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Soil Science Society of America Journal 66:324-330 (2002)
© 2002 Soil Science Society of America

DIVISION S-10 - WETLAND SOILS

Modeling of Nitrogen Sequestration in Coastal Marsh Soils

A. H. Hussein* and M. C. Rabenhorst

Dep. of Natural Resource Sciences and L A, University of Maryland, College Park, MD 20742. Contribution of the Maryland Agric. Exp. Stn

* Corresponding author (pedon{at}dnamail.com)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Extensive field-based data from two representative submerged upland tidal marsh soils in the Chesapeake Bay area were gathered to develop a predictive model for total N sequestration. The data covered the range in physiographic position and variation in marsh habitats. Sampling protocol and model validation assure the validity of the model, and placed 80% confidence, and 10% accuracy on the rate of total N sequestration and the predictive model. In coastal marsh ecosystems, total N sequestration continues to occur with time by accumulation in the organic horizons and sea-level rise is the driving force. The predictive model was a two-step linear function indicating accelerated sequestration of total N in the past two centuries. During the last 150 yr, the rate of total N sequestration averaged 4.2 ± 1.15 g m-2 yr-1, while over the last one or two millennia the rate of total N sequestration averaged 1.47 ± 0.3 g m-2 yr-1. In the next century, modeled prediction of total N sequestration in newly forming marshes averaged 20 ± 7.9 g m-2 yr-1. Present and long-term rates of organic S and total N sequestration in coastal marsh ecosystems were comparable as well as their future predictions. Sequestration of total N in terrestrial closed systems and coastal marshes showed similar long-term trends.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
BECAUSE SEA-LEVEL RISE is a continuous phenomenon, the gently sloping low-lying upland areas in the Chesapeake Bay are progressively subjected to tidal inundation with decreasing elevation. This creates a dynamic environment along the landscape, and encourages biodiversity as salinity and alkalinity increase with decreasing elevation (Hussein and Rabenhorst, 2001). Upon submergence, marsh plants progressively continue to colonize the geomorphic surfaces, entrap sediments suspended in tidal flooding and add organic matter to the soil surface. The accumulation of organic and inorganic sediments allow for the vertical accretion, and the lateral migration of marsh over the low lying coastal land.

These newly formed submerged upland tidal marsh soils are characterized by a specific set of chemical processes and properties associated with the peraquic moisture regime. The organic nature of coastal marsh soils provides an ideal environment for the sulfidization process and the subsequent accumulation of various S species (Hussein and Rabenhorst, 1999a). Other processes active in the marsh environment include those affecting the total N budget. Near the marsh surface, Cyanobacteria (blue-green algae) play an important role in the transformation of N2 gas to organic N through N fixation (Carpenter et al., 1978). Teal et al. (1979) documented N fixation by free-living bacteria and bacteria associated with the rhizosphere of marsh grasses. Ammonia uptake by marsh grasses as influenced by redox potential and salinity level has been investigated (Morris, 1984). Because of the peraquic moisture regime and the associated low redox potential, denitrification is a dominant pathway for N loss from salt marsh ecosystems (Kaplan et al., 1979). In general, waterborne N (inorganic, dissolved organic and organic particulate) enters coastal marsh ecosystems through tidal flooding, precipitation, groundwater, and surfacewater inflow, as well as N fixation by organisms. Coastal marshes export N through tidal ebb, groundwater outflow, and denitrification. The balance between these mechanisms at any given point in time determines whether total N sequestration or loss is occurring in a coastal marsh ecosystem (Fig. 1) . Generally, in coastal marsh ecosystems, the balance between addition and removal is shifted toward the addition of organic matter. This shift is because of a combination of high primary production (surface and subsurface biomass production), low organic export to adjacent estuaries, and inefficient decomposition of organic matter under a peraquic moisture regime. The primary production of coastal marshes varies greatly with latitude (Turner, 1976), nutrient availability (Patrik and Delaune, 1972), marsh environment, and chemical properties (Morris et al., 1990). Reported values for primary production in coastal marshes vary greatly with the method of determination (Hopkinson et al., 1980). Nevertheless, the net primary production of tidal marshes is generally high, especially in the southern coastal plain of North America, where it averages 8000 g m-2 yr-1 (Mitsch and Gosselink, 1993). Carbon export from coastal marshes to adjacent estuaries ranges from 100 to 200 g m-2 yr-1 (Nixon, 1980). The inefficient decomposition of organic matter in coastal marsh environments is generally related to the presence of anaerobic conditions. Under these conditions, the solubility of O in water is low (~8.7 mg L-1 under standard state conditions) and the diffusion rate is also low (~10-4 slower than that in the air). The oxygen-depleted environment in coastal marshes, encourages anaerobic decomposers that are known to decompose organic matter at slower rates than aerobic decomposers (Humphrey and Pluth, 1996; Amador and Jones, 1997).



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  		Fig. 1. Hypothetical depiction of N sequestration in coastal marsh ecosystems.

 
Coastal marsh soils with organic substrates provide a continuous supply of detritus-based food to the estuarine ecosystems. The N content of the organic substrates may correlate with the protein content of the detritus, and thus determine its food value to aquatic life. Nevertheless, published estimates of nutrient sequestrations in wetland ecosystems often failed to incorporate various marsh habitats, address the spatial variability of soil parameters within the sampling unit, and results of limited data have been often extended over vast areas (Craft and Richardson, 1998; Delaune et al., 1981). Therefore, substantiated information regarding the sequestration of essential elements in coastal marsh ecosystems is vital to understanding the productivity, and health of estuarine ecosystems, and assure better management practices. The objectives of this study were (i) to develop a predictive model for total N sequestration in the submerged upland tidal marsh soils of the Chesapeake Bay region, (ii) estimate future sequestration of N under various scenarios of sea-level rise, (iii) compare the resulting model with previously developed model depicting organic S sequestration in coastal marshes, and (iv) assess rates of total N sequestration in coastal marshes versus terrestrial closed ecosystems.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Procedures
Following a soil reconnaissance survey, two representative marshes Hell Hook (38° 21' N lat., 76° 10' W long.) and Cedar Creek (38° 19' N lat., 76° 4' W long.) were selected in the lower eastern portion of the Chesapeake Bay area in Dorchester County, Maryland. At each site, a transect was established extending from the upland across the marsh to the main stream that feeds the marsh. Along each transect, the thickness of the organic horizon was measured every 10 m using a McCauley auger. A topographic survey was carried out, and was referenced to a benchmark of known elevation (Hussein and Rabenhorst, 1999a).

To develop sampling protocol for determining soil properties within a pedon (sampling unit), a marsh soil variability study was conducted. To estimate the variability, three pedons 20 m apart were selected at Hell Hook marsh where soils were considered Histosols. In each pedon, 12 half cores (5 cm in diam.) were collected using a nested triangle scheme (Hussein and Rabenhorst, 1999b). The cores were sampled using a McCauley auger to a depth of 50 cm. Prior to chemical analyses, cores that showed no change in soil morphology with depth were divided into two increments each of which is 25 cm. Cores that showed change in soil morphology with depth were divided into two increments accordingly.

Along each transect, nine pedons (1–2 m2) were selected to represent the range in physiographic position as well as marsh habitat, and were used in developing the total N sequestration model. In addition, four pedons were selected along each transect for cross validation. Based on the results obtained from the variability study (Table 1), three cores were collected from each pedon to represent the sampling unit (pedon). The upper 125 cm of the organic horizons were sampled using 7.6-cm diam. Al tubes, 1.5 m in length. To assess vertical compaction, the thickness of the cores was compared with the sampling depth, and only those cores exhibiting minimum compaction (<=5 cm) were used. To avoid oxidation during transport, the tubes were filled with water and sealed. In minimizing vertical compaction in deeper portions of the organic horizons (>125 cm), samples were collected using a McCauley auger. In the field, samples were sectioned into 25-cm increments (unless horizon morphology suggested otherwise), placed into plastic bags, sparged with N gas and frozen using dry ice. In the laboratory, all marsh samples were frozen at -15°C until they were ready for analysis. For 210Pb dating, three cores were collected from each marsh using a McCauley sampler. Recognizing variability within marsh environments, these cores were collected to represent different physiographic positions and vegetative cover. In addition, five basal peat samples for 14C dating were collected from each marsh immediately above the dense, low n-value buried mineral soil using a McCauley sampler. This way, 14C dating and the associated long-term rates of marsh accretion are not significantly influenced by autocompaction (Hussein and Rabenhorst, 1999a).


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  		Table 1. Number of samples required within a sampling unit (pedon) to estimate total N and organic C for 95 and 80% confidence as a function of mean coefficient of variation (CV) and accuracy.{dagger}

 
Laboratory Procedures
For the most recent period (last 150 yr), marsh accretion rates were determined using 210Pb following standard methods (Flynn, 1968; Benoit and Hemond, 1988). The cores were sectioned at 3-cm intervals and each increment was weighted to determine bulk density and moisture content. Samples were oven dried at 60°C and ground in preparation for 210Pb analysis. In this method, secular equilibrium between 210Pb and 210Po is assumed. The total 210Pb activity was determined by counting the alpha-decay rate of its granddaughter 210Po. The total 210Pb activity was determined on selected samples within each core and plotted versus depth. The total activity that remained constant with depth in the deepest portion of a core was considered the indigenous background that is supported by the decay of 226Ra (supported 210Pb). The atmospheric 210Pb that reach marsh surface with rainfall is the unsupported (access) 210Pb, whose decay is used to determine the rate of marsh vertical accretion. The unsupported 210Pb activity was determined on the selected samples within each core by subtracting the supported 210Pb activity from the total 210Pb activity. The logarithmic plot of the unsupported 210Pb activity versus depth was used to estimate the marsh accretion rate in Chesapeake Bay during the past 100 to 150 yr. Accretion rates prior to the last few hundred years were estimated using 14C dating of five basal peat samples from each transect. The 14C analyses were carried out by Beta Analytic Inc. (Miami, FL). In preparation for soil analysis for total N and organic C, frozen cores were allowed to thaw under N2 gas, and sectioned into 25-cm increments (unless morphology suggested otherwise). The total weight of the wet samples was recorded to determine the bulk density, and the small magnitude of compaction was assumed to have been distributed equally along cores. The gravimetric moisture content was determined using a subsample of at least 0.5 kg of the wet sample, and was oven dried at 60°C (~48 h). The peat subsamples were then ground using a stainless steel plant mill, and ground to pass a 0.25-mm sieve. Total N and organic C analyses were carried out using high temperature combustion with IR detectors Leco 60 CNH analyzer (Leco Corp., Joseph, MI).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sampling Strategy
Scientific investigations require accurate estimation of the parameters in question. In recognizing soil variability and its impact on estimating properties, Wilding and Drees (1983) described the relationship between the number of samples (N) necessary to estimate the mean within a given confidence limits, and coefficient of variation (CV). In this mathematical relationship, T represents probability level, and accuracy (A) refers to deviation from the estimated mean value expressed in percentage,

This equation was used in conjunction with data collected from the marsh variability study to determine the number of samples that should be collected from a pedon (1–2 m2) to estimate organic C, and total N (Table 1). The number of samples that must be collected within a pedon to achieve 10% accuracy and 95% confidence is prohibitive (7, on the average) (Table 1). Therefore, to balance the desire for higher confidence and accuracy against the need for a more reasonable and practical sampling protocol, we decided to collect three cores (on the average) to represent a pedon (sampling unit). This general guideline constitutes a sampling protocol that allowed us to place 10% accuracy and 80% probability statement on the estimated parameters, and the total N predictive model (Table 1).

Site Characteristics
The Hell Hook and Cedar Creek sites are characterized by a very gentle slope from the upland to the marsh, and showed no evidence of significant human interference or change in hydrology. In general, both marshes were not disrupted, and field observations did not confirm signs indicative of marsh losses or erosion. The Hell Hook marsh occupies an area of about 2.5 km2, and the distance from the open water up the tidal creek to the main inlet that feeds the marsh is about 2.1 km. The Cedar Creek marsh occupies an area of about 3.2 km2, and the distance from the open water up the meandering tidal creek to the main inlet that feeds the marsh is about 6.3 km. The physiographic position of both marshes reflects differences in tidal energy, and sediment-loads suspended in tidal flooding (Hussein and Rabenhorst, 2001). In general, the mean high water (MHW) in this vicinity is 0.3 m (Hussein and Rabenhorst, 2001). Soils that have been mapped along the two transects generally follow a drainage sequence of Mattapex series (fine-silty, mixed, active, mesic Aquic Hapludults) or Elkton series (fine-silty, mixed, active, mesic Typic Endoaquults), Sunken (fine-silty, mixed, mesic Typic Endoaqualfs), and Honga (loamy, mixed, euic, mesic Terric Sulfihemists) (Brewer et al., 1998). The upland portion of these landscapes was generally dominated by Loblolly pines (Pinus taeda L.). Along the marsh portions of these transects at Cedar Creek and at Hell Hook, the vegetative cover was a complex mosaic of different plant species which included Salt-meadow cord grass [Spartina patens (Aliton) H. L. Mühl.], narrow-leaf cattail (Typha augustifolia L.), common threesquare (Scirpus americanus pers.), salt grass [Distichlis spicata (L.) Greene], and needle rush (Juncus roemerianus Scheele). Organic horizons of the 26 sampled pedons at Hell Hook and Cedar Creek marshes had no recognizable mineral-sediment layers, and were all underlain by dense low n-value mineral soils (Table 2). The low bulk density of the organic horizons as well as the absence of marsh losses substantiate the low mineral-sediment input, and that both marshes are keeping pace with sea-level rise by organic matter accumulation (Table 2). The upper 25 cm of the organic horizons was generally fibric in nature, whereas the subsurface and bottom tiers were generally hemic material.


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  		Table 2. Characterization data of organic soil (O), and underlying mineral soil (MS) for selected sites along Hell Hook (H) and Cedar Creek (C) marsh.{dagger}

 
Total Nitrogen Predictive Model
The regression relationship between organic C and total N indicated that the accumulation of organic C and total N in coastal marsh ecosystems are significantly related ({alpha} = 0.01 with r2 of 0.94) (C/N ratio 20:1). Marsh plants and microorganisms are considered to be among major sources of organic C, and they are likely to be among major contributors to N in coastal marsh ecosystems. Buresh et al. (1980) reported that inorganic N represents <1% of total N in Louisiana Gulf coast salt marshes. Teal et al. (1979) found that the rate of bacterial N fixation varies with seasons as well as marsh habitats, and the highest rate reported was about 500 ng N cm-2 h-1. Nitrogen fixation by Cyanobacteria is an important mechanism contributing to N sequestration in marshes with estimated rate ranging between 100 and 200 ng N cm-2 h-1 (Carpenter et al., 1978).

To develop a predictive model for total N sequestration in coastal marsh ecosystems, the time elapsed since submergence (age of the marsh) has to be estimated. For the past 150 yr, the age of the marsh was approximated using 210Pb dating. The natural logarithm of the access 210Pb activity was regressed against depth, and the rate of vertical accretion was calculated by dividing the decay constant (0.031) by the slope of the regression line (Table 3). Based on the excess 210Pb activity, the rate of marsh accretion at Cedar Creek marsh ranged between 3.2 and 1.4 mm yr-1 averaging 2.5 ± 0.9 mm yr-1, while at Hell Hook marsh the rate of marsh accretion ranged between 3.2 and 1.5 mm yr-1 averaging 2.2 ± 0.4 mm yr-1. Lack of evidence supporting erosion or marsh losses to open water, and the low bulk density data (Table 2) suggested that both Hell Hook, and Cedar Creek marshes are keeping pace with sea-level rise by organic matter accumulation. Therefore, the rate of marsh vertical accretion can also be regarded as the rate of apparent sea-level rise. In spite of differences in methods of determination and time intervals over which rates have been integrated, the 210Pb-based rates of marsh vertical accretion (apparent sea-level rise) are reasonably in agreement with observations reported by others. Based on tidal records for the last 40 yr, sea-level rise in the Chesapeake Bay has ranged between 2.5 and 3.6 mm yr-1 depending on the location (Hicks et al., 1983). Since 1990, rates of sea-level rise at Baltimore varied from 3.0 to 3.9 mm yr-1 (Kearney and Stevenson, 1991). In estimating marsh age for the last 150 yr, and to account for marsh microtopographic variation (Hussein and Rabenhorst, 1999a), elevation of the submerged geomorphic surface was subtracted from elevation of the marsh/terrestrial edge and the result was divided by the average rate of marsh accretion (apparent sea-level rise). For pedons beyond 150 yr, the long-term rate of sea-level rise (marsh vertical accretion) and age of the marsh were approximated using 14C dating. The 14C data of the five basal peat samples collected from each marsh were regressed against elevation of the submerged geomorphic surfaces, and the slope of the regression line was used to approximate the long-term rate of marsh accretion (apparent sea-level rise) (Table 3). The long-term rate of sea-level rise (marsh accretion) prior to the last century or two at Hell Hook and Cedar Creek marshes was found to be 1.12 ± 0.2 mm yr-1 (r2 = 0.94) and 0.52 ± 0.1 mm yr-1 (r2 = 0.97) respectively. Modern rates of sea-level rise showed acceleration over time. Kraft et al. (1987) have indicated that the present rate of relative sea-level rise along the U.S. Atlantic coast is as much as three to four times higher than the long-term trend over the last several thousand years. The age of the marsh prior to the last 150 yr was determined using the age elevation relationship (Table 3), and the elevation of the submerged geomorphic surface.


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  		Table 3. ln 210Pb counting rates as a function of depth, age elevation relationship and the corresponding accretion rates for selected cores at Hell Hook and Cedar Creek marshes.

 
The topography of the Chesapeake Bay area is generally characterized by low relief and gentle slopes, such that the thickness of the organic horizon overlying the mineral soil tends to increase toward the open water with decreasing elevation of the submerged geomorphic surface. The increase in thickness of the organic horizons also represents increase in the time elapsed since submergence (age of the marsh). Regression analysis between elevation of the submerged geomorphic surfaces and thickness of the overlying organic horizons, confirmed the inverse linear relationship ({alpha} = 0.01 with r2 of 0.99), and that thickness of the organic horizon is a function of the rate of sea-level rise and the age of the marsh (Fig. 2) . Because these coastal marshes are keeping pace with sea-level rise by organic matter accumulation (Table 2), and because organic C and total N are significantly related, it was postulated that the sequestration of total N (g m-2 over the entire thickness of the organic horizon) will increase with time, and that sea-level rise is the primary driving force. To verify the hypothesis, the amount of total N sequestered in the organic horizons (g m-2) at Hell Hook and Cedar Creek were normalized against sea-level rise (divided by thickness of the organic horizon) to estimate the volumetric total N content, and evaluated according to the age of the pedon (Fig. 3) . The volumetric total N content (g m-3) showed no specific pattern with time, indicating that the rate of total N sequestration in coastal marshes is driven by the rate of apparent sea-level rise.



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  		Fig. 2. The linear relationship between elevation relative to mean sea level (MSL) of the submerged mineral soil surfaces and thickness of the overlying organic horizons at Hell Hook (HH) and Cedar Creek (CC) marsh. Because elevations were measured relative to sea-level datum of 1929, some high-marsh pedons appear above MSL.

 


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  		Fig. 3. The distribution pattern of the volumetric total N content in the organic horizon of all pedons at Hell Hook and Cedar Creek marsh.

 
In most forest soils of humid regions, accumulation of organic matter is generally in the thin O and A horizons, and decreases sharply with depth. Therefore, at time zero (time of permanent submergence), the newly formed submerged upland tidal marsh soils contained some initial quantity of N. Based on samples of the O and A horizons of higher elevation upland pedons adjacent to Hell Hook and Cedar Creek marshes, the total N content at time zero was estimated to be 400 g m-2. The amount of total N sequestered (g m-2) in the upper 10 cm of the submerged mineral portion underlying the organic horizons at Hell Hook and Cedar Creek marshes showed no specific pattern with marsh age (data are not shown). Given soil variability, the initial total N content (400 g m-2), and the mean total N content (100 g m-2) within the upper 10 cm of the submerged mineral portion of the soil are not considerably different. Therefore, the initial total N content within the upper 10 cm of the submerged mineral portion of the soil was taken to be the average of these values, 250 g m-2.

In developing the total N predictive model, the predicted total N content (g m-2) within a given thickness of the organic horizon was estimated by multiplying the age of the submerged surface times the rate of sea-level rise, and the result was multiplied by the mean volumetric total N content (1795 g m-3) (Fig. 3). The total N content of the underlying submerged mineral soil was assumed to be 250 g m-2. The predicted total N sequestration for the marsh soil was considered to be the sum of the total N content in the organic horizon and in the submerged mineral portion of the soil. The resulting total N sequestration model was a two-step linear function (Fig. 4) . During the last 150 yr, the predicted rate of total N sequestration at Hell Hook and Cedar Creek marshes was 3.9 ± 0.7 and 4.5 ± 1.6 g m-2 yr-1, respectively. Prior to the last few hundred years, the predicted long-term rates of total N sequestration at Hell Hook and Cedar Creek marshes were 2.01 ± 0.4 and 0.93 ± 0.2 g m-2 yr-1, respectively. Systematic variability (increasing sequestration of total N in the organic horizon with decreasing elevation and increasing marsh age toward the open water), and random variability (marsh habitats, site characteristics, vegetation) are all integrated in the mean volumetric total N content. Therefore, the estimated rates of total N sequestration are considered to be reasonably representative of coastal marsh ecosystems. In this regard, the sampling protocol adopted in this study allowed us to place 10% accuracy and 80% confidence on the predictive model and the estimated rate of total N sequestration in coastal marsh ecosystems. To validate the predictive total N sequestration model, an additional four pedons from each marsh that were not included in the model development were utilized. The ages of these pedons were also obtained using 210Pb and 14C dating (Table 3). Total N data from these pedons compared favorably with the predicted values at Hell Hook and Cedar Creek marshes with coefficient of determination equal to 0.94 and 0.96, respectively (Fig. 5) .



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  		Fig. 4. The predictive total N sequestration model at Hell Hook and Cedar Creek marsh.

 


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  		Fig. 5. Comparison of predicted vs. observed total N values in pedons from the model validation sets at Hell Hook (HH) and Cedar Creek (CC) marsh.

 
Previous investigation in the same vicinity modeled S sequestration in coastal marsh ecosystems (Hussein and Rabenhorst, 1999a). This study confirmed that organic S is significantly related to organic C, and is the dominant S species in coastal marsh ecosystems where sediment-borne Fe inputs are limited. In coastal marshes, C/organic S ratio (20:1) is equivalent to C/total N ratio, and marsh plants as well as microorganisms are the major sources of organic S. Empirical modeling of S species indicated that organic S accumulation is mainly in the organic horizons of coastal marshes, and the rate is driven by sea-level rise. The predictive model of organic S sequestration was a two-step linear function similar to that describing total N sequestration in coastal marsh ecosystems. For the last 150 yr, rates of organic S accumulation averaged 4.3 ± 1.19 g m-2 yr-1, whereas rates of total N sequestration averaged 4.2 ± 1.15 g m-2 yr-1. The long-term rates of organic S accumulation ranged between 0.95 and 2.05 g m-2 yr-1, whereas the rates of total N sequestration averaged 1.47 ± 0.3 g m-2 yr-1.

Rates of total N sequestrations in tidal marsh ecosystems can also be compared with those of terrestrial closed systems. Okoboji soils (fine, smectitic, mesic Cumulic Vertic Endoaquolls) are very poorly drained soils that developed in depressions on till plains and moraines (Soil Survey Staff, 1998). Because of the physiographic position, these soils receive soluble and insoluble materials from up slopes during surface runoff and subsurface through flow. Therefore, these soils are characterized by over thickened A horizon in which N is sequestered. To estimate the rate of accumulation, the total N content of the A horizon (g m-2) was divided by the age of the soil (3000 yr; Walker, 1966). The rate of total N sequestration in the A horizon of Okoboji soils was 1.30 g m-2 yr-1, which is comparable with the long-term rate in coastal marsh ecosystems. However, the N content in the A horizon of mineral soils tends to reach a steady-state condition, while in coastal marsh ecosystems total N sequestration continues to occur with time because of marsh vertical accretion to keep pace with sea-level rise.

The Impact of Sea Level Rise on Nitrogen Sequestration
Major factors influencing future global warming were integrated to describe four scenarios of projected world wide sea-level rise ranging from conservative (low) to high (Hoffman and Titus, 1983). The most restrictive and moderate assumptions were linked together to generate the conservative and mid-range low scenarios. These rates were used to predict the impact of sea-level rise on total N sequestration in two soils that are very near to but slightly above MHW (8 cm above MHW at Cedar Creek and 12 cm above MHW at Hell Hook). The future total N sequestration was estimated assuming that the marsh continues to keep pace with sea-level rise, and over land migrating. Hoffman's predictions only extended through the year 2100, and most soil forming processes are so slow that their impact on soil development is evident only over longer time frames. Therefore, we decided to extend the time span of Hoffman's predictions assuming that rates of sea-level rise for both scenarios remain constant from 2100 to 2300. The future sequestration of total N (Fig. 6) occurs primarily by accumulation in the organic horizon and is mainly driven by sea-level rise. Future prediction of total N accumulation increased from the present rate of about 4 g m-2 yr-1 to range between 13 and 28 g m-2 yr-1, averaging 20 ± 7.9 g m-2 yr-1 in the next 100 yr. Future predictions of organic S accumulations for the same newly forming coastal marshes were comparable averaging 19 ± 8.2 g m-2 yr-1 (Hussein and Rabenhorst, 1999a).



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  		Fig. 6. Predicted total N sequestration in Hell Hook (HH) and Cedar Creek (CC) marshes, based on models using the conservative and midrange (MR) low sea-level rise scenarios of Hoffman and Titus (1983).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Systematic and random variability is a major factor contributing to spatial heterogeneity that compounds the complexity of coastal marsh ecosystems. The accounting for spatial variability in combination with sampling protocol allows for the development of more representative models to various ecosystems, and provides statistical parameters to describe the accuracy of the derived estimates. The predictive total N and organic S sequestration models were a comparable two-step linear functions. The models substantiated that N sequestration is mainly by accumulation in the organic horizons of coastal marshes, and sea-level rise is the driving force. For the last 150 yr, the rate of total N sequestration averaged 4.2 ± 1.15 g m-2 yr-1, whereas the long-term rate of total N sequestration averaged 1.47 ± 0.3 g m-2 yr-1. Future predictions during the next century in newly forming marshes were comparable with organic S predictions averaging 20 ± 7.9 g m-2 yr-1.


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

Received for publication December 6, 2000.


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





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