COS 33-2
Differences in nitrogen-use strategies and soil nitrogen availability throughout the growing season in native Carex stricta and invasive Phalaris arundinacea
Anthropogenic increases in soil nitrogen in freshwater wetlands favor the abundance of invaders, such as Phalaris arundinacea, compared to native sedges, such as Carex stricta. However, for these species, it is not known whether: (1) a relationship exists between leaf nitrogen content and soil nitrogen: (2) seasonal changes in leaf physiological processes relate to changes in soil nitrogen; (3) the nitrogen-use strategies differ between the native and invasive species. We hypothesized that the nitrogen-use strategy of P. arundinacea is to allocate nitrogen to carbon metabolism, supporting high productivity while invading. This strategy would be favored by high soil nitrogen but may lead to a reduction in leaf nitrogen and productivity under low soil nitrogen. We hypothesized that the strategy of C. stricta is to store nitrogen, allowing it to maintain leaf metabolism during periods of low soil nitrogen. At two sites in north-central Indiana in 2012, we collected data on total leaf nitrogen, carbon and nitrogen assimilation capacity, leaf-level soluble protein, and soil nitrogen over the growing season. In 2013, we determined leaf and soil nitrogen, leaf nitrate content, and leaf soluble protein seasonally for both species from twenty-one sites.
Results/Conclusions
Except for P. arundinacea at very low nitrate availability, no relationship between leaf nitrogen and soil nitrate occurred. Phalaris arundinacea maintained high leaf nitrogen and soluble protein contents, whereas they decreased over the season for C. stricta. Similarly, the photosynthetic capacity was higher for P. arundinacea and decreased seasonally for C. stricta. Where soil nitrate was high, the leaf nitrate assimilation capacity was higher for P. arundinacea and decreased seasonally for C. stricta. There was a general trend of increasing soil nitrate throughout the growing season, whereas soil ammonium decreased. We conclude that, throughout the growing season, P. arundinacea maintains high leaf nitrogen assimilation, allocating a high proportion of nitrogen to photosynthesis. Except at very low soil nitrogen, P. arundinacea can maintain leaf nitrogen and soluble protein contents. Proportionately less nitrogen is allocated to photosynthesis by C. stricta, and its leaf nitrogen and metabolism are unresponsive to the differences in soil nitrogen that we measured, appearing to be seasonally regulated. At most soil nitrogen concentrations, both species are capable of attaining a high content of leaf nitrogen. It is the allocation of nitrogen and the maintenance of the nitrogen pools over the growing season that differ between the two species.