YIN Wen-ya, PENG Xuan-ming, ZOU Zhao-hui, DENG Kai, DENG Gang-qiao, XIE Hong-ke, WANG Qian, WU Li-jun, ZHANG Yue-long, LI Wen-ge

1. Hunan Dule Seed Industry Co., Ltd., Changsha 410125, PRC；

2. Hunan Academy of Agricultural Sciences, Changsha 410125, PRC；

3. Hunan Institute of Nuclear Agronomy and Space Breeding, Changsha 410125, PRC；

4. Hunan Institute of Agro-Environment and Ecology, Changsha 410125, PRC

Abstract Dunuo 201, a new sorghum combination, was planted in heavy metal contaminated areas to explore the feasibility of sorghum for soil heavy metals remediation in southern China. Its yield and heavy metal contents in stems, leaves and grains were analyzed. The results showed that Dunuo 201 could accumulate heavy metals especially in stems and leaves without significant effects on its yield. The cadmium content in grains and brewed liquor of Dunuo 201 was lower than the national standard. Therefore, Dunuo 201 can be used in planting structure adjustment in heavy metal contaminated areas in southern China, which has positive significance for the rapid remediation of heavy metal contaminated soil and the improvement of farmers' economic benefits.

Key words Glutinous sorghum； Soil heavy metals； Absorption； Remove

1. Introduction

With the rapid development of world economy, heavy metal pollution is emerging endlessly, especially in Europe, Asia, Africaetc. According to statistics, about 5 million t of Pb, 15 thousand t of Hg, 3.4 million t of Cu, 15 million t of Mn and 1 million t of Ni are discharged into the soil in the world on average every year. In Switzerland, the accumulation of Cd, Cu and Zn in soil is caused by sewage irrigation. Beets, lettuces, potatoes and peanuts are heavily contaminated by heavy metals. In Australia, the cadmium content in soil is 0.11～6.37 mg/kg. At present, the soil area polluted by pesticides and heavy metals in China has reached more than 10 million hm2, the polluted cropland was about 100 million hm2, accounting for more than 10% of the cropland area, mainly in economically developed areas. As many as 12 million t of grain are polluted by heavy metals every year in China, over 10 million t of grain production reduction due to heavy metal pollution, the total economic loss is at least 20 billion yuan[1-2].

Cd and Pb are especially serious in heavy metals pollution. Cd is a very toxic heavy metals, which not only has toxic effects on crops, but also can enter the human body through the food chain and cause a variety of diseases[3-6]. Pb can cause plant dwarfing, leaves chlorosis, and also affect plant absorption of mineral elements and change membrane structure and osmotic balance[7]. Due to the high harmfulness and universality of heavy metal pollution, a lot of researches have been done on the treatment of soil heavy metal pollution in soil around the world. Phytoremediation technology has attracted wide attention of scientists due to its economic and environmental advantages. Phytoremediation refers to a general term for the technology of using plants to absorb, extract, hydrolyze, transform or fix toxic and harmful pollutants in soil, sediment, sludge or ground water. However, most of the researches on phytoremediation mainly focus on the hyperaccumulators, remediation of heavy metal contaminated soil by hyperaccumulators are characterized by low cost andin situ, and obvious ecological, social and economic benefits, hence it is worth promoting[8-11]. Meanwhile, phytoremediation also has many limitations in practical application, most of the hyperaccumulators found so far are herbs, which grow slowly, have low biomass and the remediation efficiency is low[12-13]. Hyperaccumulators have a strong specificity for heavy metal accumulation, and they often show hyperaccumulation ability for only one or two heavy metals, but they are very weak for others. Therefore, the inherent limitations of hyperaccumulators also seriously restrict their application in remediation practice[14]. For this reason, Dunuo 201 was selected as the test material, which had many good characters such as tall stems, lush leaves, high resistance to drought, waterlogging, high temperature, barren, salinity, pests and diseases, and lodging. The test site was Zhenzhutai group, Zhangqiao village, Junchu Town, Liling City, the biomass and yield of sorghum, the heavy metals content in stems, leaves and grains, and the heavy metals removal amount were studied, in order to provide theoretical support for local heavy metal remediation and agricultural planting structure adjustment.

2. Materials and Methods

2.1. Materials

The test sorghum variety was Dunuo 201, provided by Hunan Dule Seed Industry Co., Ltd. The test soil was red-yellow soil polluted with cadmium.

2.2. Test and sampling method

The test site was Zhenzhutai group, Zhangqiao village, Junchu Town, Liling City. The crop was local farmland cultivated all year round, the test area was 3.33 hm2. In early May, direct seeding after digging hole, trenching and ridging cultivation were conducted, and the planting density was 124 000 plants/hm2, once cultivating and earthing up at jointing stage were conducted, urea application rate was 300 kg/hm2, and the crop was harvested in mid-October. Field management and pest control was in accordance with local traditional cultivation management techniques.

6 representative fields were selected in the test area, then 5 m2quadrats were randomly selected in each field, which were represented by T1, T2, T3, T4, T5 and T6 respectively. The above ground parts of sorghum were collected, washed 3 times with clear water and 3 times with distilled water and then dried by airing, and determined the fresh weight of stems, leaves and grains. And then deactivation of enzymes for 30 min at 105℃ was performed, and they were dried at 80℃ to constant weight. Dry matter was measured by 1% balance. After Being crushed and sieved with 40 mesh sieve, they were ready for later use. Three areas with high yield, middle yield and low yield were selected for soil sampling in test area, which were expressed as A, B and C respectively； soil samples of plough layer were collected and dried for later use.

2.3. Heavy metal content determination method

The soil nutrients content and pH value were determined by conventional methods[15], the heavy metal content in soil was determined by ammonium acetate extraction[16]； sorghum stems, leaves and grains was hydrolyzed by HNO3-HClO4(5 ： 1), and their heavy metal contents were determined by ICPMS after constant volume and dilution. All analysis vessels were immersed in dilute nitric acid solution for 24 h, then washed with pure water for 3 times.

3. Results and Analysis

3.1. Analysis of soil nutrients and heavy metal contents in soil of Dunuo 201 sorghum demons- tration field

It can be seen from Table 1 and 2 that the soil in the demonstration filed was acidic with a pH value of 5.15 and rich in organic matter and nitrogen, which were 41.7 and 2.28 g/kg respectively, and available nitrogen was 200 mg/kg. Total and available cadmium content was 1.15 and 0.295 mg/kg respectively, total and available chromium was 91.5 and 0.025 mg/kg respectively, total and available lead was 68 and 8.911 mg/kg respectively, total arsenic and mercury was 29.0 and 0.267 mg/kg respectively. According to the analysis of heavy metal contents in soil, the cadmium content in test soil was 3.7 times higher than the standard of GB15618-2018Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land(try out), whereas chromium, lead, arsenic, mercury,etc. did not exceed the standard. As a result, the sorghum demonstration filed had been polluted by cadmium, and many crops were not suitable for cultivation in such soil.

3.2. Yield analysis and comparison of Dunuo 201

As shown in Table 3 that there was a significant difference in fresh weight of sorghum among 6 samples taken from the demonstration filed, the highest and lowest was 3 145.5 and 2 376.6 kg/667m2respectively, which may be related to geographical location of the soil, sunlight, water and fertilizer,weeds and other factors. The percentage difference of dry matter content was not significant, the highest and lowest was 34.4% and 29.6% respectively, and the average was 32.42%. While the total dry matter was significantly different, T1 and T2 were significantly higher than T3, T4, T5 and T6. The dry weight of stems and leaves was relatively higher, the highest was 648.7 kg/667m2, and the average was 530.8 kg/667m2. The average grain yield was 373.9 kg/667m2, the highest was 454.5 kg/667m2, which indicated that the crop at the demonstration site grew well, and this variety had good characteristics, which was suitable for planting in Hunan Province.

Table 2 Heavy metal contents in soil mg/kg

Table 3 Yield of fresh and dry weight of Dunuo 201

3.3. Analysis of heavy metals content in stems, leaves and grains of Dunuo 201

As can be seen from Table 4, the average cadmium content in sorghum stems and leaves was 4.279 mg/kg, significantly higher than the national standard of 0.2 mg/kg, whereas the cadmium content in sorghum grains was 0.144 mg/kg and lower than the national standard. The average lead content in stems, leaves and grains was 7.527 and 1.401 mg/kg, respectively, which were higher than the national standard of 0.2 mg/kg. The average mercury content in stems, leaves and grains was 0.021 0 and 0.007 6 mg/kg, respectively, the mercury content in stems and leaves was higher than the national standard of 0.02 mg/kg, while the mercury content in grains was significantly lower than the national standard. The average chromium content in stems and leaves, grains was 16.342 and 5.064 mg/kg respectively, which were significantly higher than the national standard of 1.0 mg/kg. The average arsenic content in stems and leaves, grains was 0.728 and 0.376 mg/kg, respectively. The arsenic content in stems and leaves was significantly higher than the national standard of 0.5 mg/kg, while the arsenic content in grains reached the national standard. Therefore, Dunuo 201 could accumulate some heavy metals in soil, especially cadmium, lead, chromium, arsenic,etc., and the heavy metals content in grains was significantly lower than that in stems and leaves, and some of them were lower than national standards, such as cadmium, mercury, arsenic,etc.

3.4. Analysis of the removal of some heavy metals from soil by Dunuo 201

It can be seen from Table 5 that the total amount of heavy metals removed from soil by the aboveground parts of Dunuo 201 was 2 416.60 mg/667m2for cadmium, 4 584.91 mg/667m2for lead, 14.50 mg/667m2for mercury, 10 677.68 mg/667m2for chromium and 530.63 mg/667m2for arsenic. The amount of heavy metals removed from stems and leaves was significantly higher than that from grains, the highest amount of heavy metals removed from stems and leaves and grains was chromium, 8 872.70 and 1 804.98 mg/667m2for stems, leaves and grains respectively, followed by lead, which was 4 090.78 mg/667m2from stems and leaves and 494.13 mg/667m2from grains. This may be related to the heavy metal contents in soil, and the growth amount of sorghum will also affect the removal amount of heavy metals. In general, when the biomass is large, the removal amount of heavy metals is high.

3.5. Cadmium content in sorghum liquor stillage and sorghum liquor

Table 4 Heavy metal contents in stems, leaves and grains of Dunuo 201

Table 5 Removal of some heavy metals from soil by Dunuo 201 mg/667m2

100 kg of Dunuo 201 sorghum grains were collected from the experimental area, after brewing then the cadmium content in sorghum liquor stillage and sorghum liquor were detected. The results showed that the cadmium content in sorghum liquor stillage was 0.0 198 mg/kg, and that in sorghum liquor was 0.182 μg/L, which were lower than the national standard. Therefore, the brewed liquor was edible.

4. Conclusions

Lead, mercury, chromium and arsenic in soil of the experiment base, Zhenzhutai group Zhangqiao village, Junchu Town, Liling City did not exceed the standard, while the cadmium content in soil was 3.7 times higher than the standard of GB15618-2018Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land(try out). The experimental field had been seriously polluted with cadmium, hence it was not suitable for the cultivation of common crops.

The new sorghum combination Dunuo 201 adapted to the growth of South China, its biomass and yield were relatively high, with an average fresh weight of 2 775.0 mg/667m2and a grain yield (dry weight) of 373.9 mg/667m2.

Dunuo 201 had a strong enrichment effect on cadmium, lead, mercury, chromium, arsenic and other heavy metals in soil, especially the stems and leaves, with an average absorption of 4.279, 7.527, 0.021, 16.342 and 0.728 mg/kg, respectively. However, the heavy metals content in sorghum grains such as cadmium, mercury and arsenic was lower than the national food safety standard (Maximum Content of Contaminants in FoodGB2762-2012), the cadmium content in sorghum liquor was 0.182 μg/L, which met the national requirements for food safety. The amount of heavy metals removed from the soil in the aboveground parts of sorghum was 2 416.60 mg/667m2for cadmium, 4 584.91 mg/667m2for lead, 14.50 mg/667m2for mercury, 10 677.68 mg/667m2for chromium and 530.63 mg/667m2for arsenic. Therefore, Dunuo 201, the new sorghum combination can be used as the extension variety of agricultural structure adjustment in heavy metal pollution areas, which will not just remedy soil, but create benefits for farmers as well.