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 200 mg kg− 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 (370 μmol mol− 1), but under the high CO2 concentration (570 μmol mol− 1) nTiO2 significantly reduced rice biomass by 17.9% and 22.1% at 50 mg kg− 1 and 200 mg kg− 1, respectively, and grain yield by 20.8% and 44.1% at 50 mg kg− 1 and 200 mg kg− 1, respectively. In addition, at the high CO2 concentration, nTiO2 at 200 mg kg− 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.