Study on the Passivation Effect of Ca-Si Soil Conditioner on Heavy Metal Absorption by Rice

Yupeng ZHANG Xiaoxiao TAN Xiaoyuan CHEN Jiahui LIANG Chongjian MA Yongjun GUO Jianbing ZHOU

Abstract ［Objectives］ In order to solve the problem of heavy metal pollution in paddy fields, field experiments were carried out to study the effect of Ca-Si soil conditioner on heavy metal remediation in paddy fields.

［Methods］ A conventional planting mode (CK) was used as the control, and a Ca-Si soil conditioner (Ca-Si) treatment was set to analyze the differences in soil pH and heavy metal contents in different treatments.

［Results］ The content of cadmium in the paddy field exceeded the soil pollution risk control value of agricultural land by 59.33%. The Ca-Si soil conditioner increased the pH value of paddy field soil by 0.58 unit. The fixation rates of Ca-Si soil conditioner on chromium, arsenic, cadmium and plumbum reached 75.96%, 14.09%, 18.93% and 7.81% compared with the CK, respectively, and the available cadmium and lead contents were reduced by 82.35% and 80.00%, respectively.

［Conclusions］ This study provides ideas and references for the remediation of heavy metal pollution in paddy soil.

Key words Conditioner; Late rice; Soil pollution; Passivation

Received: January 9, 2020Accepted: March 7, 2020

Supported by Natural Science Foundation of Guangdong Province (2018A030307075); Special Fund for Science and Technology Innovation Strategy of Guangdong Province (135-99000206, 432-99000228); Science and Technology Planning Project of Shaoguan City (2018sn038); School-level Project of Shaoguan University (SY2016KJ04); Undergraduate Innovation and Entrepreneurship Project (Sycxcy2017-041, 201810576009).

Yupeng ZHANG (1989-), Male, P. R. China, lecturer, master, devoted to research about plant nutrition.

*Corresponding author. E-mail: chenxy2@126.com.

According to the survey, the over-standard rate of polluted cultivated soil reached 19.4% in China, and the over-standard rates of cadmium, mercury, arsenic, copper, lead, chromium, zinc and nickel were 7.0%, 1.6%, 2.7%, 2.1 %, 1.5%, 1.1% , 0.9% and 4.8%, respectively. Remediation of heavy metal-contaminated soil to reduce the transmission of heavy metals in the food chain is one of the effective measures to reduce the harm of heavy metal pollution to humans［1-3］. As one of the main food crops, rice plays a decisive role in China餾 food security. With the rapid development of China餾 economy, the situation of heavy metal pollution in paddy soils is becoming increasingly serious ［4-6］.

At present, there are various techniques for the remediation of heavy metal-polluted farmland, but the cost is generally high and the remediation efficiency is low［7-9］. At present, the common soil conditioning agents can be divided into three categories: inorganic conditioning agents, organic conditioning agents, and inorganic-organic mixed conditioning agents. Common inorganic conditioning agents are mainly lime, calcium carbonate, coal ash, phosphate, bentonite and inorganic silicon fertilizers; common organic conditioning agents are mainly farm manure, peat, crop straws, etc. ; and common inorganic-organic mixed conditioning agent is mainly ludge compost［10-12］.

This study was aimed at the problem of heavy metal pollution in paddy soil. The Ca-Si soil conditioner was selected for field experiments to study the passivation effect of the Ca-Si soil conditioner on heavy metals in the paddy soil. This study provides ideas for the remediation of heavy metal pollution in paddy fields.

Materials and Methods

General situation of the experimental field

The test site is located in the farmland near Dabaoshan mine, Qujiang District, Shaoguan City. It has a humid subtropical monsoon climate with an average annual temperature of 18.8-21.6 ℃ and an annual average precipitation of 1 400-2 400 mm. The annual frost-free period is 350 d and the annual sunshine duration is 1 473-1 925 h. The tested rice variety was Maba Yinnian. Manual transplanting was adopted. The basic fertility of the paddy field is shown in Table 1.

Experimental design

The test crop was late rice, which was transplanted on July 5, 2018, and harvested on October 25. The experiment consists of two treatments, namely treatment 1 (CK): conventional fertilization, and treatment 2 (Ca-Si): conventional fertilization+Ca-Si soil conditioner. Ridges were formed manually with 5 m×5 m per plot, and each treatment was repeated 3 times. The fertilizer application was the same for each treatment, involving N 211.5 kg/hm2, P2O5 60 kg/hm2 and K2O 187.5 kg/hm2, applied in three times, respectively. Specifically, the first application was performed during seeding with N 33.33%, P2O5 50% and K2O 26%; the second application was carried out 2-3 d after pest control with N 33.33%; and at the last time, N 33.33%, P2O5 50% and K2O 74% were applied during the filling stage. The fertilizers were urea (Guangxi Hechi Chemical Industry, N 46%), calcium superphosphate (Zhejiang Zhongnong Chemical Fertilizer Co., Ltd., P2O5 12%), and potassium chloride (Heilongjiang Beifeng Agricultural Production Materials Group Co., Ltd., K2O 60%). For treatment 2 (Ca-Si), the Ca-Si soil conditioner was applied at one time when plowing the padding field, at an application rate of 3 000 kg/hm2. The calcium-silicon soil conditioner was provided by Foshan Zhibao Ecological Technology Co., Ltd. It has the technical indicators of CaO≥20% and SiO2≥10%, and the Ministry of Agriculture registration certificate number: Nongfei (2017) Linzi 13312. Other field management methods were the same in the two treatments.

Research methods

Sample collection

Soil samples: Basic soil samples were collected before rice planting, and test soil samples were collected after rice harvesting in each treatment. Samples were collected using the "S" point method. Soil collected from five points was mixed into one soil sample and the sample was divided by the quarter method to remove excessive soil. After removing impurities and air-drying, the material was crushed and sieved through a 0.1 mm sieve, and finally stored in a sealed bag at a dry and cool place for testing.

Sample testing

Soil pH was measured using a pH meter (soil-water ratio 1∶5). Soil organic matter was determined by the potassium dichromate volumetric method with external oil bath heating. Soil total nitrogen was determined by Kelvin method. Soil total potassium was determined by NaOH melting-flame photometry. Soil available nitrogen was determined by alkaline hydrolysis diffusion method. Soil rapidly available phosphorus was determined by molybdenum antimony colorimetry.Soil rapidly available potassium was determined by flame photometry (Bao Shidan, 1999). Soil lead was determined according to GB/T 17141-1997. Soil chromium was determined according to HJ 491-2009. Soil cadmium was determined according to GB/T 17141-1997. Soil arsenic was determined according to GB/T 22105.2-2008. Soil mercury was determined according to GB/T 22105.1-2008. Soil available lead, chromium, cadmium, arsenic and mercury were determined by ICP-MS (soil). The passivation rate was calculated according to Passivation rate (%) = (Treatment content-Control content)/Control content×100%.

Data analysis

Data was processed and plotted with Excel software, and t test was performed using SPSS (IBM SPSS Statiatics 20) software.

Results and Analysis

Heavy metal contents in paddy field

The contents of heavy metal elements lead, chromium, cadmium, arsenic and mercury in the paddy soil were 132.95, 29.63, 2.39, 7.95 and 0.22 mg/kg, respectively; and the available lead, chromium, cadmium, arsenic and mercury contents were 0.06 , 0.001, 0.360, 0.000 1 and 0.000 6 mg/kg (Table 2), respectively. Referring to "Soil Environment Quality Risk Control Standard for Soil Contamination of Agriculture Land" (GB15618-2018), The risk control values for soil contamination of agricultural land (pH≤5.5) were as follows: Pb 400 mg/kg, Cr 800 mg/kg, Cd 1.5 mg/kg, As 200 mg/kg and Hg 2.0 mg/kg, respectively. Therefore, only the content of cadmium among the contents of heavy metal elements lead, chromium, cadmium, arsenic and mercury in paddy soils exceeded the agricultural land soil pollution risk control value, and the over-standard rate reached 59.33%.

Effects of the Ca-Si soil conditioner on heavy metal contents in soil

The results in Table 3 showed that the Ca-Si soil conditioner could increase the pH of the paddy soil. The pH values of the control treatment and the Ca-Si soil conditioner treatment were 5.56 and 6.14, respectively, i.e. , the Ca-Si soil conditioner increased the soil pH by 0.58 unit. The calcium content of the Ca-Si soil conditioner treated soil was (37.04±0.35) mg/kg, and the fixation rate reached 75.96% compared with the control treatment. Moreover, the fixation rates of arsenic, cadmium and lead were 14.09%, 18.93% and 7.81%, respectively, though the fixation effect of mercury element was not significant.

Effects of the Ca-Si soil conditioner on the contents of soil available heavy metals

The results in Table 4 showed that the available lead contents in the CK and the Ca-Si soil conditioner treatment soil were (0.05±0.01) and (0.01±0.00) mg/kg, respectively, and the available cadmium contents were (0.34±0.01) and (0.06±0.01)  mg/kg, respectively. It could be seen that the Ca-Si soil conditioner reduced the availability of available cadmium and available lead in soil by 82.35% and 80.00%, respectively. Available chromium, available arsenic and available mercury were not detected in the test soil.

Agricultural Biotechnology2020

Discussion

In the acid paddy soil region of southern China, soil heavy metals have high activity and strong mobility, and are easy to accumulate in rice plants and rice, causing the heavy metal contents in rice to exceed the standards［13］. At present, the remediation technology of heavy metal-contaminated soil can be roughly divided into 5 categories, namely isolation, solidification, reduction of toxicity, physical separation and extraction［14］. Application of soil conditioners is the most important measure to remediate heavy metal-contaminated soil in China［15-16］. On the one hand, soil conditioners play a role in improving soil quality, and can reactivate soils that have been acidified, hardened and over-fertilized. Studies have shown that soil conditioners are effective in increasing the pH of the soil, improving the structure of the soil and enhancing the adsorption capacity, thereby reducing the absorption of heavy metals by crops, especially mineral source soil conditioners that are rich in silicon, calcium, magnesium and other ions, which are good conditioners for the treatment of soil contaminated by heavy metals. Furthermore, the effect is more significant as the application rate increases［17-18］. On the other hand, the absorption of heavy metals in soil by crops is also affected by agronomic measures such as water conditions and cultivation methods［19］. The application of soil conditioners to soils polluted by heavy metals affects the form and chemical properties of soil heavy metal ions, thereby reducing the absorption of heavy metals by crops. Studies have shown that after applying soil conditioners, the contents of water-soluble and exchangeable heavy metal ions in non-rhizosphere soil were significantly reduced, and the contents of organic bound heavy metal ions were significantly increased［20］. Therefore, the application of different types of soil conditioners can affect the physical and chemical properties of the soil, and then affect the movement and transformation of exchangeable components in the soil environment［21］. In this study, Ca-Si soil conditioners can increase the pH of paddy soil, reduce the activity of available lead and cadmium in the soil, enhance the passivation of heavy metals in the soil, and reduce the migration of heavy metals to rice plants. In this study, Ca-Si soil conditioners could increase the pH of paddy soils, reduce the activity of available lead and cadmium in the soil, enhance the passivation of heavy metals in the soil, and reduce the migration of heavy metals to rice plants.

Conclusions

On the premise that the content of cadmium in the experimental paddy soil exceeded the agricultural land soil pollution risk control value by 56.67%, the calcium-silicon soil conditioner was applied, and the calcium-silicon soil conditioner increased the pH of the paddy soil by 0.58 units. The passivation rates of the conditioner to chromium, arsenic, cadmium and lead reached 75.96%, 14.09%, 18.93% and 7.81%, respectively, and the available state contents of available cadmium and lead elements were reduced by 82.35% and 80.00%, respectively.

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