Below- and Aboveground Biomass of Spartina alterniflora: Response to Nutrient Addition in a Louisiana Salt Marsh

Abstract

The responses of Spartina alterniflora aboveand belowground biomass to various combinations of N, P, and Fe were documented in a 1-year field experiment in a Louisiana salt marsh. Five levels of N additions to 0.25 m 2 plots resulted in 18% to 138% more live aboveground biomass compared to the control plots and higher stem densities, but had no effect on the amount of live belowground biomass (roots and rhizomes; R&R). There was no change in the aboveground biomass when P or Fe was added as part of a factorial experiment of +P, +N, and +Fe additions, but there was a 40% to 60% decrease in the live belowground biomass, which reduced the average R&R:S ratio by 50%. The addition of various combinations of nutrients had a significant affect on the belowground biomass indicating that the addition of P, not N, eased the need for root foraging activity. The end-of-the-growingseason N:P molar ratios in the live above-and belowground tissues of the control plot was 16.4 and 32.7, respectively. The relative size of the belowground standing stocks of N and P was higher than in the aboveground live tissues, but shifted downwards to about half that in fertilized plots. We conclude that the aboveground biomass was directly related to N availability, but not P, and that the accumulation of belowground biomass was not limited by N. We suggest that the reduction in belowground biomass with increased P availability, and the lower absolute and relative belowground standing stocks of P as plant tissue N:P ratios increased, is related to competition with soil microbes for P. One implication for wetland management and restoration is that eutrophication may be detrimental to long-term salt marsh maintenance and development, especially in organicrich wetland soils.

The effects of N, P and crude oil on the decomposition of Spartina alterniflora belowground biomass

“…This N:P ratio is similar to the N:P ratio for dead S. alterniflora roots and rhizomes from a Terrebonne estuary that had a N:P ratio averaging 52.5:1 for 12 months, and 58.5:1 for May (Darby and Turner 2008b), which is the month when the samples for this experiment were collected.…”

Spatial response of coastal marshes to increased atmospheric CO 2

“…In fact, an analysis of the extensive literature on elevated nitrogen levels in coastal marshes reveals large uncertainties in plant responses (Table S2). Fertilization experiments usually find that increased nitrogen promotes aboveground biomass productivity (3,6), but a wide scatter characterizes the observations, even within the same study (Fig. S1).…”

“…S1). Nitrogen's effect on belowground biomass productivity is even less clear, such that even the sign of the change in productivity remains undetermined (4,5) and may be dependent on the levels of nitrogen addition (6). Such scatter can be attributed to the fact that existing studies used different methods (e.g., the forms of nitrogen adopted for fertilization, such as ammonium sulfate, ammonium nitrate, etc.…”

“…Species composition, baseline ambient nitrogen loading (19,20), salinity, flooding frequency, and the concentration of other nutrients also vary widely across the existing literature, and can significantly affect the outcome of nitrogen addition experiments (4,(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31). Hence, to isolate the potential impacts of increased nitrogen concentrations, we use the minimum (55.2 g N·m −2 ·y −1 ) and maximum (892.8 g N·m −2 ·y −1 ) nitrogen addition levels and the corresponding biomass responses from one study that explored a wide range of nitrogen addition levels at a single site (6). The minimum addition level corresponds to a 15% increase in both the aboveground and belowground biomass, whereas the maximum addition experiment caused a 134% increase in aboveground biomass and a 24% decrease in belowground biomass.…”

“…In fact, direct CO 2 effects on vegetation and marsh accretion (as opposed to its indirect effects, e.g., via the increase in temperature ) have not yet been incorporated into marsh models, and their importance relative to other leading forcings of marsh dynamics (e.g., inorganic deposition, RRSLR, nutrient levels) remains unknown. Here we use existing data and a 1D ecomorphodynamic model to assess the direct impacts of elevated CO 2 on marsh morphology, relative to ongoing [e.g., RRSLR, and suspended sediment concentration (SSC)] and emerging [nutrient levels (4)(5)(6)] environmental change.…”