Abstract Based on the literature and experimental results， three kinds of amendments， namely， rice biochar， hydroxyapatite and potassium dihydrogen phosphate （KH2PO4）， and deeper ploughing were selected to evaluate the field application effect of soil amendments and agronomic measures on the remediation of Cd contamination in greenhouses. Cd-contaminated greenhouse screening was conducted from 2015 to 2017. In September 2017， comparative tests of eight treatments were performed in September 2017， and a selective test was performed in September 2018. The results of the screening of contaminated areas indicated that the distributions of over-standard sites and Cd content were significantly different. Greenhouse No. 4， in which the highest over-standard rate was 83.33% and the average content of Cd in soil was 0.535 mg/kg， was used as a comparative test experimental site for eight treatments. Greenhouse No. 1 was selected as the optimum test greenhouse for selective testing， with three plots having average Cd concentrations of 0.530， 0.568 and 0.792 mg/kg. The 8 months following comparative treatment showed that the content of available Cd following the treatment of rice biochar+hydroxyapatite+deeper ploughing （treatment T6） was reduced by 32.55% and 24.96% compared with the CK （the control） and 2 months following treatment， respectively. The content of available Cd using rice biochar+KH2PO4+deeper ploughing （treatment T7） treatment decreased by 47.88% and 31.00% compared with the CK and 2 months following treatment. The selective test results showed that in rice biochar+hydroxyapatite+deeper ploughing： the total Cd content of decreased from 0.489 to 0.372 mg/kg （23.86% lower）， compared with the CK， and the mean content of available Cd decreased by 10.71% after 8 months， which was the lowest available Cd measured （0.133 mg/kg）. In addition， the bean plant Cd content， bioconcentration factor （BCF） and translocation coefficient （TF） decreased by 15.86%， 23.68% and 25.77%， respectively. Rice biochar+hydroxyapatite+deeper ploughing is a favoured technology for the remediation of Cd-contaminated protected vegetable fields.
　　Key words Facility vegetable fields; Cd; Contaminant remediation; Technical study
　　The accumulation of Cd in arable soil has attracted much attention due to its harmful effects on human health through the food chain of agricultural products and groundwater[1-2]. The long-term high agricultural investment and high multiple cropping indexes in greenhouse vegetable cultivation cause the risk of pollution of greenhouse vegetables. The long-term accumulation of heavy metals is significantly higher than that in open-air vegetable fields[3-4]， which may be caused by closed or semi-closed special environments， loss of rainwater， etc. The remediation of heavy metals that will not affect normal production and yield has been studied extensively[5]. The application of agronomic remediation includes water and fertilizer source control， filling， rotation， uniform tillage， deeper ploughing， etc. Physicochemical remediation includes adjusting pH， amendments， etc.[6]and bioremediation technology includes biological flora[7]enrichment or low absorption phytoremediation[8]， etc. Different restoration techniques have different principles and advantages. There is less research on the actual application of combined amendments for protected vegetables. 　　On the basis of previous experiments and references， three kinds of soil amendments， including rice biochar， hydroxyapatiteand potassium dihydrogen phosphate （KH2PO4）[9-10]， were selected to address Cd pollution in greenhouse vegetables. The changes in Cd content in soil and the absorption characteristics of vegetables were compared and analyzed.
　　Materials and Methods
　　General test conditions
　　Vegetable Rotation： Croissant Crisp+Bean， 2018; Time： January 2015-October 2019.
　　Geographical coordinates： 116°46′26″ E， 38°31′58″ N; Average greenhouse planting： 15 years.
　　Collection of soil samples
　　Soil samples were collected according to the Technical Specifications for Monitoring of Soil Environmental Quality in Farmland （NY/T395） from plough layers 0-20 cm deep. Setting： 3 repeated sampling zones in Greenhouse in which mixed samples were collected using a sampling method consisting of 5 diagonal sampling points. Each sampling point was marked so that the next sampling points were nearly as far as possible.
　　Sample evaluation
　　Soil samples were air-dried， ground and sieved through a 100 mesh nylon sieve. Vegetable samples were sequentially washed three times with tap water and distilled water， and then dried at 105 ℃ for 20 min， dried at 70 ℃ and crushed for evaluation. The soil samples were pretreated according to DD2005-01 of the China Geological Survey technical standards， and the vegetable samples were pretreated according to Food Hygiene Inspection Methods GBT5009.11-2003 and GBT5009.17-2003. The analytical quality was controlled by using an internal standard according to GBW07427 （GSS-13） and GBW10011 （GSB-2）. An Agilent 7700XICP-MS mass spectrometer was used for the determination.
　　Physicochemical properties
　　The basic physical and chemical properties of the test materials are shown in Table 1， including pH， EC， Cd content， superficial area， pore volume， porosity， organic matter， total potassium， total phosphoru， and total nitrogen.
　　Data processing and analysis
　　BCF （Bioconcentration factor）=plant Cd content （mg/kg）/soil Cd content （mg/kg）;
　　TF （Translocation factor）=Cd content in aerial part of plant （mg/kg）/Cd content in root （mg/kg）.
　　IBM SPSS Statistics 20， Excel and SAS statistical software were used for data processing and chart analysis.
　　Screening of Cd-contaminated greenhouse
　　Contaminated vegetable field screening
　　Results analysis of large-scale random sampling from 2015 to 2016 indicated that the Cd content was approximately 0.8 mg/kg near the same centralised area that was repeatedly identified when intensive sampling was performed in May 2017 in four nearby facilities. Greenhouse Nos. 1-4 are oriented in a south-north direction and were built progressively southward. They cover areas of 0.37， 0.27， 0.34 and 0.27 hm2， respectively， and 27， 18， 24 and 18 samples were collected from the respective greenhouse areas， representing sample areas of 135.87， 148.22， 138.96， 151.92 m2， respectively. Each greenhouse was divided into east， middle and west sections， using an S-shaped sampling method; the east section was sampled from south to north， the middle section was sampled from north to south， and the west section was sampled from south to north. Individual plots were sampled according to methods described in "Collection of soil samples" above. 　　Data analysis
　　According to the Environmental Quality Evaluation Standard for Greenhouse Vegetable Producing （HJ/T333-2006 and HJ/T333-2006）， the total Cd standard is ≤0.4 mg/kg （pH>7.5）. As shown in Fig. 1， the average total Cd concentration of the soil in the four greenhouses was 0.388 mg/kg. The average Cd concentrations for Greenhouse Nos. 1-4 were 0.386， 0.361， 0.264 and 0.542 mg/kg， respectively. The over-standard site ratios for Greenhouse Nos. 1-4 were 37.04%， 27.78%， 0% and 83.33%， respectively. The respective Cd over-standard rates for the east， middle and west sections were as follows： 24.14% with a mean Cd concentration of 0.328 mg/kg， 44.83% with a mean Cd concentration of 0.421 mg/kg and 31.03% with a mean Cd concentration of 0.416 mg/kg. The degree of Cd contamination in the sections increased as follows： west > middle > east. There was a significant linear correlation between the Cd content and the sampling sites in Greenhouse No. 4， R2=0.604 4， while no correlations were observed for other three greenhouses.
　　Screening results
　　The distribution of over-standard sites in each greenhouse was uneven， and the total Cd content in soil was notably different. In Greenhouse No.4， the Cd over-standard ratio was 83.33% with a mean CD concentration of 0.542 mg/kg， which presented conditions to perform a comparative test of eight treatments. In Greenhouse No. 1， there were three concentrated plots， X6+X7 （average CD concentration of 0.530 mg/kg）， X16+X17+X18 （average CD concentration of 0.568 mg/kg） and X21+X22 （average CD concentration of 0.792 mg/kg）， for use in conducting selective testing.
　　Pollution Remediation Research
　　Comparative test
　　Experimental design
　　As discussed in the literature and based on experimental results， eight treatments， including rice biochar， hydroxyapatite， KH2PO4 and deeper ploughing single or combined， were compared in Greenhouse No. 4 in September 2017. Each treatment was repeated three times， and each treatment plot was 32 m2 in area. The dosage of amendments was as follows： 10 800 kg/hm2 rice biochar， 225 kg/hm2 hydroxyapatite and 450 kg/hm2 KH2PO4 together with deeper ploughing 30 cm beneath the ground surface. After watering and before sowing， the amendment was sprinkled evenly on the soil surface. The soil was ploughed and mixed and then placed carefully. Other management practices were unified in the same manner as other vegetable greenhouses. 　　The eight treatments were as follows： CK （control）; hydroxyapatite （T1）; rice biochar （T2）; deeper ploughing （T3）; hydroxyapatite+deeper ploughing （T4）; rice biochar+deeper ploughing （T5）; rice biochar+hydroxyapatite+deeper ploughing （T6）; and rice biochar+KH2PO4+deeper ploughing （T7）.
　　Soil samples were collected 2 and 8 months following treatment in accordance with the methods described in "Collection of soil samples".
　　Data analysis
　　As shown in Fig. 2， compared with after 2 months （first sampling） following the average available Cd in soil did not change， the average total Cd concentration in soil decreased after 8 months （second sampling）. In soil designated as CK （control）， the total Cd decreased and available Cd increased. The total Cd and available Cd in soil decreased simultaneously in the treatments using rice biochar+hydroxyapatite+deeper ploughing （T6） and rice biochar+KH2PO4+deeper ploughing （T7）. The calculated least significant difference （LSD） was not significant.
　　1 is the first sampling and 2 is the second sampling. The test treatments are as follows： CK （control）; hydroxyapatite （T1）; rice biochar （T2）; deeper ploughing （T3）; hydroxyapatite+deeper ploughing （T4）; rice biochar+deeper ploughing （T5）; rice biochar+hydroxyapatite+deeper ploughing （T6）; and rice biochar+KH2PO4+deeper ploughing （T7）.
　　The change rate analysis of total Cd in soil： As shown in Fig. 3， total Cd levels after 8 months of remediation were lower than that after 2 months except for hydroxyapatite+deeper ploughing treatment （T4）. The total Cd in soil decreased by 23.31% and 21.30% following rice biochar+deeper ploughing （T5） treatment and rice biocharcoal+hydroxyapatite+deeper plowing （T6） treatment， respectively.
　　The change rate analysis of available Cd content in soil： As shown in Fig. 3， the available Cd in the second CK sampling increased by 44.30%. The available Cd in the second sampling following the hydroxyapatite （T1） treatment was higher than that in the CK second sampling and that in the first sampling following the hydroxyapatite （T1） treatment. The available Cd following the rice biochar （T2） treatment increased by 25.98% in the first sampling and decreased by 15.59% in the second sampling compared with the CK， in which the available Cd only decreased by 3.30% in the second sampling compared with the first sampling. The available Cd in soil in the second sampling following the deeper ploughing （T3） treatment increased 20.54% and decreased 19.54% in the first sampling compared with the CK in which there was only a 3.68% increase in available Cd in the second sampling compared with the first sampling. The available Cd in soil increased in both the first and second samplings following the hydroxyapatite+deeper ploughing （T4） treatment. The available Cd in soil following the rice biochar+deeper ploughing （T5） treatment decreased 12.51% and 43.40% in the second sampling compared with the corresponding CK， which was 6.65% lower than that in the first sampling following the T5 treatment. The available Cd in soil following the rice biochar+hydroxyapatite+deeper ploughing （T6） treatment increased by 29.72% in the first sampling and decreased by 32.55% in the second sampling compared with the CK， in which the available Cd decreased by 24.96% in the second sampling compared with the first sampling. The available Cd following the rice biochar+KH2PO4+deeper ploughing （T7） treatment in the first sampling was 9.00% higher than that in the CK sampling but in the second sampling was respectively 47.88% and 31.00% lower than that in the second CK sampling and the first sampling following the T7 treatment. The levels of available Cd following seven remediation treatments were 16.49% lower than that of the CK. The LSD was not significant. 　　Comparison results
　　Total Cd data were used to determine whether Cd concentrations in soil exceeded the established Cd standard and the changes in available Cd related to plant absorption and showed the application effect of the remediation agents. In the field experiment， the uncontrollable factors were complex， and the soil Cd content was in a dynamic state. After 8 months of cultivation， the effect of soil Cd remediation was stable relatively. The total Cd and available Cd contents in soil were reduced by the rice biochar+deeper ploughing （T5） treatment， but the available Cd content changed minimally between the first and second samplings， only 6.95%. The available Cd in the second sampling following the rice biochar+hydroxyapatite+deeper ploughing （T6） treatment was 32.55% and 24.96% lower than that of the CK second sampling and that of the first T6 sampling， respectively. The available Cd content in the second sampling following the rice biochar+KH2PO4+deeper ploughing （T7） treatment was 47.88% and 31.00% lower than that of the CK second sampling and that of the first T7 sampling， respectively. Therefore， the rice biochar+hydroxyapatite+deeper ploughing （T6） and rice biocarbon+KH2PO4+deeper ploughing （T7） treatments were determined as the selective treatments for further remediation experimentation.
　　Selective test
　　Experimental design
　　In September 2018， three over-standard concentrated plots， X6+X7， X16+X17+X18 and X21+X22， in Greenhouse No. 1 were initially intensively tested， and then selective tests were performed according to the screening results and dosage described in "Contaminated vegetable field screening" above. Each test plot area was 25 m×0.8 m/row×3 rows=60 m2. The remediation agent was evenly distributed on both sides of the ploughed ridge， which was 10 cm away from the root. Other management practices were unified in the same manner as vegetable greenhouses. The treatments were designated as follows： rice biochar+KH2PO4+deeper ploughing （Treatment 1）; and rice biochar+ hydroxyapatite+deeper ploughing （Treatment 2）.
　　In the second sampling in November 2018， samples of the aerial parts of bean plants （stem+leaf+fruit） and the root samples from the plants were collected according to the one-to-one principle. Soil samples were collected for the third time in April 2019.
　　Data analysis
　　Analysis of Cd content in soil
　　Total Cd analysis： Table 2 shows that the mean total Cd concentrations for the first densification samples all exceeded the established standard and were very close. In the process of restoration， the average concentration of total Cd in Treatment 1 decreased at first and then increased， and in it decreased continuously， but the difference was not obvious. After restoration， the total Cd in decreased from the highest concentration of 0.489 mg/kg reported for all three treatments to 0.372 mg/kg （23.86% decrease）， which was not over-standard. Compared with the results of the first sampling， the total Cd content of all treatments showed a downward trend， and the LSD multiple comparison analysis was not significant. 　　The maximum and minimum concentrations of total Cd in the first sampling showed that the soil Cd was over-standard and that the concentrations were greater than or close to the standard value. After restoration， the minimum concentrations did not exceed the standard and the maximum concentration decreased significantly.
　　Available Cd analysis： The difference of the mean concentrations of available Cd for the first dense sampling was small. After restoration， compared with the CK， the available Cd in decreased to 0.133 mg/kg （10.71% decrease）， which was the lowest among the three treatments. However， the available Cd in treatment 1 increased by 18.12% compared with the CK. Compared with the first sampling， the available Cd increased by 56.79% and 20.1% in treatment 1 and， respectively. The LSD multiple comparison analysis showed no significant difference.
　　Analysis of bean absorption characteristics
　　The current Chinese National Standard for Food Safety （GB2762-2017） limit for cadmium in leafy vegetables is 0.2 mg/kg and the limit for other vegetables is 0.05 mg/kg. Prior to this experiment， the measured content of Cd in the bean fruits was approximately 0.025 mg/kg， which was very low， and the beans were not vegetables that could be easily enriched[11]. Therefore， the aboveground parts （stem+leaf+fruit mixed sample） and underground parts （roots） of bean plants were sampled in this study.
　　Compared with the CK， the content of Cd in the aerial part of the plant decreased by 0.36% and 15.86% in Treatments 1 and 2， respectively. The Cd concentrations increased in the roots for both treatments and increased by 25.50% in treatment 1. The BCF and TF of the green beans decreased by 23.68% and 25.77% in treatments 2， respectively.
　　Selection result
　　The total Cd content in decreased to 0.372 mg/kg， which was below the Cd standard. The average concentration of available Cd decreased by 10.71%， which was the lowest measured （0.133 mg/kg）.In addition， compared with the CK， decreased the Cd content in the aerial plant parts by 15.86%， increased Cd content in the underground roots and decreased the BCF and TF by 23.68% and 25.77%， respectively. Rice biochar+hydroxyapatite+deeper ploughing （treatment 2） is a better remediation technology for Cd-contaminated protected vegetable fields.
　　Discussion
　　Sample collection
　　This research process indicates that the large area of random sampling has important meaning in which an over-standard spot periphery perhaps exists as a universal over-standard and distributes remarkable unevenness. The results of the Cd content evaluation appear to fluctuate， which is perhaps a normal objective in the field. Therefore， to collect as many spot mixed samples as far apart as possible is of vital significance， and the representation and scientificity of the samples need to be evaluated. 　　Cd influencing factors
　　Greenhouse vegetable land is relatively closed， and climate conditions such as atmospheric deposition and precipitation have less impact， but there are still many factors affecting the change in soil Cd in the field with complex interaction processes. Yang et al.[12]explained that different nitrogen fertilizers added organic fertilizer that significantly increased the soil pH and decreased the amount of available Cd in soil differently. Wang et al.[13]found that the distribution of heavy metals in different particles is very different. In the case of soil contaminated with heavy metals， the soil exhibits the enrichment characteristics of fine particles， such as oxides， clay minerals and humus， which have higher specific surface area and higher adsorption capacity for heavy metals. Huang et al.[14]thought that Cd content was the highest in 2-0.25-mm soil aggregates. Roth et al.[15]found that Cd adsorption by coarse silt， fine silt and clay increased with the increase in temperature and organic matter. Xu et al.[16]（2019） studied the phytoremediation of Cd and Cu combined pollution by hydroxyapatite and four plants. Each combination can improve soil enzyme activities， and different combinations have different effects on soil characteristics， organic matter transformation， microbial activity and Cd and Cu content. Jasmin et al.[17]reported that the reduction of Cd content in indoor pot experiments with different poplar biochar added in two years was different， and the field conditions decreased by 84% in the first year and did not change in the second year. Therefore， the change in soil particle size， pH value and microbial population caused by organic fertilizer status， farming operation， water content， field temperature and remediation time may cause the fluctuation of soil Cd content， distribution change and speciation conversion.
　　In this study， compared the CK remediation effect of with two months and eight months in the two experiments， the CK exhibited a normal transformation rule in which total Cd decreased and available Cd increased while applying remediation agents that can reduce total Cd and available Cd at the same time， which indicated that remediation agents can promote the transformation of total Cd into available Cd， immobilise available Cd and reduce the effectiveness of Cd， thus reducing the risk of impact to soil and products. Cd availability， soil and product risk cannot be controlled by a lack of intervention by remediation measures. 　　Treatment agents
　　In recent years， biocarbonisation technology has become a frontier hotspot in the fields of global agriculture， ecology and carbon emission reduction. Biochar affects key soil physical and chemical properties （soil pH， cation exchange capacity， hydrophobicity， aggregates， etc.）， and microbial community structures[18-19]have strong adsorption capacities for heavy metal pollutants， promote the formation of precipitation or complexation of heavy metals and reduce the bioavailability of Cd in the soil. They inhibit Cd uptake by crops and reduce toxicity to plants[20]. He et al.[21]found that both kenaf core and sludge biochar can immobilise Pb and Cu in contaminated soil， but the immobilisation effect of Zn， Cd and As is not obvious.
　　Many scholars try to reduce or alleviate the toxicity of heavy metals in plants through the antagonistic effect of phosphorus， and phosphate fertilizer and hydroxyapatite are applied in the remediation of heavy metals in soil[22]. Alina et al.[23]explained that the interaction of P and Cd was mainly the effect of P on Cd uptake. He et al.[24]maintained that P fertilizer applied to oats， ryegrass， carrots and spinach could significantly reduce the Cd content in plants. Karblene[25]studied the effect of different farmland management measures on the Cd content of potato and ryegrass plants and found that the Cd content of plants decreased by approximately 41% with the increase of phosphate fertilizer. However， the results of Sterrett et al.[26]showed that the increase of soil P supply would promote the accumulation of Cd in plants.
　　In this study， biochar， the use of phosphorus-based treatment agents combined with deeper ploughing agronomic measures during the performance of single and combined treatment tests indicated that none of the treatments affected the growth and development of beans. The results indicated that the combination of treatment effects is better than single treatments. This study should continue to track the changes of test results over a specified period of time， determine the best treatment period and perform economic evaluations.
　　Conclusions
　　Based on the comparison of protected vegetable fields and selective experiments on them， the two treatments of rice biochar+KH2PO4+deeper ploughing and rice biochar+hydroxyapatite+deeper ploughing can effectively change the absorption characteristics of heavy metals in vegetables. Rice biochar+hydroxyapatite+deeper ploughing can reduce the content of available Cd in soil and can reduce the content of Cd， BCF and TF in bean plants more effectively. Rice biochar+hydroxyapatite+deeper ploughing is a favoured technology for the remediation of Cd-contaminated protected vegetable fields. 　　References
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