[Effects of Chlorine-based and Sulfur-based Fertilizers on Rice Bioavailability of Cd in Soils].

To explore the effects of single or combined application of chlorine-and sulfur-based fertilizers on rice bioavailability of Cd in soils, pot experiments with reddish clayey soil (developed from quaternary red clay parent materials) under three exogenous Cd levels (0, 0.5, and 2.0 mg·kg-1) were conducted. Meanwhile, chlorine-based fertilizers (KCl, NH4Cl) and sulfur-based fertilizers[K2SO4, (NH4)2SO4] were added in different proportions. The soil pH, Cd morphology, and Cd accumulation in rice at different growth stages were analyzed. The results revealed that both chlorine-and sulfur-based fertilizers could acidify the soil; however, the effect of chlorine-based fertilizers was more significant. During the filling stage of rice, the soil pH value of the treatment of applying single chlorine-based fertilizer decreased by 0.28 on average compared with that of applying single sulfur-based fertilizer. At the maturity stage of rice, chlorine-based fertilizer could activate the residual Cd, whereas sulfur-based fertilizer passivated the acid-extracted Cd to its residual state. Compared with the single application of the same fertilizer, the combined application of chlorine-and sulfur-based fertilizers was more likely to promote the accumulation of Cd in rice plants. The highest Cd accumulation of brown rice was 0.21 mg·kg-1 (2.0 mg·kg-1 exogenous Cd level) in the 1:1 (mole ratios of Cl:S) treatment of chlorine-and sulfur-based fertilizers, which was 16.4% higher than that of single chlorine-based fertilizer and 113.3% higher than that of single sulfur-based fertilizer. Therefore, the combined application of chlorine-fertilizers and sulfur-based fertilizers will increase the concentration of Cd in brown rice. To ensure food quality and safety, it is more advisable to apply single sulfur-based fertilizer for rice planting.

[Effects of Chelate GLDA on the Remediation of Cadmium Contaminated Farmland by Pennisetum purpureum Schum].

In order to clarify the effects of chelate tetrasodium glutamate diacetate (GLDA) on the remediation of cadmium (Cd) contaminated farmland by Pennisetum purpureum Schum, GLDA was applied in different methods within 60 days:total application doses of 585, 1170, and 2340 kg·hm-2 were equally divided into 1-4 applications, respectively, and the time intervals of 2-4 applications were 30, 20, and 15 days, respectively. The biomass, Cd content, and amount of Cd extracted from aboveground parts of Pennisetum purpureum Schum, in addition to the pH, dissolved organic carbon (DOC) mass concentration, and other indicators in soil were analyzed. The results showed that the biomass and Cd content of the aboveground parts of Pennisetum purpureum Schum increased significantly when GLDA was applied many times at a low application dose. The number of applications and the total dose were the key factors influencing the biomass and Cd content, respectively. The highest amount of Cd extracted was 16.78 g·hm-2 at 585 kg·hm-2 (applied four times), which was 275.39% higher than the CK treatment (i.e., no GLDA was applied). There was significant positive correlations between the pH, mass concentration of DOC, and content of DTPA-Cd, and the total dose and number of applications. The mass concentration of DOC was the main factor affecting the DTPA-Cd content. The total Cd content in the soil (after at 585 kg·hm-2 was applied four times) decreased by 3.23% compared with that of the soil before planting Pennisetum purpureum Schum. Therefore, the application of GLDA is of great significance for the remediation of Cd contaminated farmland by Pennisetum purpureum Schum, and the application method should be selected reasonably.

[Effects of Water Management on Soil Properties and Cd Behavior of Typical Paddy Soils].

To explore the effects of water management mode on Cd environmental behavior in different parent-material-developed paddy soils, two parent-material-developed paddy soils (yellow clayey soil and granitic sandy soil) under three exogenous Cd levels (0.5, 2.0, and 5.0 mg·kg-1) with different water management modes (long-term flooding, moistening irrigation, and wet-dry rotation) were cultured in this study. The soil redox potential (Eh value), pH value, Cd concentration in soil solution, and Cd fractionation were also determined. The results showed that water management mode had different effects on the pH and Eh values of soils developed from different parent materials. The change rates of soil pH value were as follows:long-term flooding:-2.61% (yellow clayey soil), 2.25% (granitic sandy soil); alternation of dry and wet:-1.96% (yellow clayey soil); 0.52% (granitic sandy soil); wet irrigation:-4.08% (yellow clayey soil) and -0.52% (granitic sandy soil). The Eh value of the soils was negatively correlated with the pH value. The influence pattern of water management model on Cd mass concentration of soil solutions in two parent-material soils was consistent. The Cd mass concentration of soil solutions in granitic sandy soil was higher than that in yellow clayey soil. The mean values of Cd concentration were 1.03 µg·L-1 for yellow clayey soil and 1.07 µg·L-1 for granitic sandy soil. Water management mode had no significant effect on the proportions of organic bound Cd or Fe-Mn bound Cd in two different parent-material-developed soils. The long-term flooding mode promoted the transformation of exogenous Cd to residual Cd, and this promotion in yellow clayey soil was higher than that in granitic sandy soil. In conclusion, during the process of regulating soil Cd bioavailability through water management, the role of soil parent materials needs to be considered.

Effects of Cd-resistant bacteria and calcium carbonate + sepiolite on Cd availability in contaminated paddy soil and on Cd accumulation in brown rice grains.

A pot experiment was conducted to evaluate the effects of combined application of cadmium (Cd)-resistant bacteria (J) and calcium carbonate + sepiolite (G) on both Cd bioavailability in contaminated paddy soil and on Cd accumulation in rice plants. Adding the mixture (J + G) to the soils significantly increased soil pH, decreased extractable Cd contents, and increased Fe/Mn-oxide Cd and organic-bound Cd contents. The applying of J + G, J and G decreased Cd contents in various rice tissues (roots, stems and leaves, husks, and brown rice grains) to different degrees. Compared with those of the CK, Cd contents decreased by 17.8%-53.3% in the roots, 12.3%-27.4% in the stems and leaves, 25.4%-44.6% in the husks, and 28.8%-55.7% in the brown rice grains for the application of J + G; Cd contents decreased by 8.2%-28.5% in the roots, 11.5%-32.0% in the husks, and 27.8%-45.9% in the brown rice grains for the application of J; Cd contents decreased by 12.9%-26.5% in the roots, in the stems and leaves decreased by 4.6%-34.1% in the stems and leaves, 60.2%-79.7% in the husks, and 35.7%-47.6% in the brown rice grains for the application of G. The alone application of bacteria (J) could reduce the bioavailability of Cd in soil and the contents of Cd in brown rice grains to some extent. Moreover, when the bacteria were applied in combination with mineral (J + G), it was a more effective method than the alone application of J or G to reduce the soil Cd bioavailability. Under all the tested conditions, applications of J4+G4 (320 mL kg-1 of J + 8 g kg-1 of G) resulted in the greatest reduction in Cd contents in brown rice grains. Overall, the results indicated that the combination of Cd-resistant bacteria and mineral material could effectively reduce Cd bioavailability in paddy soils and inhibit Cd accumulation in brown rice grains.

[Cd Balance Analysis of a Typical Rice Paddy System in Central Hunan].

By conducting field positioning experiments, we studied the development trend of Cd pollution in a typical paddy system. The samples of atmospheric deposition and irrigation water were collected monthly from November 2015 to November 2018 during which fertilizer, soil, and rice samples were also collected. The Cd concentration in the samples was monitored and analyzed to conduct research on the balance between Cd inputs and outputs in a typical paddy system in Hunan Province. The results suggest that through irrigation water, atmospheric deposition and fertilizer, the average annual input of Cd in the paddy field system is 8.735 g·(hm2·a)-1, of which atmospheric deposition, the major source, accounts for 69.15%-82.04% of the total input, with an average of 76.61%. This is followed by irrigation water and fertilizer, respectively, accounting for 12.62%-23.66% and 5.34%-7.19%, with an average of 16.94% and 6.45%, respectively. Through surface runoff, soil infiltration and the rice harvest of the aboveground portion, the annual average output of Cd contained in the paddy system is 7.093 g·(hm2·a)-1. Rice harvest accounts for 85.27%-95.02% of the total output, with an average of 89.69%; surface runoff accounted for 4.57%-13.96% of the total output, with an average of 9.41%; and soil infiltration accounted for 0.41%-1.51% of the total output, with an average of 0.90%. The study indicates that Cd contained in paddy systems in Central Hunan exhibits a net input, and the soil Cd pollution is increasing as a result. Straw returning and straw removal have an important impact on the soil Cd balance, and straw removal can slow the trend of soil Cd pollution accumulation.

Cadmium and arsenic accumulation during the rice growth period under in situ remediation.

Rice (Oryza sativa L.) planted in cadmium (Cd)- and arsenic (As)-contaminated soil is considered the main source of dietary Cd and As intake for humans in Southeast Asia and thereby poses a threat to human health. Minimizing the transfer of these pollutants to rice grain is an urgent task for environmental researchers. The main objective of this study was to investigate the effects and the mechanisms of a combined amendment (hydroxyapatite + zeolite + biochar, HZB) on decreasing Cd and As accumulation in rice. In situ remediation and aqueous solution adsorption experiments were conducted. The results showed that after application of HZB, Cd and As concentrations of the exchangeable fraction and TCLP extraction in soil decreased with the growth of rice plants. Cd concentrations in rice tissues were decreased at the tillering, filling and maturing stages after in situ remediation, while As concentrations in rice tissues were decreased only at the maturing stage. When 8 kg·plot-1 (9000 kg ha-1) HZB was applied, concentrations of Cd and inorganic As in brown rice were decreased to 0.18 and 0.16 mg kg-1, respectively, lower than the levels permissible for grain in China, i.e., 0.2 mg kg-1. Application of HZB reduced Cd accumulation in rice tissues, and the suppression of Cd accumulation was significantly greater than that of As. Furthermore, HZB significantly increased rice grain yield. An aqueous solution adsorption experiment demonstrated that HZB could adsorb and covalently bind Cd and As (V) via -OH, -COOH, -Si-O-Si and CO32- groups to produce carboxylates, silicates and carbonates, thereby promoting in situ immobilization of Cd and As in soil solution.

[Effects of a Tribasic Amendment on Cadmium and Arsenic Accumulation and Translocation in Rice in a Field Experiment].

An in-situ paddy field experiment was carried out to study the influence of a tribasic amendment (QFJ, hydroxyapatite+zeolite+biochar) on Cd and As accumulation and translocation in rice grown in soil contaminated with cadmium and arsenic, with the concentrations of soil Cd and As being 3.58 mg·kg-1 and 124.79 mg·kg-1, respectively. The results showed that, after application of QFJ, the pH, CEC, and OM contents of the rice rhizosphere soil tended to increase. The exchangeable concentrations of Cd and As were reduced from 0.37 mg·kg-1 and 0.07 mg·kg-1 to 0.12 mg·kg-1 and 0.04 mg·kg-1, respectively. The concentrations of Cd and As in rice tissues decreased after in-situ restoration. When 9.00 t·hm-2 of QFJ was applied, the Cd concentration in brown rice was reduced from 0.46 to 0.18 mg·kg-1, and that of inorganic As was reduced from 0.25 to 0.16 mg·kg-1, both lower than 0.2 mg·kg-1, meeting the requirement set by the National Food Standards (GB 2762-2012). QFJ application decreased the Cd and As bioaccumulation capacity of the roots and decreased the rice plant's capacity of Cd translocation quantity from the underground parts to the aerial parts; at same time, the Cd transferring capacity of the roots and the As transferring capacity in straw and husk were also decreased.

[Dynamics of Rice Photosynthesized Carbon Input and Its Response to Nitrogen Fertilization at the Jointing Stage:13 C-CO2 Pulse-labeling].

Photosynthesized carbon (C) is an important source of soil organic C in paddy fields, and its input and distribution are affected by rice growth and soil fertility. Fertilizer application plays an important role in rice growth. The 13C pulse-labeling method was used to quantify the dynamics and distribution of input photosynthesized C in the rice-(rhizosphere-and bulk-) soil system and its response to nitrogen fertilizer (N) application. The results suggested that N fertilization significantly increased the rice aboveground and the root biomass and decreased the rice biomass root/shoot ratio. The amount of assimilated 13C gradually decreased in the rice plants but gradually decreased over 0-6 days and increased over 6-26 days in the rhizosphere and bulk soil during rice growth. N fertilization significantly increased the amount of assimilated 13C in the rhizosphere soil by 9.5%-32.6% compared with the control. In comparison to the unfertilized treatment, the application of N fertilization resulted in higher photosynthetic13C in rice aboveground and in the root by 24.5%-134.7% and 9.1%-106%, respectively. With the N fertilized and unfertilized treatments, 85.5%-93.2% and 91.3%-95.7%, respectively, of input photosynthetic 13C was distributed in the rice plants. The results suggested that N fertilization significantly affected the distribution of photosynthesized C in the rice-soil system (P<0.01). After 26 days of pulse labeling, the distribution of photosynthetic 13C into rice aboveground was increased by 13.4%, while the distribution into the rhizosphere and bulk soil were decreased by 21.9% and 52.2%, respectively, in the N fertilized treatments compared with the unfertilized treatments. Therefore, the N application increased the distribution of photosynthesized carbon in the soil-rice system but decreased the accumulation in the rhizosphere and bulk soil. The findings of this study provided a theoretical basis for our understanding of the dynamic of photosynthetic C in the plant-soil system and the assimilation of the soil organic matter pool in the paddy soil ecosystem.

[Differences in Cd Accumulation in Typical Soils Under the Double Rice System].

Pot experiments were used to study the differences of Cd uptake and accumulation in double-cropping rice in typical soil types. To analyze the soil availability of Cd (DTPA-Cd) in soils and the Cd accumulation in double-cropping rice at different growth stages of the rice, we conducted pot experiments that selected the yellow clayey soil (paddy soil developed from plate shaley parent materials) and the granitic sandy soil (paddy soil developed from granitic parent materials). Exogenous Cd was added with gradients of 0, 0.5, 1.0, 2.0, 5.0, and 10.0 mg·kg-1. Results showed that, during the rice growth period, the available Cd in the yellow clayey soil was higher than that in the granitic sandy soil, and the difference was significant (P<0.01). This showed that the content of Cd in rice (roots, shoots, leaves, rice shells, and brown rice) increased along with the treatment level and with the extension of the rice growth period. The accumulation characteristics of Cd in rice grains and other tissues of rice indicated differences between two seasons and two soil types, that is, late rice was higher in Cd than was early rice, and reddish yellow clayey soil was higher in Cd than granitic sandy soil. Significant positive linear correlations were found between the effective contents of Cd in soils and those in rice tissues (roots, shoots, leaves, and brown rice). The prediction model of Cd in rice and the characteristic equation for rice accumulation of Cd were applied to calculate the critical values of Cd:0.98 mg·kg-1 for early rice and 0.83 mg·kg-1 for late rice in reddish yellow clayey soil, and 0.86 mg·kg-1 for early rice and 0.56 mg·kg-1 for late rice in granitic sandy soil. These threshold values are higher than the National Standards given in "farmland environmental quality evaluation standards for edible agricultural products (HJ 332-2006)." The soil security threshold values and the soil environmental capacities of the two different parent materials varied greatly; therefore, different environmental quality standards may be formulated and different measures may be needed to control Cd pollution in different parent materials.

Cadmium uptake, accumulation, and remobilization in iron plaque and rice tissues at different growth stages.

Rice consumption is considered the main source of human dietary Cd intake in Southeast Asia. This study aimed to investigate Cd uptake, accumulation, and remobilization in iron plaque and rice (Oryza sativa L. cv. 'Xiangwanxian 12') tissues at different growth stages. A pot experiment was performed in two Cd-contaminated paddy soils. Cd concentrations in iron plaque and rice tissues at five different growth stages (tillering, booting, milky, dough, and maturing) were measured. Cd concentrations in iron plaque and rice tissues (roots, stems, leaves, spikelet, husks, and brown rice) varied with growth stage. Cd accumulation in rice plants increased with extending growth in both soils, reaching 15.3 and 35.4µg/pot, respectively, at the maturing stage. The amounts of Cd in brown rice increased from the milky to maturing stages, with the greatest percentage uptake during the maturing stage. Cd amount in iron plaque significantly affected the uptake and accumulation of Cd in roots and aerial parts of rice plants. Accumulated Cd in leaves was remobilized and transported during the booting to maturing stages, and the contributions of Cd transportation from leaves to brown rice were 30.0% and 22.5% in the two soils, respectively. A large amount of Cd accumulated in brown rice during the maturing stage. The transportation of remobilized Cd from leaves was also important for the accumulation of Cd in brown rice.

Effects of an additive (hydroxyapatite-biochar-zeolite) on the chemical speciation of Cd and As in paddy soils and their accumulation and translocation in rice plants.

A pot experiment was carried out to investigate the remediation of paddy soils contaminated with both Cd and As. The effects of a soil additive (HZB: hydroxyapatite + zeolite + biochar) on the chemical speciation of Cd and As in the soil and on the accumulation and translocation of these two elements in rice plants were studied. The application of HZB decreased the concentration of acid-extractable Cd in the soil by 7.3-32.6% by promoting the transformation of soil Cd from soluble to insoluble species; as a result, the concentration of organically bound Cd in the soil increased by 6.8-49.5%. The application of HZB also promoted the transformation of soil As from soluble to insoluble species, thus increasing the concentration of Ca-bound As by 34.1-93.4% and reducing the concentration of soil-exchangeable As by 12.2-55.1%. However, when the application rate of HZB was greater than 4.0 g kg-1, the concentration of soil-exchangeable As increased again. It was found that the application of HZB decreased the Cd and As bioaccumulation capacity of rice root, and among various rice organs, rice husk was the highest in Cd transferring capacity, whereas rice root was the highest in As transferring capacity. When the amount of HZB applied was 0.05-0.2%, the Cd and As concentrations in the various parts of the rice plants decreased significantly.

[Effect of Phosphorus Addition on the Abundance of Autotrophic CO2-Fixation Microorganisms in Rhizospheric Soil from a Phosphorus-Limited Paddy Field].

A rice pot experiment was conducted to investigate the effect of phosphorus addition on the abundance of autotrophic CO2-fixation microorganisms using phosphorus-limited paddy soil from the Changsha Observation and Research Station for the Agricultural Environment. Rice seedlings were transplanted in the paddy soil with or without phosphorus addition, corresponding to P-treated-pot (P) or control pot (CK), respectively. Rhizosphere soils were collected from the P and CK treatments during the tillering and shooting stages. The physical and chemical soil properties were measured and the abundance of autotrophic CO2-fixation microorganisms was quantified with a real-time PCR technique based on four functional genes (cbbL, cbbM, accA, and aclB) involved in three CO2-fixation pathways (CBB cycle, rTCA cycle, and 3-hydroxypropionate/4-hydroxybutyrate cycle). The results show that phosphorus addition improves the concentrations of DOC and Olsen-P and the pH value, whereas negative effects on the MBC and NH4+-N concentrations are revealed during the tillering stage. The effect of phosphorus addition on the NO3--N concentration in the tillering and shooting stages differs. Phosphorus addition significantly increases the abundances of the cbbL, cbbM, accA, and aclB genes, which are 156%, 99%, 110%, and 193% higher than those of the CK treatment in the tillering stage. However, this positive effect is not notable for the cbbL, accA, and aclB genes during the shooting stage. Redundancy analysis (RDA) shows that Olsen-P is the environmental factor that most significantly affects the abundance of autotrophic CO2-fixation microorganisms.

[Effects of Different Water Managements and Soil Eh on Migration and Accumulation of Cd in Rice].

A pot experiment was conducted to study the effects of four modes of water managements on soil Eh values,bioavailability of soil Cd,migration and accumulation of Cd in rice.These four modes of water managements were moistening throughout the entire period of rice growth (M),moistening before filling stage and flooding after filling stage (M-F),flooding before filling stage and moistening after filling stage (F-M),and flooding throughout the entire period of rice growth (F).The results indicated that the exchangeable contents of Cd increased firstly and then declined with the soil Eh values changing from negative to positive.Compared with M,the other three modes (M-F,F-M,and F) significantly reduced the contents of Cd in all rice tissues,including roots,stems,husks and brown rice.Meanwhile,Cd contents in brown rice due to the treatments of M-F and F were 0.19 mg·kg-1 and 0.10 mg·kg-1,respectively.These Cd contents were lower than the limits of 0.2 mg·kg-1 in national food safety standard (GB 2762-2012).Compared with M,the other three modes significantly decreased Cd accumulation amounts in the aboveground parts of rice and also decreased Cd translocation factors in rice.There were significant positive exponential relations between soil Eh values and Cd accumulation amounts in the aboveground parts of rice,Cd translocation factors in rice,or Cd contents in brown rice.The rice biomass due to M-F treatment reached the maximum among the four modes.It was worthy to be mentioned that although Weiyou 46 was considered a variety rice with high Cd accumulation,Cd content in rice brown of Weiyou 46 could be lower than 0.2 mg·kg-1 in the 5 mg·kg-1 of Cd contaminated soil through proper water managements.In summary,M-F treatment ensured high rice yield with low Cd contents in brown rice and could be recommended as the irrigation mode in rice production.Simultaneously,maintaining soil Eh value between -160--130 mV was also important after the filling stage.

[Effect of Straw Application on the Dynamics of Exogenous Nitrogen and Microbial Activity in Paddy Soil].

Returning straw to the field provides an important source of fertilizer that can increase soil fertility. However, the rate of straw carbon utilization is low and large amounts of greenhouse gases are emitted due to the high carbon to nitrogen ratio of the straw mass. In this regard, the application of inorganic nitrogen and phosphate fertilizers can control the ratio of elements in the soil, increase the activity of microorganisms and their utilization of elements, and promote the improvement of soil fertility. In this study, straw application conditions were simulated, and inorganic nitrogen fertilizer labeled with 15N was added to examine the effects of different nutrient fertilizer additions on the transformation and distribution of exogenous nitrogen in the soil, and also the characteristics of the microbial response. The results showed that application of straw increased the contents of ammonia nitrogen and total nitrogen in the soil and soil solution. When both straw and inorganic nitrogen fertilizer were applied, the 15N-TN in the soil remained at 28 to 33 µg during the 100-day culture phase. In contrast, 15N-NH4+ increased gradually during the initial 30 days of the culture phase, but subsequently decreased gradually. Application of phosphate increased the contents of 15N-TN and 15N-NH4+ in the soil, but decreased the content of 15N in the soil solution by 28%. The distribution of inorganic nitrogen in the soil showed that the proportion of 15N in the soil remained at 52%-61%. Addition of phosphate fertilizer increased the distribution ratio of 15N in the soil by up to 16.5%, whereas the proportion of 15N in the soil solution decreased from 36% on the fifth day to 30% on the 100th day, thereby the loss amount of 15N reduced by 1.2-fold. Addition of straw promoted microbial activity and significantly increased the microbial biomass nitrogen (MBN) content of the soil. Addition of inorganic fertilizer further promoted the microbial activity of the soil. After the 100-day culture experiment, the addition of straw, inorganic nitrogen, and phosphate fertilizer increased MBN to between 2.0-fold and 2.2-fold that of the control treatments. Addition of phosphate fertilizer increased the utilization of 15N by microorganisms, so that the amount of 15N-MBN was 1.5-fold higher than that of treatments where only straw and nitrogen fertilizer were added. Examination of soil enzyme activity showed that nitrogen fertilizer reduced soil enzyme activity and substrate affinity. When both nitrogen and phosphate fertilizers were added, the enzyme activity was 48.1% higher than that when only straw was added. The findings of this study thus provide a theoretical basis for furthering our understanding on the nitrogen cycle of the paddy soil ecosystem, the improvement of soil fertility, and the reduction of greenhouse gas emissions.

[Correlations Between Different Extractable Cadmium Levels in Typical Soils and Cadmium Accumulation in Rice].

Pot experiments were used to study the correlations between different extractable cadmium levels in typical soil and cadmium accumulation in rice. To analyze the pH, Cd in soil solution(SSE-Cd), TCLP extractable Cd level(TCLP-Cd), and Cd accumulation in rice at different growth stages of rice, we conducted pot experiments which selected the reddish clayey soil(developed from quaternary red clay parent materials) and purple paddy field(developed from purple sandy shale parent materials), meanwhile added with exogenous Cd with the gradients of 0, 0.5, 1, 2, 5, 10 mg·kg-1. The results showed that, during the rice growth period, the content of SSE-Cd in reddish clayey soil was in the range of 0 and 2.5 µg·L-1, and the average content was 0.57 µg·L-1; TCLP-Cd was in the range of 0 and 0.25 µg·L-1 with the average content of 0.10 mg·kg-1;The content of SSE-Cd in purple paddy field was in the range of 0 and 1.6 µg·L-1 with the average content of 0.48 µg·L-1; TCLP-Cd was in the range of 0 and 0.2 mg·kg-1, and the average content was 0.07 mg·kg-1. It showed that the cadmium concentrations in soil solution and the TCLP extractable Cd levels were both significantly reduced in two types of soil with the extension of rice growth period, and the content in reddish clayey soil was higher than that in purple paddy. The TCLP extractable Cd level was significantly positively correlated with Cd concentration in soil solution. The total Cd accumulation in rice plants gradually increased with increasing exogenous Cd concentration. There were significant positive correlations between Cd concentration in soil solution and Cd concentration in rice, Cd concentration in soil extracted by TCLP method and Cd concentration in rice and total Cd accumulation in rice plant. The soil environmental capacities of the two different parent materials varied greatly,and the safety threshold of Cd in purple paddy field was 2.06 times of that of reddish clayey soil. There were significant differences in Cd uptake and accumulation in different soils, so different measures may be needed to control Cd pollution in different parent materials. TCLP extractable Cd was more relevant with total Cd accumulation in rice, and had more extraction amount. Therefore, the TCLP method can more accurately evaluate the biological availability of soil Cd.

A three-year in-situ study on the persistence of a combined amendment (limestone+sepiolite) for remedying paddy soil polluted with heavy metals.

In order to study the persistence of a combined amendment (LS, limestone+sepiolite) for remedying paddy soil polluted with the heavy metals Pb and Cd, a three-year in-situ experiment was conducted in a paddy soil near a mining area in southern Hunan, China. LS was applied at rates of 0, 2, 4, and 8g/kg (w/w); rice was subsequently planted for the three consecutive years of 2012 (first season), 2013 (second season), and 2014 (third season). Experimental results indicated that LS significantly increased soil pH values for all three seasons, and the enhancement ranked as follows: first season>second season>third season. Under the experimental conditions, the effect of LS on decreasing exchangeable concentrations of soil Pb and Cd was as follows: first season (97.6-99.8% for Pb and 88.3-98.9% for Cd)>second season (80.7-97.7% for Pb and 28.3-88.0% for Cd)>third season (32.6-97.7% for Pb and 8.3-71.4% for Cd); the effect of LS on reducing Pb concentrations in brown rice was: first season (73.5-81.2%)>third season (29.6-68.1%)>second season (0-9.7%), and that for reducing Cd concentrations in brown rice was third season (72.7-81.0%)>first season (56.1-66.8%)>second season (20.9-32.3%). For all three seasons, the effect of LS on reducing Cd content in brown rice was better than that for Pb. The highest translocation factors for Pb and Cd were from rice straw to husk, implying that the husk of rice plants was the main organ in which heavy metals accumulated. The effect of LS for decreasing soil exchangeable Cd content was relatively persistent, but that for Pb gradually decreased with time, implying that LS was more suitable for the long-term remediation of Cd-polluted soil than Pb-polluted soil.

Accumulation of Heavy Metals in Vegetable Species Planted in Contaminated Soils and the Health Risk Assessment.

The objectives of the present study were to investigate heavy metal accumulation in 22 vegetable species and to assess the human health risks of vegetable consumption. Six vegetable types were cultivated on farmland contaminated with heavy metals (Pb, Cd, Cu, Zn, and As). The target hazard quotient (THQ) method was used to assess the human health risks posed by heavy metals through vegetable consumption. Clear differences were found in the concentrations of heavy metals in edible parts of the different vegetables. The concentrations of heavy metals decreased in the sequence as leafy vegetables > stalk vegetables/root vegetables/solanaceous vegetables > legume vegetables/melon vegetables. The ability of leafy vegetables to uptake and accumulate heavy metals was the highest, and that of melon vegetables was the lowest. This indicated that the low accumulators (melon vegetables) were suitable for being planted on contaminated soil, while the high accumulators (leafy vegetables) were unsuitable. In Shizhuyuan area, China, the total THQ values of adults and children through consumption of vegetables were 4.12 and 5.41, respectively, suggesting that the residents may be facing health risks due to vegetable consumption, and that children were vulnerable to the adverse effects of heavy metal ingestion.

[Comparison of the Persistence of a Combined Amendment Stabilizing Pb, Cd, Cu and Zn in Polluted Paddy Soil].

A three-year in-situ experiment was conducted in a paddy soil near a mining area in southern Hunan in order to study the persistence of combined amendment of limestone+sepiolite (marked as LS) stabilizing Pb, Cd, Cu and Zn in polluted paddy soil. LS with ratios of 0, 2, 4, and 8 g·kg-1 was applied once to the paddy soil, and rice was subsequently planted for three consecutive years of 2012 (first season), 2013 (second season), and 2014 (third season). The experimental results indicated that:①LS significantly increased soil pH values for all three seasons, and the enhancement ranked as follows:first season > second season > third season. ② LS obviously decreased the exchangeable contents of soil Pb, Cd and Zn for all three seasons, and the decreasing magnitude of exchangeable contents of soil heavy metals was 32.6%-97.7% for Pb, 8.3%-71.4% for Cd, and 10.9%-83.5% for Zn, respectively, in the third season; however, there was no significant decrease for Cu. The effects of LS decreasing exchangeable contents of soil heavy metals in three seasons followed the order of Pb > Zn > Cd > Cu. ③ LS decreased contents of Pb and Cd in brown rice in the third season by 26.7%-66.7% and 59.1%-80.3%, respectively, and the reduction trend increased with increasing LS application. Cu and Zn contents in brown rice did not decrease effectively. The effect of LS reducing contents of Pb, Cd, Cu and Zn in brown rice followed the order of Pb > Cd > Cu > Zn for the first season and the second season, but Cd > Pb > Zn > Cu for the third season. For all three seasons, the total effect of LS reducing heavy metal contents in brown rice followed the order of Pb > Cd > Cu > Zn. ④ The effect of LS stabilizing soil Pb and Cd emerged gradually with time. Therefore, LS was suitable for remedying soil polluted with Pb and Cd for a relatively long time because of its persistence.

[Provoking Effects of Exogenous Zn on Cadmium Accumulation in Rice].

Pot experiments were carried out to study the influences of different concentrations of exogenous Zn on accumulation of Cd in various rice organs of low Cd accumulation cultivar Xiang-Wanxian 12 (XWX12) and high Cd accumulation cultivar Wei-You 46 (WY 46) exposed to soil with medium and serious Cd pollution. The results showed that:In the soil with medium Cd pollution, Cd contents in various rice organs of two rice varieties were increased by exogenous Zn, and the Cd contents in brown rice of XWX12 and WY46 were increased by 125.0% -275.0% and 6.6% -91.2%, respectively, but still lower than 0.2 mg·kg-1. In the soil with serious Cd pollution, Cd accumulation in various rice organs were reduced by exogenous Zn. Cd contents in brown rice of XWX12 and WY46 were reduced by 16.6%-63.5%, and 15.6%-74.4%, respectively, and Cd contents in brown rice of WY 46 were gradually decreased with increasing exogenous Zn application, resulting in lower than 0.2 mg·kg-1 of Cd in brown rice. The correlations of Cd contents in brown rice and exchangeable contents of Cd and Zn in soil were different depending on the Cd pollution levels and the rice varieties. In the soil with medium Cd pollution, Cd content in brown rice of XWX12 was linearly positively related to exchangeable Zn content in soil, and Cd content in brown rice of WY46 was linearly positively related to exchangeable Cd or Zn contents in soil. In the soil with serious pollution, however, the correlation of Cd contents in brown rice of WY46 and the contents of exchangeable Cd or Zn in soil was linear negative correlation. Therefore, for improvement of the rice quality, under the condition that total Zn content in soil was lower than the pollution level, applying a certain amount of Zn to reduce Cd contents in brown rice was feasible in the soil with serious pollution.

[Transformation and mobility of arsenic in the rhizosphere and non-rhizosphere soils at different growth stages of rice].

Speciation and bioavailability of arsenic in the rhizosphere and non-rhizosphere soils at different growth stages (tillering stage, jointing stage, booting stage, filling stage and maturing stage) of rice (Oryza sativa L.) were studied using toxicity characteristic leaching procedure (TCLP) and arsenic speciation analysis. Pot experiments were conducted and the soil samples were taken from a certain paddy soil in Hunan Province contaminated by mining industry. The results showed that: (1) With the extension of rice growth period, pH values and TCLP extractable arsenic levels in the rhizosphere and non-rhizosphere soils increased gradually. Soil pH and TCLP extractable arsenic levels in non-rhizosphere soils were higher than those in the rhizosphere soils at the same growth stage. (2) At the different growth stages of rice, contents of exchangeable arsenic (AE-As) in rhizosphere and non-rhizosphere soils were lower than those before the rice planting, and increased gradually with the extension of the rice growing period. Contents of Al-bound arsenic (Al-As), Fe-bound arsenic (Fe-As) and Ca-bound arsenic (Ca-As) increased gradually after rice planting, but not significantly. Residual arsenic (O-As) and total arsenic (T-As) decreased gradually after rice planting, by 37.30% and 14.69% in the rhizosphere soils and by 31.38% and 8.67% in the non-rhizosphere soils, respectively. (3) At the different growth stages of rice, contents of various forms of arsenic in the soils were in the following order: residual arsenic (O-As) > Fe-bound arsenic ( Fe-As) > Al-bound arsenic (Al-As) > Ca-bound arsenic (Ca-As) > exchangeable arsenic (AE-As). In the pH range of 5.0- 5.8, significant positive linear correlations were found between most forms of arsenic or TCLP extractable arsenic levels and pH values, while the Ca-bound arsenic was poorly correlated with pH values in the rhizosphere soils.