Watering techniques and zero-valent iron biochar pH effects on As and Cd concentrations in rice rhizosphere soils, tissues and yield.

Zero-valent iron amended biochar (ZVIB) has been proposed as a promising material in immobilizing heavy metals in paddy fields. In this study, the impacts of pH of ZVIB (pH 6.3 and pH 9.7) and watering management techniques (watering amount in the order of CON (control, 5/72)>3/72>1-3/72>3/100>1/72, with 5/72, for example, representing irrigation given to 5 cm above soil surface in 72 hr regular interval) on As and Cd bioavailability for rice and its grain yield (YieldBR) were investigated in a pot experiment. Brown rice As (AsBR) content was irrelative to the watering treatments, while significantly decreased (>50%) with the addition of both ZVIB materials. The diminutions of brown rice Cd (CdBR) content as well as the YieldBR were highly dependent on both the soil amendment materials' pH and watering amount. Among all the watering treatments, 3/72 treatment (15% less irrigation water than the CON) with ZVIB 6.3 amendment was the optimum fit for simultaneous reduction of AsBR (50%) and CdBR contents (19%) as well as for significant increment (12%) of the YieldBR. Although high pH (9.7) ZVIB application could also efficiently decrease As and Cd contents in brown rice, it might risk grain yield lost if appropriate (e.g. 3/72 in our study) watering management technique was not chosen. Therefore, ZVIB would be an environmentally friendly option as an amendment material with proper selection of watering management technique to utilize As and Cd co-contaminated arable soils safely for paddy cultivation.

Reduction mechanism of Cd accumulation in rice grain by Chinese milk vetch residue: Insight into microbial community.

Chinese milk vetch is an efficient approach to reduce Cd accumulation in rice, nevertheless, its reduction mechanism is not well understood. In this study, we investigated the rice grain Cd, soil properties and microbial community in a Cd-polluted paddy field amended with milk vetch residue (MV) or without (CK) during rice growth period. We found that milk vetch residue averagely decreased the Cd content in rice grain by 45%. Decrease of Cd in rice mainly attributed to the inhibition of Cd activation by milk vetch residue at heading stage probably by the formation of HA-Cd (Humic Acid) and CdS. Increased pH and organic matter (OM) promoted the reduction of available Cd. In addition, nonmetric multidimensional scaling (NMDS) analysis revealed that microbial community structure was significantly different between MV and CK treatment (r = 0.187, p = 0.002), and the core functions of differentially abundant genera were mainly associated with N-cycling, organic matter degradation and sulfate-reducing. The application of milk vetch residue increased the abundance of sulfate-reducing bacteria (SRB) by 8-112% during the rice growth period, which may involve in promoting the transformation of Cd to a more stably residual Cd (CdS). Canonical correspondence analysis (CCA) and mantel test analysis indicated that available K (p = 0.004) and available N (p = 0.005) were the key environmental factors of shaping the SRB. Altogether, changes in soil properties affected microbial structure and functional characteristics, especially the response of SRB in MV treatment would provide valuable insights into reducing the bioavailability of Cd in soil.

Influence of landform, soil properties, soil Cd pollution and rainfall on the spatial variation of Cd in rice: Contribution and pathway models based on big data.

Landform, soil properties, soil cadmium (Cd) pollution and rainfall are the important factors affecting the spatial variation of rice Cd. In this study, we conducted big data mining and model analysis of 150,000 rice-soil sampling sites to examine the effects by the above four factors on the spatial variation of rice Cd in Hunan Province, China. Specifically, the variable coefficient of rice Cd in space was significantly correlated with the partition scale according to the logistic fitting. The improved random forest results suggested that elevation (DEM) and pH were the two most important factors affecting the spatial variation of rice Cd, followed by relief, soil Cd content and rainfall. Typically, variance partitioning analysis (VPA) revealed that both the soil property and the interactive effects between the soil property and Cd pollution were the principal contributors to the rice-Cd variation, with the respective contributing rates of 30.5 % and 29.0 %. Meanwhile, the partial least square-structural equation modelling (PLS-SEM) elucidated 4 main paths of specific indirect effects on rice-Cd variation. They were landform → physicochemical property → soil acidity → rice-Cd variation, landform → soil acidity → rice-Cd variation, physicochemical property → soil acidity → rice-Cd variation, and soil texture → soil acidity → rice-Cd variation. This work can provide a general guidance for scientific zoning, accurate prediction and prevention of Cd pollution in paddy fields.

More effective than direct contact: Nano hydroxyapatite pre-treatment regulates the growth and Cd uptake of rice (Oryza sativa L.) seedlings.

Cd contamination in rice urgently needs to be addressed. Nano hydroxyapatite (n-HAP) is an eco-friendly material with excellent Cd fixation ability. However, due to its own high reactivity, innovative application of n-HAP in the treatment of Cd contamination in rice is needed. In this study, we proposed a new application, namely n-HAP pre-treatment, which can effectively reduce Cd accumulation in rice and alleviate Cd stress. The results showed that 80 mg/L n-HAP pre-treatment significantly reduced Cd content in rice shoot by 35.1%. Biochemical and combined transcriptomic-proteomic analysis revealed the possible molecular mechanisms by which n-HAP pre-treatment promoted rice growth and reduced Cd accumulation. (1) n-HAP pre-treatment regulated gibberellin and jasmonic acid synthesis-related pathways, increased gibberellin content and decreased jasmonic acid content in rice root, which promoted rice growth; (2) n-HAP pre-treatment up-regulated gene CATA1 expression and down-regulated gene OsGpx1 expression, which increased rice CAT activity and GSH content; (3) n-HAP pre-treatment up-regulated gene OsZIP1 expression and down-regulated gene OsNramp1 expression, which reduced Cd uptake, increased Cd efflux from rice root cells.

[Remediation Effect and Mechanism of Biochar in Combination with Nitrogen Fertilizer on Cd-contaminated Paddy Soil].

Cadmium (Cd) heavy metal pollution has posed serious threats to soil health and the safe production utilization of agricultural products. A pot experiment was conducted to study the effects of biochar (BC) and nitrogen fertilizer with three levels, namely 2.6 g·pot-1 (N1), 3.5 g·pot-1 (N2), 4.4 g·pot-1 (N3) biochar combined with nitrogen fertilizer (BCN1, BCN2, and BCN3), on soil Cd fractions, Cd enrichment, the transport of rice, and soil enzyme activity, as well as the changes in microbial community composition and complex interactions between microorganisms through high-throughput sequencing. The results showed that biochar combined with nitrogen fertilizer led to the transformation of Cd from the exchangeable state to the residue state, and the proportion of the exchangeable state was significantly reduced by 6.2%-14.7%; by contrast, the proportion of the residue state increased by 18.6%-26.4% relative to that in CK. In addition, singular treatments of nitrogen fertilizer enhanced the accumulation capacities of Cd in roots, which increased by 22%-33.5% compared with that in CK. By contrast, the BC and BCN treatments reduced Cd accumulation in roots and the transfer capacity from stems to rice husks and husk to rice. Furthermore, the BCN treatments promoted soil enzyme activities (urease, acid phosphatase, invertase, and catalase). MiSeq sequencing showed that BCN treatments increased the abundance of the main species of soil bacterial microbes (such as Acidobacteriales, Solibacterales, Pedosphaerales, and Nitrospirales). Moreover, co-occurrence network analysis showed that the complexity of the soil bacterial network was enhanced under the N, BC, and BCN treatments. Overall, biochar combined with nitrogen fertilizer reduced soil Cd availability, inhibited the capacity of Cd accumulation and the transport of rice, and improved the soil eco-environmental quality. Thus, using BCN could be a feasible practice for the remediation of Cd-polluted agricultural soil.

Inter-annual reduction in rice Cd and its eco-environmental controls in 6-year biannual mineral amendment in subtropical double-rice cropping ecosystems.

The alkaline mineral amendment is a practical means of alleviating Cd concentration in rice grain (CdR) in the short-term; however, the long-term remediation effect of mineral amendment on the CdR and the eco-environmental controls remains unknown. Here a mineral (Si-Ca-Mg) amendment, calcined primarily from molybdenum tailings and dolomite, was applied biannually over 6 years (12 seasons) to acidic and moderately Cd-contaminated double-rice cropping ecosystems. This study investigated the inter-annual variation of Cd in the rice-soil ecosystem and the eco-environmental controls in subtropical rice ecosystems. CdR was reduced by 50%-86% following mineral amendment. The within-year reduction in CdR was similar between early rice (50%-86%, mean of 68%) and late rice (68%-85%, mean of 74%), leading to CdR in all early rice and in 83% of late rice samples below the upper limit (0.2 mg kg-1) of the China National Food Safety Standards. In contrast, the inter-annual reduction in CdR was moderately variable, showing a greater CdR reduction in the later 3 years (73%-86%) than in the former 3 years (54%-79%). Three years continuous mineral amendment was required to guarantee the safety rice production. The concentrations of DTPA-extractable and exchangeable Cd fractions in soil were reduced, while the concentration of oxides-bound Cd was increased. In addition, the soil pH, concentrations of Olsen-P and exchangeable Ca and Mg were elevated. These imply a lower apparent phytoavailability of Cd in the soil following mineral amendment. An empirical model of the 3-variable using soil DTPA-Cd, soil Olsen-P, and a climatic factor (precipitation) effectively predicted temporal changes in CdR. Our study demonstrates that Cd phytoavailability in soil (indexed by DTPA-extractable Cd) and climatic factors (e.g., temperature and precipitation) may directly/indirectly control the inter-annual reduction in CdR following mineral amendment in slightly and moderately Cd-contaminated paddy ecosystems.

[Comprehensive Risk Evaluation of Cadmium in Soil-rice System Based on Uncertainty Analysis].

Cadmium (Cd) can cause adverse health effects and is a subject of concern in rice consumption. The uncertainty analysis helps improve the accuracy in the risk assessment for Cd in soil-rice system. A regional investigation on Youxian prefecture, southern China, was conducted to analyze the Cd concentration in rice. Based on the species sensitivity distribution model (SSD), health risk assessment model, and Monte Carlo simulation, the accumulation characteristic of Cd in soil-rice system, accumulation risk of Cd in soil, and health risk of Cd concentration in rice were determined. The results showed that the plant uptake factor (PUF) of Cd of rice was well fitted by the SSD model. The mean level of PUF was 1.86, with a significant spatial heterogeneity. The rice produced in WL county tended to accumulate a high level of Cd. There was no significant relationship between concentrations of Cd in soil and rice, suggesting that of rice renders the Cd risk management very difficult. The pollution load index of Cd in soil was 2.4, which belonged to a moderate contamination level. Under current accumulation condition of Cd in soil, there would be a 90.4% probability for soil Cd concentration to be higher than the national soil quality standard after 10 years. Health risk assessment showed that the average daily dose (ADD) was 2.9 µg·(kg·d)-1, 3.5 fold higher than the WHO limit. About 93.9% of the adult populations consuming rice cropping in affected areas had the risk that the daily Cd intake was above the WHO limit. The health risk index (HRI) was around 2.1 to 4.7. The probability for health risk index (HRI) higher than 5 was 21.5%, suggesting a high health risk. When the soil pH was lower than 5.5, the probability for HRI higher than 1 was 95.3%, and when the soil pH was higher than 6, the probability for HRI higher than 1 reduced to 68.1%. An improved management of soil pH values would be needed for a better and safer rice production. The combination of uncertainty analysis, species sensitivity model and health risk assessment model was validated to be feasible and reliable in the risk analysis.