Elevated CO₂ levels modify TiO₂ nanoparticle effects on rice and soil microbial communities

Evidence suggests that CO₂ modifies the behavior of nanomaterials. Thus, in a few decades, plants might be exposed to additional stress if atmospheric levels of CO₂ and the environmental burden of nanomaterials increase at the current pace. Here, we used a full-size free-air CO₂ enrichment (FACE) system in farm fields to investigate the effect of elevated CO₂ levels on phytotoxicity and microbial toxicity of nTiO₂ (0, 50, and 200mgkg^-1) in a paddy soil system. Results show that nTiO₂ did not induce visible signs of toxicity in rice plants cultivated at the ambient CO₂ level (370μmolmol^-1), but under the high CO₂ concentration (570μmolmol^-1) nTiO₂ significantly reduced rice biomass by 17.9% and 22.1% at 50mgkg^-1 and 200mgkg^-1, respectively, and grain yield by 20.8% and 44.1% at 50mgkg^-1 and 200mgkg^-1, respectively. In addition, at the high CO₂ concentration, nTiO₂ at 200mgkg^-1 increased accumulation of Ca, Mg, Mn, P, Zn, and Ti by 22.5%, 16.8%, 29.1%, 7.4%, 15.7% and 8.6%, respectively, but reduced fat and total sugar by 11.2% and 25.5%, respectively, in grains. Such conditions also changed the functional composition of soil microbial communities, alerting specific phyla of bacteria and the diversity and richness of protista. Overall, this study suggests that increases in CO₂ levels would modify the effects of nTiO₂ on the nutritional quality of crops and function of soil microbial communities, with unknown implications for future economics and human health.