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TECHNICAL REPORTS
Ecosystem Restoration

Compost and Calcium Surface Treatment Effects on Subsoil Chemistry in Acidic Minespoil Columns

Department of Crop and Soil Sciences, Penn State Univ., 116 ASI Building, University Park, PA 16802

^* Corresponding author (rcs15{at}psu.edu)

Received for publication April 22, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Surface incorporation of a liming agent in combination with compost or biosolids is a proven way to revegetate acidic minespoils, but little is known about the effect of the surface amendments on subsoil chemistry. We conducted a greenhouse column experiment to investigate how different surface amendments affected plant growth and subsoil chemistry in highly acidic minespoil material. Columns were filled with shale minespoil material (pH 2.5), amended with CaCO₃, CaSO₄·2H₂O (gypsum), and two rates of compost, and seeded with birdsfoot trefoil (Lotus corniculatus L.) and ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.). We measured leachate and plant growth over a 170-d period with extensive irrigation. Without CaCO₃, plants could only grow at the high compost rate (68.8 g kg^-1), even though the soil pH in those treatments was below 3.5, indicating the capability of natural organic matter to detoxify Al³⁺ by forming Al–organic matter complexes. Compost had no effect on the subsoil. When CaCO₃ or gypsum was added to the surface, extractable Ca increased in the subsoil, but there was no relevant increase in subsoil pH. Even in the first 5 cm of subsoil material, extractable Al did not decrease very much, possibly because a jurbanite-like solid phase controlled subsoil Al³⁺ activities. During the reclamation of highly acidic minespoil material one should therefore not expect significant effects of the surface treatment on the untreated subsoil. A sufficient root zone would have to be achieved by incorporating the liming agent down to the desired rooting depth.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A CENTURY OF strip mining coal in the eastern United States has left more than 0.5 million ha of land surface in need of reclamation (Sutton and Dick, 1987). Oxidation of sulfide minerals in disturbed overburdens has transformed former strip mines into acidic minespoils that are often devoid of vegetation, leaving them prone to surface runoff and erosion at high rainfall intensities (Sutton and Dick, 1987) as well as increased infiltration and drainage at low rainfall intensities, because a buffering plant cover is missing. All of this can have devastating environmental effects on receiving water bodies. One of the primary objectives for reclamation mandated by the Surface Mining Control and Reclamation Act (Office of Surface Mining, 2002) is therefore sustainable revegetation. The major limitation to plant growth under acidic conditions is phytotoxic concentrations of monomeric aluminum (Al³⁺). Consequently, successful reclamation should permanently reduce acidity and Al³⁺ concentration in the soil solution. A commonly used and successful approach for reestablishing plant growth is in situ remediation through surface application and incorporation of alkaline substances such as limestone, lime, fly ash, or flue gas desulfurization (FGD) by-products (Sutton and Dick, 1987; Stehouwer et al., 1995a,b). Although a large volume of literature deals with the success of these reclamation efforts (Sutton and Dick, 1987; Sopper, 1992), little attention has been given to the effect of surface treatments on the chemistry of the untreated material below the amended layer.

Experience from amelioration of naturally occurring subsoil acidity in highly weathered soils has shown that surface incorporation of CaCO₃ alone does not significantly affect the untreated subsoil (Mathews and Joost, 1990; Pavan et al., 1984; Ritchey et al., 1980; Sumner, 1995). Surface-applied gypsum (CaSO₄·2H₂O) has efficiently and sustainably reduced Al saturation in naturally acidic subsoils through exchange of Al³⁺ by Ca²⁺ in the subsoil without neutralizing the subsoil acidity (Oates and Caldwell, 1985; Pavan et al., 1984; Ritchey et al., 1980; Sumner et al., 1986; Sumner, 1995; Toma et al., 1999; Wendell and Ritchey, 1996). Additionally, surface-incorporated organic matter has proven to beneficially affect acidic subsoils by detoxifying Al³⁺ through the formation of nontoxic complexes between Al and dissolved organic matter (DOM) in the subsoil (Hargrove and Thomas, 1981; Hue, 1992; Liu and Hue, 1996). Moreover, it has been shown that surface application of lime-stabilized biosolids decreased subsoil acidity and increased subsoil Ca saturation, probably due to increases in Ca mobility caused by Ca–DOM complexes (Brown et al., 1997; Tan et al., 1985; Tester, 1990). Stehouwer et al. (1995b) indicated that sewage sludge increased the mobility of Ca in minespoil material amended with gypsum containing flue gas desulfurization by-products, but did not examine the effect on untreated subsoil material. It is not well studied how these experiences relate to effects of surface treatments on acidic subsoil material in acid minespoils.

The objective of this study was to examine the effect of different surface-incorporated amendments on plant growth, Ca and Al mobility, and chemistry of the untreated subsoil material, using greenhouse soil columns filled with acidic minespoil material and amended with CaCO₃, gypsum, and two different rates of compost in an accelerated leaching experiment.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Minespoil and Compost Material
We collected acidic minespoil material from an abandoned mine land site in Jefferson County, Pennsylvania. The material was mainly shale and was screened during collection to eliminate all fragments with diameters larger than 20 mm. The resulting sample contained 60% rock fragments (2–20 mm). The soil separates (<2 mm) had a sandy loam texture. We thoroughly mixed the collected spoil material before it was analyzed and packed in columns. This material contained 0.8, 10.8, and 13.6 mmol_c kg^-1 extractable K⁺, Mg²⁺, and Ca²⁺ (Mehlich-3 extract). Extractable Al³⁺ and titratable acidity in 1 M KCl extracts (1:5 solid to solution suspension ratio, one hour horizontal shaker, filtered) was 43 ± 3 mmol_c kg^-1 and 77 ± 4 mmol_c kg^-1, respectively. Acidity as measured by the SMP buffer pH method (Eckert and Sims, 1995) was 285 mmol_c kg^-1.

For the compost amendments we used a biosolids compost from the University Area Joint Authority (UAJA) composting facility in State College, Pennsylvania. This compost is generated by composting undigested, dewatered sludge from the primary and secondary clarifiers of the UAJA wastewater treatment facility with hardwood sawdust. The composting process is carefully controlled and takes about 26 d. Lime is not added at any point of the process. The compost we used for our experiments had a pH of 6.8 and contained 430 g kg^-1 C, 22 g kg^-1 N, and 1.26 g kg^-1 Ca (63 mmol_c kg^-1 Ca²⁺). The ash content was 110 g kg^-1. The compost did not contain any particles larger than 20 mm and we did not screen it before use.

Column Design and Amendment Treatments
We built 36 soil columns from Schedule 40 polyvinylchloride (PVC) pipe with a 0.15-m internal diameter. Each column was 0.5 m long with a PVC plate at the bottom. We attached 0.6-m-long pieces of flexible tubing with an internal polytetrafluoroethylene lining and a 4-mm internal diameter to a hole in the bottom plate and collected the leachate from the columns in 1-L amber glass bottles that stood on the ground below the columns. The hanging tubes were intended to minimize the duration of saturated conditions at the bottom of the columns.

The lower 0.3 m of the columns was filled with untreated minespoil material (8.21 kg dry weight) and compacted to a bulk density of 1.5 Mg m^-3. These lower 0.3 m will be referred to as the "subsoil." Above this subsoil layer, amended spoil material (3.56 kg dry weight) was packed and compressed to 0.15 m thickness and a bulk density of 1.3 Mg m^-3, henceforth called the "amended topsoil." The different amendment treatments consisted of all possible combinations of adding CaCO₃, gypsum, or no Ca source with three different rates of compost (Table 1). The CaCO₃ was added so that the added Ca²⁺ equivalents equaled the previously determined total extractable acidity of the minespoil material in the soil column. Gypsum was added at the same Ca²⁺ equivalent as CaCO₃. The Ca added with compost at the high compost rate amounted to 8% of the Ca added by CaCO₃ or gypsum. The 12 different treatments (Table 1) were replicated three times in a randomized complete block design.

Every four weeks we additionally leached the columns by adding 0.1-L increments of deionized water to each column once an hour for a total of 0.9 L, equaling 50 mm of irrigation. The leachate generated within 24 h after the last 0.1-L addition was collected and analyzed. Any leachate generated afterwards and until the next leaching event was collected separately. All leachates were analyzed for pH (Orion Ross Semi-Micro pH electrode; Thermo Orion, Beverly, MA). In leachates collected from the 0.9-L leaching events we additionally measured dissolved organic carbon (DOC) as nonpurgeable organic carbon (TOC-5000A analyzer; Shimadzu, Kyoto, Japan) and Ca and Al concentrations by atomic absorption spectrophotometry.

The plants received 16 h of light per day (0600–2200 h). The average temperature was 22°C during the day and 20°C at night. After 12 weeks of growth, the plants were cut to 4 cm immediately before the 0.9-L leaching. From there on the vegetation was cut to 4 cm every four weeks for a total of four harvests. The harvested cuttings were dried (70°C, 48 h) and weighed. Fertilizer was added to all columns after the first and third harvest at the same rates as the first application. The total amount of Ca contributed by the fertilizer was 8% of the Ca added in the CaCO₃ or gypsum treatments.

The experiment ended after 170 d. At this point, the amount of water we had added to the columns with plant growth equaled almost two years of mean annual precipitation in central Pennsylvania. We disassembled the columns and sampled soil from the amended topsoil (0–0.15 m) and from three subsoil layers (0.15–0.20, 0.20–0.25, and 0.25–0.35 m). The samples were stored moist at 4°C until extraction with 1 M KCl. Because of the heterogeneity and coarseness of the material, 50 g of the moist minespoil material was extracted with 500 mL 1 M KCl (horizontal shaker, 1 h). Major cations (Al³⁺, Fe³⁺, Ca²⁺) were determined in the extracts by atomic absorption spectrophotometry; pH was measured with an Orion Ross Sure-Flow Model 81-72 combination electrode connected to an Orion Model 520A pH meter (Thermo Orion).

Statistical Analyses
All statistical analyses of the measured response variables in the subsoil were conducted with S-Plus (Mathsoft, 1999). We accounted for the fact that responses in lower subsoil layers were not independent of preceding layers by treating depth as a factor with repeated measures. Accordingly, the statistical layout was a randomized block factorial design with repeated measures on the depth factor and complete randomization for the Ca source and compost factors.

Analysis of variance followed the general procedures outlined in Neter et al. (1996) and the recommendations for repeated measures models in Girden (1992) and Kirk (1995). The analysis of variance (ANOVA) tables for these repeated measures designs contained two error strata, one for the between-columns effects (Ca source, compost) and one for the error associated with the repeated measures factor (depth), the within-columns effect. Due to a lack of degrees of freedom, no interaction was considered between block and the other factors. Main factor-level differences were tested with the appropriate error stratum mean square error. If the interaction between the repeated measures variable (depth) and the other main factors was significant, we calculated separate ANOVA tables for each layer to obtain the appropriate error variances for testing factor-level effects.

Seven simultaneous tests were conducted to test three main factor-level effects, three two-way interactions, and one three-way interaction. At a family confidence level of 0.05, each of these seven tests was significant at _i 0.0073 using the Kimball inequality (Neter et al., 1996). Mean differences were tested at a family confidence level of 0.05, with individual test levels calculated using the Bonferroni inequality (Neter et al., 1996).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Biomass Production, Leachate Volumes, and Chemistry of the Amended Topsoil
In all columns with plant growth the roots ended sharply at the boundary between the amended topsoil and the untreated subsoil. Whenever CaCO₃ was part of the topsoil amendment (L, LG), plants grew very well, and neither compost nor gypsum had any additional effect on biomass production (Table 2). Columns that did not receive CaCO₃ nor the high compost rate (Nc₀, Nc_m, Gc₀, Gc_m) did not sustain plant growth. High compost alone or together with gypsum (Nc_h, Gc_h) resulted in intermediate biomass production, even though the pH in the surface soil of those columns at the end of the experiment was below 3.5 (Table 2). In the Nc_m and Gc_m treatments the pH was not much lower, but they were not able to sustain any plant growth. Plants cannot usually grow at low soil pH because of phytotoxic levels of Al³⁺ activity (Marschner, 1995; McCray and Sumner, 1990; Pavan et al., 1982). It is, however, well known that Al³⁺ forms strong complexes with humic substances (Schnitzer and Skinner, 1964; Vance et al., 1996). Contrary to the low rate, the high compost rate provided sufficient Al sorption capacity to keep the solution Al³⁺ activity low enough to allow plant growth. However, the root system in the amended topsoil of the Nc_h and Gc_h treatments was not as well established and dense as in the CaCO₃–containing treatments and the aboveground biomass production was lower.

All Ca treatments (G, L, and LG) still contained significant amounts of Ca at the end of the experiment (Table 2). Less than 2% of the total Ca that was added to the G and L treatments and less than 1% of the Ca that was added to the LG treatments could have been removed by aboveground plant material over the course of the experiment, assuming a maximum Ca content of 15 g kg^-1 for turf grasses (P.J. Landschoot, personal communication, 2002).

Despite high Ca concentrations in the surface soil of the gypsum-only treatments (G), extractable Al was not much reduced, leaving phytotoxic Al³⁺ activities in the soil solution except for the high-compost treatment, Gc_h (Table 2). Extractable Al was higher and extractable Ca was much lower in the Nc_h treatment than in the Gc₀ treatment, yet plants grew in the Nc_h treatment, but not the in Gc₀ treatment (Table 2). This is further evidence that the large amount of compost added in the Nc_h treatment was able to significantly reduce phytotoxic Al³⁺ solution activity in the topsoil by forming nontoxic Al–organic matter complexes, while even large amounts of gypsum-derived SO^2-₄ in the Gc₀ treatments could not sufficiently reduce Al³⁺ solution activities in the amended topsoil. Even the combination of medium compost rate and gypsum (Gc_m) did not allow plant growth.

Leachate and Subsoil Chemistry
The results of this experiment were not only affected by the different treatments but also by the varying total leachate volumes (Table 2). We therefore present the leachate Al, Ca, and dissolved organic C data as cumulative losses from the columns, calculated by integrating the measured leachate concentrations over the leachate volumes (Fig. 1 and 2) .

View larger version (23K): [in this window] [in a new window]   Fig. 1. Cumulative dissolved organic carbon (DOC) leaching, calculated from the leachates collected at the bottom of the columns. Values are means of three replicates ± standard deviation. N, no Ca added; G, gypsum; L, CaCO3; LG, CaCO3 + gypsum; c0, no compost added (values for N, G, L, LG all fell on this line); cm, 34.4 g kg-1 compost; ch, 68.8 g kg-1 compost.

View larger version (34K): [in this window] [in a new window]   Fig. 2. Cumulative leaching of Ca (left panel) and Al (right panel) calculated from the leachates collected at the bottom of the columns. Values are means of three replicates ± standard deviation. N, no Ca added; G, gypsum; L, CaCO3; LG, CaCO3 + gypsum; c0, no compost; cm, 34.4 g kg-1 compost; ch, 68.8 g kg-1 compost.

Calcium
Contrary to our expectations that soluble organic matter from the compost would increase Ca movement into and mobility in the untreated subsoil material, compost application did not increase Ca leaching at the bottom of the columns (Fig. 2). Neither did compost rate affect extractable subsoil Ca in the G, L, and LG treatments (Table 3). The G and LG treatments had consistently higher total Ca leaching than all other treatments (Fig. 2). The reason for this observation may be that gypsum constituted a more mobile Ca source than CaCO₃. Overall, however, leachate Ca concentrations in treatments without a Ca source (N) were not very much lower than in the treatments with added Ca (G, L, and LG). Obviously, most of the Ca mobilized in the CaCO₃– and gypsum-amended topsoil was sorbed in the 0.3 m of subsoil material. This hypothesis was supported by the observation that at all depths more Ca was extractable in the G, L, and LG treatments than in the N treatments under no or medium compost (Table 3) and by the fact that extractable Ca generally decreased with increasing depth.

In the high-compost treatments the extractable Ca data were unexpected. There was no difference between Nc_h, Lc_h, and LGc_h treatments, while all had less extractable Ca than the Gc_h treatment. This might have been a result of the low total leachate volume in the Lc_h and LGc_h treatments (Table 2), caused by vigorous plant growth and increased water holding capacity of the surface due to the applied compost and the dense root system. This resulted in less leaching from the topsoil into the subsoil in these treatments.

Aluminum
Leachate Al concentrations initially ranged from 40 to 50 mmol_c L^-1 and generally decreased with increasing leachate volume to between 10 and 30 mmol_c L^-1. Compost did not significantly affect Al leaching. The treatments with only gypsum (G) showed the highest overall export of Al, but only slightly more than the LG treatments that received the same amount of gypsum (Fig. 2).

Extractable Al consistently increased with increasing depth (Table 3). In the absence of compost, the treatment without any Ca added to the surface (Nc₀) had the most extractable Al while LGc₀ had the least (Table 3), as would be expected if Ca and alkalinity from the surface treatment were transported into the subsoil and exchanged or neutralized Al³⁺. At the medium compost rate, however, there was no significant effect of surface treatment on extractable Al in the subsoil. At the high compost rate more Al was extractable in the subsoil of CaCO₃ treatments (Lc_h, LGc_h) than in the Nc_h and Gc_h treatments, even though one would have expected a reduction of extractable Al due to the leaching of alkalinity and Ca from the topsoil in the Lc_h and LGc_h treatments. These results could be related to low leachate volumes in those treatments and agree with the observation that extractable Ca was low in the Lc_h and LGc_h treatments. Thus, absolute leachate volume may have had a stronger effect than the type of surface treatment.

Because we collected leachate data only at the bottom of the columns, the leachates most closely represent conditions at a 0.3-m depth below the amended topsoil. Nonetheless, it is interesting that very little difference existed between the control treatments (N) and the treatments with a Ca source (G, L, and LG). Together with the observations from extractable Al this indicates that Al³⁺ solution activity was controlled by a solid phase rather than by the exchange and removal of Al³⁺ by Ca²⁺, as we had expected at the outset of the experiment. Consequently, we examined whether different solid phases could have affected Al³⁺ solution activity. For this assessment, Al³⁺ solution activity was estimated by assuming that the major cations in solution were Al³⁺ and Ca²⁺ and that the major anion, which we had not measured, was SO^2-₄, with a charge concentration equaling the sum of Al³⁺ and Ca²⁺ charges. This assumption seemed reasonable, as the pyrite oxidation that created the acidic minespoil material produces sulfate. Thus, the dominant anion in the subsoil must have been SO^2-₄, even in those treatments that did not receive gypsum in the surface amendment. Because the calculated ionic strength was above 0.1, we calculated the activity coefficients for Al³⁺ and SO^2-₄ from the Davies equation in its modified form (Stumm and Morgan, 1996). We then estimated Al³⁺ activity from total measured Al concentration, accounting for the formation of Al–SO₄ complexes. Figure 3 shows a stability diagram with lines for the aluminum hydroxy-sulfates jurbanite [AlSO₄(OH)·5H₂O], alunite [KAl₃(SO₄)₂(OH)₆], and basaluminite [Al₄(OH)₁₀SO₄·17H₂O], and diaspore (AlOOH) as a representative of the aluminum hydroxides. The SO^2-₄ activity used for the construction of the lines was the mean of the estimated SO^2-₄ activities. It is easy to see that leachate solutions were undersaturated with respect to alunite, basaluminite, and diaspore, but might have been in equilibrium with a jurbanite-like solid phase. We do not know whether our leachates were in a true equilibrium with the minespoil material, but many other studies have shown the possibility of jurbanite controlling Al³⁺ activity in acidic and acid sulfate soils (Evans, 1991; Menzies et al., 1994; Tin and Wilander, 1995; Vogt et al., 2001). Karathanasis et al. (1988), who found very conclusive solution data evidence for the presence of a jurbanite solid phase, were unable to find jurbanite peaks in X-ray diffraction experiments in the corresponding soil material. They attributed this to the presence of jurbanite in poorly crystalline or amorphous form. Apparently, mineralogical proof of the presence of a possibly amorphous, jurbanite-like solid phase is not trivial and was not attempted.

View larger version (16K): [in this window] [in a new window]   Fig. 3. Solubility diagram for jurbanite [AlSO4(OH)·5H2O], alunite [KAl3(SO4)2(OH)6], basaluminite [Al4(OH)10SO4·17H2O], and diaspore (AlOOH). The Al3+ activities were calculated from the measured Al concentration, accounting for the formation of Al–SO4 complexes. Thermodynamic data are taken from Lindsay and Walthall (1996). Lines are calculated by assuming log = -1.83. Error bars are for log = -0.83 and log = -2.83. For alunite we assumed log(K+) = -5.6. Black dots represent measured data.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

While we found a significant increase in extractable subsoil Ca in treatments that received gypsum or limestone or both, the pH remained largely unaffected by the surface amendment. Extractable subsoil Al and leachate Al³⁺ activity were clearly not controlled by exchange reactions with Ca, but possibly by a jurbanite-like solid phase. While we found some indications for a reduction in extractable subsoil Al if a Ca source was added in the topsoil, the actual differences were small and not sufficient for improving root growth conditions. Although this experiment was only performed for a short period of time, we added the equivalent of almost two years of local mean annual rainfall to the treatments with plant growth and expected to see a substantial effect on at least the first 5 cm of subsoil as a prerequisite for any significant long-term effect of the topsoil treatment on the subsoil. However, the minor changes we observed, together with the possibility for a control of Al³⁺ activity by a jurbanite-like solid phase, did not indicate any such long-term effect.

Environmental Implications
A permanent reduction of phytotoxic Al³⁺ activity in the subsoil of highly acidic minespoil materials would only be achieved by directly increasing the pH in the subsoil. A liming agent would therefore have to be incorporated to the depth of the desired root zone to eliminate phytotoxic Al³⁺ activities in the soil solution. It should not be expected that in highly acidic materials the root zone would expand significantly downward below the zone of liming. Additionally, these results indicate that the benefits of using compost in the reclamation of these highly acidic minespoils appear to be limited to amelioration of physical and chemical conditions in the zone of incorporation as summarized by Sopper (1992). However, high amounts of organic matter in the amended surface might protect plants from Al³⁺ toxicity if all buffering capacity of the applied liming agents is depleted and the pH decreases due to ongoing pyrite oxidation.

Despite these conclusions, it would also be helpful to examine the subsoil effects of gypsum and limestone in combination with compost or another source of organic matter in less acidic minespoil material, where a reduction of Al³⁺ toxicity might be achieved by reducing exchangeable Al³⁺ and exchangeable acidity and by maintaining an elevated flow of Ca and alkalinity into the subsoil to offset any ongoing pyrite oxidation.

	ACKNOWLEDGMENTS

 
This research was supported by Research Grant Award no. IS-2758-96R from BARD, The United States–Israel Binational Agricultural Research and Development Fund. We also acknowledge the helpful comments by three anonymous reviewers.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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