Qihong Zhu

Sepiolite is recommended for the remediation of Cd-contaminated paddy soil

Abstract This study aimed to assess the extractability of cadmium (Cd) and Cd uptake by rice after applying soil amendments of lime (Ca(OH)2) and sepiolite. A rice (Oryza sativa) cultivation experiment was conducted in a Cd-contaminated paddy field. After applying lime, sepiolite, and a mixture of the two as soil amendments, soil pH showed a significant increase (p<0.05), and Cd in soil extracted with NaNO3, CaCl2, and DTPA, was reduced by 61–100%, 52–98%, and 12–15%, respectively (p<0.05). The amount of exchangeable Cd showed a significant decrease in all treatments (p<0.05), and we found increases in the proportion of forms of carbonate-bound, Fe/Mn oxide-bound, and residual Cd in the soils, as measured by a sequential extraction method. The grain or rice straw biomass was neither significantly influenced by any amendment in the experiment, while the uptake of Cd by rice significantly decreased in all treatments (p<0.05). The results reveal no advantage in applying a mixture of sepiolite and lime compared with applying sepiolite alone in immobilizing Cd in the soil. The use of sepiolite or sepiolite mixed with lime was more effective than the use of lime alone in the immobilization of Cd in the soil. We therefore recommend sepiolite as a soil amendment to remediate Cd-contaminated paddy soil.

Phytoavailability of Cd and Pb in crop straw biochar-amended soil is related to the heavy metal content of both biochar and soil

Crop straw biochar incorporation may be a sustainable method of amending soil, but feedstock-related Cd and Pb content is a major concern. We investigated the effects of heavy metal-rich (RC) and -free biochar (FC) on the phytoavailability of Cd and Pb in two acidic metalliferous soils. Biochar significantly increased soil pH and improved plant growth. Pb in soil and plant tissues significantly decreased after biochar application, and a similar pattern was observed for Cd after FC application. RC significantly increased NH4NO3-extractable Cd in both lightly contaminated (YBS) and heavily contaminated soils (RS). The Cd content of plants grown on YBS increased, whereas it decreased on RS. The Cd and Pb input-output balance suggested that RC application to YBS might induce a soil Cd accumulation risk. Therefore, identifying heavy metal contamination in biochar is crucial before it is used as a soil amendment.

Effects of Peanut Shell Biochar on the Adsorption of Cd(II) by Paddy Soil

Soil from an experimental paddy field in southern China was incubated with peanut shell biochar to investigate effects of this additive on the adsorption and desorption characteristics of Cd(II) using batch methods. Incorporation of biochar increased adsorption of Cd(II) by the paddy soil at 20, 25, and 30°C; this trend was apparent both with increasing quantities of biochar and rising temperature. Incorporation of biochar primarily enhanced the non-electrostatic adsorption of Cd(II). In addition, supplementation with biochar decreased the adsorption rate of Cd(II), which decreased with increasing quantities of biochar. The Langmuir constant b and Freundlich constant kf both increased with greater quantities of biochar at different temperatures. Adsorption of Cd(II) was an endothermic process and occurred spontaneously. Incorporation of biochar decreased availability and mobility of Cd(II) to plants primarily through increased non-electrostatic adsorption of Cd(II) by paddy soil.

Horizontal and Vertical Distributions of Chromium in a Chromate Production District of South Central China

To study the horizontal and vertical distribution of chromium (Cr) in the soil of a chromate production site (CPS) and its nearby area (NA-CPS) in south central China, 61 profiles (depth: 14 m) in the CPS and 69 samples (topsoil) were excavated following a grid-sampling method. The geographic coordinates, elevation, and types of soil layers were recorded, and the total Cr in the soil and the total Cr and Cr(VI) in the leachate of the soil and in the groundwater were determined. Migration of Cr in surface soils may be represented in terms of a multiple linear regression equation (R2adj = 0.632). Distance, elevation, and pH are the primary factors that influence the horizontal distribution of Cr content in the surface soils, while the Cr concentration in different soil profiles mostly obeys the positive or negative binomial distributions. For a positive distribution, the Cr concentration decreases with increasing depth in the 0.0–8.0 m soil layer, under the fixing effect of soil. However, it shows an upward trend with a depth in the 8–14 m soil layer under the influence of Cr-polluted phreatic water. Under a negative distribution, Cr content is stable in the 0–6 m layer because of the influence of chromite ore processing residue mixed with miscellaneous fills, but it decreases obviously in the 6–14 m layer under the fixing effect of soil. Similar vertical distributions were observed for pH, LCr, LCr6+, and PCr6+. The decreasing amplitude of the Cr concentration for binomial distributions is mainly affected by the Cr concentration, pH, and LRCr of the soil. Moreover, PCr6+ of soil increases with pH, and the type of soil layer is the primary factor influencing LRCr in the soil profiles. Our results of the horizontal and vertical distributions of Cr could be used to guide investigations that are focused on reducing the number of samples in the horizontal and vertical directions at CPSs, and to improve risk assessments of CPSs and nearby areas.

Distribution and availability of cadmium in profile and aggregates of a paddy soil with 30-year fertilization and its impact on Cd accumulation in rice plant

The research was conducted to investigate the accumulation, distribution and availability of Cd in paddy soil and their relation to Cd in rice plants under 30-year fertilization regimes. Six treatments were involved in the study: control without fertilization (CK), chemical fertilizer (NPK), high nitrogen chemical fertilizer (HN), rice straw incorporation (ST), low and high dosage of manure fertilizer (LM and HM). Total and DTPA extractable concentration of Cd (T-Cd and DTPA-Cd) in bulk soils (20 cm topsoil), profiles (0-60 cm) and aggregates (>2, 1-2, 0.5-1, 0.25-0.5, 0.053-0.25 and < 0.053 mm) were investigated. The Cd concentration in relevant rice plant (roots, stems, leaves, husks and grains) were also analyzed. Manure fertilizers caused T-Cd accumulation in bulk soil with a significant increase of 36.2% in LM and 81.2% in HM. Similar impacts of manure fertilizers were observed in DTPA-Cd in the bulk soil. Further, the HM generated a further accumulation in deeper soil layers, presenting a remarkable increase of T-Cd (28.3%-225%) in 10-40 cm and DTPA-Cd (116%-158%) in 10-30 cm profiles. Moreover, the continuous application of manure fertilizers enhanced the availability of Cd in all aggregate size classes with an increase of 17.3%-87.8% in DTPA-Cd. Organic fertilizers (LM, HM and ST) heightened the content of Cd (38.0%-152%) in all parts of rice plant. The accumulation of Cd in rice plants was directly affected by fertilization regimes and Cd availability in the 10-20 cm soil layers and 0.25-0.5 mm aggregates. In conclusion, long-term application of manures resulted in increasing availability of Cd in aggregates and in topsoil and subsoil layers, which accordingly enhanced the accumulation of Cd in rice plants.

The influence of liming on cadmium accumulation in rice grains via iron-reducing bacteria

Cadmium (Cd) in soil is attracting worldwide attention, and many valuable measures and suggestions of minimizing the rice grain Cd are available. Among these methods, liming can increase the soil pH and decrease the rice grain Cd content. Here, we report that soil pH was negatively and significantly correlated with the concentration of soil extractable Fe and Cd. In addition, the iron concentration on root surface was significantly and positively associated with the available metals in soil and the rice grain Cd. However, the return of contaminated rice straw significantly increased the Cd accumulation in the rice grain, although the returned straw did not significantly influence the concentration of extracted soil Cd. Furthermore, an analysis of the functional microbe community was performed, and the response of iron-reducing bacteria (IRB) under the six treatments provides valuable insights for reducing the available Cd concentration in soil. A LEfSe (LDA coupled with effect size measurement for significant differences) analysis showed that the application of liming reduced the abundance of IRB. The results of a redundancy analysis (RDA) indicated that soil pH was significantly and negatively associated with the abundance of Proteobacteria and Geobacter and the concentration of bioavailable Fe and Cd in the soil, which could explain the reduced accumulation of bioavailable Cd in rice grain. Collectively, our results demonstrated that liming in Cd-polluted paddy soil is a reasonable strategy for minimizing rice grain Cd by increasing the soil pH, which reduces the soil available iron and Cd concentration by shifting the diversity and composition of IRB, thus ultimately resulting in decreased Cd in rice.

Strategies to Enable the Safe Use of Cadmium-Contaminated Paddy Soils in Southern China

With the rapid progression of industrialization, urbanization, and modern agriculture, large amounts of pollutants, especially heavy metals, are entering the soil in China’s agricultural areas. According to the survey by the Ministry of Environmental Protection and the Ministry of Land Resources, 19.4% for the agricultural soils are contaminated based on China’s soil environmental quality limits, cadmium being the primary contaminant. The total area of agricultural soils contaminated by cadmium is 10 million ha (7% of China’s cultivated land) and is mostly distributed in the southern rice region (Ministry of Environmental Protection P. R. C. and Ministry of Land and Resources P. R. C. 2014). Cadmium in the soil might enter the food chain relatively easily by absorption into agricultural products and therefore poses a considerable threat to human health (Toppi and Gabbrielli 1999; Chen et al. 2016). An investigation from the Ministry of Agriculture indicated that over 10% of brown rice are Cd-contaminated (Li and Xu 2015).

Foliar application of Zn can reduce Cd concentrations in rice (Oryza sativa L.) under field conditions

Cadmium (Cd) pollution in rice and its transfer to food chain are cause of global concern. Application of zinc (Zn) can reduce Cd uptake by plants, as both these metals are generally antagonistic in soil–plant systems. In a field experiment on Cd-contaminated acid soil, we investigated the effectiveness of foliar application of Zn in minimizing Cd accumulation and its effect on the content of mineral nutrient elements in rice. The treatment was done at an early grain filling stag using 0.3 and 0.5% w/v ZnSO4·7H2O solution. The spray did not affect the grain yield of rice but decreased the Cd concentration in the root, straw, husk, and brown rice to some extent and increased the Zn concentration. Foliar application of 0.5% ZnSO4 resulted in maximum Zn concentration and minimum Cd concentration in brown rice. However, the concentrations of P, K, Ca, Mg, Cu, and Mn in brown rice were not affected. The correlation between Cd and Zn concentrations in brown rice, husk, and root was significantly negative, and that between Cd and Mn concentrations in brown rice was significantly positive. The inhibition of Cd uptake resulted in a decrease in its concentration in brown rice after the treatments. Thus, the foliar application of a suitable concentration of Zn at the early grain filling stage could effectively minimize the Cd concentration while enhancing the Zn concentration in brown rice on Cd-contaminated acid soil.

Effectiveness of simultaneous applications of lime and zinc/iron foliar sprays to minimize cadmium accumulation in rice

Due to the large area of Cd-contaminated paddy soils worldwide, low-cost measures to reduce the accumulation of Cd in rice plant are necessary. A field experiment was therefore conducted to investigate the reducing effect of lime combined with foliar applications of Zn (ZnSO4) or Fe (EDTA·Na2Fe) on Cd concentrations in brown rice on a Cd-contaminated paddy soil. The results indicated that liming alone or in combination with foliar sprays of Zn or Fe increased the soil pH by 0.27-0.63 units. However, limited effects of lime or lime combined with foliar applications of Zn/Fe on soil DTPA-extractable Cd, rice grain and rice straw biomass were observed. Liming alone significantly reduced the Cd concentration in brown rice and rice straw by 31.8% and 42.3%, respectively. The Cd concentrations in brown rice decreased by 25.5% and 65.4% and in rice straw by 53.0% and 68.1% after liming combined with foliar applications of Fe and Zn, respectively. In contrast, liming combined with foliar spraying of Fe significantly increased the transfer ratio of Cd from the rice straw to the grain. As a low-cost technique, lime application combined with foliar application of ZnSO4 could be recommended for the remediation of Cd-contaminated paddy soils.

Effects of Controlled-release Ureas on Heavy-metal Mobility in Multimetal-contaminated Soil

Abstract Controlled-release N fertilizers can affect the availability of heavy metals in the contaminated paddy soil. A soil incubation experiment was conducted to investigate the effects of prilled urea (PU), S-coated urea (SCU), and polymer-coated urea (PCU) on the solubility and availability of heavy metals Cd, Pb, Cu, and Zn in a multimetal-contaminated soil. The results showed that the application of different coated urea significantly affected the solubility and availability of heavy metals. At 5 d of incubation, the application of PU, SCU, and PCU had significantly decreased the concentrations of water-soluble and available Cd, Pb, Cu, and Zn, when compared with the control. At 60 d of incubation, the depletory effects of PU on water-soluble and available heavy metals had reduced, and the initial decrease in the concentrations of water-soluble Cd, Pb, Cu, and Zn caused by SCU had changed to an increase. The concentrations of water-soluble Pb, Cu, and Zn in the SCU-treated soil were higher than those in the control. Application with PCU led to a higher water-soluble Cu than that in the control, while the available Cd, Pb, and Zn were lower than those in the control. The effect of different coated urea was much stronger on the water solubility of the heavy metals than on their availability. The effects of controlled-release urea on the transformation of heavy metals resulted in changes in the concentrations of NH+4, water-soluble SO2−4, and soil pH. The results further suggested that PCU could be used in dry farming operations in multimetal-contaminated acid soils.