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Published online 1 May 2008
Published in Soil Sci Soc Am J 72:660-669 (2008)
DOI: 10.2136/sssaj2006.0102
© 2008 Soil Science Society of America
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PEDOLOGY

Morphology and Characterization of Ditch Soils at an Atlantic Coastal Plain Farm

Robert E. Vaughana, Brian A. Needelmana,*, Peter J. A. Kleinmanb and Martin C. Rabenhorstc

a Dep. of Environ. Science and Technology, Univ. of Maryland, College Park, MD 20742
b USDA-ARS, Building 3702, Curtin Rd., University Park, PA 16802
c Dep. of Environ. Science and Technology, Univ. of Maryland, College Park, MD 20742

* Corresponding author (bneed@umd.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Some materials in drainage ditches that have traditionally been referred to and studied as sediments may be soils. In this study, we described and characterized materials found within agricultural ditches at the University of Maryland Eastern Shore Research Farm (Princess Anne, MD). Sixty-nine profiles were described in 10 ditches ranging in length from 225 to 550 m. Particle size, pH, and organic C were analyzed on 21 representative profiles. The materials met the definition of a soil in that they supported rooted vegetation. Samples of initial parent material were not collected; however, evidence was observed that horizons had formed through pedogenic processes including organic matter humification and accumulation, structure formation, Fe oxidation and reduction, sulfuricization, sulfidization, translocation, and bioturbation. Ditch soils were generally A horizons formed in loamy alluvial sediments eroded from loess-derived topsoil over gravelly and sandy C horizons formed in Coastal Plain sediments. Soil structure was described in 75% of A horizons. Redoximorphic features were described in 41% of A and 63% of C horizons. Organic C ranged from 0.4 to 124 g kg–1. Monosulfidic black oozes were observed on some soil surfaces; geological sulfidic materials were observed at depth. Shallow ditches (<1.5 m) tended to have structure and a layer in the substratum with a bright matrix color. Deep ditches (1.5–4 m) tended to have high n value, structureless sola, and Fe-depleted subsoil horizons. The presence of plants and the operation of pedologic processes may significantly affect ditch ecosystems and environmental functions.

Abbreviations: AVS, acid volatile sulfur • CRS, chromium-reducible sulfur • UMES, University of Maryland Eastern Shore


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Drainage ditches are long, linear features that are constructed to lower the water table and speed the removal of excess surface runoff to local streams and water bodies. They are used in poorly drained, tile-drained, and irrigated landscapes that require artificial drainage for agricultural and other uses. Ditches function as a hydrologic link between surface runoff, groundwater, and surface waters (Janse and Van Puijenbroek, 1998). Ditches have the potential to act as key pathways for the export of nutrients from areas of intensive agriculture (Sallade and Sims, 1997a; Vadas and Sims, 1998; Nguyen and Sukias, 2002; Needelman et al., 2007a).

Land drainage for public and agricultural use has been adopted around the world. Land drainage systems were developed in Mesopotamia around 9000 BCE (van Schilfgaarde, 1971). Networks of land-drainage systems were also developed by the Egyptians and the Greeks around 2400 BCE (Shirmohammadi et al., 1995). In the United States, organized drainage for agricultural and public purposes began in the 1600s (Evans et al., 1996). Many states rely on land drainage, in particular subsurface drainage (tile drainage) and surface drainage (open-air ditches), to control groundwater levels in both agricultural and urban areas. In the Midwest, nearly 37% of arable lands require drainage (Fausey et al., 1995). The Coastal Plain of North Carolina has >800,000 ha that rely on ditches (Evans et al., 1996). In Florida, >2.5 million ha are artificially drained (Thomas et al., 1995).

Due to periodic or continuous saturation and ponding, materials within ditches may have properties similar to wetland or subaqueous soils (Demas and Rabenhorst, 1999; Bradley and Stolt, 2003). Wetland soil properties such as high organic matter content, reducing conditions, and periodic oxidizing events may occur in ditches. Therefore, ditches are likely to function similarly to wetlands in terms of their nutrient cycling processes and retention capabilities (Bowmer et al., 1994; Nguyen and Sukias, 2002). Both ditches and subaqueous soils are influenced by overlying water bodies (Demas and Rabenhorst, 2001). Therefore, the application of select pedogenic concepts developed through the study of subaqueous soils may also apply to ditches (Needelman et al., 2007b). Situated within a drainage channel, ditch materials are also influenced by fluvial processes.

Mineral and organic materials from a variety of sources act as parent materials for ditch materials. Mineral material can accumulate from the sedimentation of suspended soil from cultivated fields in surface runoff water, from the slumping of ditch sidewalls, and from precipitates of solutes transported in groundwater such as ferrous Fe (Sallade and Sims, 1997a; Nguyen and Sukias, 2002). Organic additions to ditches include particulate and dissolved organic matter from adjacent cultivated fields in surface and subsurface runoff and in situ deposition of plant, algal, microbial, and macroinvertebrate residues (Sallade and Sims, 1997a; Nguyen and Sukias, 2002).

Most large ditches on Maryland's Eastern Shore are cleaned out periodically, approximately once every 10 to 30 yr, or when a blockage occurs (Public Drainage Task Force, 2000). Clean-outs typically involve removing materials down to the original depth of the ditch by mechanical means and placing them in an adjacent cultivated field (Public Drainage Task Force, 2000). Smaller ditches may only be cleaned out when a blockage occurs and therefore may not be cleaned out on a regular basis. Recently, ditch managers have been applying targeted clean-outs to ditch areas experiencing blockages to avoid the detrimental environmental effects of full ditch clean-outs.

Previous investigators have described the organic and mineral materials in ditches as sediments (Sallade and Sims, 1997a, 1997b; Nguyen and Sukias, 2002). Describing these materials as sediments correctly identifies their origin but does not recognize whether they are functioning as soils. To meet the definition of a soil used in Soil Taxonomy, a body of material composed of minerals, organic matter, liquid, and gases (i.e., soil or sediment material) must have the ability to support rooted vegetation or must have been altered as a result of pedogenesis (Soil Survey Staff, 1999). If sections of the in-filled sediments and underlying residual soils in drainage ditches are functioning as soils, then their environmental function would be affected by their ability to support plants, by the formation of horizons, and through pedogenic processes. For example, if there are sections of a ditch with soils that function similarly to wetland soils, it would increase the ditch's nutrient cycling and retention capacity (Bowmer et al., 1994). The ability to support rooted plants may afford these ditch sections stability and sediment retention capacity exceeding non-soil sediments. Modeling of the hydrologic, physical, and biogeochemical processes operating in ditches may change dramatically if sections of the ditch are functioning as wetlands or subaqueous soils rather than as stream-bottom sediments. Management strategies could be designed to promote stability, such as through vegetation management, and encourage environmentally beneficial pedologic processes.

The objectives of this study were to evaluate if the materials within different types of agricultural ditches at the University of Maryland Eastern Shore (UMES) Research Farm meet the definition of a soil in Soil Taxonomy and to evaluate these materials through a pedologic framework to better understand their morphologic and chemical properties.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Area
This study was conducted on the UMES Research Farm (38°12'22'' N, 75°40'35'' W) located in Princess Anne, Somerset County, Maryland (Fig. 1 ). The farm is approximately 81 ha in size with >9 km of open-air agricultural ditches. The farm has a 30-yr history of poultry litter application as fertilizer. All ditches on the farm drain into the Manokin Branch that runs along the western boundary of the farm. Approximately 2 km downstream of the farm the Manokin Branch forms the Manokin River, which ultimately drains to the Chesapeake Bay. The research farm has an average elevation of 7 m above mean sea level with relatively flat relief. The farm receives an average of 1110 mm of rainfall per year. Annual temperatures average 13°C, with monthly averages ranging from 4 to 25°C (National Water and Climate Center, 2002). Crops grown on the farm are corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and wheat (Triticum aestivum L.).


Figure 1
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  		Fig. 1. Map showing the location of the University of Maryland Eastern Shore Research Farm, Princess Anne, MD.

 
The parent materials of field soils at the site are silt loam textured loess over sandy Atlantic Coastal Plain sediments. Field soils on the farm were mapped primarily as consociations named for the dominant soil series of Othello (fine-silty, mixed, active, mesic Typic Endoaquults), Mattapex (fine-silty, mixed, active, mesic Aquic Hapludults), Matapeake (fine-silty, mixed, semiactive, mesic Typic Hapludults), and Portsmouth (fine-loamy over sandy or sandy-skeletal, mixed, semiactive, thermic Typic Umbraquults) (NRCS, 2005; Matthews and Hall, 1966). The majority of field soils are poorly to very poorly drained and would not be agriculturally productive without artificial drainage. Much of the farm is underlain with a geogenic layer containing sulfides.

Data on past ditch management operations, including clean-outs, are not available at this site. Current ditch management techniques include the use of 5-m-wide grass buffer strips and vegetation management through mowing and the selective use of herbicides. The ditches have not been cleaned out since at least 1998. We were not able to obtain information about management practices on the UMES farm before 1998.

Agronomic soil fertility tests performed on composite samples from soils at the UMES Research Farm show that soil organic matter content ranged from 1.7 to 2.4% with a mean of 2.1%. The farm soils were acidic with a pH ranging from 5.3 to 5.8 and a mean of 5.6. The Mehlich III P mean was 511 mg kg–1, ranging from a low of 439 to a high of 583 mg kg–1. Iron extracted by Mehlich III had a range of 294 to 462 mg kg–1 with a mean of 401 mg kg–1. Sulfur as SO4–S ranged from 24 to 35 mg kg–1 with a mean of 35 mg kg–1.

Ditch Selection
A total of 10 ditches were selected for this study to reflect the diversity of ditch function, size, and hydrologic properties found on the UMES Research Farm (Fig. 2 ). The term primary ditch is used to describe ditches that drain surface runoff from cultivated fields and are connected to shallow groundwater sources. Primary ditches tend to be shallow (<1.5 m), often contain stagnant water until storm flow conditions are present, and are intermittently ponded. The term collection ditch is used to refer to ditches that function to transport flow from primary ditches. Two types of collection ditches were described: shallow collection and deep collection. Shallow-collection ditches are 1.5 to 2 m deep, connect to shallow groundwater, and dry out periodically. Deep-collection ditches are >2 m in depth, connect to deep, regional groundwater, and maintain stagnant or continuous flow throughout the year.


Figure 2
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  		Fig. 2. Overview map showing drainage ditch study area and soil profile description locations on the University of Maryland Eastern Shore Research Farm, Princess Anne, MD.

 
Seven primary, one shallow-collection, and two deep-collection ditches were included (Fig. 2). The ditches ranged in length from 225 to 550 m with a mean length of 325 m. Primary ditches located within fields generally were from 0.3 to 1 m in depth (mean = 0.5 m), while collection ditches ranged from 1.5 to 4 m (mean = 2 m). Five ditches were oriented in an east–west direction, while the remaining five ditches were oriented in a north–south direction. Ditches were identified using the letter D, as well as the letter X for primary field ditches, XXS for shallow-collection ditches, and XXD for deep-collection ditches. A numerical identifier (e.g., 1, 2, 3, ...) was used to distinguish between individuals of the same type (e.g., DX1, DX2, DXXS1, and DXXD1).

Field Methods
Ditch profile description sites were located within the center of each ditch using a measuring wheel with a wheel circumference of 1 m. Description sites were located at 40-m intervals starting from the outlet of each ditch. All descriptions were performed in the summer of 2004. Each profile was excavated to at least 1 m. A spade shovel was used to excavate the materials to a depth of approximately 40 cm; the remainder of the profile was excavated using a 7.6-cm bucket soil auger. Both continuously flooded (i.e., deep-collection) and dry (i.e., primary) ditches were sampled using this technique.

For each profile description, morphological characteristics were described including horizonation, horizon boundaries, structure, moist consistence (where applicable), and redoximorphic features, based on standard soil survey techniques (Schoeneberger et al., 2002). All profile descriptions were performed by the senior author. Color was described using a Munsell soil color chart in the field; field soil texture was determined by hand. Redoximorphic features were described as a percentage of the matrix by visual estimation (Schoeneberger et al., 2002). The presence of ferrous Fe was assessed in all ditch horizons using {alpha}-{alpha}' dipyridyl dye in neutral, 1 mol L–1 NH4OAc solution (Childs, 1981; Soil Survey Staff, 1999). Samples of at least 500 g of each identifiable horizon were collected where possible. For a few horizons, however, samples of <500 g were collected if the horizon was thin. Samples were placed into labeled plastic bags and packed in coolers for transport. Samples were air dried (25°C), coarse organic debris was removed, and the samples were crushed to pass a 2-mm sieve. Ditch profiles were classified according to Soil Taxonomy (Soil Survey Staff, 1999).

A total of 69 profiles were described (Fig. 2). An average of seven profiles were described in each ditch, with the fewest in DX8 (n = 1) and the largest number of descriptions in DXXD2 (n = 10). A subset of 21 profiles that included at least one profile from each ditch was chosen for further laboratory analyses.

In addition to the profile sampling described above, five samples of ditch materials were collected for S fractionation at 40 m from the outlet in ditches DX1, DX2, and DX3 and 80 m from the outlet in DX3 in September and October of 2004. Each of these sites were selected based on their dark color, indicating the possible presence of sulfides. Surface soil samples (0–5 cm) were collected by hand with a spade shovel and subsurface samples ranging in depth from 28 to 107 cm were collected using a 7.6-cm bucket auger. Samples were placed into plastic bags and placed on ice in a cooler. The samples were then brought back to the laboratory and stored frozen at –15°C until analysis.

Laboratory Methods
Particle-size analysis was performed by pipette (Gee and Bauder, 1986) and moist (never dried) pH was performed using a soil/water ratio of 1:1. Organic C was determined using a high-temperature CNS analyzer with an infrared detector (Bremner and Tabatabai, 1971). Acid volatile S (AVS) and Cr-reducible S (CRS) fractionations were measured utilizing the Johnson–Nishita apparatus (Cornwell and Morse, 1987). Acid-volatile S consists primarily of Fe monosulfides, while CRS includes pyrite, Fe monosulfides, and elemental S (Smith, 2004). The latter method specifically reduces different S fractions to H2S gas. The H2S gas was transported through the apparatus using an N2 gas carrier that was kept at a flow rate between 40 and 70 mL min–1 (Hussein and Rabenhorst, 1999). Approximately 30 mL of sulfide antioxidant buffer (SAOBII) was used to trap the H2S gas (Cornwell and Morse, 1987). Sulfur concentration in the SAOBII was determined by potentiometric titration with Pb(ClO4)2 using an AgS electrode along with a double-junction reference electrode for end-point detection. Acid volatile S was determined by digesting a sample (with an equivalent dry weight between 1 and 2 g) for 45 min using 30 mL of cold 6 mol L–1 HCl (Cornwell and Morse, 1987). The SAOBII trap was removed after 45 min of digestion for titration. Chromium-reducible S was determined from the same sample following further treatment by 10 mL of ethanol, 40 mL of reduced Cr solution, and 20 mL of concentrated HCl. The mixture was brought to a slow boil for approximately 1 h, after which the SAOBII trap was removed and potentiometrically titrated (Canfield et al., 1986).

Data Analyses
Classical descriptive statistics were used to describe trends in the distribution of individual variables. To compare characterization data between different horizons, eight horizon classes were defined based on morphologic and genetic characteristics (Table 1 ). Kolmogorov–Smirnov tests and descriptive statistics were used to assess normality. Statistical analyses were performed using S-Plus (Insightful Corporation, 2001) and the SAS GLM procedure (SAS Institute, 2004). The CONTRAST statement in the SAS GLM procedure was used to test the following preplanned one-way comparisons between the means of morphological horizon classes for all variables: Oi > A horizons (Dark A, Reduced A); Dark A > Reduced A horizons; A horizons > subsoil C horizons (Reduced C, Bright C, and Sulfidic C); Bright C > Reduced C horizons; and Sulfidic C > Reduced C horizons.


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  		Table 1. Morphological horizon class names and descriptions. Each horizon class is a group of soil horizons with similar morphological characteristics. These classes were used to compare the means of groups of similar horizons.

 

   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Morphology
Materials at all sites supported rooted vegetation except for some thin bands of unvegetated materials in the center of the outlets of some deep ditches; these bands were overlain with organic debris and fibric materials. Ditch materials were found to contain layers that followed a sequence consistent with soil horizons (i.e., O horizons overlying A horizons overlying BC and C horizons) (Tables 2 and 3 ). For ease of reporting the results of profile descriptions, layers will be discussed in terms of soil horizons, although their equivalency was not presumed. Horizonation in general was found to include A horizons formed in recent alluvium over BC and C horizons formed from fluvial deltaic and marine sediments (Tables 2 and 3). The exceptions were nine profiles that contained an Oi horizon at the surface and three profiles described as C horizons throughout.


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  		Table 2. Profile description of a typical drainage ditch soil located in a deep-collection ditch at the University of Maryland Eastern Shore Research Farm. This profile is a coarse-loamy, mesic Humaqueptic Endoaquent.

 

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  		Table 3. Profile description of a typical drainage ditch soil located in a primary ditch at the University of Maryland Eastern Shore Research Farm. The classification of this profile is a coarse-loamy over sandy or sandy-skeletal, mesic Humaqueptic Psammaquent.

 
Ditch A horizons were, in general, dark in color (Tables 2 and 3). The hue of 90% of A horizons was 10YR. Color values ranged from 1 to 5 with a mean of 3.3. Chromas ranged from 1 to 4 with a mean of 1.8. Ditch C horizons were lighter in color than the A horizons. Hues in C horizons was primarily 10YR (64%) or 2.5Y (29%). Color values ranged from 2.5 to 8 with a mean of 5.0. Chromas ranged from 1 to 8 with a mean of 2.0.

Thirty-seven percent (69 of 185) of A horizon matrices were depleted (value 4 or more, chroma 2 or less) in color, only six of which were found at the soil surface. The mean depth at which reduced A horizons first appeared was 9 cm. Sixty-eight percent of ditch C horizon matrices were depleted. Redoximorphic features (concentrations and depletions) were found in 41% of ditch A horizons. Concentrations of Fe as soft masses, pore linings, and coatings were the only redoximorphic concentration types observed; Mn concentrations were not observed. Of the 76 A horizons described with redoximorphic features, 20 horizons (26%) contained at least two types of features and seven horizons (9%) contained all three types of features. Mean depth to first Fe concentration was 8 cm. Only six of 56 surface horizons contained redoximorphic features, four of which were described as having Fe concentrations along pore linings. Concentrations within all A horizons (nondepleted and depleted matrices) ranged from 1 to 10% of the matrix with a mean of 3%. Munsell hues of concentrations were primarily 10YR (55%) and 7.5YR (25%). Munsell values of Fe concentrations ranged from 3 to 7 with a mean of 4.0, and Munsell chromas ranged from 4 to 8 with a mean of 5.5. Iron concentrations were predominantly fine to medium (<5 mm) in size (90%), distinct in contrast (65%), and were described as masses and pore linings (90%). Only 23% of all ditch A horizons (undepleted and depleted matrices) had depletions. Depletion Munsell color hues were mostly 10YR (74%), with values ranging from 4 to 7 with a mean of 5.6 and chromas ranging from 1 to 2 with a mean of 2. Depletions were primarily medium (<5 mm) (74%) in size and faint to distinct in contrast (Schoeneberger et al., 2002).

Sixty-three percent of ditch C horizons contained redoximorphic features, of which 21% contained at least two types (soft masses, pore linings, or coatings). Redoximorphic features in C horizons were described primarily as concentrations of Fe (73%), which ranged from 1 to 40% of the matrix with a mean of 5%. Hues of Fe concentrations were primarily 10YR (67%) or 7.5YR (19%). Values ranged from 2 to 7 with a mean of 5 and chromas ranged from 4 to 8 with a mean of 6. The majority of Fe concentrations were medium (66%) in size and prominent (70%) in contrast; they were found as soft masses of Fe (97%) or pore linings or ped faces (3%). Depletions accounted for 17% of the redoximorphic features described in C horizons. Depletion hues were mainly 10YR (51%), while values ranged from 4 to 8 with a mean of 6.1 and chromas ranged from 1 to 2 with a mean of 2.0. In zones where they occurred, Fe depletions were generally medium (81%) in size and were faint (32%), distinct (32%), or prominent (35%) in contrast.

Soil structure was present in 75% of A horizons (Tables 2 and 3). In 68% of A horizons, soil structure was found to be of weak grade with 52% of the structure as subangular blocky and 48% as granular. In the 25% of A horizons that were structureless, 55% were single grained (sandy), while 45% were massive. Note that while n-value data were not systematically collected, most massive, structureless A horizons had n values >0.7. Ditch C horizons were dominantly structureless (97%) and single grained (90%).

The mean solum thickness in all ditches was 27 cm. The maximum thickness of the mineral sola in all 10 ditches was 78 cm (in profile DXXD2–6; Table 4 ). Primary ditches contained the shallowest sola (mean = 23 cm); the deepest sola were found in deep-collection ditches (mean = 32 cm; Table 4).


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  		Table 4. Depth of alluvium (A horizons), depth to the uppermost horizon containing sulfidic materials (where present), and depth to the top of the oxidized C horizon (where present) in soil profiles described in drainage ditches located at the University of Maryland Eastern Shore Research Farm.

 
Roughly 36% (25 of 69) of the profiles described contained sulfidic materials as defined by Soil Taxonomy (Table 4; Soil Survey Staff, 1999). Sulfidic materials were recognized in a total of 42 horizons. Of those, 93% were C horizons and 7% were either A or BC horizons. The mean depth to sulfidic materials was relatively shallow (64 cm; Table 4). Two profiles in DXXD3 contained sulfidic materials within 25 cm of the surface and one profile in DX3 contained sulfidic materials within 28 cm of the soil surface.

Characterization
Overall, pH values ranged from 2.6 to 6.1 with a mean of 4.7 and a standard deviation of 0.7. The mean pH of individual classes of horizons ranged from a low of 3.9 for surface C horizons to a high of 5.3 for organic Oi horizons (Table 5 ).


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  		Table 5. Chemical and physical characterization data for morphological horizon classes from samples collected at the University of Maryland Eastern Shore Research Farm.

 
The A horizons were generally found to be loamy in texture, with 46% being sandy loams and loams and 13% being clay loams and sandy clay loams. Sand concentrations of surface A horizons ranged from 96 to 954 g kg–1 with a mean of 540 g kg–1 (Table 5). The sand fraction was generally coarse and medium sands. Fine and very fine sands comprised a mean of 15% of the sand fraction (Table 5). Silt concentrations ranged from 39 to 790 g kg–1 with a mean of 283 g kg–1. Clay concentrations of the A horizons ranged from 19 to 423 g kg–1 with a mean of 178 g kg–1.

Ditch C horizons were generally coarser textured than A horizons (Table 5) and were dominated by very gravelly sands, gravelly sands, and sands (54%). Gravelly loamy sands and loamy sands accounted for 28% of C horizons (Table 5). Sand concentrations ranged from 707 to 975 g kg–1, with a mean of 877 g kg–1. Medium and fine sands comprised more than half (51%) of the sand fraction. Silt concentrations ranged from 4 to 235 g kg–1 with a mean of 73 g kg–1. Clay concentrations of the C horizons ranged from 13 to 119 g kg–1 with a mean of 50 g kg–1.

Organic C concentrations ranged from 0.4 to 124 g kg–1 (mean = 24 g kg–1) in all soil horizons analyzed. Note that coarse organic debris was removed before analysis, so these values underestimate total organic C, particularly in Oi horizons. Ditch A horizons had greater organic C concentrations than did C horizons (Table 5). Morphological horizon classes Dark A and Oi had significantly greater mean organic C concentrations than the other horizon classes (P ≤ 0.0001). Dark A horizons were found to have a greater mean organic C concentration than did Reduced A horizons (P < 0.05; Table 5). Surface C horizons had a lower mean organic C concentration than did Dark A, Reduced A, and Oi horizons (Table 5).

Acid volatile S concentrations of the four surface samples analyzed (0–5 cm) ranged from 370 to 1300 mg kg–1 and CRS concentrations ranged from 300 to 750 mg kg–1 (Table 6 ). The two samples taken at depth (28–107 cm) had AVS concentrations of 26 and 39 mg kg–1 and CRS concentrations of 1500 to 2000 mg kg–1 (Table 6).


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  		Table 6. The total acid volatile S (AVS) and chromium-reducible S (CRS) in selected drainage ditch soil samples from the University of Maryland Eastern Shore Research Farm.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Taxonomy Definition of a Soil
As defined in Soil Taxonomy, a soil is a body of minerals, organic matter, liquid, and gases that is able to support rooted vegetation in a natural environment or that is characterized by horizons or layers that are distinguishable from the initial material as a result of pedogenesis (Soil Survey Staff, 1999). The bodies of material in the agricultural ditches on the UMES Research Farm supported rooted vegetation and therefore meet the definition of a soil as defined in Soil Taxonomy. The rooted vegetation present in these soils indicates that they are stable and the overlying waters are not too deep to prevent the penetration of light for plant growth. Although it is not necessary for both portions of this definition to be met to be considered a soil, it remains a question whether these materials have formed horizons through pedogenesis. Also, the thin bands of unvegetated materials present in some ditches would only be soils if pedogenically derived horizons were present. Samples of initial material were not collected, therefore we can only speculate whether the characteristics of these materials were present in the initially deposited material or whether they have formed through pedogenesis. We speculate that the following pedologic processes are operational in these soils: organic matter accumulation and decomposition, gleization and other redox Fe transformations, soil structure formation, sulfuricization, sulfidization, translocations, and bioturbation. We also discuss the role of fluvial processes in these soils, which are both geomorphological and pedological in nature.

Pedogenic Processes
Organic Matter Accumulation and Decomposition
The organic C concentration of ditch Oi and surface A horizons appears to be enriched in organic matter relative to organic matter concentrations observed in field soils at the farm (Table 5). Organic matter enrichment is expected in saturated soils due to depressed decomposition and high net primary productivity. Elsewhere on the Delmarva Peninsula, Sallade and Sims (1997a) reported organic matter concentrations of ditch materials (0–5 cm) averaging 80 g kg–1, which is comparable to values observed in ditch surface horizons in the current study.

The organic matter enrichment observed in these soils may also have been the result of the enrichment of organic matter in eroded sediment and the erosion of macroorganic matter (agricultural residues). It is unlikely, however, that the enrichment ratios could be great enough to account for these differences (particularly since coarse organic debris was removed from ditch samples): the mean organic matter concentrations in the field soils was 21 g kg–1 while the mean C concentrations in the ditch soils were 40 g kg–1 for Oi horizons, 51 g kg–1 for Dark A horizons, and 33 g kg–1 for Reduced A horizons. Also, the distinguishable organic matter present in ditch Oi horizons was primarily nonagricultural in origin. Therefore, while we cannot prove that there is organic matter enrichment in these ditches without an initial sample, we can be confident in our speculation that these horizons have been enriched in place. The Oi horizons were present on the unvegetated bands present in some ditches; therefore it appears that these materials also meet the definition of a soil as defined in Soil Taxonomy (Soil Survey Staff, 1999).

Structure Formation
The granular and subangular-blocky structure observed in the ditch soils are not generally present in deposited alluvium and thus we can be confident that this structure has formed in place. Structure is sometimes observed in alluvial sediments, but it is generally in the form of large prisms or platy structure parting along planes of deposition. When present, the structure of ditch soils ranged from weak to moderate. Soil structure was best expressed in the A horizons of primary and shallow-collection ditches. Soil structure was rare in C horizons or deep-collection ditches. The development of soil structure in ditches is presumably the result of wetting and drying cycles as well as aggregation caused by plant roots and soil fauna (Fanning and Fanning, 1989). The high n value in deep-collection ditches, which is common in silty and loamy subaqueous soils, suggests permanent saturation by a regional groundwater (Demas and Rabenhorst, 1999).

Gleization and Other Redox Iron Transformations
Evidence of gleization, oxidation, and translocation of Fe was found in ditch surface horizons and subsurface horizons in the form of Fe concentrations, depletions, and depleted matrices. These characteristics would not be present in initially deposited material; therefore we can be certain that they have formed in place. Positive reactions to {alpha}'-{alpha} dipyridyl dye in selected ditch horizons indicated the presence of Fe2+ (Childs, 1981; Soil Survey Staff, 1999). With in situ decaying plant matter acting as a source of oxidizable organic matter for microbes, seasonally stagnant water conditions, and warm temperatures, low redox potentials are expected (McCoy et al., 1999).

The presence of a bright C horizon with visually appreciable amounts of oxidized Fe and a matrix value of 5 or more and a chroma of 3 or more was described in 18 ditch profiles (Table 4). This horizon was found in both primary and shallow-collection ditches at roughly 1 m. This horizon was also observed in field soils adjacent to primary and shallow-collection ditches at approximately 1 m in depth and within 40 m of the ditch (Vaughan, 2005). Research in North Carolina indicated that in a period of <30 yr, ditching can alter the morphology of surrounding soils by significantly increasing the quantity of concentrations within 7 m of ditches (Hayes and Vepraskas, 2000). Fluctuating water tables in primary and shallow-collection ditches on the UMES farm may be producing this hydromorphology. The connection of deep-collection ditches to deep groundwater prevents their drying out and subsequent oxidation, thereby preventing the formation of this horizon. A second explanation for the formation of this horizon is the high amounts of Fe released during the oxidation of sulfidic materials at depth. The oxidation of pyrite in soils can yield substantial amounts of Fe2+, which may move up the profile through wicking and diffusional processes (Fanning et al., 2002). This could occur as the shallow groundwater fluctuates seasonally. In the summer, as the groundwater table is lowered due to higher evapotranspiration rates, sulfidic materials may become oxidized as the ditches dry out. In winter when evapotranspiration rates are at their lowest levels during the year, groundwater containing the dissolved Fe2+ may rise toward the surface and precipitate at the shallow groundwater boundary.

Sulfidization and Sulfuricization
In addition to dark gray geological sulfidic materials found at depth in ditch profiles at UMES, Fe monosulfides were also observed (Fanning et al., 2002). Monosulfides are not present in field soils; therefore we can be certain that they have formed in place. The monosulfides were black (N 2.5/1) in color and found within or on top of ditch mineral soil surfaces, often intermixed with coarse organic debris. The formation of monosulfides as a thick black ooze, also termed monosulfidic black ooze, has been documented in ditches cut into sulfidic materials in areas of Australia (Bush et al., 2004; Smith, 2004). Monosulfides were identified in several ditches in the fall of 2004 and confirmed through AVS and CRS fractionations (Table 6). The ditches contained roughly 6 to 10 cm of water at the time of sampling. Iron monosulfides were not described on ditch soil surfaces in the summer of 2004; during this time, the ditches were dry and presumably any monosulfides had been oxidized (sulfuricization).

One possible source of S in ditches at UMES is the geologic deposits of sulfidic materials from which sulfate may be wicking up the soil profile via fluctuations in shallow groundwater. The origin of this geologically deposited sulfidic material is thought to be a past marine transgression. The last marine transgression is thought to have occurred either 82,000 or 125,000 yr BCE (Toscano and York, 1992; Groot and Jordan, 1999; Wah, 2003). The depth to sulfidic materials within these ditches ranged from 15 cm in DXXD3–6 to 151 cm in DX5–3 (Table 4). Before the construction of ditches at the site, the soils were poorly drained and existed presumably in an anaerobic environment, preserving the Fe sulfides. Upon construction of the ditches and subsequent land drainage, the sulfidic materials were subjected to fluctuating redox environments, leading to the oxidation of pyrite and the formation of FeSO4 and H2SO4. Sulfate can be transported with the soil solution or by wicking to the surface (Fanning et al., 2002).

A second possible sulfate source is poultry litter, which has been applied as fertilizer at UMES for >30 yr. A common manure application rate in this region is about 6.7 Mg ha–1 (moist) (F.J. Coale, personal communication, 2004). In 2001, the mean S concentration of all poultry manure (moist) tested (with floor litter; n = 758) at the Maryland Cooperative Extension testing lab was 5.9 g kg–1 (Maryland Cooperative Extension, 2001). At this application rate and litter S concentration for 30 yr, approximately 1200 kg ha–1 S would have been applied to soils on the farm (?40 kg S ha–1 yr–1). Because SO42– is highly susceptible to leaching through soils, with time, considerable quantities of S could migrate from field soils to ditches via shallow groundwater. Other processes by which S in litter could contribute to ditch S content include the erosion and transport of poultry litter from adjacent fields after application to ditches, as well as the direct application of poultry litter to ditches due to the absence of buffers between field edges and ditches.

Translocations and Bioturbation
Evidence of translocations and bioturbation were found in ditch soils at UMES. The high quantity of redoximorphic features described in ditch soils suggests the translocation of Fe2+ through ditch soils. Possible sources of soluble Fe2+ may be from nearby shallow groundwater inputs and from ditch soils themselves. Evidence of bioturbation by macrofauna such as crawfish and other invertebrates was observed in all ditch soils.

Fluvial Processes
Cumulization is the process whereby additions of material are made to the soil surface through hydrologic transport mechanisms and retained within the soil through entrainment and soil characteristics associated with erosion resistance (Fanning and Fanning, 1989). Sources of mineral material in ditches on the UMES Research Farm include the erosion of finer textured materials from adjacent cultivated fields and the erosion of ditch banks. Precipitation of solutes from groundwater inputs, such as Fe(II), may also occur. Profile descriptions revealed that the total quantity of alluvial materials accumulated in ditches varied considerably (Table 4). The thickness of a solum is dependent on factors such as the time from last clean-out, drainage water velocity, cultivation and management practices (e.g., vegetated buffer strips, cover crops, no-till), slope within the ditch and adjacent land, and erosion potential of adjacent field soils.

Ditch A horizon soil textures varied between ditch types. Primary ditch sola mean sand, silt, and clay contents were 560, 280, and 160 g kg–1, respectively. The shallow-collection ditch was higher in sand relative to primary ditches, with mean sand, silt, and clay contents of 780, 150, and 80 g kg–1, respectively. Deep-collection ditches were similar to the shallow-collection ditch, with a mean sand content of 720 g kg–1 and a mean silt content of 180 g kg–1. Differences in textures between ditch types (primary vs. deep collection and shallow collection) are probably the result of water velocity and the relationship between the ditch depth and the textures of the substratum. Higher water velocities are generated in collection ditches as a result of increased water inputs during storm events from primary ditches. These events may promote scouring and the suspension of fine mineral particles. The coarser texture of deeper ditches may also be a function of contrasting ditch banks. Soils at UMES are underlain by a sandy substratum. When collection ditches (>1.5 m) are constructed, it results in the exposure of these sandy soils on their banks. These materials are not as exposed in the primary ditches. As erosion and slumping of collection ditch banks occurs, sandy-coarse materials are deposited onto ditch soils in the deeper ditches.

Classification of Ditch Soils
The primary geomorphic and morphological contrasts between ditch soils in the study were between soils formed in shallow primary and collection ditches vs. those formed in deep ditches. Ditch soils formed in shallow ditches tended to have structure and a layer in the substratum with a bright matrix color and were morphologically similar to wetland soils. Soils formed in deep-collection ditches tended to have high n values, structureless sola, and subsoil horizons that were depleted in color. These soils were morphologically similar to subaqueous soils.

Categorization based on Soil Taxonomy did not lead to consistency within ditches (Table 7 ). All ditch soil profiles described were classified to the suborder level as Aquents. Endoaquents accounted for 70% of all profiles, with the subgroups being Sulfic (20%), Aeric (20%), Humaqueptic (20%), and Typic (9%) (Table 7). Family particle-size classes were fine-loamy, fine-loamy over sandy or sandy skeletal, coarse-loamy, coarse-loamy over sandy or sandy skeletal, and sandy (Table 7).


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  		Table 7. Family particle-size class and Great Group presented as the number of soil profiles described in each taxonomic class per ditch from the University of Maryland Eastern Shore Research Farm.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The materials found within sections of ditches at the UMES Research Farm meet the definition of a soil in that they are able to support rooted vegetation. There is also evidence that these materials have changed from the initial material through pedogenic processes including organic matter accumulation and decomposition, gleization and other redox Fe transformations, soil structure formation, sulfuricization, sulfidization, translocations, and bioturbation. Fluvial processes operating in these systems also may be influenced by soil characteristics such as sediment entrainment and erosion resistance.

These ditch soils are young and dynamic natural soil bodies that are undergoing pedogenesis and occur in both seasonally and permanently saturated ditch environments. Surface and subsurface pedogenic properties and processes may affect ditch environmental function. Plants may serve a variety of ecosystem services such as sediment entrainment, nutrient uptake and release, and biocycling, and may serve as a C source for ditch biological communities. The presence of organic horizons at the soil surface may influence exchanges between ditch materials and overlying waters. Fibric materials may serve as important habitat for macroinvertebrates and other organisms. Organic matter transformations and redox transformations of Fe and S may influence the nutrient and contaminant sorption characteristics of these and underlying horizons. Organic enrichment of ditch O and A horizons may serve to stabilize soils, sequester nutrients and pollutants, and stabilize soil structure. Structural development may reduce sediment losses and increase infiltration and hydraulic conductivity rates. Pedoturbation of ditch soils may serve to transport materials enriched in nutrients and pollutants from the surface to the subsurface. The presence of soils in ditches may be an important variable controlling ditch function. If so, then the management and modeling of ditches would need to integrate an understanding of the role of pedologic processes.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication March 6, 2006.


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





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