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DIVISION S-10-WETLAND SOILS

Tidal Inundation of Transgressive Coastal Areas

Pedogenesis of Salinization and Alkalinization

Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742

Corresponding author (pedon{at}dnamail.com)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Two transects were selected across submerging landscapes in Dorchester County, Maryland, to study the impact of the frequency of tidal inundation on salinization and alkalinization. The frequency of high tides of a particular elevation increases over time due to rising sea level to create dynamic environments in submerging coastal soils. The constructed frequency distribution curves of high tide at Hell Hook and Cedar Creek revealed that the frequency pattern was logarithmic and increased with lower elevation. This distribution enhances the relative importance of tidally induced pedogenic processes at elevations approaching mean high water. For pedons above mean high water, the effect of salinization and alkalinization processes follows the logarithmic frequency distribution pattern. However, for pedons below mean high water, the effect of these pedogenic processes is expected to decline toward a steady-state condition. In marsh environments, the pH-dependent acidity of submerging Al-buffered soils (Ultisols) is not replaced upon permanent inundation, and increasing electrical conductivity (EC) of the soil solution enhances the selectivity of the colloidal complex for Al. Therefore, in response to sea-level rise, low-lying Ultisols transform directly to Histosols. Due to high exchangeable acidity associated with Atlantic coastal soils, the assumptions underlying the relationship between the exchangeable sodium percentage and the sodium adsorption ratio are not valid. Thus, exchangeable sodium percentage should be used to assess soil alkalinity.

Abbreviations: CEC, cation-exchange capacity • EC, electrical conductivity • ESP, exchangeable sodium percentage • SAR, sodium adsorption ratio

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THE GENTLY SLOPING mineral soils in the Chesapeake Bay region that are within 1 m of sea level (mean high water) are influenced by tidal flooding. Along the Mid-Atlantic coast, the tidal range is generally <2 m. The tidal water is brackish or saline with electrical conductivity (EC) reaching 23 dS m^-1 in the midsection of the Chesapeake Bay (Haering, 1986). Therefore, tidal inundation imposes gradual changes across the landscapes in the chemical and physical properties of these soils with accompanying changes in the biological community structure. Brinson et al. (1985) have indicated that tidal inundation with brackish water results in osmotic effects that may stress and even kill forest species. Eventually, these forest species are replaced by marsh grasses as the soils become permanently inundated at lower elevations.

In examining the submerged-upland tidal marsh soils of Virginia, Edmonds et al. (1986) proposed the recognition of natric horizons and halmyric materials (pertaining to salt) by using the sodium adsorption ratio (SAR) and EC. Stolt and Rabenhorst (1991) documented the impact of tidal inundation on the SAR, exchangeable sodium percentage (ESP), and EC across an upland/submerged-upland catena, and the subsequent transformation of Ultisols to Alfisols and Histosols. The objectives of this study were to (i) derive long-term frequency distribution patterns of tidal inundation at the selected tidal marsh sites, (ii) evaluate the suitability of using the SAR or ESP to describe changes in the submerging soils along the Mid-Atlantic coast, (iii) investigate salinization and alkalinization processes as a function of inundation frequency across low-lying, transgressive landscapes of the Chesapeake Bay, and (iv) to examine the pedogenic transformation of Ultisols to Alfisols and eventually to Histosols in response to sea-level rise.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Procedures
Following preliminary investigations, two representative, submerged-upland marshes were selected in the eastern portion of the Chesapeake Bay region, in Dorchester County, Maryland. Hell Hook Marsh (38°21' N, 76°10' W) and Cedar Creek Marsh (38°19' N, 76°4' W) were selected for this study because they were characterized by a gradual transition from the upland to the marsh and minimal evidence of significant human disturbance or changes to hydrology. A detailed topographic survey was conducted along a transect at each site and was surveyed back to a benchmark of known elevation. Nine pedons were identified along each transect at decreasing elevations, and were sampled to a depth of approximately 2 m using a bucket auger. The soil profile was described and samples were collected from each horizon (Soil Survey Staff, 1951).

A Stevens type F water-level recorder (Stevens Water Monitoring, Beaverton, OR) was installed at each of the main tidal creeks that feed Hell Hook and Cedar Creek Marshes, and tide levels were measured for approximately 5 to 7 wk. The recorded high water levels at each site were used to establish the relationship between high tide at the research site and at the reference location at Solomons Island, MD.

Laboratory Procedures
In preparation for analysis, samples were air-dried and ground to pass through a 2-mm sieve. Soil pH was determined using a 0.01 M CaCl₂ solution (1:1 by weight). Particle-size distribution was determined by pipette (Gee and Bauder, 1986). A saturation extract was collected by preparing a soil paste using 200 g of air-dried soil (SCS, 1984). The water-soluble cations (Na⁺, Ca²⁺, Mg²⁺, and K⁺) and anion (Cl^1-) were determined using atomic absorption spectroscopy (Perkin Elmer model 5000, Perkin Elmer, Norwalk, CT) and ion chromatography (Dionex 2000i, Dionex, Sunnyvale, CA). Concentrations of water-soluble cations were used to calculate the SAR [Na/(Mg + Ca)^1/2]. The EC was also measured using a conductivity meter (YSI model 32, Yellow Springs Instrument, Yellow Springs, OH).

Cation-exchange capacity (CEC) was determined on all samples by measuring the sum of the basic cations (Ca, Mg, Na, and K) extracted using the 1 M NH₄OAC at pH 7 (minus water-soluble cations), and adding to this the exchangeable acidity at pH 8.2, measured using the BaCl₂ -TEA (SCS, 1984) by titration. Extraction for the CEC and exchangeable acidity were carried out using the procedure of Holmgren et al. (1977). The ESP was calculated using exchangeable Na and CEC.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Site Characteristics
At the Cedar Creek site, the upper portion of the transect (S3 to S9) was dominated by Loblolly Pine (Pinus taeda L.) forest (Fig. 1) . This graded through a transition zone (S9 to S11) where the forest had an understory of Salt-meadow cordgrass [Spartina patens (Aiton) H. L. Mühl.] and into the marsh zone (S11 and beyond), which was dominated by various marsh grasses. At the Hell Hook site, the highest portion of the transect (A through S1) (Fig. 2) was agricultural land (not presently farmed, but enrolled in the Conservation Reserve Program). The area from S1 through S7 was generally dominated by Loblolly pine while the transitional zone (S7 to S8) was a mixture of shrubs and Salt-meadow cordgrass as a ground cover. The marsh zone from S8 and beyond was dominated by Salt-meadow cordgrass (Fig. 2).

View larger version (22K): [in this window] [in a new window]   Fig. 1. Topographic cross section of Cedar Creek research site

View larger version (21K): [in this window] [in a new window]   Fig. 2. Topographic cross section of Hell Hook research site

The depth distribution of sand and silt indicated that the depth from the mineral soil surface at which lithologic discontinuity occurs ranges from 60 to 125 cm in both transects (Tables 1 and 2). The magnitude of variability in particle-size distribution with depth is more pronounced in Cedar Creek soils, reflecting a higher degree of heterogeneity. The depth distribution of clay demonstrates the presence of argillic horizons in nearly all pedons along Hell Hook and Cedar Creak transects (Tables 1 and 2). Analysis of the <2-µm clay indicated that Mattapex soils have mixed-clay mineralogy, where the chlorite and kaolinite content ranged between 10 and 30% (Foss et al., 1978).

View larger version (31K): [in this window] [in a new window]   Fig. 3. High-tide comparison: Solomons Island vs. Hell Hook (HH) and Cedar Creek (CC) marshes

To report high tides relative to mean high water, a conversion was made and 0.30 m (the difference between mean high water and NGVD) was subtracted from the elevation of high tides recorded. The resulting regression equation was used to derive elevation of high tides (meters above mean high water) at the research sites over the range of high tides recorded at Solomons Island. The generated data and the derived cumulative number of high-water events were used to construct frequency distributions of high tides at Hell Hook and Cedar Creek Marshes (Fig. 4) . These frequency distribution patterns were based on 50 yr of tidal records, within which the history of sea-level rise is incorporated. Based on tidal records for the last 40 yr, Hicks et al. (1983) have suggested that sea-level rise in the Chesapeake Bay has ranged between 0.25 and 0.36 cm yr^-1, depending on the location. Kearney and Stevenson (1991) have indicated that modern rates of sea-level rise at Baltimore varied between 0.30 and 0.39 cm yr^-1 since 1900. Thus the frequency of tidal inundation at a particular elevation is expected to increase over time, creating a dynamic environment in this zone of submerging coastal soils.

View larger version (28K): [in this window] [in a new window]   Fig. 4. Frequency distribution of high tides expected at Hell Hook (HH-M) and Cedar Creek (CC-M) marshes, based upon 50-yr tidal records at Solomons Island

Exchangeable Sodium, Exchangeable Acidity, and Electrical Conductivity
Alkalinization refers to the accumulation of sodium cations on the colloidal complex (Buol et al., 1989). In Soil Taxonomy, any exchangeable sodium percentage (ESP) of 15% or more or a sodium adsorption ratio (SAR) of 13 or more is a criterion to define sodicity (alkalinity) in the natric horizon (Soil Survey Staff, 1998). This definition is based on the generalization that the ESP and SAR have similar values if the ESP is <40%, and is meant to minimize the problems associated with determining the ESP (James et al., 1982). The basis for calculating the SAR from the saturation extract is the mathematical expression for the cation-exchange selectivity coefficient first derived by Gapon (1933). Given that the chemistry of Ca²⁺ and Mg²⁺ are sufficiently similar to be additive, and that the Na⁺, Ca²⁺, and Mg²⁺ cations constitute the bulk of the exchangeable cations, this provides for a direct relationship between the SAR and the ESP (Richards, 1954). This relationship has been questioned on the grounds that the calculation of SAR is based on ionic concentrations instead of activities (Van Beek and Bolt, 1973). Sposito et al. (1977) verified that there is no exact chemical relationship between the ESP and the SAR. However, statistical correlation between the two parameters is a possibility and may be of practical value. Because the soil cation-exchange capacity is the sum of the exchangeable acidity and the exchangeable bases (Peech, 1979), the depth distributions of the exchangeable acidity percentages for the sampled pedons (Fig. 5) indicate that Na⁺, Ca²⁺, and Mg²⁺ cations do not dominate the CEC in these submerging environments. In addition, the high ionic strength of the soil solution in most of the pedons (as indicated by the EC) favors ion-pair formation, which in turn affects the exchange equilibrium involving Na⁺, Ca²⁺, and Mg²⁺ in the soil solution. Since the conditions necessary to establish the exact relationship between the SAR and the ESP (Richards, 1954) are not met in these coastal soils, the SAR (Tables 1 and 2) and ESP values (Fig. 5) are weakly related with an r² of 0.52 at Hell Hook and 0.18 at Cedar Creek. Therefore, we decided to use the ESP instead of the SAR to investigate soil alkalinity. Those seeking to characterize or predict the impact of tidal inundation on low-lying mineral soils with low base status (Ultisols), such as the case along the Atlantic Coast of the United States (Soil Survey Staff, 1967), should also use the ESP instead of the SAR.

View larger version (39K): [in this window] [in a new window]   Fig. 5. The depth distribution of electrical conductivity (EC), exchangeable sodium percentage (ESP), and exchangeable acidity (EA%) for all pedons along Hell Hook (HH) and Cedar Creek (CC) transects

The soil pH measured on dry samples for the submerged pedons ranged between 4 and 6 at Hell Hook and from 3.5 to 4.5 at Cedar Creek (Tables 1 and 2). The presence of sulfide minerals and the release of sulfuric acid upon oxidation during sample preparation and/or storage had a negligible effect on soil pH and other chemical characterizations. This is because measurements of soil pH and other chemical characteristics were initiated shortly after sample preparation that lasted less than a week. Pedons above mean high water (A to S7 in Hell Hook and S3 to S9 at Cedar Creek) are generally associated with oxidizing environments in which sulfides are unstable, thus do not contain sulfide minerals. In the submerged mineral portion of both transects (S8 to S11 in Hell Hook and S11 to S15 at Cedar Creek), the amount of Fe within the soil would determine the magnitude of sulfides in the upper portion (25 cm) of the mineral soils where organic C and sulfate levels may be adequate for sulfidization (sulfide formation) (Hussein and Rabenhorst, 1999). This empirical modeling of sulfur sequestration in submerging coastal soils of the same vicinity indicated that pyrite in the upper 25 cm averaged 1 g kg^-1, and beyond this depth pyrite content is negligible. The pH and exchangeable acidity percentage for the submerged pedons at Hell Hook and Cedar Creek (Tables 1 and 2; Fig. 5) indicated that monomeric and polymeric Al species are present in the soil solution (Stumm and Morgan, 1981) and thus are major sources of exchangeable acidity. The pH-dependent acidity that is only exchangeable at pH 8.2 by hydrolysis is not replaceable at the pH occurring in marshes. Meanwhile, the dominant cation-exchange reaction in these soils involves ions of unequal charge (polymeric Al, Al³⁺ and Na⁺). Assuming ideal solution behavior of ions in the exchanger phase, the concentration valency rule should hold, that is, decreasing ionic strength (as indicated by EC) increases the preference of the ion exchanger for the ion of higher charge. However, the ionic radius and charge and the hydration behavior of Al and Na are different. Therefore, conditions pertaining to ideal solution behavior of ions in the exchanger phase are not met in these coastal environments. Accordingly, the selectivity coefficient is no longer constant and becomes a function of ionic strength of the soil solution and surface cation composition. Given soil variability along each transect and within pedons (Tables 1 and 2) and the complexity of the ion exchanger, correlation coefficients (Table 3) between EC, the exchangeable acidity percentage, and the ESP indicated that increasing soil salinity would increase selectivity of the colloidal complex for Al. Because the depth to lithologic discontinuity (Tables 1 and 2) ranged from 60 to 125 cm, correlation coefficients were calculated for the upper and the lower soil material of the submerged pedons. In a laboratory-based study, the selectivity coefficient tends to increase as Cd²⁺ exchanges greater amounts of Na from montmorillonite, and higher ionic strength (EC) creates higher values of the selectivity coefficient (McBride, 1980). While this study did not address Al³⁺ exchange reaction vs. Na⁺, a similar conclusion can be drawn, given the charges and ionic radii of Al³⁺ and Cd²⁺, and their positions in the periodic table.

View this table: [in this window] [in a new window]   Table 4. The range and the mean of the ratio of the exchangeable Mg to the exchangeable Na for all pedons along the Hell Hook and Cedar Creek transects.

At Old Point Marsh, Maryland, the relative proportion of hydroxy-interlayered vermiculite reached 10% of the clay fraction in some parts of the submerged and the upland pedons, whereas kaolinite ranged between 30 to 70% of the clay fraction in all horizons of both pedons (Stolt and Rabenhorst, 1991). Given that hydroxy-interlayered Al and acidic groups associated with kaolinite are not exchangeable at the pH occurring in marshes (which is <8.2 and used to determine exchangeable acidity), it is not anticipated that low-lying Ultisols will become fully base-saturated upon permanent submergence as reported by the authors. Therefore, it is conceivable that in response to sea-level rise, Al-buffered soils with relatively high pH-dependent acidity (Ultisols) may be transformed directly to Histosols bypassing the intermediate stage (Alfisols), as proposed by Stolt and Rabenhorst (1991). In this case, the organic horizon must be <40 cm of the upper 80 cm, and the exchangeable acidity percentage of the underlying mineral soil is >65% at 180 cm from the soil surface or at 125 cm from the upper boundary of the argillic horizon, whichever is shallower (Soil Survey Staff, 1998). Examining the depth distribution of the exchangeable acidity percentage (Fig. 5) in the submerged pedons at Hell Hook and Cedar Creek revealed that these soils still fall within the central concept of Ultisols with overlying organic horizons, averaging 25 cm. The pH-dependent acidity of Al-buffered soils (Ultisols), increasing selectivity of the colloidal complex for Al with increasing EC upon submergence, and the subsequent direct transformation of Ultisols to Histosols clearly question the existence of the Sunken soil series (Typic Endoaqualfs), as reported in the soil survey report of Dorchester County, Maryland (Brewer et al., 1998).

Pedogenesis of Salinization and Alkalinization
Inundation with brackish water adds both salts and Na⁺ to submerging coastal soils. In the very early stages of salinization and alkalinization (where the sites are at higher elevation and only rarely inundated), hydrological conditions such as fresh water input (rainfall and runoff) and water tables function as negative feedback mechanisms moderating the impact of tidal inundation and affecting the rates of these pedogenic processes. For example, during an unusually high tide, a 1-m² pedon would receive 4 cm of tidal water (EC = 19 dS m^-1). If the water percolates only downward, the saturation percentage is 30%, and the bulk density is 1.3 Mg m^-3, then the percolating water would initially be held in the upper 20 cm of the soil, raising the EC of the soil to approximately 10 dS m^-1. Assuming uniform soil material, steady-state conditions, no evapotranspiration, and only downward percolation of water, then approximately 3 yr with average rainfall of 1000 mm (about twice the pore volume) would be required to remove the accumulated salt (Cl) from the soil profile (200 cm). However, due to the sporadic nature of rainfall and resulting fluctuation of the water table, evapotranspiration during the growing season, transient type flow, and variation in hydraulic conductivity with depth, we would not expect salts to be removed from the soil so quickly and efficiently. Nevertheless, where the sites are at higher elevation and only rarely inundated, the rate of salinization and alkalinization is expected to be low.

Because frequency of inundation increases with decreasing elevation along the landscapes (Fig. 4), soils at a lower elevation experience a greater frequency of inundation, which overpowers the effects of negative-feedback mechanisms. This shift in the balance toward additions of salt and Na, rather than removals, accentuates salinization and alkalinization, the effects of which mirror the logarithmic distribution pattern of the inundation frequency (Fig. 6) . The absence of the logarithmic relationship along the Cedar Creek transect may be attributed to the heterogeneity of the soil material (Tables 1 and 2). At elevations approaching mean high water, inundation is so frequent that it is effectively continual. Because alkalinization is an exchange chemical process, its rate is determined by the ion diffusion rate, the charge and size of the other competing ions, ionic strength, selectivity coefficients, and surface cation composition.

View larger version (30K): [in this window] [in a new window]   Fig. 6. The relationship of the weighted mean electrical conductivity (EC) and exchangeable sodium percentage (ESP) vs. cumulative number of high tides for upland pedons at the Hell Hook transect

The logarithmic distribution patterns of the weighted mean EC and ESP (Fig. 6) show that salinization and alkalinization become more significant processes with increasing frequency of inundation (at lower elevations) toward the terrestrial/marsh edge (S7 in Hell Hook and S11 in Cedar Creek). The relative importance of the salinization process at Hell Hook transect, where the lithologic discontinuity had a minimal effect, becomes significant at elevations where the frequency of tidal inundation overpowered the effects of negative-feedback mechanisms. At the Hell Hook transect, the weighted mean EC for the upland pedons (A to S7) ranged from 2 to 12 Sd m^-1, and at S1 and lower elevations the EC values exceeded 4.5 Sd m^-1. Because the weighted mean EC values for the upland pedons were >4.0 Sd m^-1 at points from S1 to S7, the salinization process had clearly become significant in these soils (James et al., 1982). Because the effect of salinity on crop production and forest is detrimental, it is vital that these coastal soils that are above the mean high water but significantly influenced by salinization be recognized at the subgroup level of Ultisols for detailed soil survey and land management. The weighted mean ESP for the upland pedons along Hell Hook transect increased gradually with decreasing elevation, ranging from 2 to 5%. Because ESP was <15%, these soils are not alkaline and the alkalinization process is not significant (James et al., 1982). The insignificance of alkalinization in these coastal soils is attributed to the relatively high exchangeable acidity, low replacing power of Na, and competition from other cations of higher charges such as Mg and Al.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
For submerging coastal soils at elevations above mean high water, the degree of salinization and alkalinization is related to the frequency of inundation, which increases with proximity to sea level. Within a given tidal range, the frequency of inundation at a particular elevation is generally influenced by site characteristics such as length and tortuosity of the tidal creek and distance to the upland from the tidal creek, which may lead to different characteristics at different sites. Once the soils have become submerged, these pedogenic processes are expected to approach steady-state conditions. In marsh environments, the pH-dependent acidity of Al-buffered soils (Ultisols) is not replaced upon permanent submergence, and increasing EC of the soil solution enhances the selectivity of the colloidal complex for Al. Therefore, in response to sea-level rise, low-lying Ultisols transform directly to Histosols bypassing Alfisols. These findings question the existence of the Sunken soil series (Typic Endoaqualfs), and the mapping of Alfisols along the Chesapeake Bay area in the transitional zone between the Ultisols in the uplands and Histosols in the marsh. As a result of the low replacing power of Na in Al-buffered systems, only the salinization, and not the alkalinization, process is significant along submerging landscapes. Because of the relatively high pH-dependent acidity associated with the submerging coastal soils (Ultisols) of the Atlantic coast, the relationship between the ESP and the SAR are not valid; thus the ESP should be used to evaluate the impact of sea-level rise on these soils.

	ACKNOWLEDGMENTS

 
This work was supported by the Maryland Agricultural Experiment Station and by the USDA-NRCS. We are grateful to the reviewers and the associate editor for their valuable insights and comments.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Contribution from the Maryland Agric. Exp. Stn.

Received for publication December 1, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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