Effects of five years of nitrogen and phosphorus additions on a Zizaniopsis miliacea tidal freshwater marsh

Abstract

The purpose of this experiment was to determine if nitrogen (N) or phosphorus (P) acts as the limiting nutrient for tidal freshwater marsh vegetation. To answer this question, we added N, P, and N + P to a tidal freshwater marsh dominated by Zizaniopsis miliacea (Michx.) (giant cutgrass) in Georgia, USA, for five years to determine their effects on aboveground and belowground biomass and nutrient (N, P) uptake. Nitrogen and P were applied twice per year at an annual rate of 50 g m −2 year −1 and 10 g m −2 year −1, respectively. Aboveground biomass and leaf C, N, and P were sampled in August of each year. Belowground biomass and C, N, and P content were measured in August of year five. After two years, plots receiving N and N + P had significantly greater aboveground biomass than the control and P plots. This trend continued through the fifth year of the study and resulted in two to three times more aboveground biomass at the end of the fifth year in the N (1570 g m −2) and N + P (1264 g m −2) plots relative to P (710 g m −2) and control (570 g m −2) plots. After five years of nutrient additions, macro-organic matter (MOM), the living plus dead root and rhizome mat (0–10 cm), was significantly lower in the N (1457 g m −2) and N + P (994 g m −2) plots than the control (2189 g m −2) plots. There was less live rhizome biomass in the N + P (23 g m −2) plots than the control (1085 g m −2) plots. We observed a 31–33% increase in the N content of Z. miliacea leaves in years three through five in the N and N + P plots relative to the control plots, but observed no P enrichment of leaves. In the N-treated plots, leaf C:N decreased 20–25% whereas N:P increased 21–64% in years three through five relative to the control and P plots. These findings collectively suggest that N, rather than P, limits productivity of tidal freshwater marsh vegetation. Reduced belowground biomass that accompanies N enrichment is of special concern as it may lead to increased erosion and reduced organic matter inputs to the soil, increasing their susceptibility to disturbances associated with wind, waves, river flooding and rising sea level.