Seasonal suppression of N cycling by the invasive, clonal plant Polygonum cuspidatum

Background/Question/Methods

One cascading feature of the spread of introduced species of plants is their capacity to alter levels of resource availabilities for other species.  Few studies have tested for mechanisms by which introduced plants might alter temporal patterns of soil nutrient availability, although this could be critical in terrestrial habitats with limited growing seasons.  We hypothesized that Polygonum cuspidatum, or Japanese knotweed, can reduce soil N availability in spring in the northeastern U.S. due to the seasonal suppression of N mineralization by phenolic compounds released from the litter it deposits in fall.  To test this, we measured mineralization, standing pools of inorganic and organic N, and concentrations of phenolics in soils inside and outside stands of knotweed at four sites over the growing season.  In addition, we tested effects of adding tannins from knotweed on mineralization.

Results/Conclusions

In the uppermost soil (0-5 cm depth), both concentrations of inorganic N and rates of N mineralization were 60% lower inside than outside stands of knotweed in spring but became as high or higher inside than outside by fall.  Tannins from knotweed reduced mineralization by 30-60%.  Concentrations of dissolved organic N (DON) and phenolics showed the opposite seasonal pattern; they were nearly 3 times higher inside than outside stands in spring and similar inside and outside in fall.  Over the season, DON and phenolics showed a strong, positive relationship.  At 5-15 cm depth, concentrations of phenolics were uniformly low and differences between soils inside and outside stands were mostly non-significant.   Results strongly support the hypothesis that a high concentration of phenols in the leaves of P. cuspidatum leads to inhibition of N mineralization and availability to plants in the early growing season in this region, but indicate that effects are limited to the uppermost soil.  This is likely to affect seedlings more than established plants and could help explain how knotweed, which spreads clonally, can monopolize space.