The effects of soil nitrogen availability on the allocation of nitrogen to leaf processes for an invasive grass, Phalaris arundinacea, and native Carex species

Background/Question/Methods

Wetlands that receive a high nitrogen load are vulnerable to invasion by non-native species whose nitrogen-use strategies are favored by such conditions. The invasive grass, Phalaris arundinacea, thrives in nitrogen-rich sites that once supported Carex species adapted to lower nitrogen conditions. We hypothesized that the strategy of P. arundinacea to allocate nitrogen to photosynthetic enzymes and less to storage at high nitrogen levels would constrain photosynthesis (therefore, competitive potential) of P. arundinacea at low nitrogen. However, Carex species may maintain photosynthesis and be more competitive at low nitrogen by relying on nitrogen storage and preferential allocation of nitrogen to photosynthesis. Our findings will improve the understanding of physiological bases for the success or vulnerability of each species under variable nitrogen supply. We supplied plants of P. arundinacea, C. stricta, and C. lacustris with a complete nutrient solution containing 15 mM nitrogen for 7 weeks followed by a solution containing 0.15 mM nitrogen for 7 weeks.  After each treatment period, we measured photosynthetic parameters and determined the soluble protein content (storage and metabolic), and nitrate content of leaves. Statistical analyses were performed using mixed-effects ANCOVA, nesting the repeatedly measured plants within the ANCOVA.

Results/Conclusions

After 7 weeks of high nitrogen availability, P. arundinacea had the highest and C. lacustris the next highest carboxylation capacity, indicating strong allocations of nitrogen to CO₂ assimilation. Carboxylation data and leaf soluble protein contents suggested that a large portion of the protein was storage protein and not metabolic protein in C. stricta leaves. A higher specific leaf area (SLA) for P. arundinacea indicated less carbohydrate investment in leaves compared to the Carex species. Also, P. arundinacea accumulated more nitrate than the Carex species. Nitrogen deprivation significantly lowered the carboxylation capacity and soluble protein the most for young and old P. arundinacea leaves. Old leaves of C. lacustris had nearly the same photosynthetic capacity as the young leaves. Leaf nitrate content for P. arundinacea was reduced more than 50% under nitrogen deprivation, whereas little change in nitrate occurred for the Carex leaves. Nitrogen deprivation reduced SLA for P. arundinacea but not for the Carex species. We conclude that nitrogen deprivation causes a shift in leaf nitrogen allocation from photosynthesis and nitrate accumulation to other processes in the plant for P. arundinacea, potentially reducing its competitive advantage over Carex species, which respond less to a reduction in nitrogen supply.