Insight into the crucial role of nano-biochar in the natural formation and bioaccumulation of silver nanoparticles in the rhizosphere by single-particle ICP-MS

Abstract Nano-biochar (nano-BC) is one of the most active fractions in the BC continuum and frequently detected in terrestrial ecosystems. However, a paucity of information exists on reactivity and environmental functions of nano-BC in the rhizosphere. The present study investigated the potential of nano-BC in transforming silver ions (Ag + ) to silver nanoparticles (AgNPs) in the rhizosphere of rice. We found that the synergistic effect of nano-BC and dioxygen secreted from rice roots was essential for Ag⁺ reduction to AgNPs. In this process, nano-BC transferred electrons to dioxygen, resulting in the formation of superoxide free radicals, which subsequently donate electrons to Ag + . Notably, excess nano-BC was unfavorable to dioxygen secretion from roots and thus inhibited the formation of AgNPs. Our results highlight that although nano-BC significantly decreased the uptake of Ag by rice plants, it contributed to the accumulation of AgNPs in plant tissues. TEM and single-particle ICP-MS analyses confirmed the presence of AgNPs not only in intercellular spaces of leaf tissues but also within the interior of leaf cells. These findings indicate that nano-BC plays a critical role in regulating the chemical species and bioaccumulation of redox-active metals (such as Ag) in the rhizosphere, which has important implications for element cycling from the pedosphere to terrestrial vegetation and warrants further investigation. Graphical Abstract