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Published online 28 September 2007
Published in Soil Sci Soc Am J 71:1788-1795 (2007)
DOI: 10.2136/sssaj2006.0335
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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FOREST, RANGE & WILDLAND SOILS

Carbon Storage and Minesoil Properties in Relation to Topsoil Application Techniques

Pierre-André Jacinthea,* and Rattan Lalb

a Dep. of Earth Sciences, Indiana Univ./Purdue Univ. 723 W. Michigan St. Indianapolis, IN 46202
b Ohio State Univ. Carbon Management & Sequestration Center, 2021 Coffey Rd. Columbus OH 43210

* Corresponding author (pjacinth{at}iupui.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Reclaimed minelands could act as C sinks, but shallow soil, nutrient deficiency, and compaction could limit C accretion in these ecosystems. This study evaluated the impact of topsoil application techniques on total C storage (tree biomass and soil organic C [SOC]) in 15-yr-old experimental plots established on reclaimed land in southeastern Ohio. Treatments included topsoil (graded overburden [OV] and standard [ST] and ripped topsoil [RT]) and P fertilization (0 and 2.24 Mg ha–1 of rock phosphate). One half of each plot was planted with Austrian pine (Pinus nigra J.F. Arnold ssp. nigra) and the other half with green ash (Fraxinus pennsylvanica Marshall). A significant effect of topsoil application on tree growth and SOC was noted. In green ash plots, aboveground biomass was always <3.4 Mg C ha–1, but in Austrian pine stands it averaged 10.3, 15.2, and 2.1 Mg C ha–1 in the ST, RT, and OV plots, respectively. The pool of recent SOC (after discounting geogenic C) was in the order: ST (34.9 Mg C ha–1) > RT (29.8 Mg C ha–1) > OV (17.8 Mg C ha–1). The lower SOC in RT than in ST plots was attributed to enhanced C mineralization by soil ripping, but with the fast-growing Austrian pine, this SOC deficit was compensated by a greater (by 4.9 Mg C ha–1) standing tree biomass in the RT plots, resulting in comparable total C storage with either ST or CT. With the slow-growing green ash, however, total C storage was significantly lower in RT (27.9 Mg C ha–1) than in ST (37.4 Mg C ha–1) plots. Thus, the impact of topsoil application technique on C storage in these aggrading ecosystems is largely determined by tree growth and productivity.

Abbreviations: Abbreviations: EC, electrical conductivity • OV, graded overburden • RT, ripped topsoil • SOC, soil organic carbon • ST, standard topsoil • TOC, total organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Surface mining of coal deposits creates drastic landscape disturbance, including the removal of the vegetative cover and extensive degradation of soils. With proper restoration techniques and appropriate post-reclamation management, however, soil degradation can be reversed and mineland returned to productive use. Shallow soil depth (Andrews et al., 1998), soil physical properties restrictive to root growth (Bowen et al., 2005), soil salinity, limited biological activity, and nutrient deficiency (Barnhisel and Hower, 1997; Hüttl and Weber, 2001) are common impediments to successful rehabilitation and vegetation establishment in minelands. Phosphorus deficiency can be particularly acute, stemming in part from the high P fixation capacity of minesoils (Martinez et al., 1996; Chen et al., 1998). Improved tree survival and growth has generally been observed in P fertilization experiments (Chen et al., 1998; Praveen-Kumar et al., 2005; Casselman et al., 2006).

The Surface Mining Control and Reclamation Act of 1977 mandates that mined areas be restored to conditions similar to their premining state. This involves backfilling and grading the rock overburden to restore landscape topography, and application of topsoil to create a proper medium for plant growth. The use of heavy machinery to grade the overburden and apply topsoil during reclamation, however, has often led to minesoil compaction (Barnhisel and Hower, 1997), resulting in impaired soil drainage, poor soil aeration, and poor root development. As a result of these deleterious effects, soil compaction could negate the expected benefits of topsoil replacement on vegetation establishment and restoration of a range of soil functions. Poor growth of trees in topsoil-amended mineland has been linked to compaction (Chaney et al., 1995). To alleviate soil compaction, topsoil ripping has been implemented at several reclamation sites (Chong and Cowsert, 1997; Kost et al., 1998; Rokich et al., 2001). The impact of deep ripping on soil physical properties and tree survival has been the focus of these investigations, with limited emphasis on the effect of ripping on soil C dynamics and storage.

Restored minelands could act as C sinks, and thus could offset some of the CO2 released as a result of coal extraction and burning. In recent years, there have been numerous assessments of the impact of post-reclamation land uses and management on C sequestration in reclaimed minesoils. Although geogenic C (coal particles and dust) can be a large contributor to minesoil C stocks (10–95% of the total soil organic C; Rumpel et al., 2001; Shukla and Lal, 2005), the C storage potential of reclaimed minesoils appears quite high, with reported sequestration rates generally much greater than typically recorded in croplands (Akala and Lal, 2001; Shukla et al., 2004; Ussiri and Lal, 2005; Sperow, 2006). During the first 25 yr following mineland rehabilitation, Akala and Lal (2001) reported an accumulation of 30 Mg C ha–1 in minesoils under pasture and forest at several sites in Ohio. In fertilized reclaimed minesoils used for forage production, Shukla et al. (2004) noted a doubling in soil organic carbon (SOC) stocks in 20 yr (accretion rate: 0.75 Mg C ha–1 yr–1). Sequestration rates as high as 3 Mg C ha–1 yr–1 in reclaimed grasslands have been reported (see reviews by Shrestha and Lal, 2006; Sperow, 2006). Studies have also shown that, in several instances, SOC levels in minesoils quickly approach (Shukla et al., 2004) and sometimes exceed (Fettweis et al., 2005; Shukla and Lal, 2005; Ussiri et al., 2006) C stocks in adjacent undisturbed lands.

Few studies have investigated the impact of topsoil replacement techniques on C storage in minesoils. In restored (1–47 yr) grasslands and forests in southeastern Ohio, Shukla and Lal (2005) noted that the soil C sequestration rate was greater (0.45 Mg C ha–1 yr–1) with topsoil application than without (0.25 Mg C ha–1 yr–1). Bowen et al. (2005) also reported that both aboveground biomass and SOC storage were linearly related to the depth of topsoil applied during reclamation. These results, albeit limited, suggest that standard topsoil replacement can have positive effects on plant growth and C sequestration in minesoils. A question that remains unexplored, however, is whether additional C storage benefits could accrue from topsoil ripping. By alleviating soil compaction, soil ripping could create better soil conditions for root development and tree growth and, therefore, could result in greater C storage in the form of standing tree biomass and as SOC.

The main objective of this study was to assess the impact of different topsoil application practices on the storage of C in the biomass of trees and in minesoils. Given the context of this study, soil C was divided into mineralizable, inorganic, fossil organic, and recent organic C pools. The latter pool evolves from the decomposition of litter and the stabilization of humified C into the soil matrix. Highly responsive to changes in soil management, mineralizable C is often viewed as a store of nutrients and energy and thus could drive a range of soil processes. Although past studies (Waschkies and Hüttl, 1999; Rumpel and Kögel-Knabner, 2002) have indicated that lignite C can undergo mineralization, given the low decay rates (0.007–0.025 g lignite C kg–1 C yr–1, Rumpel and Kögel-Knabner, 2002), especially at sites where recent organic materials begin to accumulate, one can safely assume that change in the geogenic C pool size is negligible within a time frame of less than two decades. Therefore, accumulation of recent organic C was used to evaluate the impact of topsoil application practices on soil C storage.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Description of Study Sites and Sampling Procedures
The study was conducted at experimental plots established in March and April 1988 on reclaimed minelands near the village of Cumberland in Noble County, Ohio (39°47'N, 81°40'W). These plots were part of a larger investigation of the effects of topsoil application practices and P fertilization on the survival and growth of trees in reclaimed minelands (Kost et al., 1998). Before coal mining, soils (Soil Conservation Service, 1990) in the area were mapped as Morristown (loamy-skeletal, mixed, mesic Typic Udorthents), Lowell (fine, mixed, mesic Typic Hapludalfs), and Gilpin (fine-loamy, mixed, mesic Typic Hapludults). To access the coal seams, a scraper was used to remove the upper layers of earth materials (A, B, and C soil horizons), which were stockpiled and subsequently used as topsoil during the post-mining reclamation. The overburden was a mixture of calcareous claystone and sandstone, with limestone and shale as minor constituents (Kost et al., 1998). The area has a continental climate with long-term average daily temperature and annual precipitation averaging 11°C and 973 mm, respectively (Soil Conservation Service, 1990).

In the present assessment, three topsoiling practices and two P fertilizer applications (0 and 2.24 Mg ha–1 of rock phosphate spread by hand corresponding to 29 kg ha–1 of available P) were considered. At the establishment of the experimental plots, topsoil and overburden background Bray-1 P averaged 16.4 and 3.8 kg P ha–1, respectively (Kost et al., 1998). The topsoiling practices included (i) standard topsoil (ST) in which the overburden was graded and then overlaid by a 30-cm layer of topsoil, (ii) ripped topsoil (RT), which is similar to ST except that, following its application, the 30 cm of topsoil was ripped and disked to alleviate soil compaction, and (iii) overburden (OV) in which no topsoil application was made, and trees were planted directly into the graded mine spoils. The six (2 x 3) combinations of P levels x topsoiling practices were distributed into two blocks of six plots each. Each plot (11 by 6 m, slope 5–11%) was divided into two subplots, with one subplot planted with Austrian pine and the other subplot with green ash. Tree planting distance was 1.2 and 1.8 m within and between rows, respectively (Kost et al., 1998). Plots were also seeded with a mixture of grass species including perennial ryegrass (Lolium perenne L.), orchard grass (Dactylis glomerata L.), annual bluegrass (Poa annua L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), birdsfoot treefoil (Lotus corniculatus L.), and clover (Trifolium sp.) to provide an initial vegetative cover.

Soil samples were collected in March 2004 from the experimental subplots at depths of 0 to 10, 10 to 20, and 20 to 40 cm, and within tree rows. Samples were bulked to make composite samples per depth. Two soil cores (diameter 5.5 cm) per depth were taken from each subplot for determination of soil bulk density. Composite soil samples were air dried, sieved (2 mm), and used for the determination of soil chemical properties. A portion of each soil sample (<2 mm) was further crushed using a rolling grinder, and the fraction that passed through a 250-µm sieve was used for C and N analysis. Because the SOC level in the experimental plots was not determined at the time of their establishment, soil samples (18 sampling points per site within a total area of 0.2 ha and at depths 0–15 and 15–30 cm) were also collected from two newly reclaimed areas (<1 mo after topsoil application) within a 50-km radius from the experimental plots. Like the experimental plots, both of newly reclaimed sites were located within the Ohio Soil Region 12 dominated by the Gilpin–Guernsey (fine, mixed, mesic Aquic Hapludalfs)–Lowell–Upshur (fine, mixed, mesic Typic Hapludalfs) soil association (Soil Conservation Service, 1990). These samples were analyzed for recent organic C (see below), and the results used as a baseline SOC for the experimental plots.

At the time of soil sampling (March 2004), the height and diameter at breast height of every tree in an experimental plot were measured and recorded. In the OV treatment plots, because trees were short (<2 m), tree diameter was measured at the ground surface, which could possibly cause a slight overestimation of aboveground biomass in this treatment. Aboveground biomass (Bmass, kg) of trees was computed using the equation (Montagu et al., 2002; Snowdon et al., 2002)

Formula 1[1]
and

Formula 2[2]
where DBH is the diameter at breast height (cm), and A = 1.019 or 1.021, B = 2.391 or 2.589, and C = 2.413 or 2.733 for softwoods and hardwoods, respectively (Montagu et al., 2002; Snowdon et al., 2002). An average C content of 40% was assumed to convert tree biomass into C storage. The trees in each plot were also counted and the survival rate determined.

Soil Physical and Chemical Properties
Soil pH was measured with an Orion pH meter (Thermo Fisher Scientific, Waltham, MA) using a soil/water ratio of 1:2, and electrical conductivity measured with a conductivity meter using a 1:5 ratio. Particle size was determined by the hydrometer method (Gee and Bauder, 1986). Soil bulk density was determined by the core method. Soil cores were oven dried for 72 h (105°C), and the dry weight of earth materials in each core was recorded. Each soil core was then soaked overnight in water and thoroughly washed through a 2-mm sieve with tap water. The weight and volume of the materials (pebbles and gravel) retained on the sieve were recorded. Soil bulk density was computed as the ratio of gravel-free dry soil mass to core volume (corrected for pebbles and gravel volume).

Assessment of Carbon Pools
Duplicate (20-g) field-moist samples were placed in serum bottles (160 mL) and incubated at room temperature for 10 d. Air samples were taken for bottle headspace and analyzed for CO2 by gas chromatography. The amount of CO2 evolved during the 10-d incubation was taken as a measure of readily mineralizable C.

Finely ground (<250-µm) soil samples were used for total C analysis by dry combustion (960°C) using a Vario-Max C-N analyzer (Elementar Americas, Laurel, NJ). Inorganic C was determined using a procedure involving acid decomposition of carbonates (Loeppert and Suarez, 1996) in a sealed serum bottle (2 g of soil, 4 mL of 1 mol L–1 HCl) and measurement of the CO2 evolved using gas chromatography. Total organic C (TOC) was computed as the difference between total C and inorganic C. A combination of acid hydrolysis and thermal oxidation was used to fractionate TOC into thermo-hydrolyzable and recalcitrant fractions. The fractionation approach is based on the assumption that organic C associated with coal particles (fossil or geogenic C) is more resistant to hydrolysis and heat treatments than recently humified soil organic matter. Briefly, soil samples were successively treated with 10% HF to decompose silicates, NaOH (0.5 mol L–1) to remove humic and fulvic acids, and 6 mol L–1 HCl (overnight at room temperature and then for 3 h at 90°C) to decompose the hydrolyzable C fractions. Following washing and drying, the residual soil material was submitted to a heat treatment (350°C for 3 h in a furnace). The carbonaceous material that survived the acid hydrolysis and heat treatment was termed fossil C and was then determined by dry combustion using a CN analyzer. The amount of recent organic C was computed as the difference between TOC and fossil C. Further details regarding this fractionation procedure are provided in Lim and Cachier (1996) and Gelinas et al. (2001).

Carbon pools (Mg C ha–1) in a soil layer of thickness T (m) were computed as

Formula 3[3]
where C is the C concentration (% by weight) and {rho}b is the bulk density (Mg m–3).

Statistical Analysis
Data were submitted to ANOVA to assess the effect of topsoil application practices and P fertilization on tree growth and soil C pools. Analysis of soil C data was performed for each soil depth separately. The ANOVA was conducted using the GLM procedure available in SAS (SAS Institute, 2001). Unless otherwise noted, statistical significance was determined at the P < 0.05 level and Fisher's least significant difference test was used for comparison of means.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tree Growth and Standing Biomass Carbon
A significant effect of topsoil application on tree growth was noted, but the effect varied with tree species. In the Austrian pine stands, growth parameters (height, diameter, and biomass) were several times higher with topsoil application than without (Table 1 ). Austrian pine standing biomass was significantly greater in the RT (15.2 Mg C ha–1) than in the ST (10.3 Mg C ha–1) plots. Standing biomass of green ash (<3.4 Mg C ha–1), however, was always low and did not significantly differ between ST and RT (Table 1). Tree survival averaged 41 and 30%, respectively, in the plots with and without topsoil application. No effect of rock phosphate application on tree growth was noted. Averaged across topsoil application practices, standing tree biomass was 19.2 and 14.5 Mg C ha–1for the Austrian pine and 1.4 and 1.5 Mg C ha–1for the green ash stands in P-fertilized and unfertilized plots, respectively.


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  		Table 1. Tree height, diameter, and biomass C after 15 yr of growth on reclaimed mined land. Values are means with standard deviations in parentheses.

 
Physical and Chemical Properties of Soils
Soil bulk density ranged from 1.1 to 1.9 g cm–3 and increased with depth (Table 2 ). No significant difference was found, however, among the topsoil application practices with regard to bulk density. The mass and volume of coarse materials in the OV plots (10% core volume) were twofold higher than in the other treatments (Table 2). The average density (ratio of mean weight to mean volume) of the coarse materials was 2.54 g cm–3, suggesting that these coarse fragments were a mixture of mineral and organo-mineral materials. Clay content averaged 31% in the OV and 41% in the ST and RT plots. Sand content was higher in the OV plots than in the other treatments (Table 2).


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  		Table 2. Physical and chemical properties of 15-yr-old reclaimed mined land as related to topsoiling practices. Values in parentheses are standard deviations of the means (n = 8 measurements).

 
Soil pH ranged from 4.7 to 8.9 and electrical conductivity (EC) ranged from 0.01 to 0.22 dS m–1 (Table 2). A significant effect (P < 0.04) of topsoil application on soil pH and EC was detected in the 0- to 10- and 10- to 20-cm soil layers. Where topsoil was applied, soil pH was circumneutral (6.5–6.9, Table 2), whereas moderately alkaline (pH range 7.1–8.9) conditions tended to prevail in the OV plots. Likewise, EC was two to four times higher in the OV than in the ST and RT plots (Table 2).

Recent Organic Carbon Pool in Applied Topsoil
The pool of recent organic C in the newly reclaimed minesoils near Harrietsville and New Athens, OH, averaged 23.7 and 24.1 Mg C ha–1, respectively (Table 3 ). Inorganic C and fossil C accounted for 5 and 18%, respectively, of the total soil C. The C/N ratios ranged between 9 and 13 in the newly reclaimed minesoils.


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  		Table 3. Soil C in topsoil (0–30-cm depth) applied during the reclamation of minesoils at two locations in southeastern Ohio. Values are the means of 36 measurements with standard deviation in parentheses.

 
Topsoil Application and Carbon Storage
The effect of topsoil application and P fertilization on mineralizable C was limited to the 0- to 10-cm soil layer (Fig. 1 ). Mineralizable C was generally higher in the RT than in the other plots (Fig. 1); however, the effect of topsoil application technique was not significant. Compared with plots receiving no rock phosphate at the beginning of the experiment, mineralizable C was significantly (P < 0.01) higher in P-fertilized (77.3 mg C kg–1 soil) than unfertilized plots (22.4 mg C kg–1 soil).


Figure 1
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  		Fig. 1. Readily mineralizable C in reclaimed minesoils as affected by (A) P fertilization at the time of reclamation and (B) topsoil application technique. For a given soil depth, different letters over the bars indicate a significant difference between treatment means (P < 0.05).

 
The ANOVA showed significant effects (P < 0.01) of topsoil application practices on soil C pools. The total amount of C measured in the OV plots (126.3 Mg C ha–1) was two to four times that in the ST and RT plots (Fig. 2 ). Carbonates made up 73% of C in the OV treatment (Table 4 , Fig. 2), however, whereas in the ST and RT treatments, carbonate C accounted for <5% of the total C. The high concentration of carbonates reflects the calcareous overburden commonly found in southeastern Ohio coal mines. Fossil C (C fractions that are resistant to hydrolysis and thermal treatment) were also more (two to three times) abundant in the OV than in the other treatments (Table 4). On average, the fossil C pool accounted for 45% of the TOC in the OV treatment, and 17% of the TOC in the ST and RT plots. The pool (mean ± SD) of recent organic C (Fig. 2) was in the order ST (34.9 ± 9.5 Mg C ha–1) > RT(29.8 ± 5.8 Mg C ha–1) > OV (17.8 ± 8.7 Mg C ha–1) plots. In the RT and OV treatments, the C/N ratios (range 10–22) of the soil organic matter were higher than in the ST plots (range 5–13).


Figure 2
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  		Fig. 2. Carbonates and fossil and recent organic C pools in reclaimed minesoils as related to topsoil application techniques. Note that carbonates are plotted on a different scale (x3). For a given soil depth, a significant difference (P < 0.05) among topsoil application techniques is indicated by different letters.

 

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  		Table 4. Inorganic, fossil, and recent organic C concentrations in 15-yr-old reclaimed mined land as related to topsoil application techniques. Values in parentheses are standard deviations of the means (n = 16 measurements).

 
Standing biomass and recent soil C storage in the Austrian pine plots averaged 44.1, 49.5, and 19.1 Mg C ha–1 under ST, RT, and OV treatments, respectively (Fig. 3 ). Corresponding values for the green ash plots were 37.4, 27.9, and 18.8 Mg C ha–1. Across topsoil application practices and tree species, recent soil organic C accounted for >70% of the total C storage.


Figure 3
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  		Fig. 3. Organic C storage as standing tree biomass and as soil organic matter after 15 yr of mined land reclamation. For a given tree species, a significant difference (P < 0.05) among topsoil application techniques is indicated by different letters.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Phosphorus Fertilization and Mineralizable Carbon
No effect of rock phosphate application was found on tree growth or on almost any of the soil properties considered in the study. This result is in line with a previous evaluation of tree growth at this site that had also shown a lack of P fertilization effect (Kost et al., 1998).

The only significant effect of P fertilization noted in the present study was limited to the pool of mineralizable C in the surface soil layers (Fig. 1). The positive effect of P fertilization on C mineralization is probably indirect and involves litter quality. Several vegetative tissue constituents such as polyphenols and tannins have been linked to a slow rate of litter decomposition, and studies (Hattenschwiler et al., 2003; Kraus et al., 2004) have shown that their concentrations tend to increase in the tissues of plants grown in P-stressed environments. The work of Ghani et al. (2003) has also demonstrated the positive impact of long-term P fertilization of pasture on labile organic C pools. These past results suggest that the uptake of slowly released rock phosphate P by vegetation at the experimental plots may have led to improved litter quality, which, in turn, resulted in a larger pool of labile C in the P-fertilized plots.

Soil Physicochemical Properties and Tree Survival
The use of heavy machinery to grade the overburden and apply topsoil has been blamed for compaction problems in minelands. It was anticipated that the ripped topsoil would be less compacted. If bulk density can be used as a proxy for soil compaction, the data (Table 2) showed no indication that ripping had resulted in less soil compaction. These results contrasted with those of Ashby (1997) and Rokich et al. (2001), who reported positive effects of ripping on soil physical properties and root growth but, in those particular instances, the depth of soil ripping was 80 to 120 cm instead of 30 cm, as was the case at the present study site. It is also possible that topsoil ripping had alleviated soil compaction in the early years of the experiment, but that effect became undetectable as the reclaimed land settled over the years. Chong and Cowsert (1997) noted that the effect of deep tillage on water infiltration and compaction in Illinois minesoils was short lived, lasting <3 yr. Cleveland and Kjelgren (1994) found no effect of deep ripping on soil properties or tree productivity.

With an average concentration of 30%, the abundance of clay-size particles in the OV treatment was surprisingly high (Table 2). After 16 yr of loblolly pine (Pinus taeda L.) stand development on a sandstone/siltstone (2:1) overburden in Virginia, the average clay content was only 10% (Bendfeldt et al., 2001). Since climatic conditions at the two study sites are similar (mean temperature 11°C and annual rainfall 973–1150 mm), a difference in overburden chemistry and mineralogy may have contributed to these results. Kost et al. (1998) noted that claystone was a dominant overburden constituent at the present study site. Past research (Haering et al., 1993) has demonstrated that the weathering of spoil materials and soil profile development in minesoils can occur fairly rapidly (in just a few years). It is thus conceivable that the rapid weathering of claystone may have contributed to the abundance of clay-size particles in the OV treatment fine earth materials.

Electrical conductivity values at the experimental plots, including the OV treatment, were below the level (>1 dS m–1) considered restrictive to plant growth (Barnhisel and Hower, 1997). Examination of EC values from this study and from Kost et al. (1998) showed that, at this site, EC decreased with time (t, yr) according to a first-order decay model:

Formula 4[4]
with rate constants (k) of 0.28 and 0.38 yr–1 in the OV and topsoil treatments, respectively (Table 2). This gradual removal of soluble salts, probably due to weathering and leaching, should lead to more favorable soil conditions for plant growth.

Improvement in soil chemistry did not, however, translate into better tree survival (Table 1). It was also observed that even with the application of topsoil, the average rate of tree survival was only 42%, a rate much lower than the 62% reported from an earlier inventory at the site (Kost et al., 1998), indicating that tree mortality had occurred between 1998 and 2004. Since there was no weed control at the experimental plots, competition between young trees and grasses for nutrients and water is a probable explanation for the death of additional trees in the intervening 6 yr (Chaney et al., 1995; Casselman et al., 2006). As reported for other reclaimed sites in Ohio (Zeleznik and Skousen, 1996), pest infestation of young trees could also be a contributing factor to decreased survival rates.

Carbon Storage and Topsoil Ripping
An important observation in the present study is the rapid accumulation of recent organic C in the OV treatment: 17.8 Mg C ha–1 in 15 yr, with half (8.6 Mg C ha–1) of that amount stored in the top 10-cm soil layer (Fig. 2). It is unlikely that the overburden contained considerable amounts of hydrolyzable C at the beginning of the experiment. Therefore, the pool of recent organic C in the OV treatment probably originated from decomposition of vegetation biomass. Although pockets of fallen needles were observed in the Austrian pine plots, a true litter layer had not developed in either the Austrian pine or the green ash stands. Instead, and regardless of topsoil application, the surface of the experimental plots was covered with an abundant herbaceous vegetation. The extensive root system of the grass cover was best observed in the OV plots, where roots easily penetrated between the coarse gravel fragments. Annual root biomass production in the order of 13.5 Mg dry matter ha–1 has been reported in reclaimed grasslands in southeastern Ohio (Ussiri et al., 2006), suggesting that even if 10% of that amount is converted into humified soil C, root biomass input alone could be adequate to sustain the level of recent C accumulation observed in the OV plots. The significance of herbaceous vegetation to SOC buildup in OV sites was also demonstrated by the data of Shukla and Lal (2005), who reported significantly higher SOC stocks (0–30-cm depth) in grass-covered (59.3 Mg C ha–1) than in tree-covered (46.2 Mg C ha–1) OV sites in southeastern Ohio.

The rate of recent organic C accumulation in the surface layer (0–10 cm) of the OV treatment (0.6 Mg C ha–1 y–1) compared well with C sequestration rates measured under loblolly pine growing in the Appalachian region of Virginia (0.44 Mg C ha–1 yr–1; Bendfeldt et al., 2001) and in forested minesoils of southeastern Ohio (0.45 Mg C ha–1 yr–1; Shukla and Lal, 2005), but was much higher than the average rate (0.3 Mg C ha–1 yr–1) reported for Austrian pine stands growing on lignite-rich overburden in Germany (Fettweis et al., 2005). Although geographical (51 vs. 40° N) and climatic conditions may be contributing factors, the difference in C sequestration may also be related to the high acidity (from pyrite oxidation) and low nutrient status (Hüttl and Weber, 2001) of mine spoils in the Lusatian region where Fettweis et al. (2005) performed their investigation. In contrast, overburden in the coal region of southeastern Ohio is generally alkaline and rich in carbonates (Table 2). In addition, due to rapid weathering of the overburden (Haering et al., 1993), georeactive surfaces such as clay and silt particles are formed and provide a matrix for the physicochemical protection of recent organic C fractions.

In the ST and RT treatments, recent organic C included organic matter originally present in the applied topsoil as well as organic C that had accumulated during the 15 yr since reclamation. A baseline SOC is thus required to determine the net accumulation of SOC since reclamation. The pool of recent organic C measured at the newly reclaimed sites within the same soil region showed limited variability (24 ± 2.3 Mg C ha–1), suggesting that this SOC could reasonably be used as a baseline for the experimental plots. Published reports of C stocks in reclaimed mineland in the Appalachian region rarely include the initial soil C pool at the study sites. In the few instances where initial SOC was reported, considerable deviation is observed from our measured initial C pools. Bendfeldt et al. (2001) measured an initial SOC stock of 2 Mg C ha–1 (0–10 cm) in standard minesoil in Virginia. Likewise, Shukla and Lal (2005) reported a pool of organic C of 6.7 Mg C ha–1 (0–30 cm) for a newly reclaimed mineland in southeastern Ohio. These reported initial C pools (equivalent to 8–9 Mg C ha–1 if extrapolated to a depth of 40 cm) are about threefold lower than the 24 Mg C ha–1 recorded at the two newly reclaimed sites sampled for the present research. It is possible that the lower initial soil C pools in the published studies (Bendfeldt et al., 2001; Shukla and Lal, 2005) are related to differences in topsoil storage duration and differences in premining topsoil scraping techniques (varying degree of mixing of materials from different soil horizons). A most probable mechanism for the lower initial SOC in the published reports is, perhaps, the time interval between topsoil spreading and soil sampling. This information was not provided in Bendfeldt et al. (2001), but the newly reclaimed site in the study of Shukla and Lal (2005) was sampled 1 yr after topsoil spreading. In the absence of a mulch cover or a fully established vegetative cover to moderate soil temperature, mineralization of SOC released from broken soil aggregates can be substantial in the years immediately following topsoil application. Schwenke et al. (2000) reported that as much as 26% of the SOC in minesoils can be lost during the first year of reclamation.

If one accepts the 24 Mg C ha–1 value (Table 3) as the baseline C for the experimental plots, C sequestration rates averaging 0.68 and 0.35 Mg C ha–1 yr–1 can be computed for the ST and RT treatments, respectively. These rates would be two to three times higher if the baseline of 8 Mg C ha–1 suggested by the data of Bendfeldt et al. (2001) and Shukla and Lal (2005) were used. This illustrates the difficulty of estimating C sequestration rates in the absence of historical C stock for a given site, and also explains the wide range of C sequestration rates reported for minesoils (see reviews by Shrestha and Lal, 2006; Sperow, 2006).

A key motivation for this research was to determine whether C storage in minesoils could be improved through deep ripping of applied topsoil. The data in Fig. 2 and 3 clearly show that the pool of recent organic C was much lower (decrease of 5.1 Mg C ha–1) under RT than under ST. This observation suggests that the additional soil disturbance associated with deep ripping and disking may have contributed to further destruction of soil aggregates and liberation of aggregate-protected C. This, combined with increased soil aeration, could result in enhanced decomposition of organic matter in the applied topsoil. This interpretation is consistent with reports of depletion of aggregate-protected C with tillage disturbance (Beare et al., 1994; Jacinthe and Lal, 2005), a lower amount of hydrolyzable C in recently reclaimed (<1 yr) than in older minesoils (Shukla and Lal, 2005), and a rapid decline in microbial biomass C (up to 60%) in replaced topsoil during the first year of mineland rehabilitation (Schwenke et al., 2000).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Application of topsoil in the reclamation of mineland has become a common practice following enactment of the Surface Mining Control and Reclamation Act (SMRCA) of 1977. In this study, soil properties and C storage (as tree biomass and soil C) were assessed in experimental plots (15 yr old) established using both pre-SMRCA (OV) and post-SMRCA (ST and RT) reclamation techniques. In addition, the impact of P fertilization was evaluated. Except for its effect on mineralizable C on the surface layer of soils, the study showed no impact of P fertilization on tree growth or the soil parameters investigated. Tree growth was poor in the OV treatment, but in 15 yr, a large pool (17.8 Mg C ha–1) of recent soil organic C had accumulated, probably resulting from decomposition of biomass from the grass vegetation cover. The stock of recent organic C in plots with topsoil was, on average, twice that in plots without topsoil. In the Austrian pine plots, RT resulted in greater standing biomass C (increase of 4.9 Mg C ha–1) but in a lower pool of recent organic C (decrease of 5.1 Mg C ha–1) compared with ST. It follows that, in areas planted with this fast-growing tree species, comparable C sequestration (tree biomass plus SOC) was achieved with either ST (44.1 ± 4.4 Mg C ha–1) or RT (49.5 ± 3.1 Mg C ha–1). In the case of the slow-growing green ash, however, deep ripping of minesoils yielded a net C storage that was significantly less (RT: 27.5 ± 1.6 Mg C ha–1) than the C storage achievable with standard topsoil application (ST: 37.4 ± 5.4 Mg C ha–1). Thus, in these aggrading ecosystems, tree species ultimately determined the performance of topsoiling technique on C storage and the partitioning of stored C between soil and aboveground biomass.


   ACKNOWLEDGMENTS
 
This research was funded through a grant of the Ohio Coal Development Office (OCDO). The authors thank Gary Kasper and Brian Cox (American Electric Power in McConnersville, OH) for their support during our field work.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication September 22, 2006.


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




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