Xingyuan LIU, Faxiang TIAN , Yunhe XIE , , Xionghui JI , *

1. Dazhong Agricultural Science and Technology Co. Ltd., Dongguan 523169, China;

2. Soil and Fertilizer Institute of Hunan Province, Changsha 410125, China;

3. Key Laboratory of Agri-Environment in Midstream of Yangtze Plain, Ministry of Agriculture, Changsha 410125, China;

4. Longping Branch of Central South University, Changsha 410125, China

With the rapid development of industry and agriculture and due to the effects of industrial“three wastes” and agricultural activities of their own, soil heavy metal pollution in Chinese fields has been increasingly serious[1]. Hunan is the hometown of China’s non-ferrous metals. Due to the effects of continuous development of mining, dressing,smelting and other industries and increased production of agricultural products containing heavy metals, soil heavy metal pollution has become an important environmental issue affecting the economic development in Hunan Province and human health[2].The control methods for soil heavy metal pollution mainly include leaching method, soil dressing method, electrolytic method, chemical passivation method and bioremediation method.Among them, the soil in-situ chemical immobilization remediation technology which applies mud, lime and other alkaline substances to control heavy metal contamination in farmland is the most common method. It reduces heavy metal absorption by crops through increasing soil pH value,regulating the existence forms of heavy metals in soil and reducing the mobility of heavy metals in soil[3-4]. Soil chemi cal immobilization remediation in situ is a good choice for repairing largearea soil polluted by heavy metals forits simple operation, rapidness and high efficiency. Based on the pretreatment of local alkaline industrial waste for reuse,the Dazhong Agricultural Science and Technology Co.,Ltd.developed the Tianshifu soil conditioners that can effectively improve soil acidity and reduce heavy metal activity in soil, providing different choices for the repair of different types of soil heavy metal contaminations and the improvement of soil acidity. This plays an important role in the establishment of remediation system for paddy soil polluted by heavy metals and the ensurence of food quality and safety. In addition, this also has important practical significance in promoting the harmonious development between man and nature and the ecological balance.To verity the acidity-regulating and Cd contamination-controlling effects of Tianshifu soil conditioners, field tests were carried out in Beishan Town,Changsha County and Meilinqiao Town, Xiangtan County so as to provide a basis for the large-scale promotion and application of Tianshifu soil conditioners.

Materials and Methods

Test sites

According to Environmental Quality Standards for Soils (GB 15618-1995), the paddy fields slightly or moderately polluted by Cd in Beishan Town, Changsha County and Meilinqiao Town, Xiangtan County were selected for field test.

The paddy field in Beishan(113°03′28.4″ E, 28°26′22.7″ N) was slightly polluted. It was located in the test base of Agricultural Technology Promotion Station in Beishan Town,Changsha County, Hunan Province. It has a double-season rice cropping system, and the soil was paddy soil developed from granite. In the tested soil, the total Cd content and available Cd content were 0.43 and 0.15 mg/kg,respectively. The Cd contamination in the test site in Beishan was caused by contaminated irrigation water from chemical plants in the upstream during the 1980s to 1990s. The test rice cultivar was Xiangwanxian 12.

The paddy field in Xiangtan(28°26′22.7″ E, 28°26′22.7″ N) was moderately polluted, and it was located in Meilinqiao Town, Xiangtan County, Hunan Province. It also has a double-season rice cropping system,and the tested soil was paddy soil developed from quaternary red soil. In the tested soil, the total Cd content and available Cd content were 0.84 and 0.29 mg/kg,respectively.Its contamination was mainly caused by atmospheric deposition of industrial emissions from Xiangtan and Zhuzhou.The test rice cultivar was Xiangwanxian 13.

The other soil physical and chemical properties were shown in Table 1 and Table 2.

Table 1 Basic physical and chemical properties of tested soils

Table 2 Heavy metals contents in tested soils mg/kg

Test treatments

A total of five treatments were designed, and there were three replicates for each treatment. The randomized block design was adopted.The area of each plot was 20 m2, and isolation lines were arranged between every two adjacent plots. The plastic film was blanket 30 cm below the field surface.Each plot had independent irrigation and drainage to avoid mixed irrigation or drainage. For each treatment, the application amounts of N,P2O5and K2O were 180, 60 and 90 kg/hm2, respectively. Among them,70% of the nitrogen fertilizer, all of the phosphorus fertilizer and all of the potassium fertilizer were used as basic fertilizer and applied before the transplanting, and the other 30% of the nitrogen fertilizer was companied with herbicide for top dressing and was applied 7-10 d after the transplanting.Conventional water management was adopted. During the seedling to tillering stages, the paddy fields were flooded; at the tillering stage, the paddy fields were exposed to sun; at the jointing stage, the paddy fields were flooded. Pest and disease control also adopted conventional measures. The fertilizer was applied to the soils once before plowing (more than one week before rice transplanting),and then the basic fertilizer was applied. After the plowing, the paddy fields were fully harrowed. After then, the rice seedlings were transplanted.The specific design of the five treatments was as follows:

Treatment 1: conventional fertilization(CK);

Treatment 2: conventional fertilization+1 125 kg/hm2of lime(CKCa);

Treatment 3: conventional fertilization+1 125 kg/hm2of Tianshifu soil conditioner(TSR);

Treatment 4: conventional fertilization + 750 kg/hm2of Tianshifu soil bacterial conditioner(TLR);

Treatment 5: conventional fertilization + 750 kg/hm2of Tianshifu soil conditioner + 375 kg/hm2of Tianshifu soil bacterial conditioner(TSLR).

Before the starting of the test, the basic soils were sampled. The basic phydical and chemical properties and total Cd content and available Cd content of the basic soils were determined.After the harvesting of rice, thetop 0-15 cm soil of each plot was sampled for determining soil available Cd content. After the maturity, the rice in each plot was harvested independently for yield determination and sampled independently for Cd content determination in rice grains and plants.

Analysis methods

For determination of soil available Cd content, a certain amount (10 g) of soil sample was passed through a 20-mesh sieve and mixed with a certain volume (50 ml) of DTPA-TEA-CaCl2extract. After a 2-h shaking, the mixture was filtered, and the filtrate was diluted by 20 folds. Subsequently, the Cd content in the solution was determined with ICP-MS (ICAP Q,Thermo Fisher Scientific).

For the determination of soil total Cd content, the soil sample was first passed through a 100-mesh sieve,and then a certain amount (0.3 g) of sieved soil sample was placed in a boiling tube. Subsequently, 5 ml of HNO3and 1 ml of H2O2were added.The digestion was performed under microwave, and then the solution was diluted to a constant volume. The solution was then filtered, and the Cd content was determined with ICP-MS.

To ensure the reliability and stability of the data,during the determination of Cd content in rice plants, one time of parallel determination was performed every 5 samples, and one quality control sample was arranged every 40 samples. ICP-MS adopted rhodium (Rh)as the internal standard with recovery of 90%-105%.

The data was processed and analyzed using Microsoft Excel 2003 and SPSS 17.0.

Results and Analysis

Effects of different treatments on rice yield

As shown in Table 3, lime application had certain effect on rice yield.Compared with that in the control group,the rice yield in the lime application treatment was reduced by 1.25%(P＞0.05)in Beishan,and was reduced by 1.26% (P＞0.05)in Meilinqiao.The rice yields in treatment groups of Tianshifu soil conditioner, soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner were improved significantly. In Beishan, the rice yields in the treatment groups of Tianshifu soil conditioner, soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner were increased by 7.9% (P ＜0.05),5.92% (P ＜0.05) and 8.06% (P ＜0.05)respectively compared with that in the conventional fertilization treatment group; in Meilinqiao, the rice yields in the treatment groups of Tianshifu soil conditioner, soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner were increased by 5.63% (P ＜0.05),4.35% (P ＜0.05) and 9.42% (P ＜0.05)respectively compared with that in the conventional fertilization treatment group. In both of the two test sites, in terms of yield-improving effect, the three treatments all ranked as combination of soil conditioner and soil bacterial conditioner ＞soil conditioner ＞soil bacterial fertilizer, and they were all greater than the lime application treatment.

In both of the two test site,biomass amounts in rice straws of the three soil conditioner treatment groups were increased in varying degrees compared with those in the conventional fertilization and lime application treatment groups. Among the three soil conditioner treatment groups, the biomass amount in rice straw of the treatment group of Tianshifu soil conditioner and soil bacterial conditioner was increased significantly.

Table 3 Rice yields and rice straw biomass amounts of different treatment groups

Table 4 Cd contents in brown rice and rice straws of different treatment groups

Effects of different treatments on Cd content in rice

In both of the two test sites, the Cd content in brown rice of the conventional fertilization treatment group was all higher than 0.2 mg/kg (Table 4), which exceeded the national standard.The application of lime,Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all could reduce Cd content in brown rice. Compared with that in the conventional fertilization treatment group, the Cd contents in brown rice of the four treatment groups were reduced by 37.63% (P＜0.05), 37.63%(P＜0.05),32.59%(P＜0.05)and 27.95%(P＜0.05) respectively in Beishan, and were reduced by 66.67% (P ＜0.05),54.36% (P＜0.05), 50.19% (P＜0.05)and 45.22% (P＜0.05) respectively in Meilinqiao. The Cd contents in brown rice of the four treatment groups all met the national standard at both of the two test sites. There were no sig-nificant differences in Cd content-reducing effect between the lime application treatment group and the three soil conditioner application treatment groups,indicating that the Cd contamination control effects of soil conditioners were equivalent to that of lime.

In Beishan, compared with that in the conventional fertilization treatment group, the Cd contents in rice straws of the treatment groups of Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner were reduced by 11.89%,10.64% and 20.73% (P ＜0.05), respectively. The Cd content-reducing effects of soil conditioners were also equivalent to that of lime. In Xiangtan,the application of Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner showed certain inhibiting effects on Cd content in rice straw. The Cd contents in rice straws of the three treatment groups were reduced by 36.55% (P＜0.05), 34.33% (P＜0.05)and 22.35% (P ＜0.05) respectively compared with that of the conventional fertilization treatment group. In rice straws, the Cd content-reducing effects of soil conditioners were also equivalent to that of lime.

Table 5 Soil pH values, available Cd contents and microbial diversities of different treatment groups

Effects of different treatments on soil pH value,soil available Cd content and microbial diversity

At maturity stage, the soil available Cd contents were determined(Table 5). The results showed that in Beishan and Xiangtan, the soil available Cd contents were all reduced to some extent by application of Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner, as well as lime. The soil available Cd contents of the three soil conditioner treatment groups were reduced by 10.52% (P＜0.05), 6.70%(P ＜0.05) and 2.57% respectively compared with that of the conventional fertilization treatment group in Beishan,and were reduced by 12.87%(P＜0.05), 2.70% and 5.82% respectively in Xiangtan. In terms of soil available Cd content, the treatment effects of soil conditioners were also equivalent to that of lime.

At maturity stage, soil physical and chemical properties were analyzed. In Beishan and Xiangtan, the soil pH values were all significantly increased by application of Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner. In Beishan, the soil pH values of the three soil conditioner treatment groups were increased by 0.45(P＜0.05),0.26(P＜0.05)and 0.14(P ＜0.05) respectively compared with that of the conventional fertilization treatment group; and in Xiangtan, the soil pH values were increased by 0.16(P＜0.05),0.08 and 0.14 (P＜0.05)respectively. The application of lime significantly reduced soil microbial diversity (Shannon index); there were no significant differences in soil microbial diversity between the lime application treatment group and the three soil conditioner treatment groups;the application of Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner was trended to increase soil microbial diversity(Shannon index).

Discussion

The application of Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all could significantly reduce Cd content in rice straw. Meanwhile,the transport rate of Cd was also reduced.It suggested that the application of Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner inhibited the transportation of Cd from rice straw to rice grains. The application of lime reduced the Cd content in rice grains mainly through reducing the Cd absorption of Cd by rice plants from soil.

The correlation analysis between Cd content in rice grain and soil pH and soil available Cd content showed that there were no significant correlations between rice grain Cd content and soil pH (r=-0.337) or between rice grain Cd content and soil available Cd content (r=0.364) in Beishan;in Xiangtan, rice grain Cd content was negatively related to soil pH value(r=-0.587,P＜0.05),and was positively related to soil available Cd content(r =-0.160). In both of Beishan and Meilinqiao, the application of lime,Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all reduced soil pH value, reduced soil acidity and reduced soil available Cd content,thereby reducing the absorption and accumulation of Cd in rice. However,in different test sites,the treatment effects were different, which might be due to the differences in soil type and cropping system and the effects of irrigated water and atmospheric deposition. In this study, the application of lime, Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all effectively reduced soil acidity and increased soil pH values.In the premise of using low Cd absorption rice varieties Xiangwanxian 12 and Xiangwanxian 13 as the test materials, the Cd contents in rice grains of the treatment groups of lime, Tianshifu soil conditioner, Tianshifu soil bacterialconditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all met the national food safety standards. However, the application of lime and soil conditioners did not significantly reduce soil available Cd content.There were also no significant differences in functions among different types of products. Therefore,in practical production, the application parameters of Tianshifu soil conditioner and soil bacterial conditioner are needed to be studied further from the aspects of reasonable application amount and different types of soil and Cd contamination in order to better serve the agricultural safety.

Conclusions

(1) The application of Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all can promote rice growth and improve rice yield.In Beishan, the rice yields in the treatment groups of Tianshifu soil conditioner, Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner are increased by 7.90% (P＜0.05),8.06% (P＜0.05)and 5.92%respectively compared with that of the conventional fertilization treatment group; in Xiangtan, the rice yields are increased by 5.63% (P ＜0.05), 9.42% (P＜0.05) and 4.35% respectively.

(2) The application of Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all can reduce Cd content in rice grain. In Beishan, the Cd contents in rice grains of the three soil conditioner treatment groups are reduced by 37.63% (P ＜0.05), 32.59%(P ＜0.05) and 27.95% (P ＜0.05) respectively compared with that of the conventional fertilization treatment group; in Xiangtan, the Cd contents in rice grains were reduced by 54.36%(P＜0.05),50.19%(P＜0.05)and 45.22%(P ＜0.05) respectively. The Cd contents in rice grains of all the treatment groups (except control) all meet the national food safety standards. Considering the Cd content-reducing effect in rice grain, the three soil conditioner treatment groups rank as soil conditioner ＞combination of soil conditioner and soil bacterial conditioner ＞ soil bacterial conditioner.In addition to reducing Cd absorption by rice plants,as described in the lime application treatment group, the application of soil conditioners also inhibits the transportation of Cd from rice straw to rice grain.

(3) The application of Tianshifu soil conditioner,Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner all can effectively improve soil acidity, improve soil pH value and reduce soil available Cd content to some extent. The application of lime significantly reduces soil microbial diversity,but the application of Tianshifu soil conditioner, soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner show no significant effects on soil microbial diversity; the application of Tianshifu soil bacterial conditioner and combination of Tianshifu soil conditioner and soil bacterial conditioner is trended to increase soil microbial diversity(Shannon index).

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