are labeled as follows:

[0023] * indicates the results of an independent samples t-test, with a p-value < 0.05, meaning there is a statistically significant difference between the two groups.

[0024] Figure 2 , Figure 4 and Figure 9 In the bar chart, the letter labels are explained below:

[0025] NS- indicates that there are no significant differences between the included data. Detailed Implementation

[0026] This invention discloses a method for reducing the mercury content in rice from lightly to moderately mercury-contaminated paddy fields. To better explain this invention, specific implementation examples are provided below for detailed description.

[0027] Example 1

[0028] Test period: October 2021

[0029] Test location: Southern Jiangsu region

[0030] This example involves field sampling conducted in October 2021, during the rice harvest season, in the southern Jiangsu region for two different rice varieties.

[0031] ① Plot 1

[0032] Rice variety: Suxiangjing (Sujing) 1176

[0033] Soil properties: pH 6.43; Soil mercury content 1.02 mg / kg.

[0034] Timing for drying the field: 30-40 days after transplanting rice seedlings

[0035] ② Plot 2

[0036] Rice variety: Nanjing 3908

[0037] Soil properties: pH 5.96; Soil mercury content 2.32 mg / kg.

[0038] Timing for drying the field: 30-40 days after transplanting rice seedlings

[0039] Figure 1 and Figure 2 In the middle, the criteria for judging the dry field group is that the soil surface turns white and the cultivated land is dry and cracked during the rice harvest period; the criteria for judging the flooded group is that the soil is still slightly moist during the rice harvest period.

[0040] This field sampling mainly revealed that under different water and fertilizer management methods, the concentration of mercury in Suxiangjing 1176 (a type of glutinous rice) in farmland with mild to moderate mercury contamination was significantly higher than that in other crops. Figure 1 There were statistically significant differences in mercury content in brown rice; Nanjing 3908 ( Figure 2 There was no significant difference in mercury content in brown rice.

[0041] Example 2

[0042] This example is divided into two blocks. One block compares flooding and dryland treatments. The other block shows the reduction of mercury content in five rice varieties under dryland conditions. Each treatment is a randomized block experiment with three replicates.

[0043] Trial period: June to November 2022

[0044] Test location: Southern Jiangsu region

[0045] Soil properties: pH 6.68±0.17; Soil mercury content 0.89±0.13 mg / kg.

[0046] ①Water Management Zone

[0047] Rice variety: Changxiangjing 1813

[0048] Flooding treatment: After the tillering stage, the paddy field is left to dry for about 8 days, with a low degree of dryness and no soil cracking. After the waxy ripening stage, the field is left to dry, that is, after the grain filling stage, no more irrigation is needed. If it rains, the paddy field is drained naturally. During the growing season, the water level is maintained at more than 5 cm for flooding irrigation.

[0049] Field drying treatment: Dry the field for about 10 days at the end of the tillering stage. The degree of drying is such that the paddy field cracks, the rice leaves extend upwards in a sword shape, and feel prickly to the touch. After the waxy ripening stage, that is, after the grain filling is completed, stop irrigating and drain the paddy field naturally when it rains. During the growing season, keep the water layer at least 2 cm moist during irrigation and expose the field to the elements appropriately.

[0050] Within the water management blocks, under the condition that the rice variety was Changxiangjing 1813, there were significant differences in the bioaccumulation coefficient of mercury in brown rice. Figure 3 Meanwhile, there was no statistically significant difference in yield. Figure 4Compared with the flooded group, the average mercury content in brown rice in the dry-field group decreased by 59.38%; the average mercury content in brown rice in the dry-field group was 37.5% lower than the national standard limit (GB 2762—2022 "National Food Safety Standard Limits for Contaminants in Food", mercury content in brown rice <20μg / kg).

[0051] ② Field-stayed variety screening group

[0052] Rice varieties: Chunyou 927, Ningxiangjing 9, Changxiangjing 1813, Xiushui 121 and Nanjing 3908

[0053] Water management during the growing season: Same as the dryland treatment in the water management block experiment.

[0054] Under the field drying treatment, Ningxiangjing 9 had the lowest bioaccumulation coefficient compared to the other four rice varieties. Figure 5 The mercury content in brown rice of Chunyou 927, Ningxiangjing 9, Changxiangjing 1813, Xiushui 121, and Nanjing 3908 was significantly lower than that of Chunyou 927, Ningxiangjing 9, Changxiangjing 1813, and Xiushui 121 (p < 0.05). The mercury content in brown rice of Chunyou 927, Ningxiangjing 9, Changxiangjing 1813, and Xiushui 121 did not exceed the standard (mercury content in brown rice < 20 μg / kg); only Nanjing 3908 exceeded the standard under the dry field treatment, which also corroborates the result in Example 2 that there was no significant difference in mercury content in brown rice of Nanjing 3908 under dry field and flooded conditions. Figure 2 It is speculated that the technique of using the dry field treatment to reduce the mercury content in brown rice is only applicable to some rice varieties, and other rice varieties need to be verified through experiments.

[0055] Compared with the yield per mu approved in Jiangsu Province, the field-stopping treatment did not have a significant impact on the yield of the five rice varieties. Figure 6 ).

[0056] Example 3

[0057] This example is a flooding experiment of potted rice. The experimental design was to raise the ceramic pots using anti-mildew and waterproof silicone sealant under the same soil background values, ambient temperature and humidity, fertilization and pesticide application, and rice variety (i.e., Xiushui 121) to ensure that the maximum water level during the rice growth period was 3cm, 6cm and 9cm respectively. Each treatment was a 3-replica randomized block experiment, hereinafter referred to as 3cm, 6cm and 9cm.

[0058] Test location: Jiaxing City, Zhejiang Province

[0059] Soil collection location: Southern Jiangsu region

[0060] Trial period: June to November 2022

[0061] Soil properties: pH 4.76; Soil mercury content 0.68 mg / kg.

[0062] The key points of the three treatment experiments are as follows:

[0063] 3cm - Ensure that the water level of Xiushui 121 is maintained at 3cm for a long time during its growth and development period. When the water level drops to the point where the soil surface is slightly dry and cracked, re-irrigate to a water level of 3cm.

[0064] To ensure that the water level of Xiushui 121 is maintained at 3-6 cm throughout its growth and development period, the 6 cm block is also irrigated to the maximum water level of 6 cm each time the 3 cm treatment group is irrigated. This is to prevent the water level from dropping below 3 cm during the rice's growth and development period.

[0065] To ensure that the water level of Xiushui 121 is maintained at 6-9 cm throughout its growth and development period, the 9 cm block is also irrigated to the maximum water level of 9 cm during each irrigation of the 3 cm treatment group. This is to prevent the water level from dropping below 6 cm during the rice's growth and development period.

[0066] The total mercury content of rice roots, stems, leaves, brown rice, and husks was determined by collecting samples during the harvest period. Figure 7 ), and calculate the cumulative amount of mercury in the tissue according to the following formula.

[0067] Accumulated mercury content in rice tissues (μg) = Mercury content in rice tissues (μg / kg) × Dry weight (kg)

[0068] The results of this pot experiment showed no significant differences in mercury content in roots, stems, leaves, brown rice, and glumes among the three treatments. The mercury accumulation in leaf tissues in the 3cm block was significantly lower than in the 6cm and 9cm blocks, indicating that higher irrigation height significantly increased the dry weight of rice leaf tissues but not the dry weight of rice grains. Therefore, under these experimental conditions, water level was not the most significant factor affecting the total mercury content of brown rice; frequent field exposure and increased water level did not produce significant differences in mercury content in brown rice under different water control treatments.

[0069] Example 4

[0070] Trial period: June to November 2023

[0071] Test location: Southern Jiangsu region

[0072] Rice variety: Nanjing 46

[0073] Soil properties: pH 5.96±0.13; Soil mercury content 1.39±0.18 mg / kg

[0074] Experimental Design: A total of 4 plots were set up (referred to as plot 1, 2, 3, and 4, hereinafter referred to as plot 1, 2, 3, and 4), each plot having an area of ​​540m². 2(60m long × 9m wide), with 1m wide ridges between plots. The ditches between plots are 50cm wide and 40cm deep. After the fields were laid out in methods 1, 2, and 3, natural rainfall caused the fields to be flooded for 2 days after being laid out, thus failing to achieve the expected continuous laying out, but the total number of days of laying out was 10.

[0075] The specific land clearing methods for the four communities are as follows:

[0076] Method 1 - During the tillering stage, the paddy field is left to dry for 2 days, then flooded for 2 days, and then left to dry for another 8 days, for a total of about 10 days of drying. Light water control begins during the heading stage, and the field is often left exposed until the grain filling stage is over. After the waxy ripening stage, the field is left to dry, meaning that no more water is applied after the grain filling stage, and the field is drained naturally when it rains.

[0077] Method 2 - During the tillering stage, the paddy field is left to dry for 2 days, then flooded for 2 days, and then left to dry for another 8 days, for a total of about 10 days of drying. After the waxy ripening stage, the field is left to dry naturally by draining the water when it rains. During the remaining growth stages, the field is moist irrigated, with each irrigation water layer not exceeding 2 cm.

[0078] Method 3 - During the tillering stage, the paddy field is left to dry for 2 days, then flooded for 2 days, and then left to dry for another 8 days, for a total of about 10 days of drying. After the waxy ripening stage, the field is left to dry naturally by draining the water when it rains. During the remaining growth stages, the water level should be kept no higher than 5 cm.

[0079] Method 4 - Do not dry the field at the end of the tillering stage; dry the field after the waxy ripening stage, and drain the water when it rains to allow the paddy field to dry naturally. Throughout the entire growth period, maintain a water level of 5-10 cm when irrigating.

[0080] The key points for implementing Method 1 are as follows:

[0081] Field rest period 1: During the end of tillering, the field is left to dry for 2 days, then flooded for 2 days, and then left to dry for another 8 days, for a total of 10 days of field rest.

[0082] 2. Drying out the paddy field: After the waxy ripening period, the paddy field is dried out. That is, after the grouting is completed, no more water is injected. When it rains, the water is drained to allow the paddy field to dry out naturally.

[0083] Water management during the growing season: Slight water control begins at the heading stage, with frequent exposure of the field until the grain filling stage is completed.

[0084] The key points for implementing Method 2 are as follows:

[0085] Field rest period 1: During the end of tillering, the field is left to dry for 2 days, then flooded for 2 days, and then left to dry for another 8 days, for a total of 10 days of field rest.

[0086] 2. Drying out the paddy field: After the waxy ripening period, the paddy field is dried out. That is, after the grouting is completed, no more water is injected. When it rains, the water is drained to allow the paddy field to dry out naturally.

[0087] Water management during the growing season: wet irrigation should be carried out from the heading stage to the end of the grain filling stage, and the water layer should not exceed 2 cm each time. The field can be exposed appropriately.

[0088] The test results are as follows Figure 8 Because method 1 exposes the field to weeds during the growing process, resulting in abundant weeds in the paddy field, which is detrimental to rice growth, even if the yield is not reduced ( Figure 9 However, this method is not conducive to farmer management. A comparison of method 1 and method 2 shows that ( Figure 8 and Figure 9 Excessive water control during the rice growing season, leading to open fields, will significantly reduce the mercury content in brown rice. However, method 2 can already reduce it to below the national standard limit, resulting in good rice growth and healthy paddy fields.

[0089] This implementation case involved two rounds of weeding during the rice's growth cycle: routine weeding was carried out 1-2 days before transplanting; a second routine weeding was conducted 17 days after transplanting, following tillering. It is important to ensure the field is adequately dry before applying herbicides. During the implementation process, an additional herbicide application can be applied at the end of tillering when the field is dry, depending on the rice's growth condition. This herbicide can be sprayed in conjunction with pesticides for disease and pest control.

[0090] This invention has shown significant results after implementation in paddy fields of Nanjing 46 rice in southern Jiangsu Province. Primarily targeting soils with mercury concentrations of 0.6–1.6 mg / kg, it effectively reduces the mercury content in Nanjing 46 brown rice without significantly impacting the yield of this rice variety. Compared to the commonly used method of continuous flooding irrigation throughout the entire growth period, the mercury content in brown rice decreased by 48.99%–64.09%. In paddy fields with mild to moderate mercury contamination in southern Jiangsu, this technology effectively controls the mercury content in brown rice to meet the national standard GB 2762—2022 "National Food Safety Standard - Limits of Contaminants in Food" (mercury content in brown rice <20 μg / kg).

[0091] The irrigation water in the above four examples was tested by a CMA-certified testing unit in accordance with HJ 597—2011 "Determination of Total Mercury in Water - Cold Atomic Absorption Spectrophotometry", and all met the requirements of GB 5084—2021 "Standards for Irrigation Water Quality in Farmland" (total mercury ≤ 0.001 mg / L).

Claims

1. A method for reducing the mercury content of brown rice grown in a light to moderately mercury-contaminated paddy field, characterized by, The method is suitable for the south of Jiangsu, and the steps include: 30-40 days after transplanting, intermittent field is started at the end of tillering, dry 2 days, flood 2 days, dry 8 days again, regardless of whether it rains, ensure that the total dry days reach 10 days, the degree of field is determined by the fact that the farmland cracks, the rice leaves stretch upwards and touch the hand; field after the end of the ripening period, stop watering, make the rice field dry naturally, drain when it rains; water control during the growth period: wet irrigation is implemented from the heading stage to the end of grain filling, each time the water layer does not exceed 2 cm, and the rice variety is Nanjing 46.

2. The method for reducing the mercury content of brown rice in a light to moderately mercury-contaminated paddy field according to claim 1, characterized by, The light to moderate mercury pollution rice field has a soil mercury element concentration of 0.6-1.6 mg / kg.

3. The method for reducing the mercury content of brown rice grown in a light to moderately mercury-contaminated paddy field according to claim 1, characterized by, The rice is additionally applied with herbicides at least once after transplanting.

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