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Effects of salinity and sulfide on the distribution of Phragmites australis and Spartina alterniflora in a tidal saltmarsh
Aquatic Botany 62 (1998) 161±169
Effects of salinity and sulfide on the distribution of Phragmites australis and Spartina alterniflora in a tidal saltmarsh Randolph M. Chambers*, Thomas J. Mozdzer, Joelle C. Ambrose Biology Department, Fairfield University, Fairfield, CT 06430, USA Received 6 February 1998; accepted 4 August 1998
Abstract In laboratory studies, the short-term response of Phragmites australis and Spartina alterniflora to root immersion in solutions of different salinity and sulfide concentration was measured as the rate of ammonium depletion (a proxy for root uptake) from an initial concentration of 20 mM. From 0 to 20 ppt in the absence of sulfide, ammonium uptake as a function of dry root weight decreased for both Phragmites (from 29.7 3.5 s.e. to 8.2 1.5 mmol N gÿ1 hÿ1) and Spartina (from 25.5 3.5 to 9.0 1.5 mmol N gÿ1 hÿ1). With an average sulfide concentration of 582 mM at 20 ppt, the rate of ammonium uptake for Spartina (10.9 2.2 mmol N gÿ1 hÿ1) was not significantly different from the rate in the absence of sulfide. In contrast, the rate of ammonium uptake was significantly lower for Phragmites when the average sulfide concentration was increased from 0 to 375 mM (1.7 1.6 mmol N gÿ1 hÿ1). In a tidal saltmarsh, the average porewater sulfide concentration in Phragmites australis stands (124 206 mM) was almost an order of magnitude lower than that measured in stands of Spartina alterniflora (989 166 mM). These field and laboratory results support the notion that increased sulfide in the rhizosphere reduces the ability of Phragmites to take up nutrients relative to species such as Spartina that are better-adapted to sulfidic soil conditions, thus restricting the distribution of Phragmites in tidal saltmarshes. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Reed; Rhizosphere; Restoration; Wetlands
1. Introduction In contrast to the well-documented European die-off of Phragmites australis (Cav.) Trin. ex Steudel (van der Putten, 1997), the expansion of Phragmites into tidal * Corresponding author. Tel.: +1 203 254 4000/2543; fax: +1 203 254 4253; e-mail: [email protected] 0304-3770/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 8 ) 0 0 0 9 5 - 3
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and non-tidal wetlands of North America has scientists and managers working to identify its cause and, potentially, its control (Marks et al., 1994). Rapid expansion is typical of successful species introductions but is notable in this instance because Phragmites has been an existing member of wetland plant communities in North America for thousands of years (Niering and Warren, 1977). Some researchers suspect a more aggressive genotype has been introduced from Europe (e.g., Besitka, 1996), while others point to widespread environmental changes driven by human activities that may have enhanced conditions for Phragmites growth (Roman et al., 1984; den Hartog et al., 1989). In North American estuaries, Phragmites is found more commonly in tidal freshwater to mesohaline marshes (0±18 ppt) than in polyhaline marshes (18±35 ppt). Greenhouse and laboratory experiments have shown that growth is stunted when Phragmites is flooded by saltwater (Hellings and Gallagher, 1992; Lissner and Schierup, 1997). From a physiological perspective, decreased growth at high salinity may be linked to osmotic difficulties and/or competition for nitrogen binding sites between sodium and ammonium in the rhizosphere (Bradley and Morris, 1991). From a management perspective, decreased growth at high salinity may allow for restoration of Spartina-dominated wetlands recently overgrown by Phragmites (Dreyer and Niering, 1995); diked or impounded wetlands frequently are restored when restrictions to tidal flow are removed and saltwater is re-introduced. Because Phragmites has been observed growing and flowering even where salinity approaches 40 ppt (Hellings and Gallagher, 1992; Burdick and Dionne, 1994), however, high salinity on its own probably is insufficient to control Phragmites growth. Management efforts that increase tidal saltwater flow into Phragmites wetlands also alter the soil environment by increasing the depth and duration of flooding, creating anoxic and sulfidic conditions around the roots. Phragmites transports oxygen to its root and rhizome system (Armstrong et al., 1996b; Brix et al., 1996) and thus is capable of growing in saturated soils. The response of Phragmites to sulfide concentrations >1 mM, however, includes stunted growth, callus blockage of gas-pathways and bud death (Armstrong et al., 1996a). For many wetland plants, soil sulfide is a toxin (Havill et al., 1985); sulfide blocks alcohol dehydrogenase activity, thereby inhibiting fermentative metabolism, decreasing energy production and reducing nitrogen uptake and plant growth (Koch et al., 1990). For Phragmites, maintenance of a positive oxygen balance (Weisner and Graneli, 1989) and carbon balance (Cizkova-Koncalova et al., 1992) are key to its success or failure under different soil regimes. Its functional response to reducing conditions in the rhizosphere, however, has not been investigated fully (van der Putten, 1997). The present study was completed to identify environmental limits to Phragmites distribution in saltwater wetlands where it has replaced Spartina species. Field measurements of rhizosphere conditions in Phragmites and Spartina alterniflora Loiseleur stands were combined with laboratory experiments measuring the effects of rhizosphere salinity and sulfide on nitrogen uptake by greenhouse plants. Our objective was to determine the relative responses of these species to different soil environments, to assist the development of successful management strategies for removal of Phragmites and restoration of Spartina in North American tidal wetlands.
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2. Methods 2.1. Field measurements Field surveys of species distributions and rhizosphere conditions were completed during 1995 in the Charles E. Wheeler salt marsh, located at the mouth of the Housatonic River in southwestern Connecticut, USA (41.1N, 73.1W). Average soil porewater salinity ranged from 13 to 23 ppt (Chambers, 1997). Five transects, each approximately 15 m in length, were established through stands of Spartina alterniflora which graded into stands of Phragmites. Surface elevations above mean low water were determined as described in Chambers (1997). In late August at each of 8 locations along the length of each transect, species presence and flowering status were determined in 0.25 m2 quadrats. Also in 1995, suction lysimeters (Chambers and Odum, 1990) were installed to 10 cm depth in the sediments at each quadrat location. Porewater was collected from lysimeters concurrent with measurements of plant distributions. Immediately after collection, water for sulfide analysis was filtered (Whatman GF/C, 1.2 mm) directly into vials containing Cline's reagent (Cline, 1969). Porewater for ammonium analysis was acidified with 6 N HCl to a pH < 2 and refrigerated before analyzing using standard analytical techniques (Parsons et al., 1984). The interference of sulfide with color development was removed by bubbling the acidified samples with nitrogen gas, then adjusting the pH to neutrality with 6 N NaOH. 2.2. Nitrogen uptake experiments All plants for nitrogen (N) uptake experiments were collected from a mesohaline tidal marsh. In spring of 1996 and 1997, emerging shoots and associated roots and short rhizomes of Phragmites and Spartina alterniflora were dug up, rinsed free of all inorganic sediment and dead organic matter, and potted individually in clean sand under ambient light and temperature conditions of the greenhouse. Sands in 10 10 cm pots were kept wet at field capacity with freshwater, using a moisture-controlled electronic sprinkling system. A one-time application of a timed-release fertilizer (Osmocote# 10± 10±10) was completed at the time of potting. Shoot and root growth of Phragmites and Spartina in pots was rapid. Plants between 30 and 60 cm shoot height were taken from the greenhouse to the laboratory, rinsed free of all sand and placed in 500 ml incubation flasks under a photosynthetically active photon flux density of 130 mmol mÿ2 sÿ1 for the duration of each experiment. Roots were immersed in pH 6±7 solutions of different salinity, sulfide or oxygen status, with starting concentrations of ammonium of approximately 20 mM, selected based on measurements of ammonium in porewater from mixed Spartina/Phragmites stands in the field. At least five plants were used for each treatment group. Initial experiments in 1996 compared N uptake by Phragmites as a function of rhizosphere salinity; treatment solutions of 0, 10, 20, and 30 ppt were prepared using nitrogen- and phosphorus-free buffered sea salts, and the rhizosphere was oxygenated. A second experiment tested the effect of rhizosphere anoxia on N uptake by Phragmites at 0 ppt salinity by continuously bubbling the flasks with nitrogen gas.
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In 1997, additional experiments were completed on both Phragmites and Spartina alterniflora, comparing N uptake under oxic conditions at 0 and 20 ppt salinity treatments. Finally, the response of both species to sulfide in the rhizosphere was determined by amending the flasks with sodium sulfide (treatment solution at 20 ppt salinity, pH 7, anoxic conditions). The sulfide concentrations in three treatment groups were established based on field measurements of sulfide concentrations in Phragmites, Spartina and mixed-species stands. Because nitrogen bubbling to maintain anoxic conditions also removed hydrogen sulfide gas from the treatment solutions, additional amendments to the flasks were required over the course of experiments to replace sulfide removed by bubbling. Thus, only a fairly broad sulfide concentration range could be maintained for each treatment. Experiments lasted for 4±8 h after plants were immersed in treatment solutions. During the first 2 h, 5 ml water samples were withdrawn with replacement from the flasks at time intervals every 10±20 min to capture the initial response of the plants to the treatment; we sampled at roughly hourly intervals thereafter. For all water samples, dissolved ammonium and sulfide concentrations were determined using standard colorimetric analysis. At the completion of experiments, the plant roots were excised, dried and weighed. Time-course depletions in N concentrations in the flasks (our proxy for nitrogen uptake) were used to calculate rates of N uptake per gram of dry root weight. Leastsquared regression equations used the linear portions of the uptake curves for rate calculations (all regression coefficients >0.9, with exceptions noted below). 3. Results 3.1. Field measurements Single-species stands of Spartina occurred at stations with significantly lower elevations than either Phragmites or mixed-species stands (Table 1). For single-species stands, Spartina was observed flowering in 14 of 15 quadrats and Phragmites in 11 of 14 quadrats. In mixed-species stands, at least one of the species was not flowering in 9 of 9 quadrats (flowering status was not determined in 2 other quadrats), which we consider to be a result of interspecific competition along these transitional edges. Table 1 Field observations of environmental conditions in Spartina (N 15), Phragmites (N 14) and mixed-species (N 11) stands, Charles E. Wheeler Marsh, August 1995 (mean 1 standard deviation) Community
Elevation (cm)
Porewater sulfide (mM)
Porewater ammonium (mM)
Spartina Spartina/Phragmites Phragmites
118.5 9.8 126.5 7.1a 130.4 4.1a
989 166 494 470a 124 206a,b
49 66 19 21 4.2 5.8a
a
Significantly different from Spartina stands. Significantly different from Spartina/Phragmites stands. Concentrations of sulfide and ammonium were measured from porewater collected at 10 cm rhizosphere depth. Significant differences were determined using Duncan's Multiple Range Test (p < 0.05). b
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Fig. 1. Effect of rhizosphere salinity on short-term nitrogen uptake by Phragmites australis. Average rates standard error are plotted for N 15, 5, 9 and 5 at salinity 0, 10, 20 and 30 ppt. refers to significant difference from rate at 0 ppt (Duncan's Multiple Range Test, p < 0.05).
Analysis of porewater collected from lysimeters at 10 cm soil depth showed that rhizosphere sulfide and ammonium concentrations were highest in Spartina stands, intermediate in the mixed-species stands and lowest in Phragmites stands (Table 1). For Phragmites stands that were on average 12 cm higher in elevation than Spartina, the concentrations of ammonium and sulfide were about an order of magnitude lower. 3.2. Nitrogen uptake experiments Nitrogen uptake by Phragmites decreased with increasing salinity in the rhizosphere (Fig. 1). The average rates at 0 and 10 ppt were not significantly different, but rates at 20 and 30 ppt were significantly lower than the freshwater treatment (Duncan's Multiple Range Test, p < 0.05). Similar to Phragmites, nitrogen uptake by Spartina decreased with increasing salinity. The rate at 0 ppt (25.5 3.9 mmol gÿ1 hÿ1, N 5) was significantly higher than the rate at 20 ppt (9.0 1.5mmol gÿ1 hÿ1, N 5) (t-test, p < 0.01). Based on these experimental results measuring N uptake, the relative responses of Phragmites and Spartina to increasing salinity in the rhizosphere are similar. Nitrogen uptake under anoxic conditions at 0 ppt salinity was measured only for Phragmites, yielding a rate significantly lower than under oxic conditions (4.8 0.8 mmol gÿ1 hÿ1, N 8 versus 29.7 3.5 mmol gÿ1 hÿ1, N 15) (t-test, p < 0.01). To examine the combined effects of anoxia and rhizosphere sulfide on N uptake, each species was exposed to sulfide regimes typical of conditions observed in the field (Table 1). Two experiments were completed testing Phragmites exposure to low and high sulfide treatments. During the experiments, the dissolved sulfide ranged from 185± 276 mM per flask in the `low' treatment group (average 195 mM), and from 300±430 mM in the `high' treatment group (average 375 mM). Sulfide concentrations in the single, `very high' treatment group for Spartina ranged from 300±950 mM (average 582 mM).
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Fig. 2. Combined effect of rhizosphere sulfide and anoxia on short-term nitrogen uptake by Phragmites australis and Spartina alterniflora at 20 ppt salinity (N 5). Time-averaged sulfide concentrations are presented for each treatment group. refers to significant difference from rate at 0 mM sulfide, oxic controls (t-test, p < 0.05).
The responses of the species to sulfide were quite different (Fig. 2). Nitrogen uptake by Phragmites decreased non-significantly under the low sulfide treatment but dropped to near 0 under the high sulfide treatment. For the high sulfide treatment, the correlation coefficients for nitrogen uptake were less than 0.9, and for 2 of the 5 plants tested, ammonium was actually released by the plants to the rhizosphere solution. In contrast, nitrogen uptake by Spartina was unaffected by very high sulfide concentrations, and rates were as high as those measured under oxic conditions at 20 ppt (Fig. 2). 4. Discussion For Phragmites under oxic conditions, nitrogen uptake decreased with increasing salinity up to 30 ppt (Fig. 1). From both field observations and greenhouse studies, elevated salinity in the rhizosphere is known to stunt the growth of Phragmites (Hellings and Gallagher, 1992; Marks et al., 1994; Lissner and Schierup, 1997). Responses by Phragmites and Spartina at 0 and 20 ppt, however, were similar in our study, indicating the relative abilities of the species to obtain N from fresh to mesohaline salinity regimes do not change. Also, Chambers (1997) found the field distribution of Phragmites and Spartina in this marsh was not influenced by salinity, indicating their spatial separation was due to other environmental effects. Polyhaline salinity regimes can preclude Phragmites from some marshes (Lissner and Schierup, 1997), but based on our results, changes in salinity in mesohaline marshes are insufficient to alter the competitive abilities of Phragmites and Spartina to obtain N for growth. The effect of rhizosphere anoxia on N uptake by Phragmites at 0 ppt salinity (5 mmol gÿ1 hÿ1) was greater than even the highest salinity treatment under oxic
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conditions (Fig. 1). Bradley and Morris (1990) found that N-uptake by Spartina decreased from 13 to 8 mmol gÿ1 hÿ1 in oxic vs anoxic treatments at 20 ppt. Wetland plants are able to shunt oxygen to the roots to maintain aerobic respiration (Gries et al., 1990), but when this mechanism is overwhelmed they shift to fermentative metabolism. The observed decrease in N-uptake by Phragmites may reflect the decrease in energy available for N assimilation under these conditions (Koch et al., 1990). We report the first measurements of the combined effect of anoxia and rhizosphere sulfide on the ability of Phragmites to take up nitrogen relative to Spartina (Fig. 2). Sulfide as a toxin is known to influence many different metabolic processes, and most freshwater wetland species are more sensitive to sulfide than are saltwater species (Havill et al., 1985; Koch et al., 1990; Morris et al., 1996). In our experiment, N uptake by Spartina exposed to anoxia and sulfide was not significantly different from controls, whereas N uptake by Phragmites decreased to roughly one-fourth of values obtained under oxic conditions. These short-term responses are reflected in the field distributions of the plants and associated edaphic conditions (Table 1); the average porewater sulfide concentration was almost 1 mM for Spartina stands and only 124 mM for Phragmites. Howes et al. (1986) proposed a positive-feedback model of Spartina production, arguing that plant growth modifies environmental conditions (primarily, aeration of the rhizosphere and elimination of sulfide toxicity) to promote additional plant growth. Experimental research investigating various aspects of the model has yielded conflicting results. In contrast to the results of Bradley and Morris (1990) who measured decreased N uptake by Spartina as a proxy for reduced growth, we saw no combined effect of anoxia and high sulfide concentrations on N uptake. Either our experiments were too short to record the effects of sulfide toxicity or the plants were tolerant of sulfide and were able to compensate physiologically. In fact, Morris et al. (1996) measured greater Spartina growth with increasing sulfide concentrations up to 1 mM, so it is difficult to reach a consensus regarding the absolute effects of rhizosphere anoxia and sulfide on Spartina (Koch et al., 1990). Because growth stimulation of Spartina under sulfidic conditions has been measured experimentally and observed in the field, tolerance or compensation for rhizosphere sulfide must sometime occur. For Phragmites, the negative effect of rhizosphere anoxia and sulfide was dramatic (Fig. 2) and has implications for the limits to Phragmites distribution in tidal marshes (Table 1). Phragmites has out-competed Spartina for space and/or nutrients in many tidal wetlands; evidence for the competition is seen in the spatial segregation of Phragmites and Spartina to high and low tidal elevations, respectively, where sulfide concentrations are significantly different (Table 1). The absence of flowering by one or both species in transitional, mixed species stands also indicates a reduction in fitness consistent with species competition. If high concentrations of sulfide in the rhizosphere establish a physiological limit to the spread of Phragmites relative to Spartina in tidal marshes, then management for this edaphic condition should be possible. Current restoration methods have focused on enhancement of flooding by saltwater into wetlands to encourage Phragmites removal, but this approach may only work when rhizosphere sulfide concentrations are increased as a consequence. Increasing salinity and increasing anoxia do decrease the ability of Phragmites to take up nitrogen (results, Fig. 1); because Spartina responds similarly, however, no shift in competitive advantage may be realized.
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Rhizosphere sulfide inhibited N uptake by Phragmites but not by Spartina (Fig. 2), so an increase in rhizosphere sulfide concentration could constrict the current distribution of Phragmites and promote the re-establishment of Spartina in tidal wetlands. As a management goal, however, removal of Phragmites must be weighed against the risk of ongoing habitat alteration in North American wetlands via hydrologic manipulation. Acknowledgements This research was funded by a Tidal Wetlands Research Grant from the Connecticut Chapter of The Nature Conservancy. Our thanks go to M.A. Sabo for field and laboratory assistance, and to B.H. Chambers, G. Bowes and two anonymous reviewers for commenting on earlier versions of the manuscript. References Armstrong, J., Afreen-Zobayed, F., Armstrong, W., 1996a. Phragmites die-back: sulphide- and acetic acidinduced bud and root death, lignifications, and blockages within aeration and vascular systems. New Phytol. 134, 601±614. Armstrong, J., Armstrong, W., Beckett, P.M., Halder, J.E., Lythe, S., Holt, R., Sinclair, A., 1996b. Pathways of aeration and the mechanisms and beneficial effects of humidity- and venturi-induced convections in Phragmites australis (Cav.) Trin. ex Steud. Aquat. Bot. 54, 177±197. Besitka, Sister M.A.R., 1996. An ecological and historical study of Phragmites australis along the Atlantic Coast. M.S. Thesis, Drexel University, Philadelphia, PA, USA. Bradley, P.M., Morris, J.T., 1990. Influence of oxygen and sulfide concentration on nitrogen uptake kinetics in Spartina alterniflora. Ecology 71, 282±287. Bradley, P.M., Morris, J.T., 1991. The influence of salinity on the kinetics of NH4 uptake in Spartina alterniflora. Oecology 85, 375±380. Brix, H., Surrell, B.K., Schierup, H.-H., 1996. Gas fluxes achieved by in situ convective flow in Phragmites australis. Aquat. Bot. 54, 151±163. Burdick, D.M., Dionne, M., 1994. Comparison of Salt Marsh Restoration and Creation Techniques in Promoting Native Vegetation and Functional Values. Office of State Planning, Concord, NH, USA. Chambers, R.M., 1997. Porewater chemistry associated with Phragmites and Spartina in a Connecticut tidal marsh. Wetlands 17, 360±367. Chambers, R.M., Odum, W.E., 1990. Porewater oxidation, dissolved phosphate and the iron curtain. Biogeochemistry 10, 37±52. Cizkova-Koncalova, H., Kvet, J., Thompson, K., 1992. Carbon starvation: a key to reed decline in eutrophic lakes. Aquat. Bot. 43, 105±113. Cline, J.D., 1969. Spectrophotometric determination of hydrogen sulfide in natural waters. Limnol. Oceanogr. 14, 454±458. den Hartog, C., Kvet, J., Sukopp, H., 1989. Reed. A common species in decline. Aquat. Bot. 35, 1±4. Dreyer, G.D., Niering, W.A., 1995. Tidal Marshes of Long Island Sound: Ecology, History and Restoration. Connecticut College Arboretum Bulletin No. 34, 73 pp. Gries, C., Kappen, L., Losch, R., 1990. Mechanism of flood tolerance in reed, Phragmites australis (Cav.) Trin. ex Steudel. The New Phytol. 114, 589±593. Havill, D.C., Ingold, A., Pearson, J., 1985. Sulphide tolerance in coastal halophytes. Vegetation 62, 279±285. Hellings, S.E., Gallagher, J.L., 1992. The effects of salinity and flooding on Phragmites australis. J. Appl. Ecol. 29, 41±49. Howes, B.L., Dacey, J.W.H., Goehringer, D.D., 1986. Factors controlling the growth form of Spartina alterniflora: feedbacks between above-ground production, sediment oxidation nitrogen and salinity. J. Ecol. 74, 881±898.
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Koch, M.S., Mendelssohn, I.A., McKee, K.L., 1990. Mechanism for the hydrogen sulfide-induced growth limitation in wetland macrophytes. Limnol. Oceanogr. 35, 399±408. Lissner, J., Schierup, H.-H., 1997. Effects of salinity on the growth of Phragmites australis (Cav.) Trin ex Steudel. Aquat. Bot. 55, 247±260. Marks, M., Lapin, B., Randall, J., 1994. Phragmites australis (P. communis): threats, management and monitoring. Nat. Areas J. 14, 285±294. Morris, J.T., Haley, C., Krest, R., 1996. Effects of sulfide concentrations on growth and dimethylsulphoniopropionate (DMSP) concentration in Spartina alterniflora. In: Kiene, R., Visscher, P., Keller, M., Kirst, G. (Eds.), Biological and Environmental Chemistry of DMSP and Related Sulfonium Compounds. Plenum Press, New York, pp. 87±95. Niering, W.A., Warren, R.S., 1977. Our dynamic tidal marshes: vegetation changes as revealed by peat analysis. Connecticut College Arboretum Bulletin No. 22, 12 pp. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergammon Press, New York. Roman, C.T., Niering, W.A., Warren, R.S., 1984. Salt marsh vegetation change in response to tidal restriction. Environ. Manage. 8, 141±150. van der Putten, W.H., 1997. Die-back of Phragmites australis in European wetlands: an overview of the European research programme on reed die-back and progression (1993±1994). Aquat. Bot. 59, 263±275. Weisner, S.E.B., Graneli, W., 1989. Influence of substrate conditions on the growth of Phragmites australis after a reduction in oxygen transport to below-ground parts. Aquat. Bot. 35, 71±80.
Effects of salinity and sulfide on the distribution of Phragmites australis and Spartina alterniflora in a tidal saltmarsh Randolph M. Chambers*, Thomas J. Mozdzer, Joelle C. Ambrose Biology Department, Fairfield University, Fairfield, CT 06430, USA Received 6 February 1998; accepted 4 August 1998
Abstract In laboratory studies, the short-term response of Phragmites australis and Spartina alterniflora to root immersion in solutions of different salinity and sulfide concentration was measured as the rate of ammonium depletion (a proxy for root uptake) from an initial concentration of 20 mM. From 0 to 20 ppt in the absence of sulfide, ammonium uptake as a function of dry root weight decreased for both Phragmites (from 29.7 3.5 s.e. to 8.2 1.5 mmol N gÿ1 hÿ1) and Spartina (from 25.5 3.5 to 9.0 1.5 mmol N gÿ1 hÿ1). With an average sulfide concentration of 582 mM at 20 ppt, the rate of ammonium uptake for Spartina (10.9 2.2 mmol N gÿ1 hÿ1) was not significantly different from the rate in the absence of sulfide. In contrast, the rate of ammonium uptake was significantly lower for Phragmites when the average sulfide concentration was increased from 0 to 375 mM (1.7 1.6 mmol N gÿ1 hÿ1). In a tidal saltmarsh, the average porewater sulfide concentration in Phragmites australis stands (124 206 mM) was almost an order of magnitude lower than that measured in stands of Spartina alterniflora (989 166 mM). These field and laboratory results support the notion that increased sulfide in the rhizosphere reduces the ability of Phragmites to take up nutrients relative to species such as Spartina that are better-adapted to sulfidic soil conditions, thus restricting the distribution of Phragmites in tidal saltmarshes. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Reed; Rhizosphere; Restoration; Wetlands
1. Introduction In contrast to the well-documented European die-off of Phragmites australis (Cav.) Trin. ex Steudel (van der Putten, 1997), the expansion of Phragmites into tidal * Corresponding author. Tel.: +1 203 254 4000/2543; fax: +1 203 254 4253; e-mail: [email protected] 0304-3770/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 7 7 0 ( 9 8 ) 0 0 0 9 5 - 3
162
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and non-tidal wetlands of North America has scientists and managers working to identify its cause and, potentially, its control (Marks et al., 1994). Rapid expansion is typical of successful species introductions but is notable in this instance because Phragmites has been an existing member of wetland plant communities in North America for thousands of years (Niering and Warren, 1977). Some researchers suspect a more aggressive genotype has been introduced from Europe (e.g., Besitka, 1996), while others point to widespread environmental changes driven by human activities that may have enhanced conditions for Phragmites growth (Roman et al., 1984; den Hartog et al., 1989). In North American estuaries, Phragmites is found more commonly in tidal freshwater to mesohaline marshes (0±18 ppt) than in polyhaline marshes (18±35 ppt). Greenhouse and laboratory experiments have shown that growth is stunted when Phragmites is flooded by saltwater (Hellings and Gallagher, 1992; Lissner and Schierup, 1997). From a physiological perspective, decreased growth at high salinity may be linked to osmotic difficulties and/or competition for nitrogen binding sites between sodium and ammonium in the rhizosphere (Bradley and Morris, 1991). From a management perspective, decreased growth at high salinity may allow for restoration of Spartina-dominated wetlands recently overgrown by Phragmites (Dreyer and Niering, 1995); diked or impounded wetlands frequently are restored when restrictions to tidal flow are removed and saltwater is re-introduced. Because Phragmites has been observed growing and flowering even where salinity approaches 40 ppt (Hellings and Gallagher, 1992; Burdick and Dionne, 1994), however, high salinity on its own probably is insufficient to control Phragmites growth. Management efforts that increase tidal saltwater flow into Phragmites wetlands also alter the soil environment by increasing the depth and duration of flooding, creating anoxic and sulfidic conditions around the roots. Phragmites transports oxygen to its root and rhizome system (Armstrong et al., 1996b; Brix et al., 1996) and thus is capable of growing in saturated soils. The response of Phragmites to sulfide concentrations >1 mM, however, includes stunted growth, callus blockage of gas-pathways and bud death (Armstrong et al., 1996a). For many wetland plants, soil sulfide is a toxin (Havill et al., 1985); sulfide blocks alcohol dehydrogenase activity, thereby inhibiting fermentative metabolism, decreasing energy production and reducing nitrogen uptake and plant growth (Koch et al., 1990). For Phragmites, maintenance of a positive oxygen balance (Weisner and Graneli, 1989) and carbon balance (Cizkova-Koncalova et al., 1992) are key to its success or failure under different soil regimes. Its functional response to reducing conditions in the rhizosphere, however, has not been investigated fully (van der Putten, 1997). The present study was completed to identify environmental limits to Phragmites distribution in saltwater wetlands where it has replaced Spartina species. Field measurements of rhizosphere conditions in Phragmites and Spartina alterniflora Loiseleur stands were combined with laboratory experiments measuring the effects of rhizosphere salinity and sulfide on nitrogen uptake by greenhouse plants. Our objective was to determine the relative responses of these species to different soil environments, to assist the development of successful management strategies for removal of Phragmites and restoration of Spartina in North American tidal wetlands.
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2. Methods 2.1. Field measurements Field surveys of species distributions and rhizosphere conditions were completed during 1995 in the Charles E. Wheeler salt marsh, located at the mouth of the Housatonic River in southwestern Connecticut, USA (41.1N, 73.1W). Average soil porewater salinity ranged from 13 to 23 ppt (Chambers, 1997). Five transects, each approximately 15 m in length, were established through stands of Spartina alterniflora which graded into stands of Phragmites. Surface elevations above mean low water were determined as described in Chambers (1997). In late August at each of 8 locations along the length of each transect, species presence and flowering status were determined in 0.25 m2 quadrats. Also in 1995, suction lysimeters (Chambers and Odum, 1990) were installed to 10 cm depth in the sediments at each quadrat location. Porewater was collected from lysimeters concurrent with measurements of plant distributions. Immediately after collection, water for sulfide analysis was filtered (Whatman GF/C, 1.2 mm) directly into vials containing Cline's reagent (Cline, 1969). Porewater for ammonium analysis was acidified with 6 N HCl to a pH < 2 and refrigerated before analyzing using standard analytical techniques (Parsons et al., 1984). The interference of sulfide with color development was removed by bubbling the acidified samples with nitrogen gas, then adjusting the pH to neutrality with 6 N NaOH. 2.2. Nitrogen uptake experiments All plants for nitrogen (N) uptake experiments were collected from a mesohaline tidal marsh. In spring of 1996 and 1997, emerging shoots and associated roots and short rhizomes of Phragmites and Spartina alterniflora were dug up, rinsed free of all inorganic sediment and dead organic matter, and potted individually in clean sand under ambient light and temperature conditions of the greenhouse. Sands in 10 10 cm pots were kept wet at field capacity with freshwater, using a moisture-controlled electronic sprinkling system. A one-time application of a timed-release fertilizer (Osmocote# 10± 10±10) was completed at the time of potting. Shoot and root growth of Phragmites and Spartina in pots was rapid. Plants between 30 and 60 cm shoot height were taken from the greenhouse to the laboratory, rinsed free of all sand and placed in 500 ml incubation flasks under a photosynthetically active photon flux density of 130 mmol mÿ2 sÿ1 for the duration of each experiment. Roots were immersed in pH 6±7 solutions of different salinity, sulfide or oxygen status, with starting concentrations of ammonium of approximately 20 mM, selected based on measurements of ammonium in porewater from mixed Spartina/Phragmites stands in the field. At least five plants were used for each treatment group. Initial experiments in 1996 compared N uptake by Phragmites as a function of rhizosphere salinity; treatment solutions of 0, 10, 20, and 30 ppt were prepared using nitrogen- and phosphorus-free buffered sea salts, and the rhizosphere was oxygenated. A second experiment tested the effect of rhizosphere anoxia on N uptake by Phragmites at 0 ppt salinity by continuously bubbling the flasks with nitrogen gas.
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In 1997, additional experiments were completed on both Phragmites and Spartina alterniflora, comparing N uptake under oxic conditions at 0 and 20 ppt salinity treatments. Finally, the response of both species to sulfide in the rhizosphere was determined by amending the flasks with sodium sulfide (treatment solution at 20 ppt salinity, pH 7, anoxic conditions). The sulfide concentrations in three treatment groups were established based on field measurements of sulfide concentrations in Phragmites, Spartina and mixed-species stands. Because nitrogen bubbling to maintain anoxic conditions also removed hydrogen sulfide gas from the treatment solutions, additional amendments to the flasks were required over the course of experiments to replace sulfide removed by bubbling. Thus, only a fairly broad sulfide concentration range could be maintained for each treatment. Experiments lasted for 4±8 h after plants were immersed in treatment solutions. During the first 2 h, 5 ml water samples were withdrawn with replacement from the flasks at time intervals every 10±20 min to capture the initial response of the plants to the treatment; we sampled at roughly hourly intervals thereafter. For all water samples, dissolved ammonium and sulfide concentrations were determined using standard colorimetric analysis. At the completion of experiments, the plant roots were excised, dried and weighed. Time-course depletions in N concentrations in the flasks (our proxy for nitrogen uptake) were used to calculate rates of N uptake per gram of dry root weight. Leastsquared regression equations used the linear portions of the uptake curves for rate calculations (all regression coefficients >0.9, with exceptions noted below). 3. Results 3.1. Field measurements Single-species stands of Spartina occurred at stations with significantly lower elevations than either Phragmites or mixed-species stands (Table 1). For single-species stands, Spartina was observed flowering in 14 of 15 quadrats and Phragmites in 11 of 14 quadrats. In mixed-species stands, at least one of the species was not flowering in 9 of 9 quadrats (flowering status was not determined in 2 other quadrats), which we consider to be a result of interspecific competition along these transitional edges. Table 1 Field observations of environmental conditions in Spartina (N 15), Phragmites (N 14) and mixed-species (N 11) stands, Charles E. Wheeler Marsh, August 1995 (mean 1 standard deviation) Community
Elevation (cm)
Porewater sulfide (mM)
Porewater ammonium (mM)
Spartina Spartina/Phragmites Phragmites
118.5 9.8 126.5 7.1a 130.4 4.1a
989 166 494 470a 124 206a,b
49 66 19 21 4.2 5.8a
a
Significantly different from Spartina stands. Significantly different from Spartina/Phragmites stands. Concentrations of sulfide and ammonium were measured from porewater collected at 10 cm rhizosphere depth. Significant differences were determined using Duncan's Multiple Range Test (p < 0.05). b
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Fig. 1. Effect of rhizosphere salinity on short-term nitrogen uptake by Phragmites australis. Average rates standard error are plotted for N 15, 5, 9 and 5 at salinity 0, 10, 20 and 30 ppt. refers to significant difference from rate at 0 ppt (Duncan's Multiple Range Test, p < 0.05).
Analysis of porewater collected from lysimeters at 10 cm soil depth showed that rhizosphere sulfide and ammonium concentrations were highest in Spartina stands, intermediate in the mixed-species stands and lowest in Phragmites stands (Table 1). For Phragmites stands that were on average 12 cm higher in elevation than Spartina, the concentrations of ammonium and sulfide were about an order of magnitude lower. 3.2. Nitrogen uptake experiments Nitrogen uptake by Phragmites decreased with increasing salinity in the rhizosphere (Fig. 1). The average rates at 0 and 10 ppt were not significantly different, but rates at 20 and 30 ppt were significantly lower than the freshwater treatment (Duncan's Multiple Range Test, p < 0.05). Similar to Phragmites, nitrogen uptake by Spartina decreased with increasing salinity. The rate at 0 ppt (25.5 3.9 mmol gÿ1 hÿ1, N 5) was significantly higher than the rate at 20 ppt (9.0 1.5mmol gÿ1 hÿ1, N 5) (t-test, p < 0.01). Based on these experimental results measuring N uptake, the relative responses of Phragmites and Spartina to increasing salinity in the rhizosphere are similar. Nitrogen uptake under anoxic conditions at 0 ppt salinity was measured only for Phragmites, yielding a rate significantly lower than under oxic conditions (4.8 0.8 mmol gÿ1 hÿ1, N 8 versus 29.7 3.5 mmol gÿ1 hÿ1, N 15) (t-test, p < 0.01). To examine the combined effects of anoxia and rhizosphere sulfide on N uptake, each species was exposed to sulfide regimes typical of conditions observed in the field (Table 1). Two experiments were completed testing Phragmites exposure to low and high sulfide treatments. During the experiments, the dissolved sulfide ranged from 185± 276 mM per flask in the `low' treatment group (average 195 mM), and from 300±430 mM in the `high' treatment group (average 375 mM). Sulfide concentrations in the single, `very high' treatment group for Spartina ranged from 300±950 mM (average 582 mM).
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Fig. 2. Combined effect of rhizosphere sulfide and anoxia on short-term nitrogen uptake by Phragmites australis and Spartina alterniflora at 20 ppt salinity (N 5). Time-averaged sulfide concentrations are presented for each treatment group. refers to significant difference from rate at 0 mM sulfide, oxic controls (t-test, p < 0.05).
The responses of the species to sulfide were quite different (Fig. 2). Nitrogen uptake by Phragmites decreased non-significantly under the low sulfide treatment but dropped to near 0 under the high sulfide treatment. For the high sulfide treatment, the correlation coefficients for nitrogen uptake were less than 0.9, and for 2 of the 5 plants tested, ammonium was actually released by the plants to the rhizosphere solution. In contrast, nitrogen uptake by Spartina was unaffected by very high sulfide concentrations, and rates were as high as those measured under oxic conditions at 20 ppt (Fig. 2). 4. Discussion For Phragmites under oxic conditions, nitrogen uptake decreased with increasing salinity up to 30 ppt (Fig. 1). From both field observations and greenhouse studies, elevated salinity in the rhizosphere is known to stunt the growth of Phragmites (Hellings and Gallagher, 1992; Marks et al., 1994; Lissner and Schierup, 1997). Responses by Phragmites and Spartina at 0 and 20 ppt, however, were similar in our study, indicating the relative abilities of the species to obtain N from fresh to mesohaline salinity regimes do not change. Also, Chambers (1997) found the field distribution of Phragmites and Spartina in this marsh was not influenced by salinity, indicating their spatial separation was due to other environmental effects. Polyhaline salinity regimes can preclude Phragmites from some marshes (Lissner and Schierup, 1997), but based on our results, changes in salinity in mesohaline marshes are insufficient to alter the competitive abilities of Phragmites and Spartina to obtain N for growth. The effect of rhizosphere anoxia on N uptake by Phragmites at 0 ppt salinity (5 mmol gÿ1 hÿ1) was greater than even the highest salinity treatment under oxic
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conditions (Fig. 1). Bradley and Morris (1990) found that N-uptake by Spartina decreased from 13 to 8 mmol gÿ1 hÿ1 in oxic vs anoxic treatments at 20 ppt. Wetland plants are able to shunt oxygen to the roots to maintain aerobic respiration (Gries et al., 1990), but when this mechanism is overwhelmed they shift to fermentative metabolism. The observed decrease in N-uptake by Phragmites may reflect the decrease in energy available for N assimilation under these conditions (Koch et al., 1990). We report the first measurements of the combined effect of anoxia and rhizosphere sulfide on the ability of Phragmites to take up nitrogen relative to Spartina (Fig. 2). Sulfide as a toxin is known to influence many different metabolic processes, and most freshwater wetland species are more sensitive to sulfide than are saltwater species (Havill et al., 1985; Koch et al., 1990; Morris et al., 1996). In our experiment, N uptake by Spartina exposed to anoxia and sulfide was not significantly different from controls, whereas N uptake by Phragmites decreased to roughly one-fourth of values obtained under oxic conditions. These short-term responses are reflected in the field distributions of the plants and associated edaphic conditions (Table 1); the average porewater sulfide concentration was almost 1 mM for Spartina stands and only 124 mM for Phragmites. Howes et al. (1986) proposed a positive-feedback model of Spartina production, arguing that plant growth modifies environmental conditions (primarily, aeration of the rhizosphere and elimination of sulfide toxicity) to promote additional plant growth. Experimental research investigating various aspects of the model has yielded conflicting results. In contrast to the results of Bradley and Morris (1990) who measured decreased N uptake by Spartina as a proxy for reduced growth, we saw no combined effect of anoxia and high sulfide concentrations on N uptake. Either our experiments were too short to record the effects of sulfide toxicity or the plants were tolerant of sulfide and were able to compensate physiologically. In fact, Morris et al. (1996) measured greater Spartina growth with increasing sulfide concentrations up to 1 mM, so it is difficult to reach a consensus regarding the absolute effects of rhizosphere anoxia and sulfide on Spartina (Koch et al., 1990). Because growth stimulation of Spartina under sulfidic conditions has been measured experimentally and observed in the field, tolerance or compensation for rhizosphere sulfide must sometime occur. For Phragmites, the negative effect of rhizosphere anoxia and sulfide was dramatic (Fig. 2) and has implications for the limits to Phragmites distribution in tidal marshes (Table 1). Phragmites has out-competed Spartina for space and/or nutrients in many tidal wetlands; evidence for the competition is seen in the spatial segregation of Phragmites and Spartina to high and low tidal elevations, respectively, where sulfide concentrations are significantly different (Table 1). The absence of flowering by one or both species in transitional, mixed species stands also indicates a reduction in fitness consistent with species competition. If high concentrations of sulfide in the rhizosphere establish a physiological limit to the spread of Phragmites relative to Spartina in tidal marshes, then management for this edaphic condition should be possible. Current restoration methods have focused on enhancement of flooding by saltwater into wetlands to encourage Phragmites removal, but this approach may only work when rhizosphere sulfide concentrations are increased as a consequence. Increasing salinity and increasing anoxia do decrease the ability of Phragmites to take up nitrogen (results, Fig. 1); because Spartina responds similarly, however, no shift in competitive advantage may be realized.
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Rhizosphere sulfide inhibited N uptake by Phragmites but not by Spartina (Fig. 2), so an increase in rhizosphere sulfide concentration could constrict the current distribution of Phragmites and promote the re-establishment of Spartina in tidal wetlands. As a management goal, however, removal of Phragmites must be weighed against the risk of ongoing habitat alteration in North American wetlands via hydrologic manipulation. Acknowledgements This research was funded by a Tidal Wetlands Research Grant from the Connecticut Chapter of The Nature Conservancy. Our thanks go to M.A. Sabo for field and laboratory assistance, and to B.H. Chambers, G. Bowes and two anonymous reviewers for commenting on earlier versions of the manuscript. References Armstrong, J., Afreen-Zobayed, F., Armstrong, W., 1996a. Phragmites die-back: sulphide- and acetic acidinduced bud and root death, lignifications, and blockages within aeration and vascular systems. New Phytol. 134, 601±614. Armstrong, J., Armstrong, W., Beckett, P.M., Halder, J.E., Lythe, S., Holt, R., Sinclair, A., 1996b. Pathways of aeration and the mechanisms and beneficial effects of humidity- and venturi-induced convections in Phragmites australis (Cav.) Trin. ex Steud. Aquat. Bot. 54, 177±197. Besitka, Sister M.A.R., 1996. An ecological and historical study of Phragmites australis along the Atlantic Coast. M.S. Thesis, Drexel University, Philadelphia, PA, USA. Bradley, P.M., Morris, J.T., 1990. Influence of oxygen and sulfide concentration on nitrogen uptake kinetics in Spartina alterniflora. Ecology 71, 282±287. Bradley, P.M., Morris, J.T., 1991. The influence of salinity on the kinetics of NH4 uptake in Spartina alterniflora. Oecology 85, 375±380. Brix, H., Surrell, B.K., Schierup, H.-H., 1996. Gas fluxes achieved by in situ convective flow in Phragmites australis. Aquat. Bot. 54, 151±163. Burdick, D.M., Dionne, M., 1994. Comparison of Salt Marsh Restoration and Creation Techniques in Promoting Native Vegetation and Functional Values. Office of State Planning, Concord, NH, USA. Chambers, R.M., 1997. Porewater chemistry associated with Phragmites and Spartina in a Connecticut tidal marsh. Wetlands 17, 360±367. Chambers, R.M., Odum, W.E., 1990. Porewater oxidation, dissolved phosphate and the iron curtain. Biogeochemistry 10, 37±52. Cizkova-Koncalova, H., Kvet, J., Thompson, K., 1992. Carbon starvation: a key to reed decline in eutrophic lakes. Aquat. Bot. 43, 105±113. Cline, J.D., 1969. Spectrophotometric determination of hydrogen sulfide in natural waters. Limnol. Oceanogr. 14, 454±458. den Hartog, C., Kvet, J., Sukopp, H., 1989. Reed. A common species in decline. Aquat. Bot. 35, 1±4. Dreyer, G.D., Niering, W.A., 1995. Tidal Marshes of Long Island Sound: Ecology, History and Restoration. Connecticut College Arboretum Bulletin No. 34, 73 pp. Gries, C., Kappen, L., Losch, R., 1990. Mechanism of flood tolerance in reed, Phragmites australis (Cav.) Trin. ex Steudel. The New Phytol. 114, 589±593. Havill, D.C., Ingold, A., Pearson, J., 1985. Sulphide tolerance in coastal halophytes. Vegetation 62, 279±285. Hellings, S.E., Gallagher, J.L., 1992. The effects of salinity and flooding on Phragmites australis. J. Appl. Ecol. 29, 41±49. Howes, B.L., Dacey, J.W.H., Goehringer, D.D., 1986. Factors controlling the growth form of Spartina alterniflora: feedbacks between above-ground production, sediment oxidation nitrogen and salinity. J. Ecol. 74, 881±898.
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