Effect of biochar and Fe-biochar on Cd and As mobility and transfer in soil-rice system.

In this study, the effects of biochar derived from rice-straw (biochar) and iron-impregnated biochar (Fe-biochar) on Cd and As mobility in rice rhizosphere and transfer from soil to rice were investigated with different application rates. 1-3% biochar reduced porewater Cd in rhizosphere but elevated soluble As, resulting in 49-68% and 26-49% reduction in the root and grain Cd, with a simultaneous increase in root As. Unlike biochar, 0.5% Fe-biochar decreased porewater As throughout rice growth, resulting in reduced root As, which, however, increased Cd uptake by root. Biochar-induced soil As mobilization was probably through competitive desorption and Fe-biochar-induced soil Cd mobilization was probably via soil acidification. The results suggested that biochar and Fe-biochar was effective in reducing Cd and As uptake by rice, respectively, so they may be used as emergency measures to cope with single Cd or As contamination in paddy soils.

Yin D ，Wang X ，Peng B ，Tan C ，Ma LQ ... -  《-》

Effect of sulfur and sulfur-iron modified biochar on cadmium availability and transfer in the soil-rice system.

Cadmium (Cd) contamination in paddy soils has aroused global concern. Sulfur modified biochar (BC) could combine the benefits of BC and S for Cd remediation. However, no information is available on the impact of sulfur modified biochar on Cd phytoavailability in paddy soils. In this study, a pot experiment was conducted to investigate the effect of sulfur modified biochar (S-BC) and sulfur and iron (Fe) modified biochar (S-Fe BC) on Cd mobility and Cd transfer in the soil-rice system. The application of S-BC and S-Fe BC effectively reduced pore water Cd in the rhizosphere and non-rhizosphere pore water throughout the rice growth stages. S-BC and S-Fe BC addition increased the total chlorophyll content, as well as the root, shoot and grain biomasses of rice. Furthermore, S-BC and S-Fe BC amendments greatly increase the formation of Fe plaque on rice root surface, thus decreasing Cd accumulation in different rice tissues. In particular, S-Fe BC supplementation significantly reduced the Cd concentration in rice grains to 0.018 mg kg-1 in Cd-contaminated soil, which was lower than the China National standard for food contamination limit (0.2 mg kg-1 Cd). Sequential extraction results showed that S-BC and S-Fe BC can promote the transfer of exchangeable Cd to Fe-Mn oxide, organic and residual bound forms which reduce Cd in paddy soils. Thus, the amendment of S-Fe BC to Cd-contaminated paddy soil is an effective strategy to decrease Cd accumulation in rice grains and thereby protect public health.

Rajendran M ，Shi L ，Wu C ，Li W ，An W ，Liu Z ，Xue S ... -  《-》

Microbe mediated arsenic release from iron minerals and arsenic methylation in rhizosphere controls arsenic fate in soil-rice system after straw incorporation.

Arsenic (As) contamination is a global problem. Straw incorporation is widely performed in As contaminated paddy fields. To understand how straw and straw biochar incorporation affect As transformation and translocation in the soil-microbe-rice system, a pot experiment was carried out with different dosages of rice straw and straw biochar application. Results showed that both straw biochar and straw application significantly increased As mobility. Straw biochar mobilized As mainly through increasing soil pH and DOM content. Straw incorporation mainly through enhancing As release from iron (Fe) minerals and arsenate (As(V)) reduction to arsenite (As(III)). Straw biochar didn't significantly affect As methylation, while straw incorporation significantly enhanced As methylation, elevated dimethylarsenate (DMA) concentration in soil porewater and increased As volatilization. Straw biochar didn't significantly change total As accumulation in rice grains, but decreased As(III) accumulation by silicon (Si) inhibition. Straw incorporation significantly increased DMA, but decreased As(III) concentration in rice grains. After biochar application, dissolved As was significantly positively correlated with the abundance of Bacillus, indicating that Bacillus might be involved in As release, and As(III) concentration in polished grains was negatively correlated with Si concentration. The significant positive correlation between dissolved As with Fe and the abundance of iron-reducing bacteria suggested the coupling of As and Fe reduction mediated by iron-reducing bacteria. The significant positive correlation between DMA in rice grains and the abundance of methanogenic bacteria indicated that methanogenic bacteria could be involved in As methylation after straw application. The results of this study would advance the understanding how rice straw incorporation affects As fate in soil-microbe-rice system, and provide some guidance to straw incorporation in As contaminated paddy soil. This study also revealed a wealth of microorganisms in the soil environment that dominate As mobility and transformation after straw incorporation.

Yang YP ，Zhang HM ，Yuan HY ，Duan GL ，Jin DC ，Zhao FJ ，Zhu YG ... -  《-》

Simultaneous alleviation of cadmium and arsenic accumulation in rice by applying zero-valent iron and biochar to contaminated paddy soils.

The fates of cadmium (Cd) and arsenic (As) in paddy fields are generally opposite; thus, the inconsistent transformation of Cd and As poses large challenges for their remediation. In this study, the impacts of zero valent iron (ZVI) and/or biochar amendments on Cd and As bioavailability were examined in pot trials with rice. Comparison with the untreated soil, both Cd and As accumulation in different rice tissues decreased significantly in the ZVI-biochar amendments and the Cd and As accumulation in rice decreased with increasing ZVI contents. In particular, the concentrations of Cd (0.15 ± 0.01 mg kg-1) and As (0.17 ± 0.01 mg kg-1) in rice grains were decreased by 93% and 61% relative to the untreated soil, respectively. A sequential extraction analysis indicated that with increasing Fe ratios in the ZVI-biochar mixtures, bioavailable Cd and As decreased, and the immobilized Cd and As increased. Furthermore, high levels of Fe, Cd, and As were detected in Fe plaque of the ZVI-biochar amendments in comparison with the single biochar or single ZVI amendments. The ZVI-biochar mixture may have a synergistic effect that simultaneously reduces Cd and As bioavailability by increasing the formation of amorphous Fe and Fe plaque for Cd and As immobilization. The single ZVI amendment significantly decreased As bioavailability, while the single biochar amendment significantly reduced the bioavailability of Cd compared with the combined amendments. Hence, using a ZVI-biochar mixture as a soil amendment could be a promising strategy for safely-utilizing Cd and As co-contaminated sites in the future.

Qiao JT ，Liu TX ，Wang XQ ，Li FB ，Lv YH ，Cui JH ，Zeng XD ，Yuan YZ ，Liu CP ... -  《-》

Effect of calcium and iron-enriched biochar on arsenic and cadmium accumulation from soil to rice paddy tissues.

Arsenic (As) and cadmium (Cd) are nonessential toxic metal(loids) that are carcinogenic to humans. Hence, reducing the bioavailability of these metal(loids) in soils and decreasing their accumulation in rice grains is essential for agroecology, food safety, and human health. Iron (Fe)-enriched corncob biochar (FCB), Fe-enriched charred eggshell (FEB), and Fe-enriched corncob-eggshell biochar (FCEB) were prepared for soil amelioration. The amendment materials were applied at 1% and 2% application rates to observe their alleviation effects on As and Cd loads in rice paddy tissues and yield improvements using pot trials. The FCEB treatment increased paddy yields compared to those of FCB (9-12%) and FEB (3-36%); this could be because it contains more plant essential nutrients than FCB and a lower calcite content than that of FEB. In addition, FCEB significantly reduced brown rice As (AsBR, 29-60%) and Cd (CdBR, 57-81%) contents compared to those of the untreated control (CON). At a 2% application rate, FCEB reduced the average mobility of As (56%) and Cd (62%) in rhizosphere porewater and enhanced root Fe-plaque formation (76%) compared to those of CON. Moreover, the enhanced Fe-plaque sequestered a substantial amount of As (171.4%) and Cd (90.8%) in the 2% FCEB amendment compared to that of CON. Pearson correlation coefficients and regression analysis indicated that two key mechanisms likely control AsBR and CdBR accumulations. First, rhizosphere soil pH and Eh controlled As and Cd availabilities in porewaters and their speciation in the soil. Second, greater Fe-plaque formation in paddy roots grown in the amended soils provided a barrier for plant uptake of the metal(loids). These observations demonstrate that soil amendment with Fe-enriched corncob-eggshell biochar (e.g., 2% FCEB) is a prospective approach for the remediation of metal accumulation from the soil to grain system while simultaneously increasing paddy yield.

Islam MS ，Magid ASIA ，Chen Y ，Weng L ，Ma J ，Arafat MY ，Khan ZH ，Li Y ... -  《-》