The purpose of this study was to clarify the effects of biochar on the diversity of bacteria and fungi in the rice root zone and to reveal the changes in soil microbial community structure in the root zone after biochar application to provide a scientific basis for the improvement of albic soil. Rice and corn stalk biochar were mixed with albic soil in a pot experiment. Soil samples were collected at the rice maturity stage, soil nutrients were determined, and genomic DNA was extracted. The library was established using polymerase chain reaction (PCR) amplification. The abundance, diversity index, and community structure of the soil bacterial 16SrRNA gene V3 + V4 region and the fungal internal transcribed spacer-1 (ITS1) region were analyzed using Illumina second-generation high-throughput sequencing technology on the MiSeq platform with related bioinformatics. The results revealed that the biochar increased the soil nutrient content of albic soil. The bacteria ACE indexes of treatments of rice straw biochar (SD) and corn straw biochar (SY) were increased by 3.10% and 2.06%, respectively, and the fungi ACE and Chao indices of SD were increased by 7.86% and 14.16%, respectively, compared to conventional control treatment with no biochar (SBCK). The numbers of bacterial and fungal operational taxonomic units (OUT) in SD and SY were increased, respectively, compared to that of SBCK. The relationship between soil bacteria and fungi in the biochar-treated groups was stronger than that in the SBCK. The bacterial and fungal populations were correlated with soil nutrients, which suggested that the impacts of biochar on the soil bacteria and fungi community were indirectly driven by alternation of soil nutrient characteristics. The addition of two types of biochar altered the soil microbial community structure and the effect of rice straw biochar treatment on SD was more pronounced. This study aimed to provide a reference and basic understanding for albic soil improvement by biochar, with good application prospects.

Biochar is a potential amendment for the remediation of Cd-contaminated soils. Although the immobilization effect of biochar on soil Cd has been studied under indoor laboratory conditions, the effect of biochar on rice Cd uptake and soil Cd fractions under field conditions is still poorly understood. Here, the Cd content of the different organs of rice and the Cd fractions in soil were characterized for three years after the application of different amounts of biochar (0, 7.5, 15, 30 t ha⁻¹, and 3 t ha⁻¹ year⁻¹). The Cd content of brown rice, husk, leaf, stem and sheath, and root under biochar treatment could be maximally reduced by up to 26.25%, 20.16%, 20.74%, 33.2%, and 26.89%, respectively. Biochar altered the Cd fractions in soil, including the decrease in exchangeable Cd content and the increase in Fe-Mn oxide bound Cd and organic bound Cd. The concentration factor of Cd uptake by rice was reduced by 32% under biochar application, while biochar had little influence on the transfer factor and distribution factor. The immobilization effect of biochar on soil Cd lasted for at least three years, but the trend of Cd immobilization efficiency over time for different amounts of biochar treatment was different. The Risk Assessment Code (RAC) of Cd in soil with biochar amendment could be reduced to a medium risk level from a high risk level. Redundancy analysis (RDA) revealed that changes in soil pH and Fe-Mn oxide bound Cd content caused by biochar application contributed most to the reduction in the Cd content of rice organs. These findings would enhance our understanding of the immobilization effect of biochar on Cd in paddy soil under field conditions.