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Biochar can change the availability and morphology of soil Cd. However, the influence of biochar on Cd chemical form and subcellular fraction in rice is poorly understood, particularly under different irrigation methods. A pot experiment of biochar application combined with two irrigation methods (continuous flooding and intermittent irrigation, CF and II) was conducted. The Cd accumulation, chemical form and subcellular fraction in rice organs and the associated physiological responses were examined. Biochar significantly reduced soil available Cd (30.85–47.26% and 32.35–52.35%) under CF and II but increased the Cd content (30.4–63.88% and 13.03–18.59%) in brown rice. Additionally, the Cd content in shoots/grains under II was higher than that under CF. Biochar elevated the Cd soluble fraction in roots while lowered the cell wall fraction under both irrigation methods, whereas the opposite result was observed in leaves. Biochar increased water-, ethanol-, and NaCl-extractable Cd in roots meanwhile increased ethanol-extractable Cd in leaves under both irrigation methods. Moreover, the total amount of water-, ethanol-, and NaCl-extractable Cd in rice roots was higher under II than under CF. Related hormones and antioxidant enzymes may also be involved in biochar-mediated Cd accumulation in rice grains. Thus, changes in Cd chemical form and subcellular fraction in the root and leaf are the main mechanisms of biochar-induced rice grains Cd accumulation. Graphical Abstract

BACKGROUND Biochar can play a key role in improving paddy soil and productivity. However, there is limited information on the effects of biochar on rice quality and starch gelatinization. In this study, four rice straw biochar dosage treatments (0, 20, 40 and 60 g kg−1; CK, C20, C40 and C60, respectively) were set up to investigate rice yield components, rice processing, appearance and cooking quality, and starch gelatinization. RESULTS Addition of biochar increased the effective panicle, grain number per panicle and seed setting rate. However, it decreased the 1000‐grain weight, resulting in an increase in yield. In 2019, all the biochar treatments improved the head rice rate (9.13–11.42%), whereas in 2020 only the C20 treatment improved. Low biochar dosage had little effect on grain appearance. High biochar dosage significantly decreased the chalky rice rate by 21.47% and chalkiness by 19.44% in 2019. However, it significantly increased the chalky rice rate and chalkiness by 118.95% and 85.45% in 2020, respectively. Biochar significantly lowered the amylose content except for the C20 and C40 treatments in 2020, and the gel consistency. The C40 and C60 treatments significantly increased the peak and breakdown viscosities and decreased the setback viscosity compared with CK. Correlation analysis showed that starch gelatinization characteristics were significantly correlated with the head rice rate, chalky rate and amylose content. CONCLUSION A lower biochar dosage can improve the yield and milled rice rate and maintain a higher quality of appearance, whereas a higher biochar dosage can significantly improve starch gelatinization. © 2023 Society of Chemical Industry.

Biochar exhibits a good adsorption ability for heavy metals in soil and has been widely used as a remediation material in Cd-contaminated soil. However, the status of Cd uptake by rice driven by soil physicochemical properties and rhizosphere microbial communities after years of biochar application is not well understood. In this study, the relationship between the rhizosphere microbial community and soil physicochemical properties and rice Cd accumulation were investigated during the main rice growth stages. The results showed that in comparison to the non-biochar treatment (control), a noticeable reduction in Cd content in rice stem sheaths, leaves, rice husks and milled rice with different growth stages were observed in the biochar treatment after four years, which decreased by 38.76–66.18%, 40.93–70.27%, 43.64–47.92% and 31.91–34.38%, respectively. Compared to non-biochar treatment (control), the properties of the soil in different growth stages by biochar treatment of the soil pH, soil organic matter (SOM), total nitrogen (TN) and available phosphorus (AP) were significantly increased, which increased by 10.5–16.13%, 8–25%, 75–130.13% and 132.95–191.43%, respectively. The content of available Cd (ACd) concentration in different stages by biochar treatment was significantly decreased, which decreased by 26.57–44.24%. Biochar application after four years changed the rhizosphere bacterial community structure composition (phyla level) in all stages. The relative abundance of Proteobacteria, Bacteroidetes and Nitrospirae was increased, while the relative abundance of Chloroflexi, Acidobacteria and Actinobacteria was decreased. Meanwhile, the biochar application enriched Rhodocyclaceae, Burkholderiaceae, Nitrosomonadaceae, Anaerolineaceae, Ignavibacteriales and Bacteroidales, which may contribute to the reduction of Cd uptake and accumulation in rice. These results suggest that biochar treatment after four years changed the rhizosphere microbial community structure and soil physicochemical properties and promoted the colonization of specific microbial populations in the rice rhizosphere to form a special protective system in the rice rhizosphere, which reduced Cd uptake by rice.

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