Changes in fluorescent dissolved organic matter and their association with phytoavailable phosphorus in soil amended with TiO2 nanoparticles

Abstract

This study presents the impacts of TiO2 nanoparticles (TNPs) amendment on plant growth, phosphorus (P) content, and dissolved organic matter (DOM) composition in the rhizosphere. For this work, wheat plants (Galaxy-2013) were exposed to soil amended by different amounts of TNPs (i.e., 0, 50, and 100 mg TNP/kg of soil) for 40 days and harvested. The maximum increase in the shoots and roots lengths reached 15.9 ± 0.3% and 3.8 ± 0.3% respectively, which was concurrent with improved P content in the plants. Compared with the control, the P content in the shoots and roots was enriched by 23.4% and 17.9% at 50 mg TNP/kg of soil respectively. The increased electrical conductivity (EC) and decreased pH of the rhizosphere implied that the added TNPs might induce the enhancement of the P dissolution. Fluorescence spectroscopy revealed the increase of microbial activity as depicted by the humification index (HIX) changing from 0.88 ± 0.02 to 0.92 ± 0.01, with increasing TNPs amendments. Excitation-emission matrix coupled with parallel factor analysis (EEM-PARAFAC) showed the presence of four fluorescent components (C1 to C4) in the rhizosphere. Three of them (C1-C3) were related to humic-like substances, while the C4 was associated with protein-like fluorescence. EEM-PARAFAC results revealed the degradation of C4, and the enhancement of the other three components, which supported the stimulation of microbial activity by the TNPs amendment. This study provided new insights into the relation between improved phytoavailble P in plants and the changes in the rhizosphere soil solution chemistry and the DOM composition upon TNPs amendments.