Seasonal Pore Water Dynamics in Marshes of

Seasonal Pore Water Dynamics in Marshes of Barataria Basin, Louisiana T. C. FEIJTEL,* R. D . DELAUNE, AND W . H . PATRICK, JR.

ABSTRACT

Sediment redox potential (Eh), interstitial pore water pH, sulfate, carbonate, and trace metals were measured monthly from August 1984 to August 1985, in a salt, brackish, and freshwater marsh in Barataria Basin, LA. The salt marsh was characterized by low Eh during the summer and fall, resulting in pyrite formation. Early spring oxidation of the sediments created extreme acid conditions, indicating that pyrite was oxidized completely to sulfate. Brackish and freshwater marshes exhibited a much greater seasonal variability, with strong vertical redox fluctuations. In the freshwater marsh, both Fe2* and Mn!* were controlled by temporal pH changes and were relatively independent of oxidation-reduction phenomena, suggesting a possible organic control mechanism. However, in the brackish marsh, Fe2* concentrations were governed by pH (r = -0.74** significant at the 0.01 probability level) and Eh (r = 0.65**) changes, indicating that Fe!* concentrations were likely controlled by pyrite formation and oxidation. Ion activity product calculations indicated that iron and manganese phosphates and carbonates were unlikely to form in Barataria Basin marshes. However, iron and manganese sulfide supersaturation occurred in all three marshes. Brackish marshes were found to contain significantly higher pyrite concentrations (1.4% w/w), than salt (0.69% w/w) and freshwater (0.66% w/w) marshes. The dynamic pyrite cycling in Louisiana salt marshes, resulted in a low pyritic pool, characterized by single, fine grained euhedral crystals. Pyritic Fe represented 23% of the total Fe accumulation flux in the salt marsh, and 34% in the brackish marsh. Additional Index Words: redox potential, pH, Fe, Mn, accumulation, cycling.

M

ARSHES IN COASTAL LOUISIANA are character-

ized by a rapid C accumulation (DeLaune et al., 1978; Hatton et al., 1983; Feijtel, 1986), indicating that large quantities of metabolizable organic matter are passed quickly through zones of aerobic oxidation. The chemistry of these marsh sediments should be largely characterized by the input of organic matter and the consumption and regeneration of dissolved oxidizing agents, 0 2 , NOT, SO5-, and HCOj, and solid oxides of manganese and iron. The diffusion and flux of these dissolved pore water solutes and their respective reactions with solid phases control the activities of interstitial pore water constituents. The mobility of iron, manganese, and sulfur compounds is strongly influenced by redox potential (Eh) and pH (e.g., Berner, 1971; Goldhaber and Kaplan, 1974; van Breemen, 1976; Lord, 1980; Howarth and Giblin, 1983). In the absence of thermodynamically more favorable electron acceptors, sulfate will be consumed as an electron acceptor producing alkalinity and sulfides. It has been suggested that plant height and productivity may be limited by the effect of sulfides on nuT.C. Feijtel, Dep. of Soil Science and Geology. Agricultural Univ. Wageningen. P.O.B. 37, 6700 AA Wageningen, Netherlands: and R.D. Delaune and W.H. Patrick, Jr.. Lab. for Wetland Soils and Sediments, Louisiana State Univ.. Baton Rouge. LA 70803-7511. Contribution of Lab. for Wetland Soils and Sediments. Received 23 Dec. 1986. *Corresponding author. Published in Soil Sci. Soc. Am. J. 52:59-67 (1988).

59

trient uptake (King et al., 1982; DeLaune et al., 1983). Giblin and Howarth (1984) reported dynamic seasonal transformations in the Great Sippewissett Marsh. This cycle was governed by sulfate reduction and pyrite oxidation, resulting in the precipitation and release of Fe 2+ , respectively. To examine the dynamics of trace metal cycles in marshes, we measured concentrations of dissolved pore water constituents throughout the year in stands of Spartina alterniflora, Sp. patens, and Panicwn hemitomum. Quantitative pore water analysis and geochemical saturation calculations allowed the elucidation of the main governing reactions. MATERIALS and METHODS

Study Area Samples examined in this study were obtained from freshwater, brackish, and saltwater marshes located in Barataria Basin, LA (Fig. 1). Barataria Basin is a 400 000-ha interdistributary estuarine basin with well-defined vegetative units that is bound on the east by the Mississippi River and on the west by the river's most recently abandoned channel, Bayou Lafourche. Several primary vegetative marsh units that generally correlate with substrate type and salinity can be identified within the basin: saltwater marsh, brackishintermediate marsh, and freshwater marsh (Chabreck, 1972; Bahrand Hebrard, 1976). Freshwater marsh (salinity 10%») covers approximately 14% of the basin; Sp. alterniflora is the dominant vegetation. Feijtel et al. (1985) reported a C surplus of 150 to 250 g m~2 yr"'. This C is thought to originate mainly from the tidal salt marshes. Sampling and Pore Water Analysis The sampling rationale employed in this study was to collect a sequence of cores in a time series from the same marsh location within a radius of 3 m. This allowed us to minimize the effects of spatial heterogeneity and to assess seasonal pore water variations. Spatial variability of major cations and anions determined on three cores within each location was +300 mV during late winter and early spring resulted in extreme acid interstitial pH values of around 3.2 to 4.0 (Fig. 5). During the early spring oxidation rates were high enough to neutralize the alkalinity produced by sulfate reduction and substantially lower the pH. The pH minimum in March resulted in a sharp increase of interstitial Fe 2+ , indicating the dissolution of pyrite (Fig. 2). Subsurface Mn 2+ concentrations (at the 3-cm depth) also showed a slight increase in March, but maximum values were recorded in the month of July (Fig. 3). Manganese oxides are reduced at higher Eh levels than iron oxides, and as reducing conditions persisted, Fe 2 " exhibited a second maximum in the summer. The reduction of manganese and iron oxides as a result of persistent reducing conditions released significant amounts of M n - and Fe 2+ in solution. Reprecipitation as sulfides and possibly carbonates in the pH

late summer is thought to control Fe 2+ and Mn 2+ solubilities during the fall. Significant interrelation between Fe 2+ , interstitial pH, and SO2,- indicated that Fe 2+ was very likely controlled by pyrite formation and oxidation (Table 1). The consumption of H + seemed to be linked to the formation of pyrite (r = 0.48* significant at the 0.05 probability level), whereas SO 2- production is apparent when pyrite oxidation occurs (r = —0.68** significant at the 0.01 probability level). The lack of a significant correlation between pyrite and interstitial dissolved Fe is attributed to the subsequent oxidation of Fe 2+ to an oxide or oxyhydroxide phase upon sediment oxidation for prolonged periods (Fig. 2). However, a significant inverse relationship (r = —0.46*; n = 14) was found by removing the variability of the months where oxidation processes predominated. As flooding and reduction of the sediment persisted levels of dissolved Fe decreased (>Eh - Fc = 0.43*), suggesting the precipitation of iron monosulfides or pyrite. In the brackish marsh, oxidation processes dominated during the winter and summer, and reduction was highest in the spring (Fig. 4). This corresponded

pH

pH

8 -

8

-i

87 7 6 -

6 —' 6 -

5 -

S O 11 5

D J H A M J A 19 25 8 17 26 1 13 DATE

1

1

S O 11 5

1 ' 1 ' 1

1

T~

D J H A M J A 19 25 8 17 26 1 13 DATE

I

S O 11 5

1

i

1

I

1

r-

D J H A M J A 19 25 8 17 26 1 13 DATE

Fig. 5. Seasonal p'H variations at the 3-, 15-, and 29-cm depths in salt, brackish, and freshwater marshes, respectively (from left to right).

63

FEIJTEL ET AL.: SEASONAL PORE DYNAMICS IN MARSHES OF BARATARIA BASIN, LOUISIANA

to a p H m i n i m u m in December and high interstitial F e r and M n concentrations (Fig. 5). A second F e peak i n A p r i l was associated with the reduction of iron oxides, as reduction phenomena persisted. Highly correlative behavior of F e with p H , Eh, and S O . - (Table 2), suggested that pyrite oxidation and sulfate reduction were possibly also controlling Fe " solubility i n Little Lake. Pyrite content was significantly correlated to sediment E h , indicating pyrite formation upon reduction of brackish marsh sediments. Similar to the brackish marsh, the freshwater marsh was characterized by a much higher variability than salt marsh sediments. N o consistent seasonal pattern could be deduced from the vertical redox profile as redox maxima at the 1-cm depth corresponded to redox m i n i m a at the 9-cm depth. Nevertheless, interstitial p H profiles indicated severe acid conditions i n October and July, which corresponded with peaks i n interstitial F e and M n (Fig. 2 and 3). The F e and M n were subject to a p H control mechanism, and seemingly independent of Eh and S O . fluctuations (Table 3). The lack of a significant E h - p H relationship due to the limited inorganic sediment content of freshwater marshes, suggested that inorganic p H buffering was minimal, and likely governed by the accumulation and removal of organic acids. This resulted i n high p H values during the winter months, when bacterial activity was surpressed, followed by progressively lower p H with increasing temperatures throughout the summer and early fall (Fig. 5). The +

2 +

2 +

2 +

2

Table 3. Correlation matrixf for interstitial Fe, Mn, p H , SOJ", sediment Eh, and pyritic content in Lake des Allemands (freshwater marsh). Soil property Fe

Fe

Mn

pH

sor

Eh

Pyrite

1.00

0.57** 79 1.00

-0.46** 67 -0.37** 67 1.00

0.12$

-0.02$ 30 0.12$ 30 -0.07$ 28 0.14$ 49 1.00

0.01$ 20 -0.14$ 20 0.42* 20 -0.42$ 13 -0.34$ g

2_i

2 +

2 +

2

-

Table 1. Correlation matrixf for interstitial Fe, Mn, p H , SOJ", sediment Eh, and pyritic content in Airplane Lake (salt marsh).

Fe

Mn

1.00

0.30** 80 1.00

pH

S03"

P

H

SO;-

0.28* -0.38** 74 78 0.01$ 0.20$ 74 78 1.00 -0.65** 72 1.00

Eh

Pyrite

Eh

Pyrite

0.43* -0.26$ 31 21 0.04$ -0.26$ 21 31 0.48* -0.85** 30 21 0.59** -0.68** 29 20 1.00 -0.63$ 5 1.00

*,** Significant at the 0.05 and 0.01 probability levels, respectively, f With correlation coefficient and just below number of observations. J Nonsignificant.

Table 2. Correlation matrixf for interstitial Fe, Mn, p H , SO4", sediment Eh, and pyritic content in Little Lake (brackish marsh).

Fe Mn PH

so;Eh Pyrite

SOf

60 0.13$ 48 1.00

Eh Pyrite

1.00

'* Significant at the 0.05 and 0.01 probability levels, respectively, f With correlation coefficient and just below number of observations. $ Nonsignificant.

Table 4. Annual mean I A P of main Fe and M n sulfides, carbonates, and phosphates at different depths in marsh sediments. Ion activity productsf cm

FeS

MnS

n

FeCO,

n

MnCO,

Fe,(PO,)

18.4 16.9 15.8 14.8 14.5 12.3 12.0 11.1 11.0

51.2 49.7 47.1 45.0 44.3 42.1 41.0 40.5 41.9

50.8 49.1 47.2 45.2 43.3 41.8 40.9 39.7 39.6

18.1 17.4 16.1 15.1 14.3 15.2 13.9 14.0 14.2

41.1 39.9 38.3 38.0 37.3 38.0 37.2 36.8 37.3

41.3 40.0 38.3 38.1 37.5 38.1 37.0 36.9 36.9

18.9 20.6 21.1 18.9 19.2 18.2 18.0 17.5 16.5

52.1 53.0 53.9 50.1 49.8 49.7 49.6 49.9 47.3

51.0 51.8 52.2 48.9 48.5 48.4 48.3 48.5 47.1

2

Mn,(P0.),

Salt 1 3 5 7 9 14 21 29 37

28.0 24.2 24.0 21.1 20.8 18.2 16.1 15.4 15.9

28.1 24.9 24.0 20.5 19.9 18.0 16.1 15.4 15.2

1 3 5 7 9 14 21 29 37

27.6 26.7 25.9 24.2 23.0 20.1 18.9 18.4 17.7

28.0 26.3 24.5 23.9 23.0 19.9 20.0 20.6 20.2

1 3 5 7 9 14 21 29 37

28.7 28.1 27.0 26.3 24.5 20.6 20.1 20.0 18.8

29.0 28.7 27.5 25.8 23.9 20.6 20.4 19.9 1S.7

18.1 16.5 15.9 15.1 14.8 13.3 12.8 11.7 12.2 Brackish

Fe

Mn

Soil property

pH

0.20$

2 +

2 +

Soil property

Mn

60

17.S 17.1 15.9 15.0 14.3 15.1 13.6 13.6 13.8 Freshwater 19.1 20.9 21.3 19.0 19.0 18.4 18.0 17.8 16.7

1 - log (IAP) M e S : activity of elemental S = 1, and therefore, -loglFeS) n

Fe

Mn

1.00

0.41** 75 1.00

pH

SO;-

Eh

Pyrite

-0.74** 0.65* 0.56** -0.03$ 50 63 25 21 0.09$ 0.66** -0.46* 0.05$ 50 25 21 63 1.00 -0.49** -0.46* 0.25$ 52 24 16 1.00 0.47* -0.68$ 18 11 1.00 -0.68$ 6 1.00

*•** Significant at the 0.05 and 0.01 probability levels, respectively, t With correlation coefficient and just below number of observations. $ Nonsignificant.

-loglFeS,).

Table 5. Solubility products at 25 °C and 10* Pa. Mineral

Reaction products

-l0g(* p>

Reference Berner (1967) Berner (1967) Berner (1967) Berner (1967) Singer and Stumm (1970) Nriagu (1972) Mills (1974) Mills (1974) Morgan (1967) Postma (1981)

3

FeS (amorphous) FeS (mackinawite) Fe,S, Igreigite) FeS, (pyrite) FeCO, (siderite)

Fe" + S" Fe" + S" 3Fe' • + 3 S " + S Fe" + S'"+ S F e " + COf-

16.9 17.5 18.2 27.6 10.2

Fe,(PO,), (vivianite) MnS (alabandite) MnS, (hauerite) MnCO, (rhodochrosite) Mn,(PO.) (reddingite)

3Fe" + 2POJM n " + S" M n " + S" + s Mn" + cor 3Mn - + 2POJ"

36.0 17.8 20.6 10.4 31.8

s

::

64

SOIL SCI. SOC. AM. J., VOL. 52, 1988 Table 6. Annual meant and standard error of pyrite content and interstitial Fe, Mn, SOJ", pH, and saUnity. Sampling site

soj-

Fe Mn Pyrite pH Salinity n = 80 n = 80 n = 22 n = 74 n = 78 n = 68 mg L g kg"' mg L"' gLFreshwater 0.48a 0.23a 6.7a 62a 5.5a 1.0a 0.08 0.02 1.4 9.8 0.1 0.2 Brackish 0.27a 0.20a 13.7b 130a 6.6b 2.7b 0.02 0.02 1.7 18.8 0.1 0.2 Salt 1.38b 0.83b 6.9a 1643c 6.7b 16.5c 0.24 0.04 1.0 91 0.2 0.4 t Means with the same letter are not significantly different (Duncan's multiple range test (p = 0.01), SAS Inst., 19821.

Fig. 6. Single euhedral pyrite particles in the salt marsh, present on external root walls. The length of the bar equals 10 Mm.

production and degradation of organic matter during the summer and early fall resulted in a net accumulation of organic acids and release of adsorbed trace metals (Feijtel, 1986). Phase Transformations and Saturation States After Fe 2+ and Mn 2+ are produced in the subsurface sediment layer, both Fe 2+ and Mn 2+ can .reprecipitate as oxides upon exposure to 0 2 , or under anaerobic conditions, reprecipitate with anions produced during anaerobic metabolism. Ion activity products (IAP) were calculated for annual mean Fe and Mn profiles in Barataria Basin marshes (Table 4), and compared with tabulated values of selected carbonate, sulfide, and phosphate minerals (Table 5). Seasonal variation resulted in significant variation ( 2 0 cm (Fig. 7). Data indicated that the oxidation step at the surface resulted in the release of F e and SO .- and ultimately i n the precipitation of F e O O H (Fig. 2; Table 1). In the late summer ferric oxyhydroxides are reduced again with the release of F e , which initializes a new cycle of pyrite formation and oxidation. The brackish and freshwater marshes were characterized by the absence of single crystal pyrite particles at greater depths and exhibited less variation through the season. Pyrite concentrations in these marshes were not correlated with p H and S O 5 (Tables 2 and 3), suggesting a more steady-state pyrite formation. This resulted in a rather static and high pyritic pool. Tidal exchanges supply salt marshes with a high mineral input, including nutrients and dissolved salts. Freshwater marshes, on the other hand, are highly organic because of the lack of mineral sediment input, and are characterized by low bulk densities (Table 6). Accretion rate estimates were calculated from the depth of the 19 6 3 C s horizon. The 1953 horizon, representing the first year C s was artificially introduced into the environment (Pennington et a l , 1973), was 2 +

2

2 +

-

137

137

66

SOIL SCI. SOC. AM. J., VOL. 52, 1988 i

|

I

0 100 200 300 400 CESICM-137 CPO/SSCTIOH)

3 5 ,7 9 11 13 15

'

3 5 7 9 11 IS 15 17 19 21 23 25 27 29

7

21 23 25 Z7 29 0 100 200 300 CESIUK-137 CPCI/5ECTI0N>

TO

5888888888a

0 100 200 300 400 CESItJJ-137 CPCI/SSCriOK)

Fig. 8. Cesium-137 distribution in salt, brackish, and freshwater marshes, respectively (from left to right).

used to double-check the rate estimates. Sedimentation rates averaged 8.5, 9.5, and 10.5 mm yr _l in the freshwater, brackish, and salt marsh sampling sites, respectively (Fig. 8). The accumulation of trace and heavy metals also followed a seaward increase (Feijtel, 1986). With the river's sediment supply cut off, organic material produced in situ was the only material available for freshwater marsh accretion. In salt marshes, however, frequent turbid tidal washes supplied the marshes with a high mineral input. This resulted in a significantly higher Al and Zn accumulation rates along this hydraulic energy gradient. The accumulation rates of Fe in the salt marsh were significantly higher than in freshwater marshes, but not significantly higher than in the brackish marshes (Table 6). Brackish marshes accumulate about 40.9 g Fe m - 2 yr _l , whereas salt marshes accrete 53.6 g m - 2 yr _1 . About 34% of this accumulation flux in the brackish marshes occurs in the pyritic form, whereas in the salt marshes pyritic Fe only accounts for 23% of the total Fe flux. This suggests that a large part of the accretionary S flux occurs in a nonpyritic form in Louisiana salt marshes, due to the dynamic behavior of pyrite. CONCLUSIONS Strong seasonal pore water variations were observed in all three marshes, indicating nonsteady-state conditions. Nonlinear diagenetic equations are needed if pore water profiles are to be used in the estimation of rates of dissolution and precipitation. Dynamic Fe and pyrite cycling in the salt marsh resulted in a low pyritic content. Pyrite content along this salinity transect was found to be significantly higher in the brackish marsh. Subsurface pyrite of the salt marsh was characterized as single fine-grained euhedral crystals, indicating its rapid formation. At greater depths, pyrite formation seemed limited by the formation of amorphous iron sulfides. Pyrite accumulates in the salt marsh at a rate of 12.5 g m 2 - yr _1 or 23% of the total Fe flux. However, in brackish marshes, 34% of the total Fe flux accumulates in the pyritic form.

ACKNOWLEDGMENT

The authors would like to thank Prof. N. van Breemen, Dr. R. Brinkman, Dr. P.A. Moore, Jr., and three anonymous reviewers for their critical analysis of our paper. We also gratefully acknowledge financial support by the Louisiana Sea Grant College Program, a part of the National Sea Grant College Program, which is a part of The National Sea Grant Program maintained by NOAA, U.S. Dep. of Commerce, and by the State of Louisiana.

REFERENCES

Bahr, L.M., and J.J. Hebrard. 1976. Barataria Basin: Biological characterization. Louisiana State Univ., Sea Grant Publication T-76005. Begheijn, L.T., N. van Breemen, and E.J. Velthorst. 1978. Analysis of sulfur compounds in acid sulfate soils and other recent marine soils. Commun. Soil Sci. Plant Anal. 9:873-882. Berner, R.A. 1963. Electrode studies of hydrogen sulfide in marine sediments. Geochim. Cosmochim. Acta 27:563-575. Berner, R.A. 1967. Thermodynamic stability of sedimentary sulfides. Am. J. Sci. 265:773-785. Berner, R.A. 1971. Principles of chemical sedimentology. McGrawHill, New York. Chabreck, R.H. 1972. Vegetation, water and soil characteristics of the Louisiana coastal region. La. Agric. Exp. Stn., Bull. 664. Day, J.W., Jr., C.S. Hopkinson, and W.H. Connor. 1982. An analysis of environmental factors regulating community metabolism and fisheries production in a Louisiana estuary, p. 121-136. In V.S. Kennedy (ed.) Estuarine comparisons. Academic Press, New York. DeLaune. R.D., W.H. Patrick, Jr., and R.J. Buresh. 1978. Sedimentation rates determined by 137Cs dating. Nature (London) 275:532-533. DeLaune, R.D., C.J. Smith, R.P. Gambrell, and W.H. Patrick, Jr. 1985. Analytical methods and sampling procedures for environmental studies of wetland sediment-water-plant systems. Lab. for Wetland Soils and Sediments. Louisiana State Univ.. Baton Rouge. DeLaune, R.D., C.J. Smith, and W.H. Patrick, Jr. 1983. Relationship of marsh elevation, redox potential and sulfide to Spartina alterniflora productivity. Soil Sci. Soc. Am. J. 47:930-935. Feijtel, T.C., 1986. Biogeochemical cycling of metals in Barataria Basin. Ph.D. diss. Louisiana State Uiiiv. Baton Rouge (Diss. Abstr. 86-25334). Feijtel, T.C., R.D. DeLaune, and W.H. Patrick, Jr. 1985. Carbon flow in coastal Louisiana. Mar. Ecol. Progr. Ser. 24:255-260. Gambrell, R.P., R.A. Khalid, M.G. Verloo, and W.H. Patrick, Jr. 1977. Transformation of heavy metals and plant nutrients in dredged sediments as affected by oxidation reduction potential and pH. Vol I and Vol II. Contract Rep. D-77-4. U.S. Army Eng. Water. Exp. Stn., Vicksburg, MS. Giblin, A.E., and R.W. Howarth. 1984. Porewater evidence for a dynamic sedimentary iron cycle. Limnol. Oceanogr. 29:47-63.

67

PARKER ET AL.: COMPARISON OF THREE SPECTROPHOTOMETRY METHODS Goldhaber. M.B., and I.R. Kaplan. 1974. The sulfur cvcle. p. 569— 655. I n E.D. Goldberg (ed.) The sea. Vol. 5. Wiley, New York. Gosse'link, J.G. 1984. The ecology of delta marshes of coastal Louisiana: A community profile. U.S. Fish and Wildl. Serv., Office of Biol. Serv., Washington, DC. Hatton, R.S., R.D. DeLaune, and W.H. Patrick, Jr. 1983. Sedimentation, accretion and subsidence in marshes of Barataria Bay, Louisiana. Limnol. Oceanogr. 28:494-502. • Holdren, G.R. 1977. Distribution and behavior of manganese in the interstitial waters of Chesapeake Bay sediments during early diagenesis. Ph.D. diss. John Hopkins Univ., Baltimore, M A . Howarth, R.W. 1979. Pyrite: Its rapid formation in a salt marsh and its importance in ecosystem metabolism. Science (Washington. DC) 203:49-51. Howarth, R.W., and A.E. Giblin. 1983. Sulfate reduction in the salt marshes of Sapelo island. Georgia. Limnol. Oceanogr. 28:70-82. Howarth, R.W., and J.M. Teal. 1979. Sulfate reduction in a New England salt marsh. Limnol. Oceanogr. 24:999-1013. Howes, B.L., J.W.H. Dacey, and G . M . King. 1984. Carbon (low through oxygen and sulfate reduction pathways in salt marsh sediments. Limnol. Oceanogr. 22:814-832. Jorgensen. B.B. 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark). Limnol. Oceanogr. 22:189-201. Jorgensen, B.B., and T. Fenchel. 1974. The sulfur cycle of a marine sediment model system. Mar. Biol. 24:189-201. King, G.M., M . J . Klug, R . G . Wiegert, and A.G. Chalmers. 1982. Relations of soil water movement and sulfide concentration to Spartina alternifiora production in a Georgia salt marsh. Science (Washington, DC) 218:61-63. Lord, J.C., III. 1980. The chemistry and cycling of iron, manganese, and sulfur in salt marsh sediments. Ph.D. diss. Univ. of Delaware, Newark. Luther, G.W., III, A.E. Giblin, R.W. Howarth, and R.A. Ryans. 1982. Pyrite oxidation in marsh sediments. Geochim.- Cosmochim. Acta 46:2665-2670. Mendelssohn, I.A., K.L. McKee, and W.H. Patrick, Jr. 1981. Oxygen deficiency in S p a r t i n a a l t e r n i j l o r a roots: Metabolic adapta-

tion to anoxia. Science (Washington, DC) 214:439-441. Mills, K.C. 1974. Thermodynamic data for inorganic sulfides, selenides, and tellurides. Buttersworth, London. Morgan, J.J. 1967. Chemical equilibria and kinetic properties of manganese in natural waters, p. 561-622. I n S.P. Faust and J.V. Hunter (ed.) Principles and applications of water chemistry. Wiley, New York. Nriagu, J.O. 1972. Stability of vivianite and ion pair formation in the system F e ^ P O ^ - H ^ d - H ^ O . GeoChim. Cosmochim. Acta 36:459-470. Pennington, W., R.S. Cam bray, and E.H. Fisher. 1973. Observations on lake sediments using fallout 'Cs as a tracer. Nature (London) 242:324-326. Postma, D. 1981. Formation of siderite and vivianite and the porewater composition of a recent bog sediment in Denmark. Chem. Geol. 31:225-244. Rickard, D.T. 1975. Kinetics and mechanisms of pyrite formation at low temperatures. Am. J. Sci. 275:636-652. SAS Institute Inc. 1982. SAS user's guide: Statistics. SAS Inst., Inc., Cary, N C . Singer, P.C., and W. Stumm. 1970. Solubility of ferrous iron in carbonate bearing waters. J. Am. Water Works Assoc. 62:198202. Skyring, G.W., R.L. Oshrain, and W.J. Wiebe. 1978. Sulfate reduction rates in Georgia marshland soils. Geomicrobiol. J. 1:389— 400. Smith, C.J., R.D. DeLaune, and W . H . Patrick, Jr. 1983. Nitrous oxide emission from Gulf coast wetlands. Geochim. Cosmochim. Acta 47:1805-1814. Sposito, G., and S.V. Mattigod. 1979. G E O C H E M : A computer program for the calculation of chemical equilibria in soil solutions and other natural water systems. The Kearny Foundation of Soil Sci., Univ. of California, Riverside. van Breemen, N . 1976. Genesis and solution chemistry of acid sulfate soils in Thailand. Agric. Res. Rep. 848. P U D O C Wageningen, Netherlands. l3

Comparison of Three Spectrophotometric Methods for Differentiating Mono- and Polynuclear Hydroxy-Aluminum Complexes D.

R . P A R K E R , L . W. Z E L A Z N Y , * A N D

ABSTRACT Increasing interest in the relationships between aluminum (Al) speciation and biological toxicity has created a need for accurate methods for speciating aqueous A l . Polynuclear hydroxy-Al complexes are generally viewed as metastable species, and are thus more amenable to empirical than to computational approaches to speciation. The objectives of this study were to evaluate and compare the ferron (8-hydroxy-7-iodo-5-quinoline-sulfonic acid), aluminon (aurintricarboxylic acid, triammonium salt), and 8-hydroxyquinoline methods for their ability to differentiate mono- and polynuclear A l in solutions typical of those used in related phytotoxicity studies. Test solutions were 20 i i M in A1C1.„ 0.4 m M in C a C l or 40 m M in NaCI, and had basicities (molar OH/A1 ratios) ranging from -1 to 2.25. Reactions between test solutions and ferron or aluminon were kinetically modeled as two parallel irreversible reactions to yield estimates of the mononuclear fraction, / „ . The reaction of 8-hydroxyquinoline with mononuclear A l was essentially instantaneous, and use of an arbitrary cutoff was unavoidable. Estimates of / „ were in the order 8-hydroxyquinoline > ferron > aluminon at low basicities, while at a basicity of 2.25 the general order was aluminon

T. B. KINRAIDE

>: 8-hydroxyquinoline > ferron. Increasing ionic strength of test solutions increased estimates of f„ by all methods. Significant quantities of polynuclear Al were detected even in solutions without added base or with slight acidification. Aging solutions up to 32 d resulted in decreased estimates of f using aluminon, but estimates using ferron and 8-hydroxyquinoline were only minimally affected. All three methods yield results of adequate precision for most purposes, although the ferron procedure is somewhat less sensitive than the other two. Additional studies using ferron demonstrated its utility for characterizing the nonmononuclear A l fraction using kinetic analyses. Ferron may be the preferred method based on its simplicity, level of precision, and moderate reaction rate with A l . m

2

D.R. Parker and L.W. Zelazny, Dep. of Agronomy, Virginia Polytechnic Inst, and State Univ., Blacksburg, VA 24061; and T.B. Kinraide. Appalachion Soil and Water Conserv. Res. Lab., USDA-ARS, Beckley, W V 25802-0867. Contribution of the Dep. of Agronomy, Virginia Polytechnic Inst, and State Univ., and USDA-ARS. *For correspondence and reprints. Received 11 May 1987. Published in Soil Sci. Soc. Am. J. 52:67-75. (1988).

A d d i t i o n a l Index Words: aluminum speciation.

D

ferron, aluminon, 8-hydroxyquinoline,

ESPITE EVER-INCREASING INTEREST i n the

Soil

chemical factors governing aluminum (Al) toxicity to agronomic crops, many questions regarding the relationships between A l speciation and toxicity remain unanswered. Possible mobilization o f soil A l by acid deposition, and consequent adverse effects on a variety of organisms, provide an additional impetus for discerning the basic relationships between A l chemistry and biological responses. Whenever possible, A l speciation is most readily accomplished computationally, using known formation and solubility constants. While some authors have treated polynu-