The response of leaf nitrogen metabolism in competing invasive Phalaris arundinacea and native Carex stricta to seasonal changes in ammonium and nitrate

Background/Question/Methods

The invasion of Phalaris arundinacea in wetlands inhabited by the native sedge, Carex stricta, is an ideal model system to study the physiological effects of eutrophication on invasion. Previous work has determined that Phalaris exhibits higher rates of photosynthesis, higher specific leaf areas (SLA, leaf dry mass/leaf area), and greater net carbon gain across a broad range of temperatures compared to Carex, particularly in nitrogen enriched areas. What is unknown is the role nitrogen assimilation efficiency and soil nitrogen preference play in determining where each species is competitive. Present research is addressing the following questions: (1) How do seasonal changes affect leaf traits and nitrogen assimilatory processes of each species under the current climatic conditions?; (2) Does soil nitrogen vary seasonally?; (3) Does soil nitrogen form affect the performance of either species? Data were collected from a Carex-dominated sedge meadow and an adjacent Phalaris-dominated site in north-central Indiana during spring, summer, and autumn in 2011. Both species are present at both sites.  Leaf tissue was collected seasonally from each site for total activated nitrate reductase activity, leaf nitrogen, and specific leaf area for both species. Soil samples were also collected for nitrate/ammonium analysis.

Results/Conclusions

Invasive Phalaris exhibited more leaf nitrate reductase activity in both the Phalaris- and Carex-dominated sites, with higher activity in the Phalaris-dominated site. The SLA of Phalaris was constant over the year, whereas the SLA of Carex decreased significantly from spring to autumn in both sites. Leaf nitrogen was higher in Phalaris compared to Carex in both sites and did not vary seasonally in Phalaris but decreased throughout the season in Carex. The soil in the Phalaris-dominated site had significantly more nitrate than the Carex-dominated site while the Carex-dominated site had significantly more ammonium seasonally than the Phalaris-dominated site. Soil nitrate increased from summer to autumn, whereas ammonium decreased in both sites. By autumn, the Phalaris site had ~three fold the nitrate of the Carex site, consistent with the two fold greater leaf nitrate reductase activity in Phalaris compared to Carex.  Ammonium levels were similar. These data indicate that Carex slows down its incorporation of nitrogen into its leaves earlier in the year than Phalaris, potentially giving Phalaris an advantage to invade more space from the summer into the autumn.  Also, the nitrate reductase and soil nitrogen data suggest that Phalaris responds better to high nitrate than does Carex.