Compositional and predicted functional dynamics of soil bacterial community in response to single pulse and repeated dosing of titanium dioxide nanoparticles

Titanium dioxide nanoparticles (TiO₂NP) are often released into the soil through repeated discharge of wastewater and repetitive applications as fertilizers. Adverse effects of a single pulse on soil bacterial communities have been widely studied, while the impact of repeated exposure is poorly understood. This study compared the impacts of single and repeated exposure scenarios on the soil bacterial community. The repeated exposure promoted the total bacterial biomass but reduced the community diversity and induced larger alterations in community composition compared to the single exposure. Regarding the dosing frequencies of repeated exposure, community divergence increased in initial dosing cycles, and community stability was re-established and remained in subsequent dosing cycles. According to the different tolerance to dosing frequencies, the dynamic response patterns of the featured OTUs and functional genes could be classified into four types: 1) promotion, 2) suppression-recovery-promotion, 3) promotion-suppression-stable, and 4) suppression. These results suggest that chronic exposure with repetitive low-dosing of nanoparticles induced a tendency towards larger alteration of both community composition and functioning than in case of application of a single pulse of the same dosage. This study brings new insight into understanding the compositional and predicted functional dynamics of the soil bacterial community in response to nanoparticles and identifies a data gap in realistic time-variable exposure testing.