Effect of salinity on the critical nitrogen concentration of Spartina alterniflora Loisel

Aquatic Botany, 43 (1992) 149-16 ! 149 Elsevier Science Publishers B.V., Amsterdam

Effect of salinity on the critical nitrogen concentration of Spartina alterniflora Loisel.

P.M. Bradley ~'b and J.T. Morris a "Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA bUS Geological Survey, WRD Stepheoson Center, Suite 129. 720 Gracern Drive. Columbia,

SC29210-7651, USA

(Accepted 14 February 1992)

ABSTRACT

Bradley, P.M. and Morris, J.T., 1992. Effect of salinity on the critical nitrogen concentration of Spar- tina aRernijlora Loisel. Aquat. Bot., 43: 149-161.

Nitrogen was withheld from the salt marsh grass Spartina alterniflora Loisel., in order to determine the effect of salinity (sea salts) on critical tissue nitrogen concentrations (defined here as the mini- mum tissue concentralion required to sustain biomass aecumulation). The critical nitrogen concen- tration per kilogram dry weight ofabove-grouttd tissues increased non-linearly from a mean of 8.2 g kg-I at 5 g l - I and 20gl-~ salinityto 13.6gkg-~ and22.9gkg-~ atsal ini t iesof40gl-t and 50gl -I , respectively. Below-ground tissue nitrogen concemrations averaged 62% of the above-ground values irrespective of salinity treatment. These results suggest that the critical nitrogen concentration is a function of salinity and indicate that the internal nitrogen supply required in support of grovah in. creases with salinity. Above-ground tissue nitrogen concentrations reported in the literalure and the relationship between salinity and critical nitrogen concentration observed in this study were used to evaluate the nitrogen status orS. alterni.flora over a wide range of geographical locations. Compari- sons suggest that both sbott and tall forms of S. alterniflora are nitrogen limited in the majority of marshes along the Gulf and Atlantic Coasts of the US.

INTRODUCTION

Primary production in North American salt marshes is generally consid- ered to be limited by nitrogen availability. Within east coast salt marshes, primary productivity of the dominant species, the grass Spartina alterniflora Loisel., is highest along the margins of tidal creeks and diminishes with dis- tance from the creeks. This productivity gradient is thought to be a function of nitrogen limitation. The evidence for nitrogen limitation in S. alterniflora is based primarily on the in situ response to nitrogen fertilization (Valiela and

Correspondence to: J.T. Morris, Department of Biological Sciences, University of South Caro- lina, Columbia, SC 29208, USA.

© 1992 Elsevier Science Publishers B.V. All rights reserved 0304°37?0/92/$05.00

150 P.M. BRADLEY AND J.'l'. MORRIS

Teal, 1974; Gallagher, 1975; Broome et al., 1975; Patrick and Delaune, 1976; Buresh et al., 1980; Cavalieri and Huang, 1981; Morris, 1988), but these re- sults may be misleading due to possible secondary effects of nitrogen treat- ment (Smart and Barko, 1980 ). Nitrogen additions may increase rates of mi- neralization of other limiting nutrients from organic matter and alter the ionic composition and pH ofthe interstitial water (Mendelssohn, 1979; Smart and Barko, 1980). To avoid these drawbacks, Smart and Barko (1980) estimated the critical nitrogen concentration (CNC) (the minimum tissue concentra- tion needed to sustain biomass accumulation) of S. alterniflora grown at a salinity of 15 g 1- ~. A comparison of the nitrogen concentration of individuals grown on natural sediments at the same salinity to this experimentally deter- mined CNC verified that growth ofS. alterniflora was nitrogen limited (Smart and Barko, 1980).

However, the sensitivity of tissue nitrogen concentrations to osmotic stress is well established (see Bates, 1971 for a review of the factors affecting nitro- gen concentration). Moreover, Cavatieri and Huang ( 1979, 1981 ) observed an increase in the cytoplasmic concentrations of the nitrogen-containing os- motica, proline and glycine-betaine, in S. alterniflora in response to increas- ing salinity. Such a shift in nitrogen allocation from growth toward produc- tion of osmotica is expected to be accompanied by an increase in tissue nitrogen concentration provided the nitrogen supply is adequate. We tested this hypothesis by estimating the CNC ofS. alterniflora grown under varying salinity treatments, and we have used the empirical relationship between CNC and salinity observed in this study to evaluate the nitrogen status of S. alter- niflora from different field sites.

Although increased growth following nitrogen addition is generally viewed as evidence of nitrogen limitation, the lack of response to fertilization of the tall form ofS. alterniflora (Valiela and Teal, 1974; Gallagher, 1975; Broome et al., 1975; Patrick and Delaune, 1976; Buresh et al., 1980; Cavalieri and Huang, 1981 ) does not preclude nitrogen limitation if viewed at a physiolog- ical level. Nitrogen limitation means that growth is restricted by the availa- bility of nitrogen within plant tissues. Internal nitrogen availability is deter- mined by the balance between sources (primarily the uptake of external nitrogen) and sinks (e.g. growth and production of nitrogen-containing os- motica). Spartina alterniflora demonstrates Michaelis-Menten type uptake kinetics such that the rate of uptake is first-order at low external nitrogen concentrations, while at high external concentrations uptake is maximal and constant. Hence, a situation may exist where the external nitrogen concentra- tion is sufficient to saturate the uptake process while plant growth is still lim- ited by the internal nitrogen supply. Such a scenario would only require that the rate of dilution of internal nitrogen by growth and its allocation to meta- bolic processes, such as production of osmotica, be at least equal to the max. imum rate of uptake. We have used the empirical relationship between CNC

SALINITY AND INTERNAL N SUPPLY IN SPARTI3M ALTERNIFLORA 151

and salinity observed in this study to assess the potential that tall S. altemi- flora is nitrogen limited in situ even though these plants do not respond to nitrogen application.

M E T H O D S

Rhizomes ofS. alterniflora were collected in May 1990 from a salt marsh at North Inlet, South Carolina. In situ interstitial salinity averaged 40 g l- ~ at the time of collection. In the greenhouse, rhizomes were individually potted in sand and placed 12 per salinity treatment in tubs filled with artificial sea- water. The salt solution was maintained even with the surface of the potted sand to simulate the water-logged conditions of the marsh. Four salinity treat- ments of 5, 20, 40, and 50 g l- ~ were created with Instant Oceans Sea SMts. Solutions were amended with NH4CI, KzHPO4, CuSO4, ZnSO4, and Fe-EDTA to give final concentrations equivalent to 25% that of Hoagland's solution (Smart and Barko, 1980). All other essential elements were provided by the sea salts. Solutions were replaced every 10-14 days. Solution pH was moni- tored daily and adjusted with NaOH or H2SO4. Solution salinity was moni- tored daily and adjusted w.;th tap water.

The CNC ofS. alterniflora was estimated as described by Smart and Barko (1980). After an initial period in which plants were allowed to assimilate nitrogen while provided with a surplus of NH4CI, randomly selected individ- uals were transferred to N-free solutions. The period of exposure to excess nitrogen prior to transfer to N-free solution was varied to provide a range in biomass for N-limited plants. Plants were maintained in N-free culture for at least 4 weeks after above-ground growth (change in above-ground height) had ceased. This method is based on the assumption that the initially high tissue concentration will decrease with growth until further growth is pre- vented by the lack of internal nitrogen. The final tissue concentration is used as an estimate of CNC. This experimental procedure provides a greenhouse model of what occurs naturally in the marsh during the growing season. Ni- trogen in S. alterniflora in the marsh varies seasonally (Chalmers, 1979; Men- delssohn, 1979; (3allagher et al., 1980; Hopkinson and Schubauer, I984; Omes and Kaplan, 1989) with a relatively high concentration during the winter. During the growing season, tissue N concentrations decrease as the growth rate exceeds the rate of nitrogen uptake by the plant.

All plants were grown in complete nutrient conditions during a 6 week pe- riod of acclimation. Subsequently, two pots from each salinity treatment were randomly selected, rinsed repeatedly in saline solution to remove interstitial nitrogen, and placed in a separate tub containing a solution that was N free but identical in all other respects. This process was repeated every 10 days until only four plants at each salinity remained in high nitrogen treatments. All transferred plants were maintained in N-free solutions for a period rang-

152 P.M. BRADLEY A N D LT. M O R R I S

ing from 40 to 70 days, depending on the sequence in which they were trans- ferred, before they were harvested and analyzed. Exposure to 50 g !- k salinity was fatal to seven of the 12 plants. Consequently, all survivors were placed in N-free solutions for 60 days to ensure that CNC could be estimated.

Harvested plants were rinsed in tap water and separated into above-ground live, dead and total below-ground tissues. Tissues were dried for 7 days at 110°C and weighed. Since plants grown in 5 g 1- i salinity and excess nitrogen were grazed by insects, the dry mass of these plants prior to grazing was esti- mated by applying a correction for whole leaf weight to the number of partial leaves on each plant. To determine nitrogen and carbon concentrations, tis- sue was ground in a Wiley mill to pass a 60 mesh screen. Nitrogen and carbon concentrations (g kg- ! dry weight) were determined for duplicate samples of live above-ground and total below-ground tissue on a Perkin Elmer 2400 CHN elemental analyzer. The relationship between aboveground biomass and final tissue nitrogen concentration was evaluated for nitrogen-limited plants using the Statistical Analysis Systems (SAS) general linear model procedure (Sta- tistical Analysis Systems, 1985). Statistical tests of treatment means were made using the SAS general linear model procedure (PROC GLM) (Statis- tical Analysis Systems, 1985 ), one-way analysis of variance and Tukey's Stu- dentized Range test at P< 0.05.

We compared the experimental measure of CNC with the nitrogen concen- tration of S. alterniflora leaves collected from marsh sites known as Oyster Landing and Goat Island at North Inlet, SC. Aboveground tissue from three to seven individuals was harvested, rinsed with tap wa~er to remove surface mud and salt, dried for 7 days at 110°C, and analyzed as above for C and N. At Oyster Landing we collected tissues at distances of 0, 20, 40, 60, and 80 m from ~he creek bank. Tissues from Goat Island were collected at four sites representing a range of sediment salinities. Interstitial salinity at each collec- tion point was monitored with a rcfractometer over the growing season as described in Bradley and Men'is ( 1991 ).

RESULTS

In the greenhouse study, biomass was greatest in the 5 g 1- t treatment and decreased as salinity increased (Table 1 ). Biomass did not differ significantly between the 40 and 50 g 1-' treatments. Above-ground biomass also in- creased with length of exposure to excess nitrogen prior to the transfer of plants to N-free solutions (data not shown). For all salinity treatments biomass was significantly greater in plants maintained in excess nitrogen for the duration ofthe experiment (Table 1 ). In contrast, nitrogen treatment significantly af- fected below-ground biomass only at a salinity of 5 g 1- m. However, the rela- tive aUocation of growth between below-ground (B) and above-ground (A) tissues was significantly affected by nitrogen treatment. Among N-limited

SALINITY AND INTERNAL N SUPPLY IN SPARTINA ALTERNIFLORA ~ ~3

TABLE 1

Mean biomass allocation (g dry weight_+ SD) as a funct ion o f salinity t reatment for p lants provided excess nitrogen for 90 days a n d for p lants l imited by nitrogen. N-limited plants were t ransferred to N- free solutions for 4 0 - 7 0 days following periods o f 2 4 - 5 4 days, respectively, in excess nitrogen. Within each parameter group, values in a co lumn followed by the same s u ~ r s c r i p t are not significantly dif- ferent ( P < 0.05, T u k e y - K r a m e r Studentized Range Test ). Fo r a given salinity, significant differences between ni trogen t rea tments ( P < 0 . 0 5 , T u k e y - K r a m e r Student ized Range Test) are indicated by a n asterisk. N D indicates no da ta

Growth parameter Salinity Nitrogen t rea tment

Amended Limi ted

Live above-ground 5 17.4 + 2.5 ~ 11.9 + 3.2 u 20 9.5_+ 3,3 b 5.2 + 1.6 b~' 40 2.9_+ 2.0 ~ ! .2 + 0.4 c'* 50 N D 0 . 7 + 0 . 6 c

Dead above-ground 5 5.9 + I. 14 4.0 + 1.4 ~ 20 4.2___ ! .6 a 2.1 + 0 . $ b'* 40 1.3-+0.2 b 1 .4_0 .3 b 50 N D 1.0-+0.4 b

Total above-ground 5 23.3 _+ 3.5 ~ 15.9 _+ 4.0 ~'* 20 13 .7+4 .9 b 7 .4+2 .1 b'* 40 4 . 2 + 1,9 ¢ 2 . 6 + 0 . 6 ¢ 50 N D 1.8 _ 0.6 ¢

Total below-ground 5 9.7 _+ 2.3 a 15.3 _+ 3.2 a'* 20 8.9 -+ 3.4 "b 7.3 _+ 2.3 b 40 3.4__. 1.9 b 3.1 -+0.9 c 50 N D 1.6 _+ 0.3 ~

Total plato 5 33.0_+ 5.3 a 31.3 +_. 6.8 a 20 22.6 __ 8.2 a 14.7 +_ 4.3 b 40 7.7__ 3.8 b 5 .7+ 1.5 ¢ 50 N D 3.4 _+ 0.9 ~

Below/above-ground ratio 5 0.4 + 0. I b 0.9 --+ 0. P '* 20 0 .6+0 .1 ~ 0.9_+0. P'* 40 0.7 +0 .1 a I.I +0 .1 a'* 50 N D 0.9 -+ 0.2 ~

Height (¢m) 5 9 9 . 0 + 1.7" 8 8 . 0 _ + 12.6 ° 20 83.0 + 7.0" 63.5 + 9.4 ~'* 40 51.3_+ 11.0 ~ 37.4+4.8 ~'*

50 N D 36.6_+ 10.1 ~

N u m b e r o f leaves 5 70.3 _+ 13.0 • 55.1 _+ 7.2 a'* 20 5 7 . 8 + 9 . 0 a'b 3 3 . 5 + 9 . 3 b'* 40 35.0 - 16.4 b 12.8 + 3.6 ~'* 50 N D 7,8 -+ 5.0 ~

N u m b e r & e m e r g e n t s tems 5 21.3 + 1.5 ~ 10,9 _+ 2.6 a'* 20 16.0_+2.9" 8.1 ± 4 . 0 a-b-* 40 8.3_+4.6 b 5.5 + 1.5 b'c 50 N D 2.6 + 1.5 c

N u m b e r o f rh i zomes 5 26.3 + 2.3 ~ 10.9 _+ 4.2 ~'* 20 24.3_+ 7.8 ~ 9.6 _ 4.0 ~'* 40 5 . 8 + 3 . 2 ~ 3.5 + 1.3 b 50 N D !.8 + O.g b

1 5 4 P . M . B R A D L E Y A N D L T . M O R P , I S

plants, B /A was approximately 1 under all salinity conditions. In contrast, under high nitrogen the quotient increased from 0.42 at 5 g 1- i salinity to 0.79 at a salinity of 40 g 1- ~. Both plant height and leaf number were reduced by increasing salinity as well as by decreasing exposure to nitrogen. Both the numbers and weights ofemergent stems and rhizomes were sensitive to nitro- gen treatment. Numbers of stems and rhizomes were reduced by at least 50% in plants transferred to N-free solutions, but total below-ground weight was independent of nitrogen treatment except at the lowest salinity (Table ! ).

The CNC at a given salinity was estimated as the mean nitrogen concentra- tion ofplants transferred to N-free solutions. For each salinity treatment, the slope of the regression of above-ground tissue nitrogen concentration against biomass was not significantly different from zero at a significance level of 0.10 (PROC GLM) (Statistical Analysis Systems, 1985). Thus, the mean tissue concentration from all nitrogen-limited plants was taken as an estimate of CNC for each salinity treatment (Fig. 1 ). For a given salinity treatment, the below-ground nitrogen concentration under limiting conditions was al- ways less than that under nitrogen amendment and averaged 62.3_+4.6% ( _+ SD) of the above-ground tissue N concentration. The CNC was found to be a non-linear function of salinity and this relationship is shown in Figs. 2 and 3 for above- and below-ground tissues, respectively. While the nitrogen concentration of tissues increased with salinity, there was a concomitant and linear decrease in the quotient of C / N as salinity treatment increased (Fig. 4) .

To evaluate the nitrogen status ofS. alterniflora in the field, we compared

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Fig. !, Reletionship between above-ground biomass (g dry weight) and above-ground tissue nitrogen concentration (g kg- ~ dry weight) for S. alterniflora limited by nitrogen. Vertical lines represent the estimated CNC for each salinity.

SAL]NITY ~ 4 D INTERNAL N SUPPLY IN SPARTINA ALTERNIFLOR4 | 55

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Fig. 3, Relationship between the critical nitrogen concentration (g kg-~ dry w~:ight) of below- ground tissue and salinity (g ! - t ). Error bars are standard deviations.

in situ tissue N concentrations with values of CNC calculated on the basis of sediment salinity. At all North Inlet sites, ~he observed above-ground tissue N concentrations were less than or equal to the salinity-calculated CNCs (Fig. 5). Along the transect at Oyster Landing, ~o significant differences in above- ground tissue N concentration were found. Early in the growing season (May) sediment salinity varied from 32 g 1- ] near the creek margin to a high of 46 g 1-' at a distance of 80 m from the creek bank (data not shown). W~en the tissue samples were collected in September, salinity varied from 39.0 g 1 -~ at

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156 P.M. B R A D L E Y A N D J .T . M O R R I S

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Fig. 5. Above-ground tissue N concentration (g kg- i ) and interstitial salinity (g 1-1 ) reported for S. alterniflora from North America salt marshes during June-October ( b ), including data from inland and creek bank areas of the marsh, and from measurements made during Septem- ber at the Oyster Landing (O) and Goat Island (llll) marshes at North Inlet, SC. The curve ( - - ) represents the relationship betw~e~ CNC ( + SD ) and salinity observed in this study.

the creek bank to 50 .0 g i - ~ at a d i s tance o f 80 m. The calculated C N C s were 13.1 and 22 .9 g k g - 1 c o m p a r e d wi th a spatial ly averaged t issue N concentra- t ion, o f 13.26___ 1.08 ( +_ 1 S D ) . At the Goat Is land sites, the September sedi . merit sa l ini ty ranged from 35 .0 to 50 .0 g 1-1. T h e calculated C N C s were 11.3

SALIHITY AND INTERNAL N SUPPLY IN SI-:¢fcTINA ALTERNIFLf~,?A ] 57

g kg -~ and 22.9 g kg - t while the tissue N concentrations were 11.4 + 1.3 g kg -~ and 23.0+ 1.2 gkg -~, respectively.

Based on comparisons of tissue N concentration with the salinity-calcu- lated CNC, we concluded that salt marsh primary production is probably lim- ited by nitrogen throughout the range of S. alterniflora (Fig. S). These com- parisons made in the North Inlet marshes indicate that nitrogen probably limits primary production throughout the productivity gradient from creek bank to kqand sites. We also surveyed the literature for data on tissue N con- centrations in S. elterniflora and corresponding sediment salinities from North American salt marshes along the Gulf and Atlantic Coasts. Nitrogen in S. alterniflora varies seasonally (Chalmers, 1979; Mendelssohn, 1979; Gal- lagher et al., 1980; Hopkinson and Schubauer, 1984; Ornes and Kaplan, 1989), but it is during the summer period ofactive growth that the internal nitrogen supply is most likely to become limiting. Consequently, we have restricted ourselves to those studies which report nitrogen concentrations during the period of June to October and in several instances we have taken salinity and nitrogen values from different sources (Valiela and Teal, 1974; Gal):gne;, 1975; Patrick and Delaune, 1976; Chalmers, 1979; Mendelssohn, 1979; Bur- esh et al., 1980; Gallagher et al., 1980; Hopkinson and Schubauer, 1984; Broome et al., 1986; Delaune and Pezeshki, 1988; Ornes and Kapl~n, 1989). The CNC at each reported interstitial salinity was estimated using a polynom- ial regression of Fig. 2. When the reported nitrogen concentration was less than or within 1 SD of the estimated CNC (standard deviations of our esti- mates of CNC were less than _ 13%), we concluded that nitrogen limitation was probable. Comparisons of tissue N concentrations with the calculated CNCs (Fig. 5) indicate that nitrogen limits primary production, and where the data were available, we concluded that the most productive areas within salt marshes, aleng the margins of creeks, are probably also N limited.

DISCUSSION

The general decrease in the growth ofS. alterniflora with increasing salinity of the culture solution is consistent with previous findings (Adams, I963; Linthurst and Seneca, 1981 ). A negative correlation between productivity and interstitial salinity was also observed at Sapelo Island, GA (Nestler, 1977). In hydroponic culture, Haines and Dunn (1976) noted a decrease in biomass accumulation at salinities above 20 g 1 -I, although growth at 20 g 1-~ ex- ceeded that at 5 g 1-t. In contrast, others found maximum growth orS. alter- niflora between 0 and 8 g 1 -~ salinity (Phleger, 1971; Parrondo et al., 1978; Coleman, 1990; this study). Reduced survival of S. alterniflora has been as- sociated with salinities in the range of 40-50 g 1-: (Shifiet, 1963; Phlcgcr, 1971; Haines and Dunn, 1976; Coleman, 1990). In the current study, mot-

158 P.M. BRADLEY AND J.T. MORRIS

tality increased from 0% for salinity treatments less than or equal to 40 g 1- to 60% at 50 g 1- '.

Similarly, the effect of nitrogen limitation on growth of S. alterniflora is consistent with both field and greenhouse growth studies. In the field, addi- tion of nitrogen fertilizer to short S. aiterniflora resulted in increased height and aboveground productivity (Valiela and Teal, 1974; Ganagher, 1975; Broome et al., 1975; Patrick and Delaune, 1976; Chalmers, 1979; Buresh et al., 1980; Cavalieri and Huang, 1981; Morris, 1988).

The increase in tissue N concentration with salinity observed in this study is consistent with previous reports of plant responses to salt and osmotic stress. For example, the tissue N concentration in several crop species increased with the level of salt stress (Khalil et al., 1967; Bates, 1971; Frota and Tucker, t978; Broadbent et aL, 1988), and in many halophytes, increased salinity and osmotic stress stimulate the production of N-based osmotica (Flowers et aL, 1977; Storey et al., 1977; Storey and Wyn Jones, 1979; Jeffries et al., 1979; Oreenway and Munns, 1980; Flowers, 1985 ). The CNC orS. alterniflora in- creased when salinity exceeded 20 g 1- i (Fig. 2 ). The CNC estimates at 5 and 20 g 1- J did not differ significantly and were similar to the CNC of 7.3 +_ 0.7 g kg -I ( _+95% confidence limits) reported by Smart and Barko (1980) at 15 g 1-' and the minimum tissue concentration of 8.9 __+ 0.5 g kg-' ( _+ SE ) for S. alterniflora grown under nitrogen limitation at 16 g 1-~ (Morris, 1982). It has been argued that the C /N quotient can be used as an indicator of the relative demand for carbon and nitrogen during plant growth (Bloom et al., 1985). Our results, which support that argument, showed that the C/N quotient de- creased with increasing salinity (Fig. 4) and indicate that the nitrogen re- quirement increases under conditions of salinity stress.

In seeming contrast to several field studies in which creek bank plants failed to respond to N fertilization (Valiela and Teal, 1974; Gallaghcr, 1975; Broome ct ai., 1975; Patrick and Delaune, 1976; Burcsh ct al., 1980; Cavalieri and Huang, 1981 ), our results suggest that productivity of S. alterniflora is gen- erally limited by nitrogen throughout the salt marsh environment. If N limi. tation is viewed at an internal, physiological level, rather than from the point of view of the effect of concentrations of mineral nitrogen external to the plant, then wc can reconcile the apparently contradictory observations that N fertil- ization may not elicit a response when the tissue N concentration is equal to the CNC. This apparent paradox can be explained by considering the effect of edaphic conditions common to salt marshes on the nitrogen uptake kinet- ics of S. alterniflora. Under nitrogen4imiting conditions, growth of S. alter- niflora is constrained by the availability of'internal' nitrogen which is a func- tion of the rate ofnitrogen uptake by the root system. The kinetics of nitrogen uptake in S. alterniflora demonstrates Michaelis-Mcnten behavior, such that root weight-specific, N uptake is maximized (Vmax) at high external N con- centrations and is independent of concentration. Hence, an insensitivity of S.

SALINITY AND INTERNAL N SUPPLY IN SPARTINA JLTERNIFLORA 159

alterniflora to N fer t i l izat ion indicates that the uptake mechanism is satu- rated at environmental concentrat ions o f nitrogen (Morris , 1980; Bradley and Morris , 1990, 1991 ). However , this m a x i m u m rate o f ni trogen uptake is sen- si t ive to the sed iment concentra t ions o f oxygen (Morr is , ! 984; Morr is and Dacey, 1984; Bradley and Morr is , 1990), hydrogen sulfide (Bradley and Morris , 1990) and sal ini ty (Morr is , 1984; Bradley and Morris , 1991 ), and it may or may not be sufficient to yield the m a x i m u m potent ia l growth. Thus, p lant growth may be l imi ted by the in ternal ni t rogen avai labi l i ty , while in- creasing the external ni trogen concent ra t ion has no effect on growth.

ACKNOWLEDGMENTS

We gratefully acknowledge the assistance o f B. Haskin and Y.-H, Hwang with field work. This research was suppor ted by a fellowship from the SIo- c u m - L u n z Founda t ion to P.M. Bradley and by funding from the SC Sea Gran t Consor t ium.

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[60 P.M. BRADLEY AND J,T. MORRIS

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