The Nitrogen Games – the invasive success of Fallopia japonica

<p>The high invasion success of <em>Fallopia japonica</em> in Europe and North America is related to its niche construction strategy. A hotly debated and prominent possibility is that <em>F. </em><em>japonica</em> 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 <em>F. japonica </em>invaded systems. It was assumed that <em>F. japonica</em> has a higher affinity for ammonium and can therefore outcompete native plants that prefer nitrate. However, the uptake of ammonium by <em>F. japonica</em> 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 <em>F. japonica</em> against the strongest native competitor in European riparian zones <em>U. dioica</em>. We hypothesized that <em>F. japonica</em> has a greater potential to take up ammonium and that <em>U. dioica </em>would take advantage of the nitrate supply, and that <em>F. japonica</em> would have a slightly better nitrogen-use efficiency than <em>U. </em>.</p> <p>We performed combined &#185;&#179;C-CO2 and &#185;&#8309;N-NO3 and -NH4 labelling on young <em>F. japonica</em> and <em>U. dioica</em> plants. They were pulse labelled with &#185;&#179;CO&#8322; and fertilized with &#185;&#8309;N enriched nitrate or ammonium (44 mg N kg -&#185; dry soil). Atom excess of &#185;&#8309;N and &#185;&#179;C, was measured after seven days in non-rooted soil, rhizosphere, fine roots, transport roots, and shoots. Contrary to our expectations, <em>F. japonica</em> always utilized less soil mineral N independent of the type of nitrogen.Overall, our data revealed that the ability of <em>F. japonica</em> to inhibit nitrification is not based on an affinity for ammonium. Therefore, it appears that <em>F. japonica</em> 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.</p>