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Published online 24 October 2007
Published in J Environ Qual 36:1895-1903 (2007)
DOI: 10.2134/jeq2006.0488
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Wetlands and Aquatic Processes

Vertical Distribution of Phosphorus in Agricultural Drainage Ditch Soils

Robert E. Vaughana, Brian A. Needelmana,*, Peter J.A. Kleinmanb and Arthur L. Allenc

a Dep. Environmental Science and Technology, Univ. of Maryland, 1109 H.J. Patterson Hall, College Park, MD 20742
b USDA-ARS, Pasture Systems and Watershed Management Research Unit, University Park, PA 16802
c Dep. of Agriculture, Univ. of Maryland Eastern Shore, Princess Anne, MD, 21853

* Corresponding author (bneed{at}umd.edu).

Received for publication November 8, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Pedological processes such as gleization and organic matter accumulation may affect the vertical distribution of P within agricultural drainage ditch soils. The objective of this study was to assess the vertical distribution of P as a function of horizonation in ditch soils at the University of Maryland Eastern Shore Research Farm in Princess Anne, Maryland. Twenty-one profiles were sampled from 10 agricultural ditches ranging in length from 225 to 550 m. Horizon samples were analyzed for total P; water-extractable P; Mehlich-3 P; acid ammonium oxalate-extractable P, Fe, and Al (Pox, Feox, Alox); pH; and organic C (n = 126). Total P ranged from 27 to 4882 mg kg–1, Pox from 4 to 4631 mg kg–1, Mehlich-3 P from 2 to 401 mg kg–1, and water-extractable P from 0 to 17 mg kg–1. Soil-forming processes that result in differences between horizons had a strong relationship with various P fractions and P sorption capacity. Fibric organic horizons at the ditch soil surface had the greatest mean Pox, Feox, and Alox concentrations of any horizon class. Gleyed A horizons had a mean Feox concentrations 2.6 times lower than dark A horizons and were significantly lower in total P and Pox. Variation in P due to organic matter accumulation and gleization provide critical insight into short- and long-term dynamics of P in ditch soils and should be accounted for when applying ditch management practices.

Abbreviations: Alox, acid ammonium oxalate-extractable Al • DPS, percent degree of P saturation • Feox, acid ammonium oxalate-extractable Fe • M3P, Mehlich 3-P • Pox, acid ammonium oxalate-extractable P • Smax, Langmuir P sorption maximum • TP, total P • UMES, University of Maryland Eastern Shore • WEP, water-extractable P


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
THE degradation of fresh and estuarine waters in the USA as a result of eutrophication is a major environmental concern. In the year 2000, 11% of 22,000 surface waters identified as impaired by the USEPA were the result of agricultural N and P (USEPA, 2003). The largest estuary in the USA, the Chesapeake Bay, has experienced the effects of eutrophication for more than 30 yr, and it has been the focus of intensive research and monitoring efforts (Boesch et al., 2001). Despite coordinated long-term efforts to curb nutrient inputs into the Bay, the ecological, economic, and social impacts of eutrophication remain an increasing concern in the Chesapeake Bay watershed (Boesch et al., 2001).

In humid regions, poorly drained soils often require land drainage systems for profitable agricultural production (Shirmohammadi et al., 1995; Janse and Van Puijenbroek, 1998). Land drainage systems are designed to lower water tables and speed the removal of excess surface runoff to local streams and water bodies. Open-air drainage ditches are a hydrological link between surface runoff, ground water, and surface waters (Janse and Van Puijenbroek, 1998). Given the many flow paths they intercept and their proximity to agricultural fields, ditches have the potential to act as key conduits for the export of nutrients from areas of intensive agriculture to surface waters (Sallade and Sims, 1997a; Vadas and Sims, 1998; Nguyen and Sukias, 2002). There is increasing interest in the management of ditches to mitigate nutrient loss from agroecosystems (Needelman et al., 2007a).

Throughout the world, land drainage has been adopted for multiple uses. Land drainage systems are thought to have been first developed 9000 yr ago in Mesopotamia and by the Egyptians and Greeks (van Schilfgaarde, 1971; Shirmohammadi et al., 1995). In the USA, organized drainage began around the 1600s (Evans et al., 1996). From 1900 to 1985, the installation and use of surface and subsurface drainage in USA agriculture expanded greatly (Pavelis, 1987; Shirmohammadi et al., 1992; Shirmohammadi et al., 1995). The increase in land drainage has led to thousands of miles of open-air ditches around the country. Currently in the USA, many states rely on land drainage, in particular subsurface drainage (i.e., tile drainage) and surface drainage (i.e., open-air ditches), to control ground water levels in agricultural and urban areas. Thirty-seven percent (20.6 million ha) of arable land in the Midwest USA requires drainage for production purposes (Fausey et al., 1995). Ditches in the coastal plain of North Carolina have drained roughly 800,000 ha (Evans et al., 1996). Approximately 2.5 million ha of land in Florida are affected by artificial land drainage (Thomas et al., 1995).

The poultry industry of the southern Delmarva Peninsula produced more than 560 million broiler birds and more than 1.3 Tg of chicken in 2004 (Delmarva Poultry Industry, 2005). As a result, large amounts of poultry litter are produced each year on the Delmarva Peninsula; most is land applied as fertilizer for crops (Sims and Kleinman, 2005). Poultry litter has a low N to P ratio and is commonly applied in excess of crop P requirements (Sharpley, 1999). The continual application of poultry litter in excess of crop needs leads to surplus soil P and increased potential for nonpoint source P loss (Sharpley, 1999).

Investigations of nonpoint-source P loss from agricultural watersheds have primarily focused on overland flow mechanisms (e.g., surface erosion and runoff) (Sims et al., 1998). Consequently, the potential for P loss through subsurface transport to local surface waters has sometimes been underestimated (Sims et al., 1998). Subsurface transport of P requires vertical translocation of P though the soil profile and lateral flow that can effectively transfer leached P to surface water. The most significant instances of downward movement of P through the soil profile have been associated with excessive application of P in manure and fertilizer (Sims et al., 1998). A growing number of studies in regions with intensive animal agricultural production have shown the potential for subsurface soil P leaching and losses to shallow ground water in field soils. Mozaffari and Sims (1994) found that environmentally significant quantities of P had leached to depths near 75 cm in soils of a Delaware watershed with frequent applications of poultry litter. In ditch-drained systems, P reaching shallow ground water may move laterally to ditches between and during storm events and may constitute a significant transport pathway in these systems. The interaction of this P-laden ground water with ditch soils and sediments may influence surficial water quality.

When sufficiently stable to support rooted vegetation and to allow for the formation of horizons through pedogenesis, ditch sediments may form into soils (Vaughan et al.,2008). Key pedological mechanisms operating in ditch soils include the accumulation and humification of soil organic matter and the redox (oxidation–reduction) cycling of Fe and S (Vaughan, 2005). The formation of organic-rich horizons at the ditch soil surface may influence soil–water interactions at this interface (Needelman et al., 2007b). For example, P sorption capacity may be increased through the formation of the poorly crystalline Fe oxide mineral ferrihydrite under Fe redox cycling in soils with high organic matter concentrations (Bigham et al., 2002; Needelman et al., 2007b). Due to the acidic nature of soils found in the Atlantic Coastal Plain, ditch soil P is primarily found sorbed to or occluded by Fe and Al hydroxides or as organic P (Vadas and Sims, 1998). Loss of Fe through gleization (the dissolution and leaching of ferrous Fe) may also reduce P sorption capacity in ditch soils (Reddy et al., 1995; Sallade and Sims, 1997a; Sallade and Sims, 1997b; Vadas and Sims, 1998; Nguyen and Sukias, 2002; Needelman et al., 2007b). However, prolonged reducing conditions may lead to the precipitation of the ferrous-phosphate mineral vivianite (Fe3(PO4)2·8H2O), which has been documented in soils and sediments (Lindsay et al., 1989; Harris et al., 1994; Harris, 2002). Additional mechanisms controlling P uptake and release in ditches include the sedimentation and resuspension of organic matter and P-enriched soil particles and biotic cycling by plants and microorganisms (Johnston et al., 1997; Needelman et al., 2007b).

The focus of investigations regarding P in soils and sediments of open-air ditches has been limited to surficial layers (Sallade and Sims, 1997a; Sallade and Sims, 1997b; Nguyen and Sukias, 2002; Vaughan et al., 2007). These studies did not address the role of deeper layers in P transport and retention. Ditch surficial soils are the most likely to interact chemically with overlying drainage waters. However, shallow, lateral, subsurface flow pathways may bring ground water into contact with deeper ditch soil horizons, and inter-event diffusion and pedoturbation processes may mix P between surficial and deeper layers.

The objective of this study was to interpret the vertical distribution of P fractions and P sorption capacity in ditch soils at an Atlantic Coastal Plain farm in the context of ditch soil morphology.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Study Area
This study was located in Princess Anne, Somerset County, Maryland on the University of Maryland Eastern Shore (UMES) Research Farm (38°12'22''N, 75°40'35''W) (Fig. 1 ). The 81-ha farm has an approximately 30-yr history of poultry litter application as fertilizer. The farm has an average elevation of 7 m above mean sea level and relatively flat relief. Annual rainfall averages 1110 mm. Mean annual temperature is 13°C, with monthly means ranging from 4 to 25°C (USDA-NRCS, 2006).


Figure 1
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  		Fig. 1. Overview map showing drainage ditch study area and profile sampling locations at the University of Maryland Eastern Shore Research Farm, Princess Anne, MD.

 
We described three categories of ditches on the UMES Research Farm: primary, shallow-collection, and deep-collection. The term "primary ditch" describes those ditches that directly drain surface runoff from cultivated fields and are connected to shallow ground water sources. Primary ditches tend to be shallow (<1.5 m deep), often contain stagnant water under base flow conditions, and are intermittently dry. The term "collection ditch" applies to those ditches that function to transport flow from primary ditches or from an aggregation of primary and collection ditches. Two types of collection ditches were observed: shallow (1.5- to 2.0-m deep) and deep (>2.0 m deep). As with primary ditches, shallow-collection ditches are connected to shallow ground water and dry out periodically (Vadas et al., 2007). Deep-collection ditches are connected to regional ground water supplies, and they have stagnant or continuous water flow throughout the year (Vadas et al., 2007).

Parent materials of the cultivated field soils at the UMES Research Farm are silt loam loess over sandy Atlantic Coastal Plain sediments, with field soils classified as Endoaquults, Umbruaquults, and Hapludults (USDA-NRCS, 2005; Matthews and Hall, 1966). Agronomic soil testing of composite samples from fields between the ditches indicated that the soils are acidic, with pH ranging from 5.3 to 5.8 with a mean of 5.6, and are high in P with a mean Mehlich-3 P concentration of 511 mg kg–1, ranging from 439 to 583 mg kg–1. Iron extracted by the Mehlich-3 extractant had a range of 294 to 462 mg kg–1 for the field soils with a mean of 401 mg kg–1.

Ditch Soils
We refer to the materials in the ditches at the UMES Research Farm as "ditch soils" because these materials support rooted vegetation and have formed horizons through pedogenesis, thereby meeting the definition of a soil (Soil Survey Staff, 1999; Vaughan et al.,2008). The parent material for these soils is recently deposited alluvial sediments (A horizons) over the truncated original Coastal Plain sediments (C horizons). All ditch excavations removed any B horizons in the original soils. Pedogenic horizons include Oi horizons at the soil surface, A horizons formed in the recent alluvium, and horizons showing hydromorphology, including gleyed A horizons in the recent alluvium and horizons with redoximorphic features in the Coastal Plain sediments. Key pedological processes evident in ditches include organic matter accumulation and humification, structure formation, Fe oxidation and reduction, sulfuricization, sulfidization, translocations, and bioturbation. The A horizons are generally loamy due to their parent material of eroded loess-derived topsoil, although small segments of the ditch network have sandier A horizons. The C horizons are dominated by very gravelly sands, gravelly sands, and sands due to their Coastal Plain sediment parent material. Primary and shallow-collection ditch soils tended to have surface horizons with soil structure (granular and subangular blocky) and subsoil horizons with both bright and gleyed matrix colors. Deep collection ditch soils (1.5–4 m) tended to have high n values (fluid-like rupture resistance; Soil Survey Division Staff, 1993), structureless sola, and gleyed subsoil horizons (Soil Survey Division Staff, 1993). Iron monosulfides, known as monosulfidic black ooze (Smith, 2004), are commonly observed underlying and within the Oi horizon in these soils, particularly after periods of saturation. Much of the farm is underlain by a geological deposit of sulfidic materials, containing primarily pyrite. Drainage of this landscape leads to oxidation of these sulfidic materials with an accompanying release of sulfuric acid and ferrous Fe.

The ditches had not been cleaned out (dredged) since at least 1998 and are managed with mowing and herbicide application to restrict woody vegetation. We do not have access to data on the previous management history of these ditches. It is common in the region to clean out ditches to the depth of original excavation approximately every 10 to 20 yr.

Sampling
Ditch soils were sampled during the summer of 2004 from the midpoint of ditches at 40-m intervals as demarcated from the downstream intersection of the ditch. Soils were excavated to a depth of approximately 40 cm using a spade shovel and then to 1 m using a 7.6-cm bucket soil auger. This sampling methodology was used for ponded (i.e., deep-collection) and nonponded (i.e., primary and shallow-collection) ditches.

At each location, soil morphology was described based on standard soil survey techniques (Schoeneberger et al., 2002). All profile descriptions were performed by the first author. Samples were collected by horizon, placed into plastic bags, and packaged in coolers for transport. Samples were quickly (<5 d) air-dried (25°C), coarse organic debris was removed, and samples were ground to pass a 2-mm sieve. A total of 69 profiles were sampled from 10 ditches. A subset of 21 profiles was selected for further analysis comprising for a total horizon sample number of 126.

Laboratory Methods
Soil pH was determined before air-drying using a soil (moist) to water ratio of 1:1. Particle-size analysis was performed by the pipette method (Gee and Bauder, 1986). Acid ammonium oxalate-extractable Al, Fe, and P (Alox, Feox, and Pox) were extracted with 0.1M (NH4)2C2O4·H2O + 0.1M H2C2O4·2H2O (soil:solution = 1:40) and measured by inductively coupled plasma atomic emission spectroscopy (ICP–AES) (Ross and Wang, 1993). The degree of P saturation (DPS) was estimated from acid ammonium oxalate-extractable Alox Feox, Pox as:

Formula 1[1]
where Alox, Feox, and Pox are in mmol kg–1 (Breeuwsma and Silva, 1992). Ammonium oxalate–extractable Mn analysis was performed on a subset of samples, but only trace quantities were found (data not shown).

Mehlich-3 P (M3P) extractions were conducted by shaking 2.5 g of soil in 25 mL of Mehlich-3 solution (0.2 N CH3COOH + 0.25 N NH4NO3 + 0.015 N NH4F + 0.013 N HNO3 + 0.001 M EDTA) for 5 min (Mehlich, 1984). The supernatant was filtered through a Whatman #1 paper, and the filtrate was analyzed for P by a modified version of the colorimetric method of Murphy and Riley (1962), with a spectrophotometer wavelength of 712 nm. Water-extractable P (WEP) analyses were performed using a deionized water extraction of 2.0 g of soil in 20 mL of distilled water (soil:solution = 1:10) for 1 h followed by colorimetric analysis of the filtered extract (Kuo, 1996). Total P (TP) was analyzed by a modified semimicro-Kjeldahl procedure with P in digests determined colorimetrically (Patton and Kryskalla, 2003). Organic C was determined using a high-temperature CNS analyzer with an infrared detector (Bremner and Tabatabai, 1971).

Short-term P sorption characteristics were further analyzed on a subset of samples by constructing Langmuir sorption isotherms (Nair et al., 1984). One gram of soil was shaken with various additions of P in 25 mL of 0.01 M CaCl2 on an end-over-end shaker at 25°C. Phosphorus in the solutions was added as KH2PO4 to achieve initial solution concentrations of 0 to 40 mg L–1. After 24 h, the soil suspensions were centrifuged and filtered (0.45 µm), and the solution P concentration (C) was determined colorimetrically as described previously. The amount of P sorbed was calculated as the difference between the initial and final solution P contents. By using the Langmuir sorption equation, we calculated the soil P sorption maximum (Smax) from untransformed data by the optimizing routine developed by Bolster (2007). In addition, the equilibrium P concentration at which no net short-term sorption or desorption occurs (mg L–1) was determined by interpolating between points above and below the abscissa.

Data Analyses
Statistical analyses were performed using S-Plus (Insightful Corporation, 2001) and the SAS GLM procedure (SAS Institute, Inc., 2004). To compare the means of characterization data among horizons, eight horizon classes were defined based on similar morphological and genetic characteristics (Vaughan et al.,2008) including Oi (fibric organic), Dark A (A horizons with value and chroma ≤3), Gley A (A horizons with value ≥4 and chroma ≤2), Bright C (C horizons with value ≥5 and chroma ≥3), Oxidized C (C horizons with value ≥5 and chroma ≥4 plus visual evidence of Fe concentrations), Reduced C (C horizons with value ≥4 and chroma ≤2), Surface C (C horizons at the ditch surface), and Sulfidic C (C horizons containing sulfidic materials). Kolmogorov-Smirnov tests and descriptive statistics were used to assess normality. The DPS, pH, and organic C variables were found to be normally distributed. Other variables were found to be normal after log-transformation; for those variables, log-transformed data were used for statistical inferences (Press et al., 1989). The CONTRAST statement in the SAS GLM procedure was used to test pre-planned one-way comparisons between the means of morphological horizon classes: Oi greater than A horizons (Dark A, Gley A), Dark A greater than Gley A horizons, A greater than subsoil C horizons (Gley C, Bright C, and Sulfidic C), Bright C greater than Gley C horizons, and Sulfidic C greater than Gley C horizons.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Phosphorus concentrations were highly variable among profiles and across depths, with ranges of 27 to 4882 mg kg–1 for TP, 4 to 4631 mg kg–1 for Pox, 2 to 401 mg kg–1 for M3P, and 0 to 17 mg kg–1 for WEP. Oxalate-extractable Fe and Al and organic C were also highly variable (Table 1 ). Across all samples, Pox comprised a mean of 56% of TP, M3P 11% of TP, and WEP <1% of TP (Table 1). As a percentage of TP in individual ditch soil horizons, Pox and M3P were greater in A horizons than in subsoil C horizons, whereas the percentage of TP that was water extractable was lower in A horizons than in subsoil C horizons (Table 1). Across all horizons, Feox concentrations were on average about three times Alox concentrations (Table 1). Soil pH values ranged from 2.6 to 6.1 (mean, 4.7; SD, 0.7). The mean pH of our samples was somewhat lower than the mean pH of samples from surface horizons of ditch soils on the Delmarva Peninsula, as reported by Sallade and Sims (1997a).


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  		Table 1. Chemical characterization data for morphological horizon classes comprised of soil horizons with similar morphological properties.

 
Depth Distribution
Data from three representative primary ditch profiles are presented in Table 2 , and three representative profiles from shallow-collection and deep-collection ditches are presented in Table 3 . In primary ditches, Pox, Alox, Feox,, and DPS generally decreased with depth, with the greatest decreases being observed between the alluvial A horizons and the subsoil C horizons (Table 2). However, the pattern of decreasing P, Alox, Feox concentrations and DPS with depth was not consistent within A horizon or C horizon layers. For example, in profile DX2–2, TP and M3P within the four C horizons increased with depth (Table 2). In profile DX1–3, the greatest Pox concentration was found in the deepest A horizon (A'3) at 23 to 30 cm. In profile DX2–2, the surface Ag1 horizon (0–5 cm) had substantially lower TP and Pox concentrations than did the underlying A (5–15 cm) and A'g2 (15–23 cm) horizons.


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  		Table 2. Soil characterization data of three representative ditch soil profiles located within primary ditches.

 

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  		Table 3. Soil characterization data of three representative ditch soil profiles located within shallow-collection and deep-collection ditches.

 
Within shallow-collection and deep-collection ditches, decreases in P, Feox, and Alox as a function of depth were more consistent than those found in primary ditches (Table 3). The greatest decreases were observed at the transition between Oi and A horizons and between A and C horizons. Decreases in DPS as a function of depth were not consistent in all pedons. There were several exceptions to the trend of decreasing concentrations with depth. In the upper soil horizons, profile DXXS1–2 contained high concentrations of TP in the upper soil horizons that steadily decreased down to a depth of 44 cm below the soil surface. However, the Ag2 horizon (17–26 cm) was higher in WEP than the overlying Ag1 horizon (13–17 cm). Trends similar to those of DXXS1–2 were found in profile DXXD2–4, although there was a slight increase in TP of the A'2 horizon versus the overlying Ag2 horizon (Table 3). Oxalate-extractable Fe in DXXS1–2, DXXD2–4, and DXXD3–6 generally decreased with depth. In DXXS1–2 and DXXD2–4, Alox also generally decreased with depth. Organic C in profile DXXD2–4 was found to be irregular with depth, whereas in DXXS1–2 and DXXD3–6 organic C was found to consistently decrease with depth (Table 3).

Several mechanisms can be used to explain the irregular P distributions in the ditch soils of this study. Inputs of P are surficial and subsurface at this site; subsurface solute concentrations (P and Fe) may vary substantially between shallow and deep ground water, resulting from variability in redox chemistry and source area parent materials (Vadas et al., 2007). Nonpedogenic soil heterogeneity may also influence P distributions, particularly particle-size variability between A and C horizons. Alluvial horizons are formed from sedimentation of suspended soil from cultivated fields in surface runoff water, from the slumping of ditch sidewalls, and from the formation of precipitates from solutes transported in ground water (Sims et al., 1998; Nguyen and Sukias, 2002). Alluvial horizons may then be re-worked by fluvial processes within the ditch network during periods of instability. Burrowing macroinvertebrates can also vertically mix soil horizons. The third set of mechanisms that may affect P distributions at the study site is pedological processes, particularly the formation of organic-rich horizons at the soil surface, the formation of concentrations of Fe oxides, and gleization.

Morphological Horizon Class Differences
Alluvial A Horizons versus Subsoil C Horizons
The most substantial differences were observed when contrasting A horizons and subsoil C horizons. Mean P fraction concentration; Alox, Feox, and organic C concentrations; and DPS were significantly greater in alluvial A horizons than in subsoil C horizons (Tables 1 and 4 ). These differences were expected due to the sharp textural contrast. The finer-textured A horizons likely retained greater quantities of P than did the coarse-textured C horizons due to greater surface area, which facilitates the binding of more Fe and Al hydroxides to mineral surfaces, providing for more binding sites for P. The surficial A horizons have also been exposed to P-laden surface runoff and direct poultry manure inputs. As the ditches accrete through mineral and organic debris deposition, P may be retained in these alluvial layers. The mean DPS of A horizons was greater than that of subsoil C horizons (Tables 1 and 4), suggesting that there may not be sufficient sorbable P passing through these C horizons to saturate all available P sorption sites. There is also a sharp contrast in organic C content between these layers, but we are not able to distinguish the role of organic matter from that of the strong texture contrast with this data set.


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  		Table 4. One-way probability values for contrasts on differences between morphological horizon class means.

 
These differences in predicted P sorption are confirmed by the data obtained from sorption isotherms performed on a select set of horizons (Table 5 ). The five A horizons analyzed had a mean Smax of 219 mg kg–1, whereas the four C horizons had a mean Smax of 97 mg kg–1. Langmuir P sorption maximum does not include the slower and nonreversible occlusion of P within Fe and Al oxide mineral structures (McGechan and Lewis, 2002); this occluded P is included in Pox.


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  		Table 5. Sorption isotherm coefficients for selected horizon samples.

 
Oi Horizons versus A Horizons
Only four of the 21 profiles possessed Oi horizons sufficiently thick for sampling. These Oi horizons were thin (<9 cm), containing coarse organic debris, algal mats, decomposing organic materials, and, in some cases, monosulfidic black oozes. The Oi horizons may be of particular importance to P transfers in ditches because they lie at the soil–water interface. Despite the low sample size, contrasts indicated that mean TP, WEP, Pox, Alox, and Feox concentrations were significantly greater in Oi horizons than in A horizons (Dark A and Gley A). These differences were not significant for M3P, DPS, or organic C (Tables 1 and 4). Coarse organic debris was removed from these samples, and therefore the total C present in Oi horizons is underestimated.

The high concentrations of TP, Pox, and WEP in Oi horizons were associated with a greater P sorption capacity of these horizons as represented by Feox and Alox. The mean Alox concentration of Oi horizons was nearly twice that of Dark A horizons, whereas the mean Feox concentration was over four times the mean concentration of Dark A horizons (Table 1). Phosphorus would be expected to be found most closely associated with Al and Fe hydroxides in the acidic soils of the ditches at UMES (Vadas and Sims, 1998). The extra sorption sites provided by reactive Fe and Al in humic compounds may be retaining P (Petrovic and Kastelan-Macan, 1996). Intensive redox cycling and the presence of organic matter in these horizons may be responsible for their high concentration of Feox (Bigham et al., 2002). The similar DPS in the Oi and A horizons suggests that these horizons experience similar P loading and desorption pressures. This trend of greater estimated P sorption capacity was confirmed with the Smax of the Oi horizon, which was more than two times greater than the mean Smax of A horizons (Table 5).

Dark A Horizons versus Gley A Horizons
The low-chroma colors that characterize Gley A horizons are presumably the result of gleization. We hypothesized that gleization would reduce P concentrations and sorption capacity. This hypothesis was supported by significantly greater mean Feox, Alox, and organic C concentrations observed in Dark A horizons than in Gley A horizons (Tables 1 and 4). The greatest difference was in Feox, with a mean concentration 2.6 times greater in Dark A than in Gley A horizons. Total P and Pox means were significantly greater in the Dark A horizons than in Gley A horizons; however, significant differences of M3P, WEP, and DPS means were not observed (Tables 1 and 4). These differences support the hypothesis that reducing conditions lower the P sorption capacity and TP and Pox concentrations of ditch A horizons through the dissolution of Fe-bound P. Management practices that extend periods of saturation within ditch systems, such as water-control structures, may increase gleization rates, thereby reducing P sorption capacity.

Bright C versus Gley C Horizons
The bright C horizon class encompasses C horizons that do not have Fe-depleted matrixes or significant accumulations of Fe concentrations. In contrast, Gley C horizons did have reduced matrixes. We hypothesized that gleization would reduce P sorption capacity and retention of Gley C horizons relative to Bright C horizons. However, no significant differences in Alox and Feox were observed between Gley C and Bright C horizons; nor were differences observed in M3P, Pox, DPS, or organic C (Tables 1 and 4). Significantly greater concentrations of TP and WEP were found in Bright C than in Gley C horizons. The reason for the higher mean concentrations of TP and WEP in the Bright C horizon class is not clear given the absence of differences in other P fractions and in P sorption capacity indicators.

Oxidized C horizons
The Oxidized C horizons are layers with a visible accumulation of Fe concentrations, thought to be the result of an oxygenated ground water table at this depth or of the oxidation of sulfidic materials at this depth, which may produce appreciable quantities of Fe when oxidized (Fanning et al., 2002). We hypothesized that these Fe concentrations would act as significant P sinks within subsoil C horizons. However, no significant differences between the Oxidized C horizons and other C horizons were detected for any variables, including Feox. The Fe oxides visible in this horizon may be crystalline and therefore not extractable with ammonium oxalate, or they may be poorly crystalline but of low overall quantity despite being sufficient to coat the sand grains.

Sulfidic C horizons
The presence of sulfidic C horizons in the ditch soil profiles may pose a water-quality risk due to acidity released on oxidation of the Fe sulfides. A commonly found Fe-sulfide mineral in geologically deposited materials in this region is pyrite. Ferrous Fe is also released on Fe sulfide oxidation; this ferrous Fe can move in solution or may be oxidized to insoluble ferric Fe forms. The only statistically significant difference observed between Sulfidic C horizons and Gley Dark C horizons was a slightly greater Alox concentration (Table 1). Although the acidity released on oxidation of Fe sulfides in this horizon may affect general ditch soil properties, it does not appear that these horizons differ significantly from other C horizons in their P-retention characteristics.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Substantial differences in the distribution of P were observed between Oi horizons and A horizons, A and C horizons, and dark and gleyed A horizons in the ditch soils at the UMES Research Farm. The greater TP, WEP, and Pox concentrations in Oi horizons than in A horizons were associated with greater P sorption capacity as evidenced by isotherm data and greater Feox and Alox concentrations. The high Feox concentrations indicate that in these horizons ferrous Fe is precipitated and possibly maintained as ferrihydrite under redox cycling in the presence of organic matter. The Oi horizons lie at the soil–water interface and may play a critical role in P transfers and transformations in these systems. The contrast between the A and C horizons may have been affected by the pedological process of organic matter accumulation in the A horizons; however, we were not able to discern this effect against the nonpedological texture contrast between these horizons. Nonetheless, the A horizons and associated high P contents extended to depths below 15 cm in these soils, which is a common sampling depth. To more accurately estimate the total P load in these ditch soils, sampling would need to extend at least to the contact between the alluvial and underlying Coastal Plain sediments. Total P and Pox were greater in Dark A horizons than in Gley A horizons, likely the result of gleization, as evidenced by the greater Feox concentrations in these horizons.

Ditch management practices that alter rates of pedological processes operating in ditch soils will likely affect their P retention capacity. The installation of water-control structures and the concomitant extension of periods of soil saturation are likely to increase gleization rates. The extension of periods of soil saturation may substantially decrease the P sorption potential of loamy A horizons and cause an associated P release. Phosphorus release through gleization was not a significant factor in the sandy ditch C horizons in this study because the sandy horizons had little capacity to store P; finer-textured C horizons are likely to have greater P sorption capacities and therefore may be affected more by gleization.

Full and partial ditch clean outs may be used to remove P-saturated soils and sediments from ditches. However, if a ditch has the trend of greater P sorption capacity in Oi and A horizons than in C horizons, as was observed at this study site, then the truncated soils remaining after a clean out will have a substantially reduced P sorption capacity (Smith et al., 2006). Ditch clean-outs will only mitigate net P loss from a watershed if P inputs to ditch surface waters are substantially limited. Existing ditch soils may become P sources due to a reversal in the P concentration gradient between ditch soils and overlying waters. Otherwise, it may be better to retain the P sorption capacity of ditch surficial soils and restrict clean-outs to those necessary to maintain hydraulic capacity.


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F. Shigaki, J. P. Schmidt, P. J. A. Kleinman, A. N. Sharpley, and A. L. Allen
Nitrogen Fate in Drainage Ditches of the Coastal Plain after Dredging
J. Environ. Qual., October 29, 2009; 38(6): 2449 - 2457.
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