Abstract Heavy metal pollution and remediation of soils have drawn much attention. More and more scholars tend to remediate soil heavy metals without affecting the normal agricultural production. By analyzing the mechanism of action of organic materials to remediate Pb pollution in soil， the effect of different organic materials on the passivation of available Pb in soil was summarized， and the prospect was proposed for the future application of organic materials， in order to provide reference for the remediation of Pb and other heavy metals by organic materials.
　　Key words Organic materials； Pb； Available； Soil； Remediation
　　Soil is the material basis for human survival， an indispensable natural resource for humans， and an important part of the human environment. With the acceleration of industrialization and urbanization， various kinds of pollution problems such as the atmosphere， water， and soil are also becoming increasingly prominent. At present， the overall situation of soil environment in China is not optimistic. The total standard exceeding rate of pollution has reached 16.1%[1]. The main types of pollution are inorganic pollutants. Contaminated soil is mainly distributed in the south， especially in the southwest and south central regions of China.
　　Pb （Lead） is a heavy metal that is toxic to the human body and is widely found in human environment and food chain. Pb sources can be divided into natural sources and man-made sources. Natural sources are mainly soil parent materials， and man-made sources are mainly the use of various Pb-containing substances in industrial and agricultural production， automobile exhaust emissions， and irrational use of Pb-containing wastes[4]. When excessive Pb enters the soil， it will not only affect the quantity and activity of microorganisms in the soil， but also affect the growth and quality of crops after being absorbed by roots of crops， and eventually enter into human body through the food chain， threatening human health[5-6]. Soil Pb pollution has the characteristics of concealment， long-term duration and irreversibility. Studies have shown that excessive Pb can cause damage to the digestive system， hematopoietic system， nervous system， immune system， and reproductive system of human body[7]. Therefore， efficient remediation technology of Pb-contaminated soil has been a hot and difficult research topic.
　　Pb-contaminated soil remediation technologies can be roughly divided into physical remediation， biological remediation， and chemical remediation. The physical remediation methods include soil dressing method， soil turning method， soil replacement method， electric remediation method， heat treating method， and adsorption method[8-11]. These methods can significantly reduce the content of heavy metals in the soil， but the cost is high and the implementation is difficult. Moreover， they can also affect the soil structure and microbial activity. Therefore， physical remediation is suitable for implementation in the small-area， high-pollution regions. Biological remediation methods include the microbial remediation method and plant remediation method， which make use of certain special biological functions to control heavy metal pollution in soil[12-13]. Microbial remediation method is simple to operate， but it requires the selection and domestication of microbial species in the early stage， and the activities of the microorganisms are also easily affected by soil properties. Plant remediation method requires low costs and is not difficult to implement. It is the cleanest remediation technology at present， but its efficiency is not high[14]. Moreover， there may be a risk of secondary pollution for the treatment of enriched heavy metal plants. Chemical remediation is to reduce the availability of heavy metals in soil using chemical substances， including leaching method and modifier application method[15-16]. Although the leaching method has low cost and strong processing capacity， it will destroy soil structure and cause the loss of nutrients in the soil. Modifiers include organic materials， lime materials， silicon-rich materials， phosphorus-containing materials， clay minerals， and the application of modifiers is to change the forms of heavy metals in soil and reduce their bio-availability mainly through ion exchange adsorption， precipitation and other inactivation. 　　In this paper， through the summary of inactivation mechanisms of organic materials to soil Pb， the effects of different organic materials on soil Pb availability were compared with the aim to provide scientific bases for the remediation of Pb-contaminated soils.
　　Sources and Components of Organic Materials
　　China is a great country in agriculture with the total area of arable land ranking 4th in the world， and China is rich in organic material resources. The wastes generated during the production and processing of crops， the feces produced by livestock and poultry， and the organic wastes generated by industrial production and residential life are all sources of organic materials. In China， there is an annual output of crops stalks of 500 million t （dry weight）， forestry wastes like sawdust， wood shavings of 16 000 t， and livestock and poultry manure of 13.4 billion t， which are increasing at a rate of 5%-10% per year[17]. Although China has abundant straw resources， there are still problems such as low utilization rate， low conversion rate， low economic benefit， and serious environmental pollution[18]. Appropriate application of organic materials in the soil can not only improve the soil quality， but also reduce the environmental pollution caused by the irrational use of organic materials. Common organic materials include crop stalks， livestock and poultry manure， agricultural production and processing wastes， and domestic sludge. Different organic materials have different components and physical and chemical properties， and the same kind of organic materials also has differences due to different environmental conditions. The main components of common organic materials[19-23] are shown in Table 1.
　　Mechanisms of Organic Material Remediation of Soil Pb Pollution
　　Increasing soil adsorption complexity
　　After entering the soil， the organic materials are converted into humus under the decomposition of microorganisms. Humus is a complex macromolecular aromatic polymer compound containing a large number of oxygen-containing active groups such as carboxyl groups， alcoholic hydroxyl groups and phenolic hydroxyl groups. It has a high reactivity and a large specific surface area， so humus can be used as a good carrier to absorb heavy metals[24]. At the same pH （4.4）， the adsorption of heavy metal ions in soils with high organic matter content is 30 times that of soils with ordinary minerals[25]. Therefore， increasing the content of organic matter in soil can increase the adsorption capacity of Pb of soil， and effectively reduce the activity of Pb in soil. At the same time， these active groups can also have complex reactions with heavy metals in soil[26]， generating the compounds with low solubility， and can also reduce the activity of Pb in soil. 　　Changing soil pH
　　After the organic materials are applied to the soil， the organic acids produced during humification and CO2 generated during mineralization can produce acids when dissolved in soil water， which results in a decrease in soil pH， thereby increasing the activity of heavy metals in the soil[27]. However， Zhan et al.[28] found that when applying pig manure to soil， soil pH was positively correlated with the application amount of pig manure and negatively correlated with the application time， and soil pH was stabilized after 60 d with a slight increase. Zhu et al.[29] also proved that the addition of organic material （Astragalus sinicus） increased soil pH， which increased with the increase of addition amounts， and when the addition amount of organic materials was 3% of soil quality， the pH values of paddy soil， red soil and latosol increased by 1.55， 0.8， and 1.33， respectively. When adding organic materials to acidic soils in the culture， the increase of soil pH was positively correlated with the content of ash alkali in organic materials[30].
　　The pH of soil solution is the main factor affecting the adsorption of heavy metals by humus. pH can influence the adsorption of heavy metals by humus by changing the occurrence state of heavy metals in soil， and can also influence the adsorption of heavy metals by humus by affecting the existence of active groups on humus. It is generally believed that the under high pH conditions， the dissociation of H from the carboxyl and phenolic hydroxyl groups in humic acid becomes great， which is more favorable to form a stable complex with the metal ion[31]. Liang et al.[32] studied 2 kinds of variable-charged soils and found that the adsorption amount of Pb2+ in soil increased with the increase of pH， and the desorption amounts also increased with the increase of pH. The desorption amount was the largest when the pH was 4.8 of ferrallitic soil and 4.2 for alfisol， and then the desorption amount decreased with the increase of pH. Chen et al.[33] studied the effects of 3 sources of humic acid on the adsorption rate of Pb2+ at different pH conditions， finding that pH could significantly affect the complexation reaction of humic acid with Pb2+. Qiao et al.[34-35] studied the existing forms and adsorption-desorption of Pb2+ under the conditions of different pH levels. The results showed that bioavailability coefficient decreased with the increase of pH， and the adsorption capacity increased with the pH values. The desorption quantity increased with adsorption capacity， while the amplitude decreased with pH values. 　　Changing soil redox potential （Eh）
　　When applied to soils， organic materials can consume a large amount of oxygen for its decomposition， which cause the soil to be in an anaerobic reduction state[36]， and reduce the Eh of soil. Even in the presence of oxygen， humus produced by the decomposition of organic materials still contains the functional groups with reducibility[37-38]. Sulfur is a good ligand for Pb in the soil. In the reduced state， the oxidation of sulfur in the soil is inhibited， which can form insoluble sulfide with the carbonate-bound Pb in the soil[39]， thus reducing the content of available Pb in soil.
　　Affecting soil cation exchange capacity （CEC）
　　CEC refers to the total number of exchangeable cations per unit mass of dry soil absorbed under a certain pH condition， that is， the amount of negatively charged soil[40]. The application of organic materials to the soil can increase soil CEC[41]. Under similar conditions of other environmental conditions， the reducing effect of soil modifiers on Pb and other heavy metals under effective state enhances with the increase of soil CEC[42].
　　Affecting soil microorganisms
　　Microorganisms can reduce the available content of heavy metals in soil by adsorption， redox and mineralization[43-44]. The application of organic materials to the soil provides a large amount of carbon and nitrogen sources for the activity of microorganisms， resulting in great changes in the microbial flora and quantity in the soil[45]. Zhu Lin et al.[46] studied that straw added to the soil increased the amount of microorganisms in the soil. Xu Yanbo et al.[47] found that the addition of microorganisms to soil significantly reduced the concentration of exchangeable Pb in the soil， with a removal rate of 71%.
　　Effect of Organic Materials on the Available Pb
　　Since soil environment is complex and heterogeneous， the chemical reaction mechanisms are complex， and there are various states of heavy metals in the soil. Moreover， different forms of heavy metals have different bioavailability. Therefore， the biological toxicity of heavy metals depends to a large extent on their occurrence patterns[48]. There is no uniform definition of the available states of heavy metals. From the point of view of environmental science， the available states of heavy metals in soil are their environmental bioavailability and environmental availability， that is， the states of heavy metals which can be absorbed and utilized by creatures or produce toxic effects in soil[49]. There are a variety of detection methods for the available state of heavy metals in soil， of which chemical extraction and plant indication are commonly used methods[50-52]. Different extraction methods and agents used in chemical analysis can result in differences in the results， and the differences in the enrichment factors of indicative plants can also have different results. In this paper， the effects of different organic materials on available Pb was mainly indicated by the changes of available Pb in soil or the changes of enriched Pb in plants after the application of organic materials. 　　Organic materials can reduce available Pb content
　　The inhibitory effect of organic materials on available Pb is shown in Table 2[53-59]. As shown in Table 2， the inhibitory effect enhanced with the increase of the application amounts of organic materials of mushroom residues， coconut coir and silkworm excrement. Original soil pH could also affect the inhibition of organic materials to available Pb. Compared with low pH， the decrease of available Pb by organic materials was greater under high soil pH. However， pH was not the only factor affecting the available Pb， which might also be affected by soil Pb content， soil physical and chemical properties， and soil components.
　　Organic materialUse amountSoil pHRemediation effect
　　Naturally decomposed cowdung2% of soil quality5.22Compared with the control blank after deactivation， natural decomposed cow dung had a certain stabilizing effect on Pb with a stable efficiency of 13. 61%.
　　Pig manure500kg/hm26.3The available Pb content in soil decreased by 28.5% and 16.7% in the 1st and 2nd seasons of crops compared with the CK， respectively. Compared with CK， the cumulative volume of crops in the 1st and 2nd seasons of crops decreased by 27% and 13.9% respectively.
　　Mushroom residues1%， 5%， 10% and 20% of soil quality8.7The addition of mushroom residues significantly reduced the DTPA-Pb content with the reduction rate of 25.5-35.4%， and the reduction range increased with the increase of use amount.
　　Coconut coir， silkwormexcrement2%， 4% and 6% of soil quality7.17The inhibitory effect on available Pb increased with the increase of use amount， and the effect of 6% silkworm excrement was the best with a drop of 49.19%.
　　Decomposed pig manure1%， 2% and 3% of soil quality6.48The application amount of organic materials was significantly negatively correlated with the available Pb content in the soil. Compared with the control， the application of 3% decomposed pig manure decreased by 40.65%. At the same time， the application of organic material had a good inhibitory effect on the migration of Pb to roots and stems and leaves.
　　Cow manure and olive-leaf mixed compostTOC equaled to 10% of soil quality7.7Olive-leaf mixed compost made the total content of Pb decreased from 42.7% to 37.3%， and cow manure application made the total content of Pb decreased from 42.7% to 32.3%
　　Pyrolytic bamboo and ricestalks1% and 5% of soil quality-All treatments reduced the acid-extractable Pb content in the soil and decreased with increasing use amount. The effect of rice stalks was more significant， and the 5% rice stalk treatment had the decrease reached 34%. 　　"-" indicates there is no relevant data.
　　Organic material can increase available Pb content
　　The addition of organic materials may also increase the content of soil available Pb. Feng et al.[23] found that the combined application of leaves， rice bran， straws with chemical fertilizers increased the content of available Pb in soil， which was related with the combined application with chemical fertilizers. The application of nitrogen fertilizers can result in soil acidification， and applying urea to soil can have hydrolytic action， which can increase soil pH first. However， 1 week later， nitrification begins， which decreases soil pH to the level lower than that before the application. The application of ammonium nitrogen fertilizer can have nitrification， so soil pH drops[60]， causing the fixed Pb in soil to be released again. Tang et al.[19] also found that the addition of organic materials increased the available Pb content in soil and also increased the Pb content in the overground part of lettuce， which may be related to the complex root environment and root exudates.
　　For a certain period of time， the application of organic materials to the soil can enhance the adsorption and fixation of Pb in soil， reduce its content in available state， and reduce plant absorption. However， the mineralization and decomposition of organic materials in soil makes it possible that the adsorbed and fixed heavy metal ions can be released again in the second or third year. The heavy metal ions contained in the organic materials themselves can also enter into soil， thereby increasing the total amount of heavy metals and the available content[61]. Wang et al.[62] found that although the application of rice straw and pig manure significantly reduced the Pb content in plants and brown rice during the early cropping of rice， the effect of organic materials on available Pb fluctuated significantly during late rice， and straw application significantly increased the Pb content in straws and brown rice. Moreover， the increase was greater in the application with straw of high Pb content than of low Pb content.
　　Problems and Prospects
　　Heavy metal pollution in soil is becoming more and more serious， and its threat to the development of sustainable agriculture and human health issues cannot be ignored. As one of the 8 heavy metals， Pb deserves our attention. Soil Pb pollution in farmland is different from metal tailings and abandoned mine pits， which are mostly mild to moderate pollution[63]. Organic materials have a wide range of sources and low prices. They can be applied to the soil as passivating agents for Pb and other heavy metals， which can not only increase soil nutrients， thereby realizing resource reuse， but also reduce the availability of heavy metals in soil. However， most of the domestic studies on the remediation of organic materials to soil are still in the stage of laboratory culture， pot experiment and plot test. The research on in-situ remediation of field crops needs to be strengthened. At the same time， most scholars only study the effect of organic materials on soil pH after soil application， while ignoring the impact on soil Eh， CEC， and microorganisms. These indicators also affect the available state of Pb， and should be detected in future experiments. 　　After the application， organic materials can fix soil Pb， but the fixation gradually weakens with the passing of application time. Lime can reduce the harm of heavy metals by increasing the pH of soil， but long-term application of lime can destroy soil aggregate structure， which can affect the abundance of soil microorganisms and community structure， thereby resulting in soil erosion and nutrient loss[64]. Phosphorus-containing materials have a good remediation effect and a low price， and are listed as one of the best measures to manage Pb-contaminated soil by the US Environmental Protection Agency. However， the excessive use of phosphorus can cause crop nutrient deficiency[65]. In future experiments， it is suggested to make combined application of organic materials with lime or phosphorus-containing substances， so as to achieve efficient fixation of available Pb without compromising soil quality.
　　The remediation of organic materials of Pb-contaminated soil is in-situ passivation remediation， which can not reduce the total amount of Pb in soil， but may introduce foreign pollutants with the addition of organic materials， increasing the risk of soil contamination. However， the addition of organic materials can promote plant growth and absorption of nutrient elements. In future， studies can be carried out to the effect of organic material addition on the growth and enrichment of hyperaccumulators of Pb， and reduce the total amounts of soil Pb and other heavy metals through the cooperation of the two.
　　References
　　[1] Ministry of Environmental Protection， Ministry of Land and Resources. National soil pollution survey communique of China[N]. China National Land Resources News， 2014-04-18（002）.
　　[2] SU QJ， WANG J， LU XH， et al. Analysis and study on heavy metal content in the suburban vegetable garden soil and vegetables[J]. Journal of Guizhou Normal University （Natural Sciences）， 2016， 34（6）： 27-32.
　　[3] YANG JY， YANG XE， HE ZL. Resource and bioavailability of lead in soils[J]. Chinese Journal of Soil Science， 2005， 36（5）： 765-772.
　　[4] YAO Z， LIU J. Investigation and analysis of soil Pb contamination at two sides of arterial traffic in Yellow river delta area[J]. Agricultural Science & Technology， 2016， 17（12）：2722-2725.
　　[5] MARKUS J， MCBRATNEY A B. A review of the contamination of soil with lead ： II. Spatial distribution and risk assessment of soil lead[J]. Environment International， 2001， 27（5）：399-411. 　　[6] YAN W， MAHMOOD Q， PENG D， et al. The spatial distribution pattern of heavy metals and risk assessment of moso bamboo forest soil around lead–zinc mine in Southeastern China[J]. Soil & Tillage Research， 2015， 153 ：120-130.
　　[7] ZHOU QX， KONG FX， ZHU L. Ecotoxicology[M]. Beijing： Science Press， 2004：318-319.
　　[8] BOLAN N， KUNHIKRISHNAN A， THANGARAJAN R， et al. Remediation of heavy metal（loid）s contaminated soils-to mobilize or to immobilize[J]. Journal of Hazardous Materials， 2014， 266（4）：141-166.
　　[9] XIANG J， CHEN YH， XIANG M， et al. Comparative study on soil heavy metal pollution remediation technology[J]. Journal of Anhui Agricultural Sciences， 2014（22）：7367-7369.
　　[10] OTTOSEN L M， JENSEN P E， KIRKELUND G M， et al. Electrodialytic Remediation of Different Heavy Metal-Polluted Soils in Suspension[J]. Water Air & Soil Pollution， 2013， 224（12）：1707-1716.
　　[11] FAN T， YE WL， CHEN HY， et al. Review on contamination and remediation technology of heavy metal in agricultural soil[J]. Journal of Eco-environment， 2013， 22（10）：1727-1736.
　　[12] FARHADIAN M， VACHELARD C， DUCHEZ D， et al. In situ bioremediation of monoaromatic pollutants in groundwater： a review[J]. Bioresource Technology， 2008， 99（13）：5296-5308.
　　[13] ZHANG X， ZHOU AG， GAN YQ， et al. Advances in bioremediation technologies of contaminated soils by heavy metal in metallic mines[J]. Environmental Science and Technology， 2010， 33（3）：106-112.
　　[14] YANG QL， WU ZZ， CHEN JL， et al. Research status of phytoremediation of heavy metals contaminated soil and prospects of water and fertilizer control technologies[J]. Ecology and Environment Sciences， 2015， 24（6）：1075-1084.
　　[15] YANG JY， HE LP. Progress on the research of remediation technology of soil contaminated by heavy metals[J]. Environmental Science Survey， 2014， 33（2）：87-91.
　　[16] CAO XD， WEI XX， DAI GL， et al. Combined pollution of multiple heavy metals and their chemical immobilization in contaminated soils： a review[J]. Chinese Journal of Environmental Engineering， 2011， 5（7）：1441-1453.
　　[17] LI P， WANG WJ. Utilization status and prospect of agricultural wastes in China[J]. Tianjin Agricultural Sciences， 2009， 15（3）：46-49.
　　[18] PENG CY， LUO HL， KONG J. Advances in estimation and utilization of crop residues resources in China[J]. Chinese Journal of Agricultural Resources and Regional Planning， 2014， 35（3）：14-20.
　　[19] TANG MD， AI SY， LI MJ， et al. Effect of organic materials on the growth and Pb concentration of Lactuca sativa[J]. Journal of Agro-Environment Science， 2011， 30（11）： 2178-2184. 　　[20] YAO GH， XU HZ， ZHU LG， et al. Effects of different kinds of organic materials on soil heavy metal phytoremediation efficiency by Sedum alfredii Hance[J]. Environmental Science， 2015， 36（11）： 4268-4276.
　　[21] ZHU XW， HUANG GL， ZUO WG， et al. Effects of different organic materials on heavy metal contents and availability in mudflat soil[J]. Jiangsu Agricultural Sciences， 2016， 44（11）： 467-470.
　　[22] SHAO Y， CHEN JW， LI XM， et al. The remediation of growth and yield characteristics of wheat under Pb stress by four kinds of organic materials[J]. Ecology and Environment Sciences， 2017， 26（2）：315-322.
　　[23] FENG NN， WU JG， HU J， et al. Dynamic changes of Pb in soils using natural organic materials[J]. Journal of Northeast Agricultural Sciences， 2016， 41（1）：64-67.
　　[24] SACHS S， BERNHARD G. Influence of humic acids on the actinide migration in the environment： suitable humic acid model substances and their application in studies with uranium—a review[J]. Journal of Radioanalytical & Nuclear Chemistry， 2011， 290（ 1）： 17-29.
　　[25] SAUVE S， MANNA S， TURMEL MC， et al. Solid-solution partitioning of Cd， Cu， Ni， Pb， and Zn in the organic horizons of a forest soil[J]. Environmental Science & Technology， 2003， 37 （22）： 5191-5196.
　　[26] WANG YK. Remediation of heavy metal contaminated soils using organic materials and clay minerals[D/OL]. Nanjing： Nanjing Forestry University， 2009：2-3[2017-11-25]. http：//tsweb.lib.gznu.cn：8000/rwt/06/http/P75YPLUDN3WXTLUPMW4A/KCMS/detail/detail.aspx？dbcode=CMFD&QueryID=25&CurRec=1&dbname=CMFD2010&filename=2009201194.nh&urlid=&yx=&uid=WEEvREcwSlJHSldRa1FhdXNXa0hHbm45YlhTMjhvcGY4MytwTUpVSVBlZz0=$9A4hF_YAuvQ5obgVAqNKPCYcEjKensW4IQMovwHtwkF4VYPo HbKxJw！！&v=MDE1MjR1eFlTN0RoMVQzcVRyV00xRnJDVVJMS2 ZadVptRnlqblVMdk5WMTI3RjdHNEg5REZxNUViUElSOGVYMUw=.
　　[27] WANG YK， ZHANG HC， HAO XZ， et al. A review on application of organic materials to the remediation of heavy metals contaminated soils[J]. Chinese Journal of Soil Science， 2010（5）：1275-1280.
　　[28] ZHAN SJ， YU H， FENG WQ， et al. Effects of different organic materials and lime on soil pH and available cadmium[J]. Southwest China Journal of Agricultural Sciences， 2011， 24（3）： 999-1003.
　　[29] ZHU YX， MA L， ZHU LM， et al. Effect of organic materials incorporation on pH， iron species and adsorption and desorption of copper in acid soils under redox condition[J]. Soils and Fertilizers Sciences of China， 2011， （5）： 65-68.
　　[30] SPARLING GP， MCLAY CDA， RAPHAEL C， et al. Effect of short-term legume residue decomposition on soil acidity[J]. Australian Journal of Soil Research， 1999， 37（3）： 561-573. 　　[31] XIAZHADAN A， AIKEBAIER Y， TURSUM T， et al. Effect of pH on the complex reaction of humic acid， fulvic acid with Cd2+[J]. Xinjiang Agricultural Sciences， 2012， 49（9）： 1701-1706.
　　[32] LIANG J， XU RK， JIANG X， et al. Adsorption and desorption of Cu（II）， Pb（II） and Cd（II） in two variable charge soils at different pH[J]. Soils， 2007， 39（6）： 992-995.
　　[33] CHEN Y， ZHANG ML， GUAN LZ， et al. Effects of pH on the adsorption of Pb2+ and Mn2+ on humic acids from three different sources[J]. Chinese Agricultural Science Bulletin， 2010， 26（12）： 67-69.
　　[34] QIAO DM， QI XB， PANG HB， et al. The effect of pH level on existing form of Pb2+[J]. Journal of Soil and Water Conservation， 2010， 24（6）： 173-176.
　　[35] QIAO DM， QI XB， PANG HB， et al. Effect of pH level on adsorption characteristics of Pb2+[J]. Chinese Journal of Soil Science， 2011， 42（1）：38-41.
　　[36] HAO YX， LIU JX， YUAN MX， et al. Effects of long-term organic amendments of soil N2O emission from winter wheat-maize cropping systems in the Guanzhong Plain[J]. Environmental Science， 2017， 38（6）： 2586-2593.
　　[37] AESCHBACHER M， SANDER M， SCHWARZENBACH R P. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances[J]. Environmental Science & Technology， 2010， 44（1）：87-93.
　　[38] BAUER I， KAPPLER A. Rates and extent of reduction of Fe （III） compounds and O2 by humic substances[J]. Environmental Science & Technology， 2009， 43（13）： 4902-4908.
　　[39] YANG F， DING KQ， LIU TF. Research advances of distribution and conversion of heavy metal chemical forms and their affecting factors[J]. Journal of Anhui Agricultural Sciences， 2014， 42（29）：10083-10084，10096.
　　[40] LI JF， QU MK， HUANG B， et al. Spatially non-stationary relationships between cation exchange capacity and related control factors[J]. Acta Pedologica Sinica， 2017， 54（3）：638-646.
　　[41] WANG LQ， LUO L， MA YB， et al. In-situ immobilization remediation of heavy metals-contaminated soils： a review[J]. Chinese Journal of Applied Ecology， 2009， 20（5）： 1214-1222.
　　[42] NING JY， ZHOU GD， ZHOU CE， et al. Researches on the passivation of heavy metals in agricultural soils： a review[J]. Journal of Hangzhou Normal University （Natural Science Edition）， 2016， 15（2）：156-162.
　　[43] WU M， WANG R， GUAN Y， et al. Soil microbial remediation of heavy metal pollution mechanism research[J]. Natural Science Journal of Harbin Normal University， 2014， 30（3）： 147-150.
　　[44] CHENG Y， ZHAO XQ. Study on the consolidation and mineralization of Pb2+ by carbonate-mineralization bacteria[J]. Environmental Sciences Research， 2016， 29（10）： 1513-2020. 　　[45] GUO XW， ZHANG LH， LI K， et al. Effect of organic materials on available nutrients and microbial numbers of grape replant soi[J]. Chinese Agricultural Science Bulletin， 2009， 25（19）：147-151.
　　[46] ZHU L， ZHANG CL， SHEN QR. The influence of applying organic materials in the continuous cropping soil on the soil pH and EC value and microbe[J]. Journal of Anhui Agricultural University， 2001， 28（4）： 350-353.
　　[47] XU YB， QIAN CX， LU ZW. Remediation of heavy metal contaminated soils by bacteria biomineralization[J]. Chinese Journal of Environmental Engineering， 2013， 7（7）：2763-2768.
　　[48] HAN CM， WANG LS， GONG ZQ， et al. Chemical forms of soil heavy metals and their environmental significance[J]. Chinese Journal of Ecology， 2005， 24（12）：1499-1502.
　　[49] ZHOU WH， ZHANG JJ， ZOU MM， et al. The detection and monitoring of available heavy metal content in soil： a review[J]. Chinese Journal of Eco-Agriculture， 2017， 25（4）：605-615.
　　[50] HOUBA VJG， LEXMOND TM， NOVOZAMSKY I， et al. State of the art and future developments in soil analysis for bioavailability assessment[J]. Science of the Total Environment， 1996， 178（1–3）：21-28 .
　　[51] FAN SX. Soil heavy metal pollution and control[M]. Beijing： China Environmental Science Press， 2011：28-49.
　　[52] LIU YR， DANG Z， SHANG AA. Study on bioindicability of heavy metals in polluted soil using phytoindicating[J]. Environmental Pollution and Control， 2003， 25（4）： 215-217.
　　[53] WU LS， ZENG DM， MO XR， et al. Immobilization impact of different fixatives on heavy metals contaminated soil[J]. Environmental Science， 2015， 36（1）： 309-313.
　　[54] LI RM， WANG G， FANG L. Effects of lime complexation organic manure on uptake of Cd， Pb by crops[J]. Journal of Agro-Environment Science， 2003， 22（3）：293-296.
　　[55] ZHANG XJ， YANG SX， DUAN C， et al. Amelioration of lead-zinc tailings by spent mushroom compost： effects on growth of Lolium perenne L. and physico-chemical properties of tailings[J]. Journal of Agro-Environment Science， 2014， 33（3）： 526-531.
　　[56] LI QJ， LI DR， WANG YH， et al. Effects of three kinds of organic materials on physicochemical properties and available heavy metals in soil[J]. Journal of Soil and Water Conservation， 2013， 27（6）： 182-185.
　　[57] XU YM， LIN DS， LV JB， et al. Effects of organic manure and molecular sieve material on speciation of Cd， Pb and Cu in soil and bioavailability[J]. Journal of Agro-Environment Science， 2006， 25（2）： 326 -330. 　　[58] CLEMENTE R， áNGELES E， BERNAL M P. Heavy metals fractionation and organic matter mineralisation in contaminated calcareous soil amended with organic materials[J]. Bioresource Technology， 2006， 97（15）：1894-1901.
　　[59] LU K， YANG X， GIELEN G， et al. Effect of bamboo and rice straw biochars on the mobility and redistribution of heavy metals （Cd， Cu， Pb and Zn） in contaminated soil[J]. Journal of Environmental Management ， 2017， 186： 285-292.
　　[60] ZHOU J. Impact of nitrogen fertilizer and simulated acid rain on soil acidification[D/OL]. Hangzhou： Zhejiang University， 2013：19-20[2017-12-14]. http：//tsweb.lib.gznu.cn：8000/rwt/06/http/P75YPLUDN3WXTLUPMW4A/KCMS/detail/detail.aspx？dbcode=CMFD&QueryID=0&CurRec=1&dbname=CMFD201301&filename=1013153446.nh&urlid=&yx=&uid=WEEvREcwSlJHSldRa1 FhdXNXa0hHbm45YlhTMjhvcGY4MytwTUpVSVBlZz0=$9A4hF_ YAuv Q5obgVAqNKPCYcEjKensW4IQMovwHtwkF4VYPoHbKxJw！！&v=MjAyMzk4ZVgxTHV4WVM3RGgxVDNxVHJXTTFGckNVUkxLZl p1Wm1 GeWpnVXI3SlZGMjZIYks5SGRYSXFaRWJQSVI=.
　　[61] MCGRATH S P， SANDERS J R， SHALABY M H. The effects of soil organic matter levels on soil solution concentration and extractabilities of manganese， zinc and copper[J]. Geoderma， 1988， 42（2）： 177-188.
　　[62] WANG KR， ZHANG YZ， HU RG. Effects of different types of soil amelioration materials on reducing concentrations of Pb and Cd in brown rice in heavy metal polluted paddy soils[J]. Journal of Agro-Environment Science， 2007， 26（2）： 476-481.
　　[63] CUI B， WANG L， ZHANG GY， et al. Status and harm of heavy metal pollution in soil and research progress in remediation technology[J].Journal of Anhui Agricultural Sciences， 2012， 40（1）：373-375.
　　[64] LI JR， XU YM， LIN DS， et al. In situ immobilization remediation of heavy metals in contaminated soils： a review[J]. Ecology and Environment Sciences， 2014， 23（4）： 721-728.
　　[65] WANG JH， ZHANG F， MA L. Research progress of Stabilization binder for heavy metals contaminated soil remediation[J]. China Resources Comprehensive Utilization， 2016， 34（2）：49-52.