The Nitrogen Games – the invasive success of Fallopia japonica

The high invasion success of Fallopia japonica in Europe and North America is related to its niche construction strategy. A hotly debated and prominent possibility is that F. japonica uses weapons for chemical niche construction, which could have considerable consequences for plant nutrition and ecosystem functioning. At least one of its phenolic compounds is capable of inhibiting nitrification and nitrification is actually lower in F. japonica invaded systems. It was assumed that F. japonica has a higher affinity for ammonium and can therefore outcompete native plants that prefer nitrate. However, the uptake of ammonium by F. japonica has only been minimally studied and it has been shown that nitrogen-use efficiency seems to be the main trait. In a lab study using stable isotope labelling we tested nitrogen and carbon uptake of F. japonica against the strongest native competitor in European riparian zones U. dioica. We hypothesized that F. japonica has a greater potential to take up ammonium and that U. dioica would take advantage of the nitrate supply, and that F. japonica would have a slightly better nitrogen-use efficiency than U. .

We performed combined ¹³C-CO2 and ¹⁵N-NO3 and -NH4 labelling on young F. japonica and U. dioica plants. They were pulse labelled with ¹³CO₂ and fertilized with ¹⁵N enriched nitrate or ammonium (44 mg N kg -¹ dry soil). Atom excess of ¹⁵N and ¹³C, was measured after seven days in non-rooted soil, rhizosphere, fine roots, transport roots, and shoots. Contrary to our expectations, F. japonica always utilized less soil mineral N independent of the type of nitrogen.Overall, our data revealed that the ability of F. japonica to inhibit nitrification is not based on an affinity for ammonium. Therefore, it appears that F. japonica constructs its biogeochemical niche in a way to benefit from nitrogen-use efficiency, which we found to be higher, by supressing nitrification in nutrient rich habitats.