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Published online 28 June 2005
Published in Soil Sci Soc Am J 69:1288-1297 (2005)
DOI: 10.2136/sssaj2004.0221
© 2005 Soil Science Society of America
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Soil & Water Management & Conservation

Physical and Chemical Properties of a Minespoil Eight Years after Reclamation in Northeastern Ohio

M. K. Shuklaa,*, R. Lala and M. H. Ebingerb

a School of Natural Resources, FAES, The Ohio State Univ., Columbus, OH 43210-1085
b Los Alamos National Lab., Los Alamos, NM

* Corresponding author (shukla.9{at}osu.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The potential of using flue gas desulfurization by-products (FGD) for the reclamation of acid minespoil was assessed in Tuscarawas County of Ohio, USA. In Treatment 1, 280 Mg ha–1 of FGD was incorporated into the graded spoil to a depth of 20 cm. In Treatment 2, 280 Mg ha–1 of FGD and 112 Mg ha–1 of yard waste compost were incorporated into the graded spoil (FGDC). In Treatment 3, 112 Mg ha–1 of limestone was incorporated into the graded spoil and was covered with 20 cm of graded borrowed topsoil (BTS). Six cores and six bulk soil samples were obtained from each treatment for the 0- to 10-cm depth in summer 2002. From the 10- to 20-cm depth, only bulk soil samples were obtained. Bulk and core soil samples were also collected from an unreclaimed spoil (SP) and a nearby unmined soil (UMS). Among the three reclamation treatments, BTS showed better soil quality with higher soil organic C (28.5 Mg ha–1), water-stable aggregation (556 g kg–1), and mean weight diameter (3.2 mm) of aggregates than FGDC or FGD treatments. The FGDC had higher soil inorganic and organic C than FGD. However, saturated hydraulic conductivity (Ks), cumulative infiltration (I), infiltration rates at 5 min (i5) and 2.5 h (ic), and soil pH were similar among three treatments. Among treatments and controls, soil bulk density ({rho}b) was lower for FGD, FGDC, and UMS than BTS and SP; and water-stable aggregation and mean weight diameter of the aggregates was higher for UMS and BTS than FGDC, FGD, and SP for both depths. The I and ic were similar among UMS and treatments. Reclamation improved the soil quality with higher soil pH ({approx}7) and inorganic and soil organic C than in the SP. With respect to FGD, the soil organic C in FGDC increased at the rate of 0.64 Mg ha–1 yr–1 for the 0- to 10-cm depth. Overall, BTS was the best reclamation treatment. However, if topsoil is unavailable or transport is expensive, FGDC can be used as an effective reclamation material.

Abbreviations: {rho}b, bulk density • {theta}, volumetric water content of soil • {theta}0, antecedent volumetric water content of soil • AWC, available water content • BTS, borrowed topsoil • EC, electrical conductivity • FGD, flue gas desulfurization by-products • FGDC, flue gas desulfurization by-products + yard waste compost • fa, aeration porosity • fe, effective porosity • I, total infiltration in 2.5 h • i5, infiltration rate at 5 min • ic, infiltration rate at 2.5 h • Ks, saturated hydraulic conductivity • SMCRA, Surface Mining Control and Reclamation Act • SP, unreclaimed spoil • UMS, unmined soil


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SURFACE MINING causes drastic and immediate soil degradation and decline in soil quality (McSweeney and Jansen, 1984). Evidence of degradation of soil quality as a result of mining includes loss of structural properties (McSweeney and Jansen, 1984), increase in {rho}b (Chong et al., 1986; Guebert and Gardner, 2001), loss of soil organic C (Akala and Lal, 2001; Shukla et al., 2004a), and reduction in total porosity (Silburn and Crow, 1985). Reclamation of mining-affected lands can also exacerbate soil compaction and alter physical and structural characteristics that restrict root development and full recovery (Dunker et al., 1995; Underwood and Sutton, 1992). Appropriately conducted reclamation operations can improve soil quality (Shukla et al., 2004a). The process of restoring minespoil after the Surface Mining Control and Reclamation Act (SMCRA) of 1977 can involve (i) application of topsoil (20- to 30-cm depth) either from the same location or brought in from elsewhere; (ii) use of amendments such as alkaline FGD; (iii) use of organic amendments such as compost or sludge; and (iv) application of chemical fertilizers (N–P–K) with topsoil application (Korcak, 1995; Shukla et al., 2004b).

Opinions differ about the rate of soil development in reclaimed soils, but there is a consensus that the initial high rate of soil weathering decreases rapidly with time (Struthers, 1964). Mine spoil material represents the properties of parent material and the rate of weathering and/or soil development is an important edaphic factor of successful reclamation operations (Barnhisel and Gray, 2000). In some studies where reclamation consisted of topsoil application, weak granular structure of soil was reported (Pedersen et al., 1980; Schafer et al., 1980), which increased infiltration rates and total infiltration in the first year after tillage followed by a decrease in the subsequent years (Thomas and Jansen, 1985; Chong and Cowsert, 1997). In addition to enhancing the development of soil structure, organic matter enrichment is an important part of soil reclamation operations, which needs to be investigated further (Haering et al., 1993; Thomas et al., 2001).

The acid mine drainage resulting from the oxidation of iron disulfide minerals (pyrite) is a common water quality problem in the coal region of eastern Ohio. It is characterized by low pH (1.9–4.8) and high dissolved metal loads (Westover and Eberle, 1987). Soil reclamation post-SMCRA generally involves complete regrading of spoil material, incorporation of topsoil and/or amendments, and increased pH of acidic soils (ranging from 6 to 9; Evangelou and Sobek, 1988). Under some situations, topsoil may not be readily available and its transport can be expensive. Drastic soil disturbance and loss in aggregation also results in severe loss of soil organic C due to the removal and transport of soil (Johnson and Skousen, 1995; Korcak, 1995). Therefore, use of less costly amendments such as FGD by-products can be a useful alternative. Currently, 80% of boiler slags, 34% of fly ash, 31% of bottom ashes, and 10% of FGD are used for minespoil reclamation (American Coal Ash Association, 1998). The potential of a variety of FGD products applied with stabilizing materials (e.g., fly ash, limestone, and alkalizing agents) on mitigating soil acidity, increasing soil nutrient status, enhancing water infiltration and aggregation, and improving soil quality needs to be studied further (Dick et al., 1999; Clark et al., 2001). The properties of FGD products are variable due to techniques and extractants used in their production, and trace element concentration can vary depending on amount of fly ash graded with other amendments or spoil (Lammine et al., 2001; Laperche and Bigham, 2002). The residual alkalinity of FGD products provides the potential for ameliorating the adverse chemical conditions of acid minesoils. Organic amendments have long been used for reclamation; comparisons of treatments, such as FGD with and without compost and resoiling are not well illustrated. While the ultimate goal of reclamation is to recover the site in a manner that maximizes the rate of soil improvement across time, site-specific investigations designed to measure soil properties of reclaimed minespoil are critical (Barnhisel and Hower, 1997).

This study was designed to assess and compare soil physical, chemical, and hydrological properties of reclaimed minespoil using FGD, FGDC, and topsoiling. The main hypothesis for the study was that the reclamation of minespoil by FGD with or without compost results in greater soil quality with time than soils reclaimed by topsoiling. Our objectives of the study were to (i) evaluate the effects of reclamation treatments on soil physical and chemical properties, (ii) compare the soil properties of reclaimed spoils to that of SP and UMS, and (iii) assess the C sink potential of the reclaimed minespoil.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Descriptions and Reclamation Options
Field experiments were conducted during June 2002 at an abandoned surface coal mined site located in Franklin Township, Tuscarawas County, Ohio (40°33' N and 81°31' W). The experimental site was placed on reclamation priority due to the flooding and sedimentation on nearby roads. Before reclamation in 1994, the study area consisted of approximately 10 ha of exposed, highly erodible underclay; 18 ha of mine spoil, and 2 ha of coal refuse located on the periphery. The spoil was derived from Pennsylvanian age rocks of Allegheny Formation and consisted of sandstone and shale interbedded with coal, clay, and limestone. The spoil and underclay were extremely acidic, and pH ranged from 2.4 to 3.9. The soil was classified as the Bethesda soil series (loamy-skeletal, mixed, acid, mesic Typic Udorthent) (Waters and Roth, 1986).

The long term average annual precipitation for Tuscarawas County, Ohio, is 1016 mm, and average annual summer (June–August) air temperature is 21°C (Ohio Department of Development, 1997). The total precipitation for Year 2002 was 965 mm. The study area was reclaimed in 1994 in conformity to Ohio's pioneering 1972 legislation, which preceded the 1977 Surface Mining Control and Reclamation Act.

Six plots approximately 0.4 ha each were constructed in the reclaimed minespoil in the autumn of 1994 by first grading the exposed underclay to a 4% slope and then compacting it into an aquitard by Warren Dick and coworkers (Fig. 1) . Graded acid mine spoil was placed to a depth of 1.2 m on top of the underclay. Three treatments were applied to the reclaimed minespoil plots: (i) FGD at 280 Mg ha–1, (ii) FGD (applied at 280 Mg ha–1) + yard waste compost (applied at 112 Mg ha–1) (FGDC), and (iii) lime + BTS treatment of 20 cm borrowed soil + 45 Mg ha–1 agricultural limestone placed over 112 Mg ha–1 of limestone incorporated into the graded spoil. The average slope in these plots after reclamation is about 2%.



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  		Fig. 1. Schematic of treatment plots and sampling locations. BTS, borrowed topsoil; FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; SP, unreclaimed spoil; UMS, unmined soil.

 
The FGD consisted of atmospheric fluidized bed combustion by-product from the General Motors plant in Pontiac, MI. The FGD by-product (pH = 12.4) contained dolomite (percentage not known), Anhydrite (36%), periclase (27%), and coal ash (10%), with 95% of the particles within the size range of 2.0 to 0.1 mm. Detailed physical and chemical characterization of FGD are given in Dick et al. (1999). The yard waste compost consisted of composted leaves, twigs, and grass clippings (pH = 7.4). For FGD and FGDC treatments, the incorporation of reclamation material into graded spoil was done by a chisel plow. No tillage operations were performed thereafter. The depth of incorporation or borrowed soil application was kept constant and equal to 20 cm. The borrowed soil (pH = 4.3) was from a designated area located north of the experimental site. The soil was dug and piled in a heave before transport. It was graded to crush the clods before spreading over the graded limestone and spoil and later compacted. Therefore, it can be safely assumed that the excavation, transport, grading, and compaction resulted in severe loss of natural structure and soil organic C of the borrowed soil. Predisturbance classification of borrowed soil is unknown. The average clay content of undisturbed soil ranged from 16 to 25%, {rho}b from 1.3 to 1.5 Mg m–3, and available water content (AWC) from 0.16 to 0.20 cm3 cm–3 in the 0- to 20-cm depth (Waters and Roth, 1986).

The application rate of limestone was selected based on the lime test index (Sims, 1996) and CEC for the spoil, which were 44 and 22.8 C-molc kg–1, respectively. The calcium carbonate equivalent of FGD was approximately 40%; therefore, 2.5 times as much FGD was applied to achieve the pH of 7. The reclaimed minespoil plots were seeded to a grass–legume sward consisting of orchard grass (Dactylis glomerata L.), timothy (Phleum pratense L.), annual ryegrass (Lolium multiflorum Lam.), ladino clover (Trifolium repens L.), birdsfoot trefoil (Lotus spp.) and winter wheat (Agropyron spp.) after reclamation in 1994 (Dick et al., 1999).

Four additional plots were selected as controls: two of SP and two of UMS (Fig. 1). The plots under unreclaimed graded spoil (SP) were located on the northeast edge of the reclaimed minespoil and were constructed in the year 1994. The SP represented the original state of spoil at the experimental site and was bare without any type of vegetation. Plots under UMS were under continuous forage for the last 25 yr and were located about 1.5 km away from the experimental site (40°33' N and 81°34' W). There were no other unmined fields under no-till (no-till duration at least 8 yr) and continuous forage in the closer vicinity of the experimental plot. The UMS soil was classified as Coshocton (fine-loamy, mixed, active, mesic, Aquultic Hapludalfs)–Guernsey (fine, mixed, superactive, mesic, Aquic Hapludalfs) (Waters and Roth, 1986).

Soil Sampling
Six bulk and six core soil samples were obtained from the 0- to 10-cm depth during June 2002 from each of the three reclamation treatments (i.e., FGD, FGDC, and BTS) and two controls (UMS and SP). Only bulk soil samples were obtained from each treatment and control for the 10- to 20-cm depth (Fig. 1).

Lab Methods
Soil {rho}b was determined by the core method using 7.6-cm-long and 7.6-cm-diam. cores (Blake and Hartge, 1986). The Ks was measured on soil cores by the constant head method (Klute and Dirkson, 1986). Soil moisture characteristic curves were determined on intact soil cores for suctions of 1 kPa, 3 kPa, and 6 kPa using a tension table (Leamer and Shaw, 1941), and for suctions of 10 kPa, 30 kPa, 100 kPa, and 300 kPa using a pressure plate (Klute, 1986). The gravimetric water content was converted to volumetric water content ({theta}) following the procedure of Gardner (1986). The {theta} at 1500 kPa was determined on soil sample passing through 2-mm sieve. The difference in {theta} at saturation and 6 kPa was defined as aeration porosity (fa) and between saturation and 10 kPa was defined as effective porosity (fe) (Ahuja et al., 1984). The AWC was assessed as the difference in {theta} at 30 and 1500 kPa suction and was expressed as equivalent depth of water for the specific soil layer.

Larger clods, if present, were broken by hand into smaller segments along natural cleavage before air-drying. About 800 g of bulk soil samples were thoroughly mixed, spread on a tray, and air-dried. Each air-dried sample was divided into two subsamples, and the first subsample was sieved through 8- and 4.75-mm sieves. About 50 g of the aggregates retained on a 4.75-mm sieve were used for the determination of the water-stable aggregation and geometric and mean weight diameters of aggregates by a wet sieving technique using nested sieves (4.75, 2.00, 1.00, 0.50, and 0.25 mm) (Yoder, 1936; Youker and McGuinness, 1957).

The second subsample was passed through wooden rollers to break up the remaining clods (Gee and Bauder, 1986). The material coming out of the rollers was passed through a 2-mm sieve and 51 g of sieved soil sample (<2 mm) was used for the determination of particle size distribution by the hydrometer method using distilled water at a room temperature of about 22°C (Gee and Bauder, 1986). Gravel contents were low (<2%) of the total soil sample.

A portion of the soil (<2 mm) was ground to pass through 0.5-mm sieve for the determination of total soil C and N concentrations by the dry combustion method. Inorganic C concentration was determined using the procedure of Bundy and Bremner (1972) with some modifications. Briefly, 1.5 to 2 g of soil (<2 mm) was weighed in a serum bottle that was crimp-sealed. A glass syringe was then used to inject 4 mL of HCl (2 M) into the bottle to decompose the carbonates. The carbon dioxide (CO2) produced was injected into gas chromatograph. Using a thermal conductivity detector, the concentration of CO2 was obtained and was converted to inorganic C concentration. The soil organic C concentration was calculated as the difference between total soil C and inorganic C. The soil organic C and total soil N concentrations were converted to soil organic C and total soil N pools by multiplying the concentration by {rho}b and soil depth. Soil pH and electrical conductivity (EC) were determined on 1:1 soil/water ratio by a hand-held conductivity meter and pH electrode (McLean, 1982; Rhoades, 1982).

Six ponded water infiltration tests were conducted in June 2002 at the soil surface at each sampling location using tap water and a double ring infiltrometer (27-cm diam. of the outer ring and 15-cm diam. of the inner ring; Bouwer, 1986). Each water infiltration test was conducted for 2.5 h, and i5, ic, and I were monitored. The antecedent volumetric water content of soil ({theta}0) and the water content 24 h after the infiltration experiment, assumed to be the field capacity, were gravimetrically determined. The sorptivity (cm h–0.5) was computed by the Philip (1957) model.

Statistical Analysis
Analysis of variance was computed using the Statistical Analysis System (SAS Institute, 1989) for treatment x replicate interactions. Data for each soil depth were analyzed separately. To make multiple comparisons among treatments, the Bonferroni t test method was used. Pair-wise comparisons were done with t tests, and then to account for the compounding risk of doing many statistical comparisons, the P values were multiplied by the number of comparisons made. The significant interactions (P ≤ 0.05) and mean separations were calculated separately for the effects within the three reclamation treatments, and among reclamation treatments and controls.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Texture and Bulk Density
Soil texture in the reclaimed minespoil depends primarily on the properties and degree of weathering of spoil, soil, and amendments used for reclamation (Pedersen et al., 1980). Soil texture was loam for FGD and FGDC treatment plots, and clay loam for the BTS at both depths (USDA texture triangle). Among three reclamation treatments, BTS had the least amount of sand (291 g kg–1) and the highest clay (370 g kg–1) concentration at both depths (Table 1). The soil texture for the SP was clay loam and for the UMS silty clay loam for both depths (Table 1). The UMS had the lowest sand concentration among all five treatments at both depths (130 and 126 g kg–1) (P < 0.05). The clay concentration did not vary among SP, BTS, and UMS, and was higher than that for FGDC and FGD treatments at both depths. Silt concentration was similar between FGD and UMS, and significantly different from BTS and SP at both depths (Table 1).


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  		Table 1. Variations in sand, silt and clay concentrations among reclamation treatments and controls. Mean separation was done by the Bonferroni multiple comparison method (BSD).{dagger}

 
The {rho}b of reclaimed soil is a function of soil texture and aggregation, amendments used for reclamation, and type and amount of heavy equipment used during land forming operation (Barnhisel and Hower, 1997). Among three reclamation treatments, soil {rho}b varied in the order BTS (1.34 g cm–3) > FGD (0.94 g cm–3) = FGDC (0.88 g cm–3) (Fig. 2) , and the variations in soil {rho}b were directly related to clay concentrations. Soil {rho}b for the SP (1.48 g cm–3) was similar to the BTS (1.34 g cm–3). No significant differences in soil {rho}b were observed among UMS, FGD, and FGDC treatments (Fig. 2), although clay concentrations were significantly different among UMS and FGD or FGDC. Bulk densities from reclaimed mine-spoils were much lower to the values reported by Johnson and Skousen (1995) for minesoils developed from Kittaning coal seam spoil. The soil {rho}b for the UMS was lower (1.28 g cm–3) than BTS and SP even though the clay concentrations were similar. The lower soil {rho}b for UMS was due to the continuous dense grass cover.



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  		Fig. 2. Variation of soil bulk density among reclaimed and control plots. The SE was 0.07 among three reclamation treatments and 0.19 among all plots in the 0- to 10-cm depth. The bar indicates mean separation using Bonferroni multiple comparison. BTS, borrowed topsoil; FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; SP, unreclaimed spoil; UMS, unmined soil.

 
Soil Electrical Conductivity and pH
Among the three reclamation treatments, soil EC was higher for BTS (0.47 dS m–1) than FGD (0.11 dS m–1) or FGDC (0.03 dS m–1) for 0- to 10-cm depth only (Table 2). The EC for SP (0.03 dS m–1) and UMS (0.18 dS m–1) were similar to those from FGD and FGDC treatments for the 0- to 10-cm depth (Table 2). In year 1995, the EC values for the 0- to 10-cm depth ranged from 0.9 to 2.8 dS m–1 (Dick et al., 1999). For the 10- to 20-cm depth, no differences in EC were observed among FGD, FGDC, BTS, SP, and UMS. Soil EC values from three reclamation treatments as well as SP and UMS were much lower than the 4 dS m–1 normally considered a threshold for salinity problems. The FGD are normally comprised of large amounts of soluble salts, primarily gypsum; therefore, a higher EC was expected for the FGD and FGDC treatments. Contrary to our expectations, the EC was low for all treatments and for both depths. Overall, EC from reclaimed minespoil was in the nonsaline range (<0.47 dS m–1), probably due to the leaching and natural weathering during the last 8 yr after reclamation. The reported risks involved with FGD application are those associated with trace elements leaching from the material (Klein et al., 1975), and in the process increasing the EC of the soil (Punshon et al., 2001). We did not observe high values of EC for FGD or FGDC treatment, but believe that the trace metal concentrations in soil and plants need to be studied separately.


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  		Table 2. Variations in electrical conductivity (EC), pH, total soil nitrogen (TN), and geometric (GMD) and mean weight diameters (MWD) of aggregates among reclamation treatments and controls.{dagger}

 
Soil pH for reclamation treatments did not differ significantly for either depth (P < 0.05; Table 2). Soil pH for UMS was close to neutral ({approx}7) for both depths. The natural weathering and leaching for the last 7 to 8 yr, and application of lime increased soil pH in the reclaimed minespoil to close to neutral (=7). The pH for SP was low (=3.4) for both depths and was highly acidic. The high acidity for the SP treatment was evident from zero vegetation (i.e., no grass and/or weed) on these plots and showed that even after 8 yr of natural weathering processes, the acidity did not change much from previously measured values ranging from 2.4 to 3.9 (Dick et al., 1999). The improvement in soil quality due to reclamation was evident from the increases in soil pH, which has increased from 3.4 in the SP to >6.5 in the reclaimed minespoil for both depths.

Soil Organic Carbon and Nitrogen Pool
The soil inorganic C concentrations were similar among FGDC (4.12 g kg–1) and BTS (5.33 g kg–1) and were higher than FGD (3.13 g kg–1) for the 0- to 10-cm depth (Fig. 3) . For the 10- to 20-cm depth, the soil inorganic C was higher for BTS (9.16 g kg–1) than FGDC (3.12 g kg–1) and FGD (2.72 g kg–1). Among three reclamation treatments, the soil organic C varied in the order BTS (28.5 Mg ha–1) > FGDC (24.9 Mg ha–1) > FGD (19.8 Mg ha–1) for the 0- to 10-cm depth (P < 0.05; Fig. 4) . For the 10- to 20-cm depth, the soil organic C for BTS (20.6 Mg ha–1) and FGDC (29.0 Mg ha–1) was similar but higher than FGD (19.5 Mg ha–1). This type of a variation among reclamation treatments was consistent as borrowed soil, although drastically disturbed, was expected to have higher antecedent soil organic C. With respect to FGD (assuming soil organic C of FGD as a baseline), the soil organic C increased at the rate of 0.64 Mg ha–1 yr–1 in FGDC for the 0- to 10-cm depth, and 1.64 Mg ha–1 yr–1 for the 10- to 20-cm depth. Higher soil organic C for the FGDC than FGD for both depths was due to the application of compost, which increased total soil N concentration and decreased the C/N ratio. The total soil N concentrations for FGDC (2.04 g kg–1 for the 0- to 10-cm and 1.43 g kg–1 for 10- to 20-cm depth) were higher than FGD (1.39 g kg–1; 1.20 g kg–1) or BTS (1.38 g kg–1; 1.02 g kg–1), consequently increasing the nitrogen availability to plants (Table 2). However, Dick et al. (1999) have reported no significant differences in plant biomass among plots reclaimed with FGDC, FGD, or BTS in 1998.



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  		Fig. 3. Variation of soil inorganic C among reclaimed and control plots. The SE was 0.12 among three reclamation treatments and 0.19 among all plots in the 0- to 10-cm depth, whereas it was 0.12 among three reclamation treatments and 0.10 among all plots in the 10- to 20-cm depth. The bar indicates mean separation using Bonferroni multiple comparison. BTS, borrowed topsoil; FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; SP, unreclaimed spoil; UMS, unmined soil.

 


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  		Fig. 4. Variation of soil organic C among reclaimed and control plots. The SE was 1.3 among three reclamation treatments and 5.1 among all plots in the 0- to 10-cm depth, whereas it was 1.6 among three reclamation treatments and 5.1 among all plots in the 10- to 20-cm depth. The bar indicates mean separation using Bonferroni multiple comparison. BTS, borrowed topsoil; FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; SP, unreclaimed spoil; UMS, unmined soil.

 
The SP was not seeded to grass or any other vegetation and consequently had the least soil organic C pool (11.6 Mg ha–1 for the 0- to 10-cm and 15.9 Mg ha–1 for the 10- to 20-cm depth) (Fig. 3). The soil organic C pools for the FGDC and BTS were more than two times higher than that for the SP for both depths. Among the three reclamation treatments and two controls, the soil organic C pool varied in the order UMS > BTS ≥ FGDC ≥ FGD > SP for the 0- to 10-cm depth (P < 0.05). For the 10- to 20-cm depth, the soil organic C pools (which were calculated using the soil {rho}b for the 0- to 10-cm depth) were in the order UMS = BTS = FGDC > FGD > SP (P < 0.5). The high soil organic C for the UMS (37.8 Mg ha–1 for the 0- to 10-cm depth and 37.0 Mg ha–1 for the 10- to 20-cm depth) was expected because it was consistently under forage grass cover and also received cow manure. In general, 8 yr after reclamation of the spoil, the soil organic C for FGD, FGDC, and BTS was higher than that for the SP for both depths (Fig. 4), which indicated the potential of C sequestration in reclaimed minespoil through conversion to a restorative land use and improved management practices. Since soil organic C content for the SP represents the initial soil organic C content for the acid minespoils, the soil organic C in reclaimed minespoil increased at the rate of 1.03 Mg ha–1 yr–1 for FGD to 1.67 Mg ha–1 yr–1 for FGDC for the 0- to 10-cm depth and 0.45 Mg ha–1 yr–1 for FGD to 1.64 Mg ha–1 yr–1 for FGDC for the 10- to 20-cm depth.

The effect of manure application on UMS was clearly evident from the higher total nitrogen pool for the UMS than reclaimed minespoil or SP for both depths (P < 0.05). The reclaimed minespoil plots were seeded to a grass–legume mixture; however, similar total soil N pools for reclamation treatments and SP showed that leguminous sward did not fix significant nitrogen into the soil. The average C/N ratios for the three reclamation treatments were 18.9:1 in FGD, 17.4:1 in FGDC, and 18.5:1 in BTS for year 2002; therefore, no fertilizer or manure application is necessary at this time to facilitate decomposition of residues and maintaining an upward trend for C sequestration in the soil. However, with increase in soil organic C at the rates given above, manure or fertilizer application may be required in the future to keep the C/N ratio equal to or less than 30:1.

Water-Stable Aggregation and Aggregate Diameters
Among the three reclamation treatments, water-stable aggregation was higher for BTS (555.9 and 513.4 g kg–1) than FGD (161.7 and 134.3 g kg–1) or FGDC (220.8 and 253.5 g kg–1) treatments for both depths (P < 0.05; Fig. 5) . The geometric mean weight diameter of aggregates was similar for FGD (0.90 mm for the 0- to 10-cm and 0.88 mm for the 10- to 20-cm depth) and FGDC (0.97 and 0.87 mm) treatments and was lower than BTS (1.40 and 1.21 mm) (Table 2). The mean weight diameter of aggregates was also higher for BTS than FGD or FGDC treatments for both depths. The higher water-stable aggregation and geometric or mean weight diameter for BTS were in accord with higher clay concentration and soil organic C for the BTS than FGD or FGDC treatments.



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  		Fig. 5. Variation of water-stable aggregation among reclaimed and control plots. The SE was 49 among three reclamation treatments and 103 among all plots in the 0- to 10-cm depth, whereas it was 47 among three reclamation treatments and 109 among all plots in the 10- to 20-cm depth. The bar indicates mean separation using Bonferroni multiple comparison. BTS, borrowed topsoil; FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; SP, unreclaimed spoil; UMS, unmined soil.

 
The water-stable aggregation for the SP (166.4 g kg–1 for the 0- to 10-cm depth and 142.3 g kg–1 for the 10- to 20-cm depth) was similar to FGD or FGDC treatment for both depths (Fig. 5) mainly due to the higher clay concentration for the SP than FGD or FGDC treatments. However, in spite of the lower soil organic C for SP than FGD or FGDC for both depths, the geometric and mean weight diameters of aggregates were not significantly different among FGD, FGDC, or SP for either depth (Table 2).

The water-stable aggregation was high for UMS for both depths (609.2 and 630.5 g kg–1) and was consistent with the high soil organic C concentration, silt + clay fraction or the lower sand. The water-stable aggregation did not vary between BTS and UMS for both depths, but geometric mean diameter was higher for BTS than UMS for both depths and mean weight diameter for the 0- to 10-cm depth. The lower geometric or mean weight diameters for the UMS than BTS was possibly due to the dispersion of clay caused by the surface application of manure in the UMS plots (Haynes and Naidu, 1998). The similar water-stable aggregation and higher geometric mean diameter of aggregates for both depths for BTS than UMS indicated that the soil structure in BTS improved rapidly (i.e., within 8 yr) thus contradicted our hypothesis. This was also in contrast to an earlier study in nonacid reclaimed minespoil in southeastern Ohio, which reported smaller values of water-stable aggregation and geometric and mean weight diameters of aggregates for reclaimed minespoil than UMS 25 yr after reclamation (Shukla et al., 2004a).

Soil Porosities and Water Infiltration
The standard deviations of {theta} for a given suction were higher for the FGD and FGDC treatments than BTS and UMS treatment (Fig. 6) indicating that the soil profile of BTS and UMS were more homogeneous. The standard deviations of {theta} were the smallest for the SP. The values of fa from reclaimed minespoils were low (<0.07) and from an agriculture point of view, the AWC (<1.7 cm) for the 0- to 10-cm layer was also low for the reclaimed minespoil (Table 3). Waters and Roth (1986) also reported low AWC (0.5–1.6 cm) for Bathesda soil. The fe for FGD and FGDC were similar to the BTS mainly due to the higher macroporosity or coarse fragment contents associated with FGD by-products. The AWC was also similar for FGD, FGDC, and BTS, which could be due to the higher water-holding capacity introduced by fine particles in FGD (Punshon et al., 2001). The water availability to plants was high for all treatments and controls due to the low EC, which increases the negative osmotic potential.



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  		Fig. 6. Mean and standard deviations (bars) of soil water characteristic curves for different treatments (FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; BTS, borrowed topsoil + lime; SP, unreclaimed spoil; UMS, unmined soil) for the 0- to 10-cm depth. SWC = soil water characteristic curve.

 

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  		Table 3. Variations in porosity, infiltration rates, saturated hydraulic conductivity (Ks), sorptivity (S), available water content (AWC), and cumulative infiltration (I) among reclamation treatments and controls.{dagger}

 
The infiltration rate and time [i(t)] curves in Fig. 7 showed that standard deviations were low for infiltration tests conducted on FGD and FGDC plots indicating low spatial variability. The standard deviations of infiltration rate at different times were higher for BTS and especially for UMS plot indicating higher spatial variability. The ic did not vary among FGD, FGDC, BTS, SP, and UMS (P < 0.05; Table 3). Since the infiltration tests were run for 2.5 h, therefore, ic values were expected to be close to the Ks values, which also did not vary among reclamation treatments and controls. However, we did not observe any agreement between Ks and ic values, and believe that the constant head method on soil cores was not adequate for measuring Ks for the soil profile. The I was significantly different between UMS (52.9 cm) and SP (6.4 cm) only (P < 0.05; Table 3). The values of I for three reclamation treatments were similar but high (I > 30.6 cm). Jorgensen and Gardner (1987), Guebert and Gardner (2001), and Shukla et al. (2004a) have also reported high I values for minespoils reclaimed with topsoil application.



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  		Fig. 7. Mean and standard deviations (bars) water infiltration rate curves for different treatments (FGD, flue gas desulfurization by-products; FGDC, flue gas desulfurization by-products + yard waste compost; BTS, borrowed topsoil + lime; SP, unreclaimed spoil; UMS, unmined soil).

 
The i5 was high (>31 cm min–1) but statistically similar among the three reclamation treatments (P < 0.05; Table 3). The i5 was higher for UMS (72.0 cm min–1) than SP (9.1 cm min–1), but was not significantly different than BTS (38.4 cm min–1), FGD (33.0 cm min–1), or FGDC (31.0 cm min–1) (P < 0.05). The sorptivity was also higher for UMS (47.9 cm min–1) than SP (8.5 cm min–1) and varied similar to the i5 from reclamation treatments. The high values of i5, ic, I, and sorptivity for UMS were consistent with the high water-stable aggregation. The i5 or sorptivity is strongly influenced by antecedent moisture content of soil as well as root density, macropore, and biopore channels. The similar values of i5, I, ic, or sorptivity were in accord to the similar antecedent soil water content (20 ± 4%), fe, and AWC values for three reclamation treatments, but contrary to the higher water-stable aggregation for BTS than FGD or FGDC. Large values of sorptivity also indicated that it was not the true sorptivity, but a combination of soil soptivity and flow through the macropores or biopores formed by roots through soil profile. The reclaimed minespoil sites were seeded to a grass-legume mixture; therefore, ameliorative effects of natural pedogenesis and/or macropores formed by roots also contributed to large I. This was further supported by the fact that i5, ic, I, and sorptivity did not vary according to the soil {rho}b or water-stable aggregation (r2 < 0.2).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Reclamation treatments increased soil pH, and the post reclamation acid minespoils are now neutral and within nonsaline range. The reclamation also increased soil organic C content in the soil profile. At the end of 8 yr, the reclamation with borrowed topsoil was clearly the best, and soil development in BTS plots was evident by high water-stable aggregation, mean weight diameter, and soil organic C. Among FGDC and FGD treatments, although most soil physical and chemical properties remained similar, soil organic C content was higher for the FGDC than FGD treatment for both depths. Therefore, FGDC can be used as an effective reclamation material, especially when topsoil is unavailable and costs of transport of FGD is low. Soil organic C in the FGDC was much higher in both depths than FGD (5.1 Mg ha–1 in 0- to 10-cm and 9.5 Mg ha–1 in 10- to 20-cm depth). With respect to SP (or time zero), about two-fold increase in soil organic C was observed in FGDC for both depths. Increases in the soil organic C content for reclaimed plots during the 8-yr period clearly demonstrated their C sink potential.


   ACKNOWLEDGMENTS
 
This project was funded by Los Alamos National Laboratory, New Mexico. We also appreciate the help and support of Mr. Chris Zoller, county extension agent, and landowners Ms. Julie Randolf and Mr. Jim Loveday.

Received for publication July 1, 2004.


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




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