Investigation and Evaluation on Heavy Metal Content and Pesticide Residues of Three Edible Forest Products in Central Sichuan Province

Yan PENG Juan YAO

Abstract Taking three mains edibles products （walnut， prickly ash and bamboo shoots） and origin soil in central area of Sichuan Province as the study objects， this research aimed at revealing the level of heavy metals and pesticide residue and comprehensive evaluation the soil heavy metal pollution in the region. The results showed that：  Soil organic matter， total nitrogen， total phosphorus and heavy metal contents of mercury， arsenic in the south area were significant higher than that in the north area， while the heavy metal contents of lead and copper were significantly higher in the north area;  The pollution index of six heavy metals of cadmium， mercury， arsenic， lead， chromium and copper in the producing area was less than 1.0. However， the integrated pollution index was at a warning level， and there was a potential risk of cadmium contamination;  The walnut samples exceeded the lead limit value over 75% in the north area and over 35% in the south area， and the same as 30% of bamboo shoot samples;  There was a significant negative correlation of mercury content between walnut samples and production soil， and a significant positive correlation of arsenic content between bamboo shoots and production soil. In addition， there is no significant correlation of heavy metal contents between prickly samples and production soil;  Fenthion residues were detected in walnuts， malathion samples were detected in prickly ash， and chlorothalonil， dimethoate and chlorpyrifos residues in bamboo shoots samples. None of theses pesticide residues exceeded the standard limit value. In general， the soil is in well sanitary condition and the edible forest products are not exposed to the contamination of pesticide residues in the study area. The quality and safety of the prickly ash is good while the walnut and bamboo shoots have certain risks in terms of high lead level.

Key words Edible forest product; Heavy metal; Pesticide residues; Pollution index; Comprehensive evaluation

The economic forest is a forest with the main purpose of producing forest products such as fruits， edible oils， industrial raw materials and medicinal materials other than wood. The edible forest is within the scope of economic forests， and edible forest products refer to plants， animals， fungi and their products that are grown in forests for direct or indirect human consumption. The 13th FiveYear Plan for Forestry Development in Sichuan Province clarifies the requirements to promote modern forestry industry projects， enhance the development benefits of the forestry industry， implement forestry ecological construction and poverty alleviation， guide and organize the masses to cultivate special economic forest fruits or wood bamboo industry bases such as walnuts and prickly ash according to the local conditions， and vigorously introduce and cultivate food processing enterprises such as forest vegetables. At present， there are more than 2≠104 forestrelated enterprises in Sichuan[1]. Central Sichuan is dominated with the landforms of hills and plains， and it is also characterized for its large population density， small amount of forest resources per caPita， staggered agriculture and forestry， single tree species， and frequent occurrence of forestry pests. The main edible forests are mainly walnut， prickly ash and bamboo shoots， and the safety problems are mainly caused by pesticide residues， harmful heavy metal residues and the presence of harmful microorganisms. Therefore， in this study， evaluation and risk assessment were made to the special edible forest soils and products in central Sichuan， in order to provide a theoretical basis for the regional forestry departments to guide farmers in planting， which is of important significance to further promote forestry ecological construction poverty alleviation， guide the safe production of pollutionfree edible forest products， and improve the market competiveness.

Materials and Methods

Study area overview

The study area was in the hilly area of central Sichuan. Located at 30°01′-31°51′N， 105°27′-107°19′E， it had a midsubtropical humid monsoon climate with 4 distinct seasons and coincided rainy hot season. The land surface had small undulating， and the area was dominated with hills with gullies divided vertically and horizontally. The soil was mainly purple soil.

Sample collection

According to the distribution of resource characteristics of edible forest products， combined with geographical location and administrative division， a total of 14 administrative regions which had typical producing areas were selected from the northern part （north area， namely the 9 administrative regions under the jurisdiction of Nanchong City） and the southern part （south area， namely the 5 administrative regions under the jurisdiction of Guang]an City）. Samples were collected randomly， and GPS was used to accurately locate each sampling point. The walnut resources were widely distributed within the region， so a total of 190 walnut samples （including 154 from the north and 36 from the south） were collected for the analysis and evaluation， while 90 samples of prickly ash （including 22 from the north and 68 from the south） and 20 bamboo shoot samples （all from the north） were collected in the study. Soil samples were collected from the 0-30 cm cultivated soil layer， and a total of 300 soil samples were collected， including 196 from the north and 104 from the south.

Sample determination and evaluation methods

Safety factors of edible forest products and soil related measurement indicators

The safety factors of edible forest products included the heavy metals and pesticide residues in walnut， prickly ash and bamboo shoots. The measurement indicators related with the soil of the producing areas were total N， total P， total K， organic matter， pH value， arsenic， mercury， lead， cadmium， chromium and copper.

Sample detection method

The monitoring standards and limits of edible forest products were referred to the methods of Fu et al.[1]. The determination of basic physical and chemical properties of soil was referred to the Soil Agrochemical Analysis of Bao[2]. The methods and limits for the determination of heavy metal content in the soil of the place of production were referred to the General Environment Requirements for Edible Forest Products （LY/T 1678/2014）[3]. Among them， chromium， copper and cadmium were measured on a PE5000 atomic absorption spectrophotometer， and mercury， arsenic and lead were measured on an AFS1201 atomic fluorescence spectrometer.

Risk assessment of heavy metal contamination in the soil of the producing area

The single factor pollution index method （Pi） and the Nemerow comprehensive index method （Pcom）[4]were used to comprehensively evaluate the risk of soil heavy metal contamination in the study area. The limit standard adopted that in General Environment Requirements for Edible Forest Products （LY/T 1678/2014）. Pi<1.0 indicated no contamination from itype pollutants; Pcom0.7 indicated that it was in the environmental safety level， 0.7<Pcom1.0 indicated that it was at an alert level and the soil was still clean， and Pcom>1.0 indicated that the soil was contaminated.

Results and Analysis

Comprehensive evaluation of soil environment of edible forests in central Sichuan

The soil pH in the study area ranged from 4.82 to 8.16. The average value of soil organic matter， total N， total P and total K was respectively （13.64÷0.86）， （1.23÷0.03）， （2.65÷0.10）， （19.07÷0.64） mg/kg in the north area， and （20.70÷2.83）， （1.32÷0.08）， （3.38÷0.18）， （20.55÷0.88） mg/kg in the south area. As shown in Fig. 1， the nutrient content of the production soil was higher in the south area than in the north area， and Pearson correlation analysis showed that there were significant differences except in total K content between the south and north areas （P<0.05）.

As shown in Table 1， the Pearson correlation analysis showed that the differences in the content of soil heavy metals excluded cadmium and chromium were significant between the south area and north area （P<0.05）， but the pollution indexes Pi of soil heavy metals were all smaller than 1.0. The pollution indexes Pi of cadmium and chromium were high， while that of arsenic was the lowest. The content of soil cadmium was 0.00-1.18 mg/kg in the producing area， and there was no significant difference between the north and the south. The cadmium exceeding rate in the north was larger than 30%， and large than 10% in the south. Moreover， the single factor pollution index of cadmium was higher （0.95<PCd<1.0）. Although the soil in the producing area was at a clean level that was not contaminated with cadmium， there was still a potential risk of cadmium contamination.

The contents of mercury， lead and chromium in the soil of the producing area were 0.01-0.40 mg/kg， 0.00-76.72 mg/kg and 5.73-64.08 mg/kg， respectively. There were significant differences in the average values of soil mercury and lead contents in the producing areas between the south and the north， but there was no significant difference in the content of chromium （P<0.05）. The soil lead and chromium in the north area had 1.02% of the samples exceeding the standard rate， and the single factor pollution indexes Pi for the two were 0.21 and 0.53， respectively， both less than 1.0， indicating that both were still in the clean level without contamination. In the south， both mercury and lead also had 1.92% of samples exceeding the standard rate， and the single factor pollution indexes Pi for the two 0.48 and 0.52， respectively， both of which were less than 1.0， indicating that the soil in the south area was not contaminated by mercury or chromium.

The contents of arsenic and copper in the soil were 0.00-11.17 mg/kg and 3.01-79.93 mg/kg， both within the standard limits. The differences in soil arsenic content and soil copper content were significant between the south and the north areas （P<0.05）.

According to the Pi， the calculated Pcom（north） and Pcom（south） were 0.79 and 0.75， respectively， both greater than 0.7 and less than 1.0. Indicating that it was in the alert level， and there was potential heavy metal contamination in the soil.

Analysis of quality and safety status of edible forest products in different areas

Analysis of the quality and safety of walnuts in different areas

As shown in Table 2， 128 of the 190 walnut samples had the lead content exceeded the standard， and the standard exceeding rate was 75.32% in the north area and 38.89% in the south. None of the contents of arsenic， mercury and cadmium exceeded the standard， and the product qualification rate was 32.63%. As for the 8 pesticide indicators， only fenthion residues were detected but did not exceed the standard limit， and the detection rate was 14.29% in the north area and 5.56% in the south.

Analysis of the quality and safety of prickly ash in different areas

As shown in Table 3， the prickly ash had no samples with exceeding heavy metal contents or pesticide residues， showing good quality safety status. The average content of malathion in prickly ash samples was < 0.05 mg/kg， which was far below the standard limit （8 mg/kg）. The detection rate was 45.45% in the north area and 11.76% in the south.

Analysis of the quality and safety of bamboo shoots

As shown in Table 4， 30% of the 20 bamboo shoot samples （north area） had lead content exceeding the standard （standard limit was 0.1 mg/kg， and the actual measured value was 0.117 mg/kg）. The other heavy metal indicators and pesticide residues were all within the standard limits， and the sample pass rate was 70%. Most of the pesticide residues were not detected in bamboo shoot samples， and there were few pesticide residues， mainly highefficiency and lowtoxic chlorothalonil. And 30% of the samples were found to have dimethoate residues， and 10% of the samples were found to contain chlorpyrifos residues， but neither exceeded the standard.

Correlation analysis between edible forest products and soil heavy metal content in the producing areas

As shown in Table 5， the Pearson correlation analysis results showed that there was a extremely significant negative correlation between the mercury content of the walnut sample and the soil mercury content （r=-0.429， P<0.01）. There was no significant correlation between the heavy metal content of the prickly ash samples and the heavy metal content of the soil in the producing area， while the arsenic content of bamboo shoot samples was significantly positively correlated with the soil arsenic content in the producing area （r=0.674， P<0.05）.

Conclusion and Discussion

Risk assessment of heavy metal cadmium safety in edible forest soil of the producing areas

The soil of edible forests in central Sichuan is still clean （Pi<1.0）， and it is not polluted by heavy metals. However， the contamination of heavy metal mercury is at the warning level  （0.7<Pcom<1.0， Pi（Cd） >0.95）. He[6]studied the soil samples in walnut production filed of Weining County， Guizhou Province， which had a walnut planting history over 200 years， finding that cadmium was the major factor for the soil heavy metal contamination in walnut production areas. Jia et al.[7]found the adsorption rate of walnut peel and its polyphenol extracts to cadmium increased with the increase of pH values， and the absorption had good stability. All the soil samples from the edible forests in central Sichuan had high pH values （7.6 in the north and 6.89 in the south）. Although the cadmium content does not exceed the standard limit in the 190 walnut samples for the spot check， neither has it shown significant correlation with the soil cadmium content （Table 5）， the related research suggests that the heavy metal element pollution index in the fruit is related to the soil corresponding heavy metal element pollution index to some extent[8]. In addition， it should not rule out the impact of the sample size shortage. Therefore， we believe that there is potential cadmium contamination risk in the soil of central Sichuan region， which should be taken seriously by the production units.

Reasons and ways of lead contamination in edible forest products in the producing areas

The lead content in the soil of the regional edible forests basically meets the general requirements of the environment for the production of edible forest products. The results of soil heavy metal pollution risk assessment also indicate that there is no potential lead contamination （Pi（Pb）<0.25<1.0）. However， the samples of walnut and bamboo shoots for spot check all have exceeding lead detected， and the standard exceeding rates are as follows： walnut samples in the north>75%， walnut exceeding rate in the south>35%， bamboo shoot samples of 30%. Pearson correlation analysis shows that there is no significant correlation between edible forest products and lead content in the soil （Table 5）， which is related to the accumulation of lead in trees， the fertilization system， the traffic location of the producing area and the selection of evaluation criteria. Therefore， further study is needed. Chai et al.[9]believed that dried fruit such as walnut has relatively long tree ages， and the tree may have certain accumulation or biotransformation effects on heavy metals such as lead. Moreover， the surroundings （such as waste gas， waste water， waste residue） could also bring about lead accumulation through the adsorption of walnut trees， and the absorption and accumulation capacity of walnut was 87% higher than that of Torreya grandis. Wang et al.[10]found that most of the sampling sites for fruit with exceeding lead content were near the highway main lines， emphasizing that the qualified fertilizer should be properly applied， while the urban and rural garbage and sludge which may contain a lot of heavy metal elements should be used with caution. In addition， the difference in evaluation criteria will also lead to differences in the standard exceeding rates. For example， in the General Principles of Forest Food Quality and Safety （LY/T 1777/2008）[5]， the lead content of walnuts is  0.2 mg/kg， while in Pollutionfree Food Deciduous Fruit Tree Nuts （NY 5307/2005）[11]， the lead content of walnuts is  0.4 mg/kg. In this study， the average content of lead in the walnut samples is 0.42 mg/kg in the north， and 0.20 mg/kg in the south. If the latter standard is used， the exceeding rate of lead is only 1.30% in the walnut samples collected from the north， while the content of lead is not exceeding the standard in the walnut samples collected from the south.

Risk assessment of pesticide residues in the edible forest products of the producing areas

The detection rates of pesticide residues in the edible forest products are low， and they are all within the standard limits. Among them， the main pesticide residue is fenthion in walnut， malathion in prickly ash， and chlorothalonil， dimethoate and chlorpyrifos residues in bamboo shoots. Pesticide residues in edible forest products are mainly caused by human factors， that is， there is a problem of excessive spraying of pesticides in the region. Therefore， it is recommended that local forestry departments enhance technical guidance and supervision to reduce the risk of pesticide residues.

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