Application of Silica Nanoparticles Induces Different Effects on Inorganic and Methylated Arsenic Accumulation in Rice Grains: Insights from Arsenic Transformation in Soil

Silica nanoparticles (SiNPs) are promising agrochemicals for reducing pollutant accumulation in plants. However, their applications sometimes fail to reduce arsenic in rice grains. Here, we explored the mechanisms using pot experiments via foliar and soil applications, respectively. Both applications significantly decreased As(III) (19.2-35.1%) and DMA (74.4-81.3%) in grains but increased As(V) by 1.2-1.8-fold. SiNP applications reduced As(III) but promoted As(V) in porewater during the flooding period. The As(III)-oxidation gene <i>aioA</i> showed a 1-3 orders of magnitude higher abundance than arsenic-reduction/methylation genes, and its abundance significantly increased after both applications. SiNP applications facilitated the release of Fe-bound organic carbon into porewater and promoted bacterial growth and As(III) oxidation, leading to high As(V) availability for rice uptake. Nevertheless, foliar spraying performed better than soil amendment in reducing total arsenic in grains by inhibiting stem/nodes-to-grain translocation. Our findings highlight the microbial arsenic transformation in soil regulated by SiNPs and demonstrate the advantages of foliar spraying in the arsenic-contaminated paddy field.