This study aimed to explore natural aging effect of distiller’s grain-derived biochar (DGB) at various amendment rates (2%, 4%, w/w) on wheat (Triticum aestivum L.) growth, development and Cd uptake in soil, and provide novel insights in effect mechanisms from views of soil Cd fractions and rhizospheric microbiota. Results showed that DGB amendment promoted wheat growth. Rising DGB rate progressively increased soil pH, soil organic matter (SOM), total carbon (TC), total nitrogen (TN), NH4-N, available K, and residual Cd content to more greatly promote chlorophyll content and decrease Cd uptake of wheat. With 6-month aging, soil TN, available K and residual Cd content continuously increased to decline Cd bioavailability, which further restricted Cd uptake by wheat roots, stems, and leaves, and did not obviously change Cd uptake by wheat grains. Contrarily, soil NO3-N content progressively decreased with rising DGB rate and aging, partly due to progressively decreased nitrifier abundances of Nitrosomonadaceae and Nitrospiraceae with rising DGB rate and aging, according to rhizospheric bacterial composition. Statistical analysis verified that DGB rate and aging were synergistic factors to jointly involve soil nutrient increase and Cd fractions re-distribution. Rising DGB rate and aging jointly increased the abundances of Actinobacteria, Cyanobacteria, and Fibrobacteria phyla, and Lysobacter, Massilia, Pseudarthrobacter, and Iamia genera that positively correlated to soil residual Cd, TN, SOM, TC, and available K content, suggesting that such bacterial groups also drove soil fertility improvement and Cd bioavailability decrease. Consequently, amending 4% DGB with aging was proposed as appropriate for improving soil fertility and blocking Cd-induced health risk.
