Molecular transformation of dissolved organic carbon of rhizosphere soil induced by flooding and copper pollution

Abstract Dissolved organic carbon (DOC) in rhizosphere soil is critical for multiple geochemical processes. Soils that suffer from heavy metal pollution accompanied by flooding are present worldwide, while little research has explored their effects on rhizosphere DOC structures, especially at the molecular level. Therefore, this study was designed with the aim of improving the understanding of rhizosphere DOC fate in copper (Cu) amended and/or flooded soils. A 180-day laboratory incubation including soils with different Cu levels with and without flooding was conducted. Fourier transform ion cyclotron resonance mass spectrometry analysis showed that the number of formulas detected only in the rhizosphere DOC of flooded and Cu-treated soil was as high as 528–1375. Flooding and Cu pollution reduced molecular weight, aromaticity and compounds containing CHON2 and CHON3 of rhizosphere DOC, but increased its oxidation degree. Furthermore, Cu pollution resulted in DOC with more abundant lignin-, carbohydrate- and protein-like components but less condensed aromatic-like components. Overall, due to Cu pollution and flooding, the rhizosphere DOC pool contained more chemically and biologically labile components. Structural equation modeling indicated that this change in DOC structure was partly explained by soil enzyme activities (decrease in invertase, cellulase, urease activity and the increase in polyphenol oxidase and peroxidase activity). The findings highlighted the ecological risk of Cu pollution and flooding to soil health from the perspective of rhizosphere carbon.