Daoyou Huang

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 land cover on soil organic carbon stock in a karst landscape with discontinuous soil distribution

Land cover type is critical for soil organic carbon (SOC) stocks in territorial ecosystems. However, impacts of land cover on SOC stocks in a karst landscape are not fully understood due to discontinuous soil distribution. In this study, considering soil distribution, SOC content and density were investigated along positive successional stages (cropland, plantation, grassland, scrubland, secondary forest, and primary forest) to determine the effects of land cover type on SOC stocks in a subtropical karst area. The proportion of continuous soil on the ground surface under different land cover types ranged between 0.0% and 79.8%. As land cover types changed across the positive successional stages, SOC content in both the 0–20 cm and 20–50 cm soil layers increased significantly. SOC density (SOCD) within 0–100 cm soil depth ranged from 1.45 to 8.72 kg m−2, and increased from secondary forest to primary forest, plantation, grassland, scrubland, and cropland, due to discontinuous soil distribution. Discontinuous soil distribution had a negative effect on SOC stocks, highlighting the necessity for accurate determination of soil distribution in karst areas. Generally, ecological restoration had positive impacts on SOC accumulation in karst areas, but this is a slow process. In the short term, the conversion of cropland to grassland was found to be the most efficient way for SOC sequestration.

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).

Arsenic availability and uptake by edible rape (Brassica campestris L.) grown in contaminated soils spiked with carboxymethyl cellulose-stabilized ferrihydrite nanoparticles

This study investigated arsenic (As) availability and uptake by rape (Brassica campestris L.) during two harvest periods of carboxymethyl cellulose (CMC)-stabilized ferrihydrite (HFO) nanoparticles for in situ treatment As-contaminated soil. Application of modified HFO nanoparticles in soils not only provided a larger specific surface area but also markedly improved stability against aggregation and recrystallization. For 90-day incubation, bare HFO particles were gradually converted to the crystalline Fe(III) oxide form, although this was not observed for the 0.5% CMC-HFO nanoparticles. CMC-modified HFO nanoparticles could be more effective in lowering the As uptake by rape and available As in soils than bare HFO particles. Compared the control without amendments, As contents in rape and available As in soils decreased 69.7 and 59.0%, respectively, during the second harvest when soils were amended with 0.5% HFO nanoparticles. And the soil-solution distribution coefficient (Kd) increased by 2.6 and 2.8 times for the first and second harvest. Furthermore, the ratio of amorphous and free Fe-oxides (Feo/Fed) showed significant negative linear correlations with Asplant (P < 0.01), available As (P < 0.05), and nonspecifically sorbed As in soil (P < 0.01). In contrast, Feo/Fed was positively correlated with Kd and amorphous crystalline Fe/Al oxide-sorbed As, which suggests that a larger amount of As is associated with Fe(hydr)oxide in the amorphous phase or smaller particles.

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.