Melanie Brunn

Background The invasive plant species Fallopia japonica is suspected to use polyphenols as a novel weapon to inhibit nitrification in soil. Both specific polyphenols and their entry pathways are yet to be determined. As plants may increase the production of polyphenols under copper (Cu) stress, an additive effect can be expected in contaminated (riparian) areas. Aims This study aimed to identify the entry pathways of total and specific polyphenols in F. japonica and to test whether polyphenols inhibit nitrification with Cu contamination. Methods Combining F. japonica and a Cu gradient in a 2‐year mesocosm experiment, total polyphenol, emodin, and resveratrol concentrations were analyzed in the plant (roots, root exudates, vital and senescent leaves) and soil (rhizosphere, non‐rooted soil) representing different entry pathways. We measured the potential nitrification rate (PNR) under stress caused by F. japonica and Cu as well as the response of PNR under resveratrol and emodin addition. Results Emodin and resveratrol were detected in all plant tissues. Emodin concentrations significantly increased in senescent leaves under Cu stress, while no Cu effect was observed for resveratrol or total polyphenols. Resveratrol decreased the PNR. The stressors had neither a synergistic nor additive effect. Nitrification inhibition was lower in the rhizosphere compared to the non‐rooted soil, suggesting that F. japonica reduced nitrate availability for co‐occurring plants. Conclusions Joint occurrence of F. japonica and Cu did not amplify the PNR inhibition over the individual effect. Our study emphasizes the effectiveness of F. japonica in inhibiting the PNR in invaded riparian ecosystems with potentially negative effects on biodiversity.