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Soil Science Society of America Journal 65:932-941 (2001)
© 2001 Soil Science Society of America

DIVISION S-10 - WETLAND SOILS

Modeling the Impact of Tidal Inundation on Submerging Coastal Landscapes of the Chesapeake Bay

A.H. Hussein and M.C. Rabenhorst

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
 
Chronofunctions describing changes in electrical conductivity (EC) and exchangeable sodium percentage (ESP) were derived for upland and submerged pedons along two coastal soil transects in Dorchester County, Maryland. The weighted mean EC and ESP were regressed against time in order to derive models that describe changes in salinization and alkalinization. This was best approximated using exponential and linear models. During the early and intermediate stages of pedogenesis, the exponential model estimated the average rate of salinization for the whole profile to increase from 0.02 to 0.06 dS m-1 yr-1, while the average rate of alkalinization increased from 0.02 to 0.05% yr-1. The linear model estimated the average rate of salinization to be 0.04 dS m-1 yr-1, while it estimated the average rate of alkalinization to be 0.03% yr-1. Because only data from early and intermediate stages of the idealized chronofunctions of salinization and alkalinization were available, the use of the models to predict late-stage impact on EC and ESP is limited. To address the limitation, the measured data were joined with theoretical limits to derive sigmoidal chronofunctions. The future changes in soil salinity and alkalinity were projected using the sigmoidal models and various sea level rise scenarios.

Abbreviations: CEC, cation-exchange capacity • EC, electrical conductivity • ESP, exchangeable sodium percentage • NGVD, national geodetic vertical datum of 1929


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN TRANSGRESSIVE COASTAL AREAS sea level rise is the driving force responsible for the transformation of upland soils into tidal wetlands (Gardner et al., 1992). On the basis of tidal records for the last 40 yr, Hicks et al. (1983) suggested that sea level rise in Chesapeake Bay ranges between 0.25 and 0.36 cm yr-1, and Kearney and Stevenson (1991) indicated that sea level rise at Baltimore has been {approx}0.30 to 0.39 cm yr-1 during the last century. Because sea level rise is generally a continual function within a time frame of centuries or millennia (Hussein and Rabenhorst, 1999), soils that at one time were beyond the influence of storm tides gradually will become more frequently inundated with brackish water and eventually become transformed to tidal marsh soils as they become permanently inundated (Darmody and Foss, 1979; Gardener et al., 1992; Brinson et al., 1995; Rabenhorst, 1997).

Salinization is a pedogenic process related to the accumulation of soluble salts in soils. The accumulation of Na on the colloidal complex is generally referred to as alkalinization (Buol, et al., 1989). Soil salinity is quantified by measuring EC and soil alkalinity is quantified by ESP (Soil Survey Staff, 1998). In the mid-Atlantic region of the USA, the tidal range is generally <2 m, and the salinity of the tidal water in the mid section of Chesapeake Bay is high, reaching 23 dS m-1 (Haering, 1986). Because the frequency of tidal inundation increases with decreasing elevation, the salinity and alkalinity of low-lying upland soils in the Chesapeake Bay region increases with proximity to mean high water. The effects of these pedogenic processes follow the frequency distribution patterns of tidal inundation (Hussein and Rabenhorst, 2001). Other investigators have documented the impact of tidal inundation with brackish or saline waters on soil properties, such as Na adsorption ratio and EC (Edmonds et al., 1986; Stolt and Rabenhorst, 1991). While electrical conductivity of the soil solution is the correct parameter to document soil salinity, ESP rather than Na absorption ratio should be used to document soil alkalinity, especially in soils along the Atlantic Coast of the USA (Hussein and Rabenhorst, 2001).

Because sea level rise is generally a continuous phenomenon, the frequency of tidal inundation at a particular elevation is expected to increase with time, creating dynamic ecological environments along submerging landscapes. Therefore, in order to quantify the impact of tidal inundation along transgressive coastal landscapes, and to be able to predict future ecological changes, the history of sea level rise has to be incorporated into the modeling process. Increasing soil EC and ESP adversely affect the productivity of forest and agricultural land. Therefore, understanding future ecological changes in response to sea level rise is vital to regional management of forest and agricultural land along the Atlantic Coast of the USA. The objectives of this study were to quantify changes in soil EC and ESP with time in a transgressive coastal environment, and to develop mathematical functions to describe future effects on soil properties in response to present and predicted rates of sea level rise.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Procedures
Following reconnaissance efforts, two sites were selected in the eastern portion of Chesapeake Bay, in Dorchester County, Maryland, to represent submerging coastal landscapes: the Hell Hook site (38°21' N latitude, 76°10' W longitude) and the Cedar Creek site (38°19' N latitude, 76°4' W longitude). At these sites the upland portions of the landscapes are within 1 m of the mean sea level, slope gently toward marsh soils, and show no significant evidence of human alteration of the landscape or hydrology. At each site a transect was established extending from the upland to the marsh. A topographic survey was conducted and was referenced to a benchmark of known elevation relative to the national geodetic vertical datum of 1929 (NGVD) (formerly the sea level datum of 1929). Because the mean high water level is 0.3 m above NGVD (Hussein and Rabenhorst, 2001), 0.3 m was subtracted from the recorded elevations at each transects. Along each transect, nine sampling locations were selected at decreasing elevations. At each location, the soil was sampled to a depth of {approx}2 m with a bucket auger, the soil profile was described, and samples were collected from each horizon (Soil Survey Staff, 1951).

Laboratory Procedures
Within 1 wk, samples were air dried and ground to pass through a 2-mm sieve. The soil pH was measured on 0.01 CaCl2 solution (1:1 by weigh) and particle-size analysis was run using the pipette method (Gee and Bauder, 1986). A saturation extract was collected from soil paste using 200 g of air-dried soil. The electrical conductivity of the saturated soil extracts were determined using a conductivity meter (model 32, Yellow Springs Instrument, Yellow Springs, OH). Exchangeable bases were determined from neutral 1 M NH4OAC extracts and adjusted by subtracting water-soluble cations (Soil Conservation Service, 1984). Cation-exchange capacity (CEC) was determined by sum of exchangeable bases plus exchangeable acidity measured using BaCl2-TEA (triethanol-amine) (Soil Conservation Service, 1984). Extractions for CEC and exchangeable acidity were carried out using the procedure of Holmgren et al. (1977). The ESP was calculated using exchangeable Na and CEC.

The weighted mean EC and ESP for the soil profile were calculated by summing the products of the parameter value times the thickness of the soil horizon over the entire soil profile, and the sum was divided by the thickness of the soil profile.

The most recent marsh accretion rates (or apparent sea level rise) (100–150 yr) were determined using 210Pb techniques following standard methods (Flynn, 1968; Benoit et al., 1988). In this regard, three cores were collected from each marsh to cover the range in physiographic positions and variation in marsh grasses. A McCauley sampler was used so that there was negligible vertical compaction. The cores were sectioned into 3-cm increments and weighted to determine bulk density and moisture content. Samples were oven dried at 60°C and ground in preparation for 210Pb analysis. The total 210Pb activity was determined by counting the alpha decay rate of its granddaughter 210Po. In this method, secular equilibrium between 210Pb and 210Po is assumed. The unsupported 210Pb activity was determined on selected samples within each core by subtracting the supported 210Pb activity from the total 210Pb activity. The logarithmic plot of the unsupported 210Pb activity vs. depth was used to estimate the marsh accretion rate, or apparent sea level rise, in Chesapeake Bay during the past 100 to 150 yr.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Site Characteristics
The forested portion of both transects (S1 to S7 in Hell Hook and S3 to S9 in Cedar Creek) was dominated by Loblolly pine (Pinus taeda L.) (Fig. 1 and 2) . In the transitional zone of Cedar Creek site (S9 to S11), the vegetation was a mixture of Loblolly pine and saltmeadow cordgrass [Spartina patens (Ait.) Muhl.] as understory. This graded into a complex mosaic of various marsh grasses in the submerged portion of the landscape (S11 to S15). At Hell Hook, the vegetative cover in the transitional zone (S7 to S8) was a mixture of shrubs and saltmeadow cordgrass as ground cover that graded into a monotypic stand of saltmeadow cordgrass in the submerged portion of the landscape (S8 to S11).



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  		Fig. 1. Topographic cross section of Cedar Creek research site showing elevation of sampled pedons relative to mean high water (MHW)

 


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  		Fig. 2. Topographic cross section of Hell Hook research site showing elevation of sampled pedons relative to mean high water (MHW)

 
The upland soils (S3 to S9 at Cedar Creek and A to S7 at Hell Hook) of both sites were mostly mapped in consociations named for the Mattapex series (fine-silty, mixed, active, mesic Aquic Hapludults) or the Elkton series (fine-silty, mixed, active, mesic Typic Endoaquults). These soils have formed in loess deposit (silty material) overlying fluvial coastal plain deposit (sandy-textured material) (Matthews, 1963; Brewer et al., 1998). The transitional zones (S9 to S11 at Cedar Creek and S7 to S8 at Hell Hook) were mapped in consociations named for the Sunken series (fine-silty, mixed, mesic Typic Endoaqualfs). The marsh portion of both transects were mapped in a consociation named for the Honga series (loamy, mixed, euic, mesic Terric Sulfihemists) (Brewer et al., 1998). In the marsh, the mineral soils occur below mean sea level and are overlain by organic horizons, the thickness of which reflects the period of submergence.

The depth distribution of clay-free sand indicates that the depth (from the mineral soil surface) at which the lithologic discontinuity occurs ranges from 60 to 125 cm (Table 1 and 2). The depth distribution of clay content indicates the presence of an argillic horizon in essentially all soils along both transects (Table 1 and 2). At Hell Hook site, the exchangeable bases percentage averaged 27% in the upland pedons and 43% in the submerged mineral soils (Table 1). At Cedar Creek site, the exchangeable bases percentage averaged 21% in the upland soils and 28% in the submerged mineral soils (Table 2). The presence of argillic horizons and the relatively low base status indicate that these coastal soils are Ultisols (Soil Survey Staff, 1998). In general, the acid soil reaction (3.5–6) suggests the presence of monomeric and polymeric Al in these coastal soils (Stumm and Morgan, 1981) (Table 1 and 2). At Hell Hook site, the exchangeable acidity percentage averaged 73% in the upland pedons and 57% in the submerged mineral soils. At Cedar Creek site, the exchangeable acidity percentage averaged 79% in the upland pedons and 72% in the submerged mineral soils. The acid soil reaction, and the associated high exchangeable acidity percentage indicated that these coastal environments are Al-buffered systems.


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  		Table 1. Characterization data for selected pedons along the Hell Hook transect

 

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  		Table 2. Characterization data for selected pedons along the Cedar Creek transect

 
Idealized Electrical Conductivity and Exchangeable Sodium Percentage Chronofunctions
Upland pedons of low elevation near Chesapeake Bay are frequently inundated with brackish water, and will become progressively more so, as sea level continues to rise. Eventually, these coastal mineral soils will be transformed to tidal marsh soils upon permanent submergence. Therefore, for pedons at elevations above mean high water, the time until submergence decreases with decreasing elevation. Thus, for a given point on the upland portion of the landscape, both the time until submergence and the frequency of tidal inundation could be described. Meanwhile, for pedons near or below mean high water that have developed organic horizons over mineral profiles in response to submergence by rising seas, the elevation (or depth below mean high water) can be described in relation to the time elapsed since submergence. Within a marsh transect, the time elapsed since submergence tends to increase away from the upland and towards the open water where the organic horizons thicken.

Because the time until submergence decreases with decreasing elevation and the time elapsed since submergence increases away from the upland, each point along submerging landscapes can be understood to represent a point in time. In this regard, the marsh represents the future condition of upland soils that presently may be even beyond the influence of unusual storm tides.

When the frequency of tidal inundation at various elevations that is best described by a log function (Hussein and Rabenhorst, 2001) is joined with the rate of sea level rise (which is essentially a linear function) the initial stages of salinization or alkalinization should show increasing rates of change with time. Once the soil becomes permanently submerged (middle stage), a constant rate of change in EC and ESP would be expected. Because these soil processes are diffusion driven, we expect that as the soil begins to approach a theoretical maxima for EC or ESP, the rates of change should decrease (late stage) (Fig. 3) . This S-shaped curve possesses segments that demonstrate increasing, constant, and decreasing rates of salinization or alkalinization in submerging coastal landscapes. While the S-shaped curve has been proposed by many investigators to describe the development of a wide variety of soil properties, (Yaalon, 1975; Sondheim et al., 1981; Birkeland, 1984; James, 1988), this is the first attempt to use the same curve to model idealized chronofunctions describing changes in rates of salinization or alkalinization in submerging coastal environments.



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  		Fig. 3. Hypothetical depiction of changes in soil properties (such as salinization or alkalinization) in a submerging coastal landscape with time

 
Mathematical Modeling of Electrical Conductivity and Exchangeable Sodium Percentage
In modeling changes in soil properties with time using a statistical best-fit approach, Schaetzl et al. (1994) demonstrated that selection of the chronofunction model should be based on pedogenic theories as well as on statistics. Several possible model types were evaluated, including linear, exponential, and second-order polynomial models. The weighted mean (of the whole soil and of the upper 50 cm) EC and ESP were used as dependent variables and regressed against time as the independent variable. For pedons below mean high water, the time elapsed since submergence was approximated by subtracting the elevation of the submerged surface from the elevation of the marsh edge, and the result was divided by the average rate of sea level rise (2.2 mm yr-1 at Hell Hook and 2.5 mm yr-1 at Cedar Creek) (Table 3). For pedons above mean high water, the time until submergence (a negative value) was approximated by dividing the elevation above mean high water by the average rate of sea level rise. The results of the regression analyses are presented in Tables 4 and 5. In comparing these statistically derived models with the idealized S-shaped chronofunction, some of the models were clearly inconsistent with the pedogenic theories and thus were inappropriate. For example, the polynomial model predicted a decrease in salinization and alkalinization during the early stages along Cedar Creek, rather than showing an increase in EC and ESP with time (Fig. 4 and 5) . The linear model also had limitations in that it predicted constant rates of salinization and alkalinization and did not accommodate the gradual increase and decrease in these pedogenic processes during early and late stages. Therefore, the linear model probably best represents only the middle portion (intermediate stage) of the idealized S-shaped chronofunction (Fig. 3). The exponential model predicted initial increases in the rates of salinization and alkalinization, consistent with the increase in the frequency of inundation with proximity to sea level. However, the exponential function did not accommodate the constant and decreasing rates of salinization and alkalinization during the later stages as the soils begin to approach a theoretical maximum. Therefore, the exponential model best depicts that portion of the idealized S-shaped chronofunction during the early and intermediate stages (Fig. 3.)


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  		Table 3. ln210Pb counting rates as a function of depth and the corresponding accretion rates for selected cores at Hell Hook and Cedar Creek marshes

 

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  		Table 4. Mathematical chronofunctions relating the weighted mean electrical conductivity (EC) and exchangeable Na percentage (ESP) for the soil profile and time for Hell Hook (HH) and Cedar Creek (CC) research sites

 

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  		Table 5. Mathematical chronofunctions relating the weighted mean electrical conductivity (EC) and exchangeable Na percentage (ESP) for the upper 50 cm of the soil to time for Hell Hook (HH) and Cedar Creek (CC) research sites

 


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  		Fig. 4. Chronofunctions for electrical conductivity (EC) and exchangeable sodium percentage (ESP) for the Hell Hook (HH) and Cedar Creek (CC) research sites, constructed using the weighted mean values for the entire soil profile

 


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  		Fig. 5. Chronofunctions for electrical conductivity (EC) and exchangeable sodium percentage (ESP) for the Hell Hook (HH) and Cedar Creek (CC) research sites, constructed using the weighted mean values for the upper 50 cm of the soil

 
Using the exponential function for the entire soil profile at Hell Hook site, the rates of salinization ranged from 0.03 to 0.08 dS m-1 yr-1, and rates of alkalinization ranged from 0.02 to 0.06% yr-1 (Fig. 4). At Cedar Creek site, the rates of salinization ranged from 0.01 to 0.03 dS m-1 yr-1, and rates of alkalinization ranged from 0.01 and 0.03% yr-1 (Fig. 4). Higher rates were generally observed in the upper 50 cm when compared with the entire soil profile (Fig. 4 and 5). Other derived rates using the linear model are shown in Tables 4 and 5.

While these chronofunctions follow Jenny's approach (Jenny, 1980), variations in other soil forming factors (climate, parent material, topography, and organisms) may contribute to variability in the data. Increasing frequency of tidal inundation with decreasing elevation and the associated progressive increase with higher sea level creates a dynamic ecological gradient that affects the spatial distribution of the biotic factor. Although these low-lying submerging landscapes are gently sloping, the hydrologic gradient set by the frequency patterns of tidal inundation masks the relief effect. The soil parent material is heterogeneous, and the degree of variability differs from one pedon to another (Tables 1 and 2). In the Chesapeake Bay region, Kearney (1996) has documented the climatic changes and their implications during the little ice age (1400–1700 AD). The time of permanent submergence marks the beginning of a new set of soil forming factors. Upland soils experience fluctuation of water table, extending periods of aerobic condition, and forest vegetation cover. Upon permanent submergence, former upland soils experience a new set of conditions, including peraquic moisture regime, extending periods of anaerobic condition, and salt tolerant marsh grasses. Therefore, submerged mineral soils are polygenetic rather than monogenetic. There may also be other factors (beyond the scope of this study) that may interact to affect soil EC and the concentration of cations in the soil. These might include the chemical composition of the tidal water, including temporal variability, magnitude and composition of runoff and rainfall, fluctuation of the water tables, soil texture (affecting hydraulic conductivity), lithologic discontinuities within the profile, evapotranspiration, and plant species. The relative importance of these factors will vary both within sites and also from site to site, which could lead to both systematic and random variation in the data and can contribute to variations in chronofunctions of pedogenic processes from one site to another. However, at the Hell Hook site, the coefficients of determination for the exponential and the linear chronofunctions (for the entire profile and upper 50 cm) indicated that {approx}96% of the variability in EC was explained by time, whereas {approx}85% of the variability in ESP was explained by time (Tables 4 and 5). At Cedar Creek site, the EC exponential and linear chronofunctions for the entire soil profile explained between 51 and 57% of the variability in soil salinity, whereas within the upper 50 cm of the soil material these models explained between 75 and 92% of the total variability. The ESP exponential and linear chronofunctions for the entire soil profile explained between 40 and 43% of the variability in ESP, whereas within the upper 50 cm of the soil material these models explained between 58 and 95% of the total variability. The improved coefficients of determination of these chronofunctions (for the upper 50 cm of the soil material) suggest that the lithologic discontinuity at the Cedar Creek site significantly contributed to variation in EC and ESP within and among sites.

Modeling Future Impact of Sea Level Rise on Electrical Conductivity and Exchangeable Sodium Percentage
The exponential and linear models for EC and ESP represent only a portion (early and intermediate stages) of the idealized chronofunction (Fig. 3). Therefore, these models are not fully adequate to predict the future impact of sea level rise on salinity and alkalinity, especially for periods beyond the maximum time observed in this study. As suggested earlier, the idealized chronofunction could be described mathematically by a sigmoidal function (Yaalon, 1975; Sondheim et al., 1981; Birkeland, 1984; James, 1988), which is initially formed by joining exponential and linear functions, but in the later stages approaches an asymptotic value. This type of model could then be used to predict the future impact of sea level rise on soil EC and ESP values.

In constructing the sigmoidal function, it was necessary to identify a ceiling above which the weighted mean EC or ESP will not climb. Due to the complexity of trying to determine theoretically the appropriate maximum for EC and ESP, we decided to use maximum values for EC or ESP observed in long-submerged A or B horizons near to the mineral soil surface in Hell Hook marsh. The highest value for EC was 35 dS m-1 and for ESP was 30% (Table 1). These values are thought to represent the best choice for a ceiling because they tend to integrate all the soil, plant, climate, and hydrologic factors that may affect EC and ESP.

The uniform depth distribution of EC and ESP of most of the upland pedons at Hell Hook and cedar Creek suggests that salt and cations added during tidal inundation have been redistributed throughout the soil profile (Tables 1 and 2). Due to the complexity and the interrelationships among all the controlling factors throughout the soil profile, only the weighted means of EC and ESP for the entire soil profile were considered in the development of sigmoidal chronofunctions. The sigmoidal models were derived using TableCurve software (TableCurve, 1991) by combining the data from each transect with the specified ceiling values for EC and ESP (Fig. 6) . Although sigmoidal models for the entire soil profile were derived using the original data and the theoretical ceiling, their long-term predictive value may therefore be limited, especially for projections beyond the next 200 yr. However, the most dramatic changes in soil properties occur during the early stages as tidal inundation occurs more frequently.



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  		Fig. 6. Sigmoidal chronofunctions for electrical conductivity (EC) and exchangeable sodium percentage (ESP) for the Hell Hook (HH) and Cedar Creek (CC) research sites, constructed using the weighted mean values for the entire soil profile. Early and intermediate stages were based on site data, and two possible scenarios for the late stage were projected using theoretical maxima for EC and ESP

 
On the basis of global warming models, various rates of sea level rise have been estimated to range between 4 and 10 mm yr-1 (Hoffman et al., 1983). Using these accelerated rates as well as the current average rates for the study areas (2.2 mm yr-1 Hell Hook and 2.5 mm yr-1 Cedar Creek), the future impact on soil EC and ESP at the study sites was examined. If we were to select a point on the landscape 0.5 m above mean high water, and apply the derived sigmoidal chronofunction models, the projected impacts of various sea level rise scenarios on the soil properties (EC and ESP) are depicted in Fig. 7 . Assuming that the controlling factors remain constant, projections up to the time of submergence indicate that the rates of salinization and alkalinization are mainly driven by frequency of inundation, which tends to increase with increasing rates of sea level rise. Once the soil becomes submerged, the rates of these pedogenic processes are no longer determined by sea level rise but rather by water chemistry, soil properties, rates of diffusion, and exchange processes that took a secondary role prior to submergence.



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  		Fig. 7. Estimates of weighted mean electrical conductivity (EC) and exchangeable sodium percentage (ESP) for a pedon located 0.5 m above mean high water at the Hell Hook (HH) and Cedar Creek (CC) sites, developed using the sigmoidal chronofunction and three possible sea level rise scenarios

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
An idealized S-shaped chronofunction was conceived following theoretical constructs for low-lying upland areas along the coastal fringes of Chesapeake Bay to describe the impact of sea level rise on EC and ESP. The derived exponential and linear chronofunctions for EC and ESP described the early and intermediate stages of the pedogenic theory of salinization and alkalinization. During the early and intermediate stages, the exponential model estimated the average rate of salinization for the whole profile to increase from 0.02 to 0.06 dS m-1 yr-1, while the rate of alkalinization increased on the average from 0.02 to 0.05% yr-1. The linear model predicted that the average rate of salinization was 0.04 dS m-1 yr-1, and the average rate of alkalinization was 0.03% yr-1. Neither the exponential nor the linear functions were fully adequate to predict the future impact of sea level rise on soil salinity and alkalinity. Data from each transect were joined with theoretical limits for EC and ESP to derive sigmoidal chronofunctions for EC and ESP. The models indicated that up to the time of submergence the rates of salinization and alkalinization were mainly related to sea level rise and the associated frequency of inundation. As the soils became submerged the rates of these pedogenic processes were independent of sea level rise and were controlled by the chemistry of water and soil properties that took a secondary role prior to submergence. It is anticipated that these predictive models are tools to provide information regarding future soil conditions in coastal landscapes of the Chesapeake Bay area that is essential to long-term planning for forest and land management.


   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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