Nitrogen and phosphorus cycling strategies in two tidal freshwater macrophytes, Peltandra virginica and Spartina cynosuroides

Monotypic stands of the macrophytes Peltandra virginica and Spartina cynosuroides, which shared a common boundary, were studied at Sweethall Marsh, a tidal freshwater marsh located on the Pamunkey River within the Chesapeake Bay estuarine system, Virginia. Compartmental models indicate that both Peltandra and Spartina take up, internally cycle, and release to the environment, significant levels of nitrogen and phosphorus. Models also suggest that Peltandra and Spartina and their associated sediment compartments are capable of regulating nitrogen and phosphorus fluxes through their uptake and storage capacity. Nitrogen use efficiency was significantly higher in the shoots and roots of Spartina compared to Peltandra, while rhizome use efficiency was slightly higher in Peltandra. Phosphorus use efficiency was significantly higher in shoots, roots, and rhizomes of Spartina than in Peltandra. Lower use efficiency in Peltandra demonstrates a greater demand of nutrient per unit biomass than Spartina. Nitrogen and phosphorus recovery efficiency was higher in Peltandra. Efficiency indexes suggest that although Spartina appears to use nitrogen and phosphorus more efficiently for growth, Peltandra recovers and stores these nutrients more efficiently. Peltandra shoot, root, and rhizome tissue nutrient levels appear independent of sediment nitrogen and phosphorus availability. Spartina shoot, root, and rhizome nitrogen levels, however, appear dependent on sediment total nitrogen and total phosphorus. The relationship of Peltandra tissue nutrient levels to sediment availability is explained in terms of the rhizome storage capacity and reallocation of nitrogen and phosphorus to support productivity patterns. Spartina, however, must rely more on de novo root uptake to meet nutrient demands. Shoot, root, and rhizome nitrogen and phosphorus standing stocks were strongly correlated in both Peltandra and Spartina while sediment standing stocks were not. Nitrogen to phosphorus ratios were higher in the shoots than the roots and rhizomes of both Peltandra and Spartina reflecting the levels of nitrogen required to support photosynthesis. Nitrogen to phosphorus ratios varied over the sampling period, however appeared to converge on an optimum ratio. The correlation of nitrogen and phosphorus suggests an interaction between these nutrients although this relationship is unclear. Apparently both Peltandra and Spartina reallocate, as well as require, nitrogen and phosphorus in certain ratios which maximize productivity, uptake, and carbon assimilation