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 root:shoot ratio of grassland plants may be lower in high fertility sites than in low fertility sites as plants modify their root, rhizome and shoot morphologies (and masses) to suit prevailing nutrient availability. We conducted geographically diverse and regionally specific field sampling and measured above-and belowground plant biomasses in western Atlantic and Gulf of Mexico salt marshes to determine whether there is a similar morphological response in Spartina alterniflora, the dominant salt marsh plant. Coastal nutrient addition/enrichment, which is widespread and ongoing, may lower root and rhizome biomass, belowground production and organic accumulation in this species. Higher soil respiration and a lower Eh (redox potential) are expected additional soil property changes. The addition of P, more than of N, seems to reduce root and rhizome biomass accumulation. The cumulative effects of increased nutrient loadings on salt marshes may be to decrease soil elevation and accelerate the conversion of emergent plant habitat to open water, particularly on the lower marsh. The recommendations for management practices intended to conserve coastal marshes include reducing nutrient loading to coastal zones, solving water quality problems with a multiple nutrient approach, and choosing monitoring metrics that are based on both above-and belowground plant biomass.KEY WORDS: Salt marsh · Nutrients · Spartina alterniflora · Belowground biomass · Die-back Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 363: [63][64][65][66][67][68][69][70] 2008 accretion is too low or too high, respectively. An aggressive new form of Phragmites australis apparently out-competes Spartina alterniflora on the landward side of cordgrass distribution; in part, the dominance of P. australis results from its greater belowground biomass production, with a consequent rise in soil elevation that has negative effects on S. alterniflora (Rooth & Stevenson 2000, Bertness et al. 2002. Soil elevation may be lowered to the detriment of cordgrass survival if either root production is decreased or the decomposition of accumulated organic matter is accelerated sufficiently that net organic accumulation is less than relative sea level rise. Additional evidence for the influence of belowground biomass on marsh vigor is that the amount of live root + rhizome (R+R) biomass is a diagnostic of plant health in Louisiana salt marshes (Turner et al. 2004). The R+R biomass there was 7 times higher in marshes categorized as 'healthy' than in deteriorating 'unhealthy' marshes.An implication of these observations is that the increases in nutrient loading to coastal systems, which are widespread (National Research Council 2000, Cloern 2001, Rabalais 2002, Howarth & Marino 2006, may alter marsh ecosystem functions and perhaps compromise the long-term stability of salt marshes by reducing root production and causing a consequential decline in soil organic accumulation. Morris & Bradley (1999), for example, f...

The monthly variations of below-and aboveground biomass of Spartina alterniflora were documented for a south Louisiana salt marsh from

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