Nitrate reductase activity (NRA) in the invasive alien Fallopia japonica: seasonal variation, differences among habitats types, and comparison with native species

Nitrate reductase activity (NRA) was studied in the invasive alien plant F. japonica (Japanese knotweed) during the vegetation season and among natural, semi-natural, and human-made habitats and compared with NRA in selected native species. NRA was measured directly in the field from the beginning of May until the beginning of October. NRA was much higher than in the plant’s native range, i.e., East Asia, and showed a high degree of variation over time with the highest values being reached at the stage of fast vegetative growth and at the beginning of fruiting. NRA was highest on dumping sites probably due to the high nitrogen input into soils and near traffic and the emission of NOx by vehicles. A comparison of the enzyme activity in four selected native plant species indicated that NRA in F. japonica was the highest with the exception of Urtica dioica, which exhibited a similar activity of the enzyme. A detailed comparison with this species showed that differences between these species on particular dates were influenced by differences in the phenology of both plants. The initial results that were obtained suggest that nitrogen pollution in an environment can contribute to habitat invasibility and a high level of NRA, which in addition to the many plant traits that are commonly accepted as characteristic of invasiveness features, may be an important factor that enhances invasion success