Toxicity of nano-CuO particles to maize and microbial community largely depends on its bioavailable fractions.

Abstract

The environmental consequences of nano-CuO particles have gained significant attention in recent decades. Identification of the mechanisms for soil and plant responses with respect to the chemical speciation of nano-CuO (mainly the exchangeable and reducible fractions) remains scarce. Here, we analyzed different chemical speciation of Cu and DTPA-extractable Cu over 42 days in (1) control soil without Cu addition; (2) soil treated with nano-CuO particles; and (3) soil treated with CuSO4 solution. The applied dose was 500\u202fmg Cu kg-1 and maize was grown in these soils. Plant growth was inhibited, but the inhibition by nano-CuO was slightly weaker compared to CuSO4. Cu accumulations were similar in the roots for CuSO4 and nano-CuO treatments, but significantly higher in the shoots for CuSO4 treatment. This indicates that Cu from nano-CuO-treated soils mainly accumulated in roots but rarely transferred to shoots. Enzyme activities on the rhizoplane visualized by zymography were strongly depressed by CuSO4 but slightly inhibited by nano-CuO. Microbial community diversity measured by 16S rRNA was the lowest in CuSO4-treated soils among three treatments. These results were explained by the following mechanisms: (1) Gradual increases of DTPA-extractable and exchangeable Cu were found in nano-CuO-treated soil, and the final concentrations at day 42 were only half of those in CuSO4-treated soil; (2) Enzyme activities on the rhizoplane were positively related to soil pH and negatively correlated with DTPA-extractable and exchangeable Cu; (3) Even though reducible Cu in nano-CuO-treated soils was 1.3 times higher than in CuSO4-treated soils, indicating stronger nano-accrued oxidative stress in nano-CuO-treated soils, the toxicity induced by nano-CuO particles was still weaker than CuSO4. Nevertheless, the toxicity of Cu particles to plants and microbes mainly depends on the gradually-released bioavailable Cu. This demonstrates the greater importance of bioavailable Cu concentrations for toxicity modulation rather than the scale of Cu particles.