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Soil & Water Management & Conservation

Soil Properties and Carbon Sequestration of Afforested Pastures in Reclaimed Minesoils of Ohio

Carbon Management and Sequestration Center, School of Environment and Natural Resources, The Ohio State Univ., 2021 Coffey Rd., Columbus, OH 43210. P.A. Jacinthe, current address: Dep. of Geology, Indiana Univ.-Purdue Univ. at Indianapolis (IUPUI), 723 W. Michigan St., SL122, Indianapolis, IN 462302

^* Corresponding author (ussiri.1{at}osu.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Land-use change affects many soil properties, including soil organic carbon (SOC) pool, and the transfer of atmospheric CO₂ to terrestrial landscapes. The objective of this study was to evaluate the effects of converting pastureland to Australian pine (Casuarina spp) and Black locust (Robinia pseudoacacia L) forest on selected soil physical and chemical properties and SOC sequestration in reclaimed minesoils (RMS) of southeastern Ohio. The study sites were surface mined for coal, reclaimed and managed as pasture, and then converted into woodland 10 yr before the present study. Soil pH and electrical conductivity (EC) were higher in the RMS than in a nearby undisturbed hardwood forest. Conversion to Australian pine decreased soil pH and EC in the top 20 cm. Bulk densities of the RMS ranged from 1.24 to 1.82 Mg m^–3, and only minor changes were observed in soil bulk density after land-use conversion. Mean weight diameter (MWD) and root biomass increased significantly (P < 0.05) with conversion of pasture to Australian pine or Black locust. In addition, aggregate stability was greater in RMS under hardwood forest than under pasture. Conversion to the Australian pine forest increased the SOC pool in the top 50 cm by 6 Mg ha^–1 (11%) in 10 yr. However, the N pool in the top 50 cm was not affected by the land-use conversion from pasture to Australian pine. Conversion to Black locust increased the SOC pool in the top 50 cm by 24 Mg ha^–1 (42%), while the N pool increased by 10% under Black locust in 10 yr. The increase in the SOC pool was accompanied by an increase in the C/N ratios and root biomass in both Australian pine and Black locust sites in the 20- to 50-cm depth. Establishment of tree plantation has a greater potential for SOC sequestration than pastures in the RMS.

Abbreviations: EC, electrical conductivity • MWD, mean weight diameter • RMS, reclaimed minesoils • SOC, soil organic carbon • SOM, soil organic matter • TN, total nitrogen • TOC, total organic carbon • WSA, water-stable aggregates

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
SURFACE MINING of coal causes drastic landscape disturbance and severe soil degradation. Minesoils exhibit soil profile characteristics, physical, chemical, and biological conditions that reflect anthropogenic perturbations rather than natural soil forming processes (McSweetney and Jansen, 1984). Adverse physical, chemical, and biological conditions such as low soil organic matter (SOM) content, salinity, poor soil structure and aggregation, increased bulk density, low soil fertility and reduced microbial activity, often limit vegetation establishment and constrain restoration of RMS. Loss of SOC occurs due to reduced plant litter input, accelerated soil erosion, topsoil removal, and increased mineralization. In addition to mechanical mixing of soil horizons during removal and storage of topsoil, surface mining results in the breaking of soil aggregates and exposure to microbial attack of C fractions, which otherwise would have been inaccessible to decomposers (Six et al., 2000a). Low SOC concentration in these soils indicates that there is a great potential to increase the SOC pool with appropriate management practices.

Reclamation of minesoils seeks to return the disturbed soil ecosystem to a productive state (Barnhisel and Hower, 1997; Bradshaw, 1997). This involves restoration of minesoils' fertility, improvement of soil quality, reestablishment of vegetation, and enhancement of biomass productivity. Vegetation plays a major role in improving the physical, chemical, and biological properties of minesoils (Bradshaw, 1987). Enhanced biomass production increases SOC concentration in RMS (Lal et al., 1998; Akala and Lal, 2000, 2001). The potential of SOC sequestration in RMS depends on biomass productivity, root development in the subsoil, and changes in properties resulting from overburden weathering (Haering et al., 1993). Time since reclamation, antecedent soil properties, climate, vegetation and post-reclamation land-use, and management also affect SOC sequestration rate in RMS (Merrill et al., 1998).

Increases in the SOC pool also enhance soil structure. There is a strong direct relationship between soil aggregation and SOC concentration. For example, SOC promotes soil aggregation and aggregates encapsulate SOC, reducing the rate of SOC decomposition. The SOC encapsulated within soil aggregates has lower decomposition rate than that located outside of aggregates (Oades, 1984; Elliott and Coleman, 1988; Six et al., 2000b). Aggregation provides physical protection to SOC against microbial processes (van Veen and Kuikman, 1990; Gregorich et al., 1991; Golchin et al., 1994). Soil management exerts a strong influence on the formation and stabilization of aggregates and SOC sequestration (Tisdall, 1996; Golchin et al., 1995; Jastrow et al., 1998; Six et al., 1998).

Land-use and land-use changes are key drivers of SOC dynamics affecting the quantity and quality of incoming litter, soil structure, and SOC concentration and pool size (Houghton et al., 1999; Schimel, 1995; Glaser et al., 2000; Franzluebbers et al., 2000, 2001; Guo and Gifford, 2002; Conant et al., 2004). However, a critical review of the available literature shows contradictory results with regard to SOC changes following conversion of pasture to forest land-use. Based on a meta-analysis of available data, Guo and Gifford (2002) observed that conversion from pasture to plantations leads to SOC losses with slightly higher loss for sites planted to conifer (12%) than other tree species. In contrast, Ross et al. (1999) observed that conversion of pasture to pine did not change the SOC pool, but decreased the N pool. Murty et al. (2002) reported that the magnitude and direction of change in the SOC pool on conversion from forest to pasture depends on management and biomass inputs.

Few studies exist that have evaluated SOC dynamics in RMS, which differ from normal soils by lacking distinctive soil horizons, having poor soil structure and generally low fertility (Sencindiver and Ammons, 2000). Thus, the objective of this study was to assess the effects of conversion of pastures to forests on soil physical and chemical properties and the rate of SOC sequestration in RMS.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Soils
The experimental sites are located in Muskingum (High Hill, 39°59' 21'' N and 81° 46' 11'' W) and Morgan (Mount Carmel, 39°73' 50'' N and 81°79' 44'' W) Counties in southeastern Ohio. The predominant soil series in the area include Lowell silty clay loam (Fine, mixed, mesic Typic Hapludalfs), and Gilpin silt loam (Fine-loamy, mixed, mesic Typic Hapludults) (USDA-NRCS, 1996; 1998). Soils of this region are mainly derived from sandstone, siltstone, and shale parent materials. However, some soils are derived from limestone and limy shale parent materials (USDA-NRCS, 1996, 1998). The regional climate is temperate continental with an average annual air temperature and precipitation of 13°C and 965 mm, respectively. The predominant land-use in the region is agriculture and forestry. The reclaimed lands are predominantly used for pasture, forest, and recreation.

Coal mining is an important economical activity of the region. The sites used for this study were surface-mined for coal and reclaimed with topsoil application in 1978. Mining process involved clearing the secondary forest established in the 1930s, scraping and storing the topsoil, and removing the overburden for access to coal seams. Following coal extraction, the topography of mined sites was restored to their original physiography by grading the overburden materials, spreading the stored topsoil (20- to 30-cm depth) on top of the graded mine spoil, and establishing vegetative cover (e.g., forage grasses or trees). The Mount Carmel site was planted to forest after reclamation in 1978 and was comprised of mixed hardwood tree species including sagebrush (Artemisia tridentata L.), birch (Betula spp.), crabapple (Malus spp.), pine (Pinus spp.), fir (Arbies spp.), poplar (Populus spp.), spruce (Picea spp.), oak (Quercus spp.), and locust (Robinia spp.). The High Hill site was sown to pasture with forage species including rye grass (Lollium perenne), orchard grass (Dactylis glomerata), blue grass (Poa annua L.), alfalfa (Medicago sativa), timothy (Phleum pratense), and birds' foot trefoil (Lotus corniculatus L.). The High Hill site was grazed at a controlled stocking rate of approximately 0.8 to 1.3 cows per ha from May to November until 1993 when the pastureland was split into three parts. One part was maintained under pasture, while the other two parts were converted to Australian pine (Casuarina spp) and Black locust (Robinia pseudoacacia L) stands. A nearby undisturbed secondary hardwood forest (>70 yr old) with the same premining soil series was sampled as a reference site.

Soil samples were collected between May and July 2003 within each land-use from the summit, shoulder, and foot-slope landscape positions where the approximate land area for each land-use was 3 ± 2 ha. Bulk soil samples were collected from the 0- to 5-, 5- to 10-, 10- to 20-, 20- to 30-, 30- to 40-, and 40- to 50-cm depths at each landscape position. In addition, core samples were obtained in duplicate for each landscape position and depth interval for bulk density and root biomass determination. Core samples were collected using a 3-cm long by 5.5-cm diam. cores for the top 10 cm, and 6 cm long by 6 cm diam. cores at 10-cm interval to a 50-cm depth. The slope gradient ranged from 0 to 10% at these sites.

Sample Preparation and Analyses
Bulk soil samples were air-dried and 50 g of aggregates between 5 and 8 mm in diameter were sieved for aggregate analysis by the wet-sieving technique (Yoder, 1936; Youker and McGuiness, 1957). The aggregate analysis was conducted by using a nest of five sieves (5, 2, 1, 0.5, and 0.25 mm) oscillated in water for 30 min following gentle wetting. Water-stable aggregates retained on each sieve were backwashed from the sieve with deionized water, oven dried at 60°C for 72 h and weighed. The MWD was computed as the sum of the weighted average diameter of all aggregate-size classes, where weighting factors were the proportions of the mass of each size class to the total sample weight (Nimmo and Perkins, 2002). A 2-g subsample of each aggregate-size fraction was finely ground to pass through a 0.25-mm sieve for total organic carbon (TOC) and total nitrogen (TN) determinations. The remainder of the bulk soil was ground and sieved through a 2-mm sieve. About 10 g of sieved soil (<2 mm) was ground with a ball mill and passed through a 0.25-mm sieve for TC and TN analyses. Concentrations of C and N were determined by the dry combustion method at 900°C using a C and N elemental analyzer (Vario Max, Elementar, Germany). The SOC concentration was obtained by subtracting inorganic C (IC) and coal C from the TC. Inorganic C was determined by treating 2 g of finely ground soil with 1M HCl in a sealed serum bottle and determining the C concentration from the CO₂ evolved using gas chromatography (Loeppert and Suarez, 1996). Coal C was estimated after removal of organic matter C (humus C) through chemical extraction and heat oxidation. Briefly, acid-treated soil was demineralized with 10% HF to release metal- and silica-bound SOC. Repeated extractions with 0.5 M NaOH were then conducted until the suspension became colorless. Highly recalcitrant SOC was oxidized by heating an oven-dried sample in a muffle furnace at 320°C for 3h. Non-extractable and non-oxidizable C (coal-C) was subtracted from the TOC.

Soil cores were trimmed at both ends and bulk density was computed as the weight to volume ratio of oven-dried soil (Grossman and Reinsch, 2002). The SOC pool for a specific soil layer of thickness d (m) was calculated using Eq. [1]:

[1]

where _b is the bulk density (Mg m^–3) of the soil layer corrected for coarse fragment content and SOC concentration expressed as a weight-based percentage (%).

Separate soil cores were obtained from each landscape position at 5-cm intervals for the top 10 cm and at 10-cm intervals thereafter to the 50-cm depth during the active growing period (May-June) for root biomass determinations. Roots were separated from cores by hand washing each core sample through a 0.1-mm screen. Roots were hand-picked, oven-dried at 60°C for 48 h, and weighed.

Forest litter was collected using litter baskets (30 cm x 30 cm x 24 cm) placed at the summit, mid-slope, and foot slope landscape positions in the nearby undisturbed forest, reclaimed hardwood forest, Australian pine, and Black locust sites. At each landscape position, three litter baskets were placed directly under the tree crown canopy, crown canopy edge, and outside tree crown canopy (i.e., inter-tree space), respectively. This arrangement was in accordance with the observed fresh litter distribution at the beginning of this study. The dry matter collected was combined for one sample per landscape position. Litter baskets were set out in March, after snow melt, and litter was retrieved monthly until the end of November. Leaf and woody litter were not separated. At the pasture site, biomass was sampled from three 1-m² quadrants at the summit, mid-slope, and foot-slope landscape positions. Samples were collected at 2-mo interval between April and November. Biomass samples were oven-dried at 40°C for 72 h and weighed to compute dry matter yields.

Soil pH was measured electrometrically in a 1:2 soil mass/water volume mixture (Thomas, 1996). Electrical conductivity was determined using a conductivity meter (VWR Scientific) on air-dried soil at a soil mass/water volume ratio of 1:5 (Rhoades, 1996).

Data Analysis
All data were analyzed by an analysis of variance (ANOVA) using SAS (SAS, 1994) to assess the effects of land-use change and soil depth. Means were compared using the least significant difference (LSD) test when the ANOVA showed significant land-use effects (P 0.05).

	RESULTS

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pH and Electrical Conductivity
Soil pH and EC ranged from 6.7 to 8.5, and 0.05 to 0.7 dS m^–1, respectively (Table 1) across all soil depths, landscape positions and land-uses. Landscape position did not significantly influence pH and EC at these sites. Soil pH of the pasture and reclaimed hardwood forest increased with increasing soil depth (P < 0.05), but did not differ in Australian pine, Black locust, and undisturbed hardwood forest (Table 1). Conversion of pasture to Australian pine decreased soil pH and EC in the top 20 cm (P < 0.05), but there were no significant changes below 20 cm. In comparison, few changes in pH and EC were observed on conversion of pasture to Black locust (Table 1). The EC of the reclaimed hardwood forest was higher than other sites for all depths (P < 0.05, Table 1).

View larger version (50K): [in this window] [in a new window]   Fig. 1. Effects of land-use change on water-stable aggregates (WSA) by soil depth in reclaimed minesoils of southeastern Ohio. Error bars represent standard error of the means. HWF = hardwood forest. For each soil depth, different capital letters indicate significant differences between sites (P < 0.05). For each site, different lower-case letters indicate significant differences between depths (P < 0.05).

View larger version (42K): [in this window] [in a new window]   Fig. 2. Effects of land-use change on mean weight diameter (MWD) by soil depth in reclaimed minesoils of southeastern Ohio. Error bars represent standard error of the means. HWF = hardwood forest. For each depth, different capital letters indicate significant differences between sites (P < 0.05). For each site, different lower-case letters indicate significant differences between the depths (P < 0.05).

View larger version (53K): [in this window] [in a new window]   Fig. 3. Effects of pasture afforestation on soil organic carbon (SOC) concentration in soil macroaggregates by depth. Error bars represent standard error of the means. HWF = hardwood forest. For each soil depth and aggregate size, different capital letters indicate significant differences between sites (P < 0.05). For each site and depth, different lower-case letters indicate significant differences between aggregate sizes (P < 0.05).

View larger version (34K): [in this window] [in a new window]   Fig. 4. Effects of land-use change on root biomass in reclaimed minesoils of southeastern Ohio. Error bars represent standard error of the means. HWF = hardwood forest. Different capital letters indicate significant differences between sites for the 0- to 50-cm depth (P < 0.05). Different lowercase letters indicate significant differences between sites for each soil depth (P < 0.05).

In the 0- to 10-cm depth, SOC concentration was the lowest in the 0.25- to 0.5- and 0.5- to 1.0-mm aggregate size classes in Australian pine site (P < 0.01), but SOC concentration in the aggregates did not differ significantly in the 1- to 2-, 2- to 5-, and >5-mm aggregate sizes (Fig. 3b). In addition, SOC concentration in different aggregate sizes did not differ in pasture, reclaimed hardwood and undisturbed hardwood forests. In the Black locust site, significantly higher SOC concentration was obtained in 1- to 2- and 2- to 5-mm fractions. In the 10- to 20-cm depth, SOC concentration increased with increasing aggregate size under pasture, reclaimed hardwood forest, and undisturbed hardwood forest. However, under Australian pine and Black locust, SOC concentration did not differ significantly among aggregate-size classes (Fig. 3c). Soil organic C concentration in different aggregate size fractions did not differ below 20 cm in all land uses (data not presented). Conversion from pasture to Black locust increased aggregates SOC concentration in all size classes in the 20- to 50-cm depths. However, little change in SOC concentration was observed when pasture was converted to Australian pine.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soils under pasture, Australian pine, and Black locust differed markedly in WSA, MWD, SOC, and TN pool sizes and aggregate SOC concentration within the soil profiles. Soil EC in RMS was greater than that in the undisturbed sites, but below the threshold range (1–3 dS m^–1) considered detrimental to plant growth and productivity (McFee et al., 1981). Pasture to Australian pine conversion significantly decreased the SOC pool by about 22% in the top 5 cm, but resulted in a slight increase in the 10- to 50-cm depth. Overall, the SOC pool in the top 50 cm increased by 11% or 6 Mg ha^–1 over the 10 yr under Australian pine. Land-use change from pasture to Black locust increased the SOC pool in the top 50-cm depth by 42% or 24 Mg ha^–1 over the same10 yr, where most of the increase occurred in the 20- to 50-cm depth (Table 3). Therefore, conversion to Australian pine and Black locust increased C sequestration rates of the top 50 cm by 0.6 and 2.4 Mg ha^–1 yr^–1, respectively over the 10-yr period. These rates are much higher than those reported for agricultural soils, but are not uncommon during the first years following restoration of highly disturbed soils. For example, Akala and Lal (2001) reported C sequestration rates of 2 to 3 Mg ha^–1 yr^–1 in the first 20 yr after reclamation, decreasing to 0.4 to 0.7 Mg ha^–1 yr^–1 by 20 to 30 yr after reclamation. In agricultural soils, C sequestration rates ranging from 0.5 to 1.5 Mg ha^–1 yr^–1 were reported at the Sanborn plots in Columbia, MO with improved land management practices (Buyanovsky and Wagner, 1998). The rate of SOC sequestration depends on the amount of biomass C returned annually. Sequestration rates of 0.41 to 1.30 Mg SOC ha^–1 yr^–1 were reported in tropical forest re-established on abandoned agricultural lands (Silver et al., 2000). Similarly, a sequestration rate of 1.0 Mg SOC ha^–1 yr^–1 was reported for the first 40 yr in pine forest established on abandoned agricultural land at Calhoun Experimental Forest in North Carolina (Richter et al., 1999).

Increases in SOC on conversion of pasture to Australian pine were not accompanied by a parallel increase in the N pool, which resulted in increased soil C/N ratios. However, the N pool in the top 50 cm increased by 10% with conversion from pasture to Black locust (Table 3). These trends in N pool size may be attributed to differences in quality of the litter input. Black locust is a leguminous tree species, which fixes N, while Australian pine is not. This observation is also consistent with lower N concentrations in pine litter compared with that in broadleaf litter (Prescott et al., 2000; Ussiri and Johnson, 2003). Decreases in soil C/N ratios with increasing soil depth (Table 4) were probably due to increases in SOM decomposition with increasing soil depth (Johnson et al., 1995).

Soil organic matter is central to the formation and stabilization of soil aggregates. Soil aggregation plays dominant role in SOC sequestration by physically protecting SOM through incorporation into soil aggregates (Gregorich et al., 1991, Golchin et al., 1994). The SOM pool is stabilized through association with silt and clay particles within macro- and microaggregates (Oades, 1984; Elliott and Coleman, 1988; Six et al., 2000a, 2000b). This association provides for the physical protection against decomposition of SOM fractions, which may not be chemically resistant to microbial processes.

Aggregate stability, an indicator of soil structure (Six et al., 2000b), is another mechanism of SOM stabilization. In this study, WSA and MWD in the top 50 cm increased on conversion from pasture to Australian pine and Black locust (Fig. 1 and 2). Similar to Golchin et al. (1995) findings, WSA and MWD were strongly correlated with SOC concentration (R² = 0.69, P < 0.001), suggesting that SOC plays an important role in formation and maintenance of soil aggregates in RMS.

The SOC concentration and pool sizes in the RMS under pasture, Australian pine, and Black locust were statistically similar in the top 30 cm. In the 30- to 50-cm soil depth, however, conversion to forest increased the SOC pool by 57% under Australian pine and 133% under Black locust. Increases in the subsoil (30- to 50-cm depth) SOC pool under Australian pine and Black locust was probably due to increased SOC incorporation associated with increased root biomass, since tree species generally have deeper, more extensive root system than forages (Trumbore et al., 1995; Jackson et al., 1996; Jobbagy and Jackson, 2000).

In accordance with this hypothesis, root biomass was significantly greater (P < 0.05) under Australian pine and Black locust than under pasture, with the greatest increase in the 10- to 50-cm (Fig. 4). In addition, the SOC pool was strongly correlated with root biomass and differences in root biomass explained about 62% of variations in the SOC pool at these sites (R² = 0.62, P < 0.001).

The contribution of roots to the SOC pool depends on root productivity, root turnover rates, root exudation, mycorrhizal colonization, and soil characteristics. Roots provide a path for C movement from plant canopies to the soil. Therefore root biomass production and turnover directly influences C cycling in terrestrial ecosystems.

In general, root C has longer residence time in soil than shoot C (Gale et al., 2000; Rasse et al., 2005). More C is generally returned to the soil through root growth and turnover than through aboveground biomass in temperate forests (Rasse et al., 2001) and grassland ecosystems (Gill et al., 2002). Preferential preservation of root C in soils has been attributed to: (1) higher chemical recalcitrance of root C (Rasse et al., 2005); (2) reduced decomposition in deeper soil layers due to prevailing environmental conditions, which are detrimental to decomposition of plant tissues (Gill and Burke, 2002); (3) physical protection from microbial decomposers through aggregation (Jastrow et al., 1998); and (4) physicochemical protection of root exudates and decomposition by-products through interactions with minerals (Jones and Edwards, 1998) and metal ions, such as Al and Fe, which inhibit microbial degradation (Kinraide and Sweeney, 2003). Roots contain greater concentrations of aromatic compounds, with polyphenolic molecules such as lignin, tannin, suberins, and cutins than shoots, which increase their chemical recalcitrance (Rasse et al., 2005). Only a limited number of soil microorganisms, namely white rot fungi, can mineralize these recalcitrant structures (Kraus et al., 2003; Rasse et al., 2005).

Roots also improve aggregation by enmeshing soil particles together and through exudates, which increase microbial biomass and produce polymers that serve as aggregates binding agents (Jastrow et al., 1998). This hypothesis is supported by the increased MWD under Australian pine and Black locust than that under pasture (Fig. 2). Factors and processes important to increasing the SOC pool on change in land-use and management include: (1) increases in biomass C input; (2) decreases in the decomposability of SOM with an attendant increase in free particulate OM, in particular light fraction SOC, which can be physically stabilized by macroaggregates as intra-aggregates particulate C (Cambardella and Elliott, 1993); (3) increases in the SOM placement in the subsoil either directly by increasing belowground inputs or indirectly by enhancing surface mixing by soil organisms; and (4) increases in physical protection of SOM through encapsulation within intra-aggregate space or formation of organomineral complexes (Post and Kwon, 2000). Conditions favoring these processes generally occur when soils are converted from cropland to perennial vegetation. The data presented herein suggest that these processes were set in motion on conversion of pastures to Australian pine or Black locust in RMS.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Conversion of pasture to pine and Black locust in RMS improved soil structure, enhanced soil quality, increased root biomass, and SOC sequestration. Rates of SOC sequestration averaged 0.6 and 2.4 Mg ha^–1 yr^–1, in the top 50 cm in Australian pine and Black locust, respectively. Tree plantation establishment has greater potential for SOC sequestration than pasture in RMS because of high biomass C inputs to deeper soil depths due to more extensive tree root system.

	ACKNOWLEDGMENTS

 
This research was funded by the Department of Energy (DOE) and the Ohio Coal Development Office (OCDO), Columbus, Ohio. Authors are grateful for their support. Authors also thank the American Electrical Power (AEP) for providing access to the study sites. Special thanks to Gary Kaster and Brian Cox for their support and help during our fieldwork.

Received for publication October 18, 2005.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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