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Published online 24 October 2007
Published in J Environ Qual 36:1775-1783 (2007)
DOI: 10.2134/jeq2007.0071
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Soil Carbon and Nitrogen in 28-Year-Old Land Uses in Reclaimed Coal Mine Soils of Ohio

Raj K. Shrestha* and Rattan Lal

School of Environment and Natural Resources, The Ohio State Univ., 2021 Coffey Road, Columbus, Ohio 43210

* Corresponding author (Shrestha.10{at}osu.edu).

Received for publication February 7, 2007.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
Carbon (C) and nitrogen (N) play an important role in the restoration of ecosystem functions of reclaimed mine soils (RMSs). Postreclamation land use in RMSs affects soil C and N pools and fluxes. We compared the effects of 28-yr-old postreclamation land uses (forest, hay, and pasture) on selected chemical properties of soil, and C and N pools in reference to undisturbed forest and moderately disturbed agricultural land use in southeastern Ohio. The electrical conductivity was higher in RMSs under hay than that in pasture and forest land uses. The RMSs under pasture, hay, and forest had moderately acidic, neutral to slightly alkaline, and slightly alkaline pH, respectively. In the 0- to 5-cm soil depth, soil organic C (SOC) was higher in RMSs under pasture by 99% and under hay by 52% over that under forest. Similarly, total nitrogen (TN) was higher in RMSs under pasture by 98% and under hay by 43% over that under forest. Aggregate-associated SOC concentration in the 0- to 5-cm depth decreased in the order of RMSs under hay > RMSs under pasture > RMSs under forest. The SOC pools in the 0- to 30-cm depth decreased in the order of RMSs under hay = RMSs under pasture > RMSs under forest = undisturbed forest = agriculture land use. Nitrogen pools followed a similar trend. Hay land use has a better potential for improving soil quality in RMSs by enhancing chemical properties and SOC and TN pools than forest or pasture land uses.

Abbreviations: BD, bulk density • EC, electrical conductivity • RMS, reclaimed mine soil • SIC, soil inorganic carbon • SOC, soil organic carbon • TC, total carbon • TN, total nitrogen


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
RESEARCH on soil quality assessment has been primarily focused on agricultural and forest soils. There is a strong need to monitor changes in the soil quality of reclaimed mine soils (RMSs). Mining causes drastic disturbances in soil properties. Restoration of soil quality and ecosystem function of RMSs depends on the reclamation methods and re-establishment of vegetation, which can enhance and sustain plant growth (Shrestha and Lal, 2006). Successful establishment of a vegetative cover on RMSs depends on understanding the chemical and physical properties of disturbed soils. The RMSs are pedogenically young soils, and their properties are strongly influenced by the parent material, mining procedure, reclamation method, and postreclamation land uses.

Some adverse physicochemical properties of RMSs may inhibit soil-forming processes and adversely affect plant growth. These soils usually contain low concentrations of soil organic carbon (SOC), nitrogen (N), and phosphorus (P); high bulk density (BD) and rock fragments; unfavorable soil pH; poor structure; low porosity and water-holding capacity; and low biomass productivity (Indorante et al., 1981; Boerner et al., 1998; Hearing et al., 2000; Burger, 2004). Growth-limiting acidity in the surface-mined soils is caused by deposition of low-base-content overburden consisting primarily of sandstone or shale parent material and sulfur-bearing (FeS2) overburden layers (Mays et al., 2000). Acidic conditions limit root growth and establishment of plants. Applying fertilizers and using lime to raise soil pH above 5.5 can enhance plant growth. A yearly rate of 140 to 170 kg ha–1 actual N, split applied, can provide economical yield increases with tall grasses for adequate plant growth and ecosystem function in RMSs (Underwood et al., 2006). Plant-available P may also be low in mine soils due to high P fixation capacity of the spoil (Roberts et al., 1988).

Disturbance during mining results in loss of SOC and N pools. Thus, reclamation of mined soils is associated with an increase in SOC and N concentrations and improvement in soil fertility. Factors that affect SOC and N concentrations and pools include biomass inputs, roots, mycorrhizal fungi, degree of physical disturbances, introduction of plants and animal species, atmospheric deposition of materials, and effects on soil decomposers and primary producers (Pouyat and McDonnell, 1991; Carreiro et al., 1999).

There are about 3.2 Mha of RMSs in the USA and 0.04 Mha in Ohio, which have the potential of sequestering C in the terrestrial ecosystem by as much as 8.19 Tg yr–1 under forest land use (Shrestha et al., 2007; OSM, 2003). Predominant postreclamation land uses in Ohio are forest, pasture, and hay, in accord with the 1972 Ohio Surface Mine Law and the 1977 Federal Surface Mining control and Reclamation Act. The 1972 Ohio Surface Mine Law required topsoiling, grading back to the approximate original contour, and an immediate establishment of grass cover and/or legumes, with the objective of controlling erosion and protecting water quality. The implementation of the 1972 law led to a drastic improvement in the quality of soil and water resources. However, the effects of different land uses and degree of soil disturbances on SOC and N pools are not adequately understood (Brye et al., 2002).

Several studies have assessed the effects of mining (Indorante et al., 1981), the use of soil amendments (Bendfeldt et al., 2001; Shukla et al., 2005), conversion of pasture to forest (Ussiri et al., 2006a), and reclamation followed by establishment of forest (Fettweis et al., 2005) and pastures (Evanylo et al., 2005) on the chemical properties of RMSs. However, information on the evaluation of different postreclamation land uses on soil chemical properties and SOC and N pools in RMSs is limited. Therefore, the objectives of this study were to assess the effects of 28-yr-old postreclamation land uses such as forest, pasture, and hay on chemical properties and on SOC and total nitrogen (TN) pools of RMSs.


   Materials and Methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Study Sites and Soils
Study sites were located in Morgan County, southeast Ohio (39°59'21''N, 81°79'44''W). The elevation of the study sites ranged from 244 to 262 m above sea level. The predominant soil series of the study area are Morristown silty clay loam (loamy-skeletal, mixed, mesic Typic Udorthents), Westgate silt loam (fine-silty, mixed, mesic Typic Hapludalfs), and Elba silty clay loam (fine, mixed, mesic Typic Hapludalfs). These soils were formed from sandstone, siltstone, and shale parent materials (USDA, 1998). Mean temperature of the study area was 22°C in the summer and –1°C in the winter. The average annual precipitation was 1039 mm, with 430 mm received during the growing season between May and September.

Three 28-yr-old postreclamation land uses common in the region, such as hay (where aboveground biomass is harvested and exported for livestock feeding as hay), pasture (where the aboveground vegetation is grazed onsite by animals such as cattle), and forest (hardwood forest), were evaluated for their long-term effects on selected soil chemical properties in reference to undisturbed forest (uncultivated, unmined) and moderately disturbed agricultural land use (tilled, unmined, under hay–corn [Zea mays L.] rotation). The RMS sites were surface mined and graded to the approximate original contour. Thirty centimeters of topsoil was applied on the top of graded spoil, and the sites were planted to pasture, hay, and forest in 1977. The 30 cm of topsoil applied was the same soil stripped before coal mining. The hay sites were never grazed and were harvested once or twice every year during May to June. The pasture sites were used for cattle grazing at a stocking rate of 1.2 cattle ha–1 during May to November. The forest sites in RMSs were never harvested or disturbed for any purpose since reclamation. Unmined forest sites available in the vicinity were included in the study as reference site, which were near a cemetery and established in the same year as RMSs, left undisturbed, and were called as undisturbed forest. The soil and management practice for undisturbed forest were similar to RMSs under forest. The agricultural sites that were unmined but cultivated for hay and occasionally corn (Zea mays L.) in rotation were selected adjacent to RMSs as a moderately disturbed reference site.

Detail on vegetation species in different land uses in RMSs are described by Shrestha and Lal (2007). Briefly, the common forest tree species established in RMSs were white pine (Pinus strobus L.), white ash (Fraxinus americana L.), tulip poplar (Liriodendron tulipifera L.), sycamore (Platanus occidentalis L.), and autumn olive (Elaeagnus umbellata L.). Tree species in the undisturbed forests were white pine, green ash (Fraxinus pennsylvanica L.), white ash, and tulip poplar. Forest densities ranged from 70 to 90%, and the height of the trees ranged from 8 to 14 m. The RMSs under hay was never grazed and was mowed once in June. The grass species in the hay field were mostly kentucky fescue (Festuca megalura Nutt.) mixed with birdsfoot trefoil (Lotus corniculatus L.), bromegrass (Bromus inermus L.), and clover (Trifolium pratense L.). The grass species in RMSs under pasture site were clover mixed with kentucky fescue, birdsfoot trefoil, and bromegrass. In agriculture land use, grass species established in hay year were clover, kentucky fescue, birdsfoot trefoil, and bromegrass. The samples from agricultural sites were collected in the hay year (hay–corn rotation) before harvesting hay.

The RMSs in Ohio received 30 cm of topsoil during reclamation. The properties of the spoil material below the 30-cm depth are heterogeneous and highly variable (Ussiri et al., 2006a). Thus, any effect of land use on soil quality was assessed in the top 0- to 30-cm layer.

Soil and Biomass Sample Collection and Preparation
This study is neither a replicated field plot experiment nor an established design. Therefore, because of the lack of true replication, three sites with similar slope, elevation, and soil type were selected for each land use to serve as three pseudo replications.

Bulk soil samples were obtained for the 0- to 5-, 5- to 15-, and 15- to 30-cm depths in August 2005. Three subsamples were randomly collected from each replication, providing a total of nine replications for each land use. Soil sampling in the forests was done at 1-m distance from a tree trunk. Sites disturbed by uprooted trees and trails were avoided (ICP Forests, 2006). Soil samples were sealed in a plastic bag and transported to the laboratory in a cooler. The composite soil samples were air-dried under shade. Large clods were gently crushed and sieved through 8-, 5-, and 2-mm sieves. The bulk soil samples, gently ground and passed through a 2-mm sieve after removing visible roots, were used for measurement of pH and electrical conductivity (EC). Soil aggregates between 8 and 5 mm were used for assessment of aggregate-associated SOC and TN concentrations. Soil aggregates between 5 and 8 mm were fractionated by the wet sieving technique using a nest of sieves (4.75, 2, 1, 0.5, and 0.25 mm) (Nimno and Perkins, 2002). Aggregates retained on each sieve were dried at 60°C for 72 h and weighed. Dried aggregates were gently ground, and the skeletal fractions (>2 mm) were removed, oven dried, weighed, and used to correct water-stable aggregate. A 2-g subsample from 4.75- to 8.00-, 2.00- to 4.75-, 1.00- to 2.00-, 0.50- to 1.00-, 0.25- to 0.50-, and <0.25-mm aggregate fractions was ground to pass through a 0.25-mm sieve for SOC and TN determination. About 10 g of bulk soil samples (<2 mm) was ground using a ball mill and passed through a 0.25-mm sieve to determine total carbon (TC), SOC, soil inorganic carbon (SIC), and TN concentrations.

The undisturbed intact soil cores (5.3 cm diameter, 5 cm deep) were obtained in duplicate from 0- to 5-, 5- to 15-, and 15- to 30-cm depths in August 2005 for the measurement of soil BD and root biomass. One of the two cores was washed in a 2-mm sieve after drying and weighing. The gravels collected in the 2-mm sieve were dried in an oven and weighed, and volume was measured. These weight and volume values were subtracted from the total soil weight and volume, respectively, to correct bulk density for gravels. The second core was used for root biomass determination (Makkonen and Helmisaari, 1999). Roots were collected from 0- to 30-cm depth and separated from soil cores by washing manually and passing through a 0.1-mm sieve. Roots were hand picked, oven dried at 60°C for 48 h, weighed, and reported as Mg ha–1.

A quadrat (1 x 1 m) was used for collecting the aboveground biomass from forest, pasture, and hay fields. In the forest, forest litter was collected to estimate the contribution of aboveground forest vegetation on mine soil, from three landscape positions (summit, shoulder, and foot-slope). Three quadrats were set (under the tree crown, canopy edge, and outside of the canopy) for each landscape position. Litter inside the quadrats (including woody dead fallen biomass, leaves, undergrowth) was collected every month between April and November at the same locations, and composite samples were prepared for each quadrat. For the pasture, hay, and agricultural land uses, biomass was collected from three locations for each land use using a 1 x 1 m quadrat. All biomass samples were dried in an oven at 50°C for 72 h and weighed to calculate the total dry weight (Mg ha–1).

Soil and Plant Analyses
Soil pH was measured electrometrically using an Orion pH meter (model 420A; Orion Research Inc, Boston, MA) in 1:2 soil/water extract using a modified method based on Thomas (1996). Electrical conductivity was determined using a conductivity meter (YSI 3100 Conductivity Instrument) in 1:5 soil/water extracts (Rhoades, 1996).

The concentrations of TC and TN were determined by the dry combustion method at 900°C using a CN analyzer (Elementrar Vario MAX; GmbH, Hanau, Germany). The SOC concentrations in the bulk soil and aggregates were determined using a CN analyzer after removing soil inorganic carbon (SIC) with acid treatment and then subtracting coal-C. The SIC was removed by 1M HCl treatment (Kennedy et al., 2005). The coal-C was determined by the chemo-thermo method (Ussiri and Lal, 2006 and 2007). The concentration of SIC was determined by the Bundy and Bremner (1972) method. Briefly, 1.5 to 2 g of soil (<2 mm) was weighed in a crimp-sealed serum bottle. A glass syringe was used to inject 4 mL of 2M HCl into the bottle for carbonate dissolution and CO2 development. The CO2 evolved was injected into a gas chromatograph (Shimadzu GC-14A) equipped with a thermal conductivity detector for CO2 analysis. The volume of CO2 evolved was converted to SIC concentration. The SOC was also determined by subtracting SIC and coal-C concentrations from TC to validate the SOC concentrations determined directly in a CN analyzer after removing SIC. The SOC and TN concentrations were converted to SOC and TN pools by multiplying concentration with BD and soil depth. Dried plant samples were ground for C and N determinations by the dry combustion method using a CN analyzer.

Statistical Analyses
Data were tested for normality and statistical validity of the results. An ANOVA was computed to study the effects of postreclamation land uses on soil properties and of SOC and TN pools by using SAS (SAS Institute, 2003). Separation of means was tested using the LSD test at a level of P < 0.05. Regression analyses were conducted for bulk and aggregate-associated SOC and TN with aggregate-size distributions, BD, clay particles, and EC.


   Results and Discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Vegetative Biomass and Carbon and Nitrogen Contribution
Aboveground input of biomass and litter to RMSs under hay, pasture, and forest land uses ranged from 3.38 to 5.62 Mg ha–1 yr–1 (Table 1 ). The contribution of aboveground biomass input from forest as forest litter was 66% higher than that from aboveground biomass of hay and 21% higher than that of pasture land use. However, there were no significant differences in aboveground biomass among hay and pasture land uses (P < 0.05). The belowground dry biomass in the 0- to 5-cm depth of reclaimed land uses were 12 to 63% higher than that of agricultural land uses. However, the belowground biomass for 0- to 15-cm soil depth of hay land use was 46% higher than that of forest land use in RMSs (P < 0.05). The belowground biomass was similar for hay and pasture land uses. The higher belowground biomass in hay than hardwood forest in the upper soil depth of RMSs may be due to more fine roots in grass at shallow depths (Table 1). This observation is in agreement with the study by Ussiri et al. (2006a).


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  		Table 1. Above- and belowground biomass for different land uses in Morgan County, Ohio.

 
Nitrogen contributions from aboveground biomass or litter fall in RMSs were higher than those of agriculture and lower than that of undisturbed forest. The input of biomass N was 65% higher in pasture than that of hay land use (Table 1) (P < 0.05). High N input from pasture biomass may be related to high soil N (Table 2 ) from feces and urine of grazing cattle (Bos taurus), which are rich in N (Cole et al., 2003). However, the contribution of aboveground biomass C to RMSs were in the order of RMSs under forest > RMSs under pasture = RMSs under hay land use (P < 0.05).


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  		Table 2. Organic carbon and total nitrogen concentrations and C/N ratio of different land uses at Morgan County, Ohio.

 
Soil pH and Electrical Conductivity
The soil pH in RMSs under forest and hay tended to increase with increase in soil depth but not in pasture and undisturbed forest. The soil pH in agriculture soils tended to increase with depth (Table 3 ). Soil pH was >5.5 in all land uses, which is favorable for root development and biomass production (Haynes and Naidu, 1998). The soil pH was neutral to alkaline in RMSs under forest and hay and acidic under pasture throughout the 0- to 30-cm depth. It was slightly acidic to neutral in the undisturbed forest. In general, pH was significantly higher in RMSs under forest and hay soils than those under pasture soils for all three depths (P < 0.05). The high pH in RMSs under hay and forest may be due to the presence of carbonaceous (CaCO3/MgCO3) minerals in the overburden materials (Fig. 1 ), which tend to increase pH on weathering and dissolution (Barnhisel and Hower, 1997). Lower pH in pasture soil may be a result of the parent material and ammonium N (NH4+) from urine and feces deposition from grazing animals. Cattle urine has a high N concentration of approximately 10 g N liter–1, of which 75% is urea (Doak, 1952). The urea is hydrolyzed in soil to NH4+–N. During the nitrification process, conversion of NH4+ to NO3– causes a progressive decrease in soil pH (Haynes and Williams, 1992), resulting in the decline in pH under pasture field. In the forest land use, there is recycling of bases through litter fall decomposition, which buffers soil pH. This does not occur in pasture and hay land uses.


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  		Table 3. Soil pH and electrical conductivity (EC) for different land uses in reclaimed mine soils, Morgan County, Ohio.

 

Figure 1
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  		Fig. 1. Soil inorganic carbon concentration at different land use areas in Morgan County, Ohio. Land uses with the same letters are not different at 0.05%.

 
In general, there was a decrease in EC with increase in soil depth. The EC in RMSs was higher than those for undisturbed forest (P < 0.05) (Table 3). However, it was within the maximum limit of 2000 µS cm–1 for plant growth (Marcelis and Van Hooijdonk, 1999). The higher EC in RMSs than that of agricultural soils is due to high soil pH and presence of carbonates (Table 3; Fig. 1).

Soil Organic Carbon Concentrations and Pools
Bulk Soil
The SOC concentrations determined directly in a CN analyzer after removing SIC correlated well with the SOC determined indirectly by subtracting SIC concentration determined in a gas chromatograph from TC concentrations determined in a CN analyzer (Fig. 2 ). This trend indicates that any of the two methods followed in this study can be used safely to estimate SOC concentration in RMSs. However, the correlation was good at the 0- to 5-cm depth (R2 = 0.995; P > 0.001) and decreased with increase in soil depth.


Figure 2
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  		Fig. 2. The soil organic carbon (SOC) determination for reclaimed mine soil by direct method by analyzing in a CN analyzer after removing inorganic C and indirect method by subtracting inorganic carbon (IC) from total carbon (TC).

 
The land use had significant effects on SOC concentrations and pools (Table 2; Fig. 3 ). Most of the SOC in RMSs were concentrated in the 0- to 5-cm depth, and the increase in SOC concentration ranged from 71 to 241% higher than that of agriculture soils (P < 0.05). The SOC concentrations in pasture and hay land uses increased by 99 and 51%, respectively, over that of RMSs under forest. Grasslands (pasture and hay) possess a significant underground biomass component that serves as a large carbon storage sink for atmospheric CO2. Therefore, the high SOC concentration in the upper depths of soil profile in grassland is associated with high root biomass of 57 to 74% over that in forest soil (Table 1). However, SOC concentration in RMSs under forest after 28 yr of reclamation was similar to undisturbed forest, indicating a restorative effect of reclaiming mine soil.


Figure 3
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  		Fig. 3. Soil organic carbon pool for different land uses in reclaimed mine soil in southeastern Ohio. Land uses with the same letters are not different at 0.05. NS, not significant.

 
The proportion of SOC pools in the 0- to 5-cm depth of RMSs out of the 0- to 30-cm depth ranged from 44 to 62%. The SOC pools in the 0- to 5-cm depth significantly differed in different land uses in RMSs (P < 0.05) (Fig. 3). The SOC pool in pasture was 36% higher than in hay and 103% higher than in forest land use. High SOC pools in the 0- to 5-cm depth of pasture may be due to C input from feces of grazing cattle. The SOC pool in RMSs under forest was similar to undisturbed forest but 49% higher than agricultural soil. Lesser accumulation of SOC in the agricultural soils may be because of plowing, which enhances mineralization of soil C. At 5- to 15-cm depths, differences in land use did not affect SOC pools in RMSs. This indicates that a 28-yr period is not long enough to cause measurable changes in SOC pools of RMSs under any land use (forest or hay or pasture). However, at 15- to 30-cm depth, the SOC pools for RMSs under hay were significantly higher than those under pasture and forest land uses. The total SOC pools in the 0- to 30-cm depth of hay and pasture land uses were higher by 104 and 97%, respectively, than that of forest (P < 0.05). Although harvesting of hay removes aboveground biomass from the system, the SOC pools were higher in the lower depths, which may be because of fertilizer addition in the hay field (Gollany et al., 2006) and its deep root system. Increases in aboveground and belowground biomass in hay land use due to fertilizer addition may have increased the SOC pools. These rates of SOC sequestration imply that establishment of forest or hay in RMSs of Ohio can sequester up to 64,000 Mg C yr–1.

Aggregate Size Fractions
The aggregate-associated SOC concentration was significantly affected by land uses in RMSs (P < 0.05) (Table 4 ). Differences were more pronounced in the surface layer (0–5 cm) and decreased with increase in soil depth. In general, the aggregate-associated SOC concentration in most of the aggregate size fractions in the 0- to 5-cm depth decreased in the order of RMSs under hay > RMSs under pasture > RMS under forest (Table 4). Thus, RMSs under hay land use had a significant influence on aggregation and aggregate-associated SOC especially in the 0- to 5-cm depth, which may be due to high root biomass (Table 1), which increased aggregate stability (Shrestha and Lal, 2007), and the residence time of sequestered SOC. Some studies have shown that root C has a longer residence time in soil than shoot C (Rasse et al., 2005). Other biological factors that moderate soil aggregation are arbuscular mycorrhizal fungal hyphae and microbially derived C compounds (Batten et al., 2005). The low aggregate-associated SOC concentration in agricultural soils may be due to an increase in cultivation intensity, which led to a loss of SOC-rich macro-aggregates (>0.25 mm). In ecosystems with frequent soil disturbance, accelerated turnover rates of macro-aggregates limits the physical stabilization of aggregates (Six et al., 1999). In the present study, the macro-aggregates dominated the aggregate size fractions, which on average accounted for 92, 85, and 69% of the dry soil weight in the 0- to 5-cm, 5- to 15-cm, and 15- to 30-cm depths, respectively (Shrestha and Lal, 2007). In the 0- to 5-cm depth, SOC concentration increased with an increase in aggregate size. This observation is in accord with that of Six et al. (2000). This trend of increase in aggregate-associated SOC with increase in aggregate size class was not observed in the 5- to 15-cm and 15- to 30-cm depths.


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  		Table 4. Aggregate-associated soil organic carbon concentration in different soil depths for different land uses in reclaimed mine soils, Ohio.

 
Total Nitrogen Concentrations and Pools
In general, TN concentrations and pools decreased with increase in soil depth, irrespective of land use (Table 2; Fig. 4 ). The average TN concentrations in RMSs ranged from 3.16 to 6.26 g kg–1 for 0- to 5-cm, 0.87 to 1.45 g kg–1 for 5- to 15-cm, and 0.43 to 0.78 g kg–1 for 15- to 30-cm depths (Table 2). Land uses in RMSs showed significant differences in their ability to retain TN in soil (P > 0.05). In the 0- to 5-cm depth of RMSs, pasture and hay have 98 and 55%, respectively, higher N concentrations than that under forest. After 28 yr of reclamation, N concentration in RMSs under forest was similar to undisturbed forest.


Figure 4
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  		Fig. 4. Soil nitrogen pool for different land uses in reclaimed mine soil in southeastern Ohio. Land uses with the same letters are not different at 0.05. NS, not significant

 
The TN pool followed a trend similar to that of the C pool. Most of the TN pools in RMSs, especially in grassland, were concentrated in the surface layer (0–5 cm). The land uses in RMSs had significant effect on TN pools (P > 0.05). Increase in TN pool was 102% higher under pasture and 53% under hay than under forest land use in the 0- to 5-cm depth of RMSs. The SOC and TN pools in the 0- to 5-cm depth were higher in pasture land use than hay, which is probably because of the high frequency of organic deposition from cattle feces and urine (Franzluebbers et al., 2000). The N contribution from urine of grazing animals can be 500 kg N ha–1 (Saunders, 1984) because urine contains approximately 10 g N liter–1, of which 75% is urea (Doak, 1952). However, TN concentrations and pools in the 5- to 15-cm and 15- to 30-cm depths were higher in hay land use, which may be due to N contribution from the high root biomass (Table 1). This resulted similar TN pool in the 0- to 30-cm depth and was in the order of RMSs under hay = RMSs under pasture > RMSs under forest (P < 0.05). This trend indicates that there are no differences in the TN pool between pasture and hay land uses in the top 30-cm depth. This lack of significant difference is attributed to higher TN and SOC pools in hay than pasture land use at lower depths (5–15 cm and 15–30 cm). The TN pool in RMSs under forest was similar to that under undisturbed forest, as was the case with the SOC pool (P < 0.05).

The aggregate-associated TN concentrations in all three depths tended to increase with increase in aggregate size fraction, irrespective of land uses (Table 5 ). These results are in accord with those reported by Six et al. (1998). The aggregate-associated TN for macro- and micro-aggregates in 0- to 5-cm was affected by land use. However, land use did not have any effect on aggregate-associated TN concentration in 5- to 15- and 15- to 30-cm depths. After 28 yr of reclamation, aggregate-associated TN concentrations in macro- and micro-aggregates in the surface layer (0- to 5-cm depth) of RMSs under hay were higher than that in RMSs under forest and agricultural soil but similar to that under an undisturbed forest (P < 0.05).


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  		Table 5. Aggregate-associated total nitrogen concentration in different soil depths for different land uses in reclaimed mine soils, Ohio.

 
Carbon/Nitrogen Ratio and Inorganic Carbon
The C/N ratios in the 0- to 5-cm and 5- to 15-cm depths of RMSs did not differ among postreclamation land uses because these soils were not disturbed for 28 yr since reclamation (Table 2). However, C/N ratio at lower depth (15–30 cm) was affected by land use. C/N ratio was lower in soil under forest than under grassland (pasture and hay). The C/N ratios in RMSs were similar to the soil under undisturbed forest but higher than that of agricultural soil. High C/N ratios in RMSs may be because of undisturbed soil since reclamation in 1978, unlike soil that is plowed in agricultural land use every year, encouraging mineralization of C and decrease in C/N ratio. Fertilizer application in the agricultural site may have caused an increase in plant N concentration, resulting in a decrease in C/N ratios. The C/N ratios decreased with increase in soil depths, which may be due to increase in SOM decomposition with increasing soil depth (Johnson et al., 1995). Knowledge of the C/N ratio indicates whether microbial processes in RMSs are constrained by deficiency of C or N. For adequate soil functions, a C/N ratio of 8:1 to 12:1 is optimal.

There was a sizable concentration of SIC in the RMSs, especially in hay and forest land uses. The SIC concentration was strongly stratified with depth (Fig. 1) and increased with increase in soil depth. The RMSs under hay and forest land uses had significantly higher SIC throughout the soil profile than that of pasture land use. Soil SIC in the 15- to 30-cm depth under forest (8.5 g kg–1) was significantly higher than that under pasture (0.01 g kg–1) (P < 0.001). The presence of higher SIC, especially at the lower depths of RMSs, is due to contamination with spoil materials at 30-cm depth, which needs to be considered while estimating organic C in RMSs. The SIC pools in 0- to 30-cm depth were 17.0, 12.0, and 0.07 Mg ha–1 in RMSs under forest, hay, and pasture, respectively. Low SIC pool in RMSs under pasture may be due to soil acidification effect of pasture land use. Little is known about SIC, and there are no known studies on SIC dynamics in RMS ecosystems. However, Mikhailova and Post (2006) observed significant effect of land use on SIC pools in the Russian Chernozem.

Relationship between Soil Organic Carbon and other Soil Properties and Root Biomass
The data on positive correlation between aggregate-associated SOC and TN concentrations with aggregate size fractions are in agreement with those reported by Grandy and Robertson (2006) (Fig. 5 ) and are indicative of the importance of aggregate size and stability on SOC pool and its stabilization. An increase in SOC and TN concentrations was associated with the increase in aggregate size fractions for all land uses (Fig. 5). The aggregate-associated SOC concentration was exponentially related to aggregate-size fractions in soil under undisturbed and agricultural land uses. The correlation coefficient was stronger in pasture (r = 0.93; P < 0.01) land use, followed by that in hay (r = 0.91; P < 0.05) and forest (r = 0.88; P < 0.05). A similar relationship was observed for aggregate-associated TN concentration and aggregate size class for all land uses (Fig. 5). The SOC concentration in RMSs was also related to dry root biomass, BD, and EC (Fig. 6 ). The increase in dry root biomass in RMSs was associated with greater accumulation of SOC (SOC concentration in g kg–1 = 5.34 x dry root biomass in Mg ha–1 + 16.6; P > 0.01). These results are in agreement with the study conducted in RMSs by Ussiri et al. (2006b). Fine roots from vegetation play a large role in the belowground C pool after disturbance (Schilling et al., 1999). However, Skinner et al. (2006) did not observe any relationship between SOC concentration and belowground productivity in 6-yr-old unmined soil under mixed pasture. Soil BD, a strong determinant of the SOC pool, was negatively correlated with SOC concentration in RMSs (BD in Mg m–3 = –0.003 x SOC in g kg–1 + 1.29; P > 0.01). These data indicated that changes in the BD influence changes in soil C fluxes among land uses (Gifford and Roderick, 2003). The increase in SOC concentration of RMSs in Ohio was also associated with the increase in EC (EC in µS cm–1 = 4.22 x SOC in g kg–1 + 48.2). Terra et al. (2004) reported for coastal plain field soil that EC could be used to differentiate zones of variable SOC concentration.


Figure 5
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  		Fig. 5. Relationship of soil aggregate size fraction (0- to 5-cm depth) with total nitrogen and organic carbon concentrations for different postreclamation land uses. *Significant at 0.01. **Significant at 0.02.

 

Figure 6
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  		Fig. 6. Relationship of soil organic carbon concentration with root biomass, bulk density, and electrical conductivity.

 

   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
Due to the significant area covered by mining activities and their environmental impacts, the ecosystem restoration of RMSs has important policy implications in Ohio and elsewhere. The present study showed that all land uses, such as hay, pasture, and forest, established in RMSs restored soil quality similar to or better than undisturbed natural forest over a period of 28 yr, and land uses in RMSs differed in their restorative effectiveness. The potential of SOC accumulation (for 30-cm depth) in grassland (hay and pasture) is about 62 Mg ha–1 over a period of 28 yr, which is 72% more than it would have been under a forest land use. If RMSs are converted to grasslands and the rate of SOC sequestration is applicable over 0.04 M ha of mine land in Ohio and 3.2 M ha in the USA, the potential C sequestration is 4.42 Tg in Ohio and 353.6 Tg in the USA over a 50-yr period. In addition to restoring degraded ecosystems, C sequestration in RMSs have ancillary benefits of reducing soil erosion, improving water quality, increasing biodiversity, and enhancing soil productivity. The quantification of the source-sink relationship of SOC pool needs to be studied for a better understanding of RMSs ecosystems and their restoration potential.


   ACKNOWLEDGMENTS
 
We thank Gary Kaster and Brian Cox, American Electrical Power (AEP); Chris Penrose, Associate Professor and Extension Educator, Agriculture, and Natural Resources and 4-H Youth Development; and Bill Maghes and Carl William, farmers, McConnelsville, OH, for providing access to the study sites and help during experiment. This research was supported by a grant from the Ohio Coal Development Office, Ohio Air Quality Development Authority.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 




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