Exposure Risk Of Cadmium To Local Residents And Mitigation Strategies In A Typical Cd-Contaminated Area In Southern China

Cadmium(Cd)is a toxic trace element that is a ubiquitous contaminant in soils.Cadmium is readily taken up by plants and transferred to the food chain.Long-term dietary Cd intake at elevated levels can result in serious health problems including irreversible renal damage,low bone density,and increased risk of cancers in the lung,endometrium,bladder,and breast.Rapid industrialization in China over the last 30 years together with inadequate environmental protection has led to serious soil contamination with heavy metals and metalloids.According to the national survey of soil contamination in China,Cd ranks the first in the percentage of soil samples(7.0%)exceeding the environmental quality limit.Soil contamination with Cd coupled with soil acidification has resulted in elevated Cd accumulation in rice grain in some areas of southern China,threateung food safety and human health.So far it remains unclear how soil contamination affects Cd accumulation in food crops and the associated exposure risk to local farmers who consume rice as the staple food.In order to address the issues associated with soil contamination,China' s State Council issued the Action Plan on Soil Pollution and Control in 2016.This Action Plan sets up targets to make 90%of the contaminated agricultural soils safe for use by year 2020 and 95%by year 2030.To meet this need,it is imperative to investigate the exposure risks associated with soil contamination with Cd and mitigation strategies.In the present study,we first conducted a survey of Cd concentrations in polished rice grains from local markets in different provinces of China to estimate dietary Cd intake from consumption of rice.Next we selected a typical Cd contaminated area in southern China and conducted a survey of soil-rice and soil-vegetable paired samples to estimate dietary Cd intake for the local farmers and the associated exposure risk.We also carried out a series of field experiments and pot experiment to investigate the effects of agronomical and biotechnological measures on mitigating grain Cd concentration,and finally propose a number of mitigation strategies.The main results are summarized as follows:(1)Survey of rice Cd concentration in Chinese markets.We collected 160 polished(white)rice samples from local markets in 20 provinces in China and determined total Cd concentration.Total Cd concentration in polished rice ranged from below the detection limit(0.004 mg kg-1)to 0.77 mg kg-1,with a mean of 0.093 mg kg-1 and a median of 0.045 mg kg1.Among the 160 samples,16 samples(10%)had a Cd concentration exceeding the Chinese maximum permissible limit of Cd for rice(0.2 mg kg-1).Rice Cd concentration showed a distinct geographical pattern,increasing from low levels in the north to high levels in the south of China.The north-south difference in Cd intake from rice was amplified due to the differences in both rice Cd concentrations and rice consumption.Median daily Cd intake from rice was 0.61 ?g kg-1 body weight in the south of China,representing 73%of the tolerable daily intake(TDI)recommended by FAO/WHO,but very low for the population in the north region of China,approximately 1%of TDI.(2)Survey of Cd concentration in soil-crop paired samples a typical Cd-contaminated area in Hunan,China.A total of 200 soil-rice and 142 soil-vegetable paired samples were collected in Xiangtan county of Hunan province,southern China.Among the 342 soil samples,soil Cd concentrations ranged from 0.04 to 8.20 mg kg-1,with a mean of 1.00 mg kg-1 and a median of 0.54 mg kg-1.According to the risk screening values for soil contamination of agricultural land,75%of the soil samples had a Cd concentration exceeding the risk screening values.grain Cd concentrations varied from 0.02 to 3.61 mg kg-1,with a mean of 0.69 mg kg-1 and a median of 0.54 mg kg-1.Approximately 88%of the rice samples had a Cd concentration exceeding the Chinese maximum permissible limit of Cd.Concentrations of Cd in vegetables ranged from 0.01 to 1.28 mg kg-1 fresh weight(fw),with a mean of 0.17 mg kg-1 fw and a median of 0.08 mg kg-1 fw.Among vegetables,29%of the samples had a Cd concentration exceeding the limits.Based on the concentrations of Cd in dietary items and the amounts of consumption,it is estimated that the median dietary Cd intake varied from 66.5 to 1 16 ?g kg-1 BW month-1,representing 2.7-4.6 times the tolerable dietary intake of 25 ?g kg-1 BW month-1 recommended by JECFA.Among the subgroups.children exhibited the highest dietary Cd intake,being 116 ?g kg-1 BW month-1,which is 4.6 times the recommended tolerable intake by JECFA.Actions are needed to reduce the risk of high Cd exposure for the local residents.(3)Large spatial variation in grain Cd concentration within paddy.The survey in study(2)showed a poor correlation between rice grain Cd and soil total Cd and even incorporating other soil properties did not improve the correlation.Meanwhile,we observed large variations in grain Cd concentrations collected from paddy fields,not only at large regional scale but also at an individual field scale.We hypothesized that the within-field variation in grain Cd concentration is attributed to the farmers' practice.Farmers often prepare paddy fields with a slight slope to facilitate paddy water flow from the inlet to the outlet.We conducted a field experiment to simulate a gentle slope gradient to investigate how spatial variation in paddy water status affects grain Cd concentrations.As a result of gentle slope of 0.15° along the length of the plot.there was a 6-7-fold spatial within-field variation in grain Cd concentrations.Grain Cd concentrations ranged from 1.71 to 2.02 mg kg-1 in the irrigation inlet and from 0.28 to 0.72 mg kg-1 in the outlet area,with a distinct spatial pattern of decreasing from the irrigation inlet to the outlet.In the second year,two parallel plots of the same dimension were created,one without liming and the other received lime as CaCO3,increasing the initial soil pH from 5.5 to 6.5.Liming decreased grain Cd concentrations and largely removed the spatial variation pattern in grain Cd concentrations.The results show that a slight slope can cause large within-field variations in soil moisture content,Eh,pH and soluble Cd concentration during the grain filling stage,resulting in large spatial variation in grain Cd concentration.This spatial variation should be considered when carrying out survey and sampling.(4)Liming as an efficient way to reduce grain Cd concentration.A continuous threeyear liming trial was set up in Xiangtan county.Four rates of lime were applied and two hybrid rice cultivars were used in the present study.Different from the burnt lime(CaO)recommended in Hunan province,CaCO3 was used in the field trial as it reacts with soil more slowly and also safer for crops.The liming model RothLime was used to predict the doses of lime required.Lime(2.25-7.5 t ha-1)was applied only in 2016 and its effects on grain Cd concentrations were investigated in subsequent three consecutive years(2016-2018).Application of 7.5 t ha-1 CaCO3 increased soil pH from the initial 5.5 to the target value 6.5.Liming decreased CaCl2-extractable Cd in the rhizosphere soil and Cd accumulation in rice grain markedly.Grain Cd concentrations were closely correlated with CaCl2-extractable Cd in rhizosphere soil during the grain maturing stage.Liming at 7.5 t ha-1 decreased grain Cd concentration by 70-80%compared to the control soil in three seasons without affecting grain yield,with the grain Cd concentrations in the three seasons in the 7.5 t ha-1 liming treatment all meeting the food safety limit.This study shows that application of CaCO3 raising soil pH to 6.5 was highly effective at reducing Cd accumulation in rice grain.(5)Developing a predictive model of lime doses.Liming to a target pH 6.5 can be used as an efficient way to decrease grain Cd concentration.To increase soil pH to the target value,the application doses of lime vary among soils,depending on soil type,initial pH and the buffer capacity.Therefore,it is necessary to develop a predictive model to calculate the application doses.A total of 23 acidic paddy soils were collected from southern China and four rates of lime were applied to investigate the relationships between the changes in soil pH and the application rates.These soils were incubated at room temperature for 120 days.A quantitative model for the relationship among the changes in soil pH(?pH),the application doses of lime,soil initial pH and soil properties was developed.This model is suitable for acidic paddy soils in southern China and can predict the lime dose that are required to increase soil pH to a target pH.(6)Comparisons between liming,water management and biotechnological strategies in reducing grain Cd concentrations.Recent progress in identifying the transporter genes responsible for Cd uptake and translocation in rice plants has provided potential solutions for reducing grain Cd through biotechnology.A pot experiment with four Cd contaminated paddy soils was carried out to compare the effects of lime application(0,15 g CaCO3 pot-1),water management(continuous flooding,flooding and drainage after grain filling),transgenic plants including OsNramp5 knockout and OsHMA3 over-expression lines on grain Cd concentration.For acidic soils,liming and continuous flooding were both highly effective in reducing grain Cd concentration,but the single agronomical treatment was not enough to limit grain Cd concentration to below the Chinese food limit.For alkaline soils,water management was also highly effective for decreasing grain Cd concentration.Biotechnological measures using OsNramp5 knockout and OsHMA3 overexpression transgenic plants were the most effective method to reduce Cd accumulation in rice grain not only in acid soils but also in alkaline soils,without penalty on grain yield.OsHMA3 overexpression decreased grain Cd concentrations by 48-92%,to levels below the Chinese limit for Cd,even when rice plants were grown in serious Cd-contaminated soils.In conclusion,soil contamination with Cd is a serious environmental problem in some parts of southern China and can result in elevated dietary Cd exposure to local residents.Oneoff application of CaCO3 to increase Ph in acidic paddy soils to a target pH 6.5 is a simple,economical and effective agronomic method to reduce Cd accumulation in rice,and our results have shown that it is efficient over at least three years.A lime model was developed to calculate the application dose and the model is suitable for Chinese acidic paddy soils.Delaying soil drainage to the late grain filling stage can also decrease grain Cd accumulation.Comparing with agronomical methods,biotechnology is also an effective method to reduce Cd accumulation in rice.These methods can used alone or in combination,depending on the status of contamination and soil properties,to tack the issue of Cd contamination in rice grain in some areas of southern China.