Oxygen Nanobubble-Loaded Biochars Mitigate Copper Transfer from Copper-Contaminated Soil to Rice and Improve Rice Growth

Copper (Cu)-contaminated paddy soil has caused substantial concerns for human health through biomagnification in the soil–rice–human chain. In this study, oxygen nanobubble-loaded biochars (ONBC), including pyrochars (NPC) and hydrochars (NHC), were used to amend Cu-contaminated paddy soils and reduce rice Cu toxicity. The ONBC amendment improved the rhizospheric redox potential and dissolved oxygen concentration via slow, continuous oxygen release, promoting root iron plaque formation and effectively inhibiting root Cu influx. Moreover, the ONBC amendment improved soil microbial diversity. The abundance of bacterial genes involved in cytosolic Cu complexation and Cu export to the periplasm space for Cu stabilization markedly improved. The sequestered Cu in root iron plaque and soil bacteria inhibited Cu influx into the shoots to avoid Cu overaccumulation in the edible parts. This was reflected by the Cu content decreasing by 1.91-fold and 2.38-fold in shoots under NPC and NHC treatments, respectively. Furthermore, the improved oxygen-enriched rhizosphere environment via the ONBC amendment promoted root activity and the abundance of bacterial genes involved in nutrients mineralization, further promoting shoot biomass by 1.46-fold and 2.29-fold under NPC and NHC treatments, respectively. This study proposes an environmentally friendly strategy to mitigate Cu transfer from soil to crops and improve crop productivity.