Influence of light-irradiated Noccaea caerulescens on the characteristics of dissolved organic matter in its rhizospheric soil during phytoremediation

It has been observed that suitable light irradiation can improve the phytoremediation efficiency of various plants by enhancing their growth rate and metal uptake capacity. However, the mechanisms underlying the effects of light irradiation on metal mobilization and translocation in soils have rarely been reported. This experiment was conducted to evaluate the variation in dissolved organic matter (DOM) in the rhizosphere of Noccaea caerulescens (formerly Thlaspi caerulescens J. & C. Presl) when irradiated with different combinations of red (0, 25, 50, 90, and 100% red) and blue light. N. caerulescens was induced to secrete significantly more DOM, relative to the control, into its rhizosphere after being irradiated with pure red light and other red light combinations; this increased the bioavailability of soil Cd. Moreover, the concentrations and proportions of the hydrophilic DOM fractions, particularly hydrophilic acid, which exhibited a high affinity for Cd, increased with increasing ratios of the red light. Furthermore, DOM secreted because of the light irradiation treatments exhibited a significantly higher Cd extraction ability compared with that of the untreated control; this consequently increased the Cd uptake capacity of N. caerulescens. The results demonstrated that the secretion of more DOM, particularly hydrophilic acid, plays a pivotal role in improving the phytoremediation efficiency of N. caerulescens.