Competition and nitrogen availability affect nitrogen-use strategies in native and invasive wetland plants

Background/Question/Methods

Understanding how native and invasive species of freshwater wetlands use excess nitrogen has become important as the availably of inorganic nitrogen increases.  The invasive wetland grass, Phalaris arundinacea, requires high soil nitrogen for competitive success due to its allocation of nitrogen primarily to photosynthetic processes as opposed to storage or defense.  Native sedge species, such as Carex lacustris and Carex stricta, tend to store excess nitrogen rather than use it immediately to increase photosynthesis.  What is unknown is how the nitrogen-use strategies for these species differ when in close competition with each other under different nitrogen availabilities?

For 16 weeks (June-October), individuals of P. arundinacea, C. stricta, and C. lacustris were grown under high or low soil nitrogen alone and, for the Carex species, in competition with P. arundinacea.  One liter of the complete nutrient solution with either 15 or 1.5 mM nitrogen was applied twice a week to each 70-L mesocosm. Every 4 weeks, photosynthetic rates were measured, and leaves were collected for the analysis of nitrogen content and specific leaf area (SLA).  Photosynthetic nitrogen use efficiency (PNUE) was calculated as the amount of CO₂ fixed divided by the nitrogen content of the leaf. 

Results/Conclusions

Phalaris arundinacea had high PNUE and SLA under both nitrogen treatments, indicating that this species’ nitrogen use strategy and its leaf construction are not greatly affected by these concentrations of supplied nitrogen.  The PNUE for P. arundinacea was highest when it grew alone and increased over the growing season under high nitrogen, but not low nitrogen.  This increase in PNUE was greater when P. arundinacea grew with C. lacustris than when it grew with C. stricta.  Growth with C. stricta decreased SLA for P. arundinacea more than when it grew with C. lacustris.

When growing with P. arundinacea, C. lacustris increased its PNUE over time while leaf nitrogen remained relatively constant, suggesting that competition causes C. lacustris to allocate more nitrogen to photosynthesis over storage compared to when it grows alone.  When C. stricta grew alone, its PNUE was similar between high and low nitrogen treatments over time.  However, when growing with P. arundinacea, the PNUE of C. stricta decreased.  These results suggest that C. lacustris may be able to tolerate a P. arundinacea invasion better than C. stricta.