Surface coal mining in the eastern USA disturbs hundreds of hectares of land
every year and removes valuable and ecologically diverse eastern deciduous
forests. Reclamation involves restoring the landscape to approximate original
contour, replacing the topsoil, and revegetating the site with trees and
herbaceous species to a designated post-mining land use. Re-establishing an
ecosystem of ecological and economic value as well as restoring soil quality
on disturbed sites are the goals of land reclamation, and microbial
properties of mine soils can be indicators of restoration success.
Reforestation plots were constructed in 2007 using weathered brown sandstone
or unweathered gray sandstone as topsoil substitutes to evaluate tree growth
and soil properties at Arch Coal's Birch River mine in West Virginia, USA.
All plots were planted with 12 hardwood tree species and subplots were
hydroseeded with a herbaceous seed mix and fertilizer. After 6 years, the
average tree volume index was nearly 10 times greater for trees grown in
brown (3853 cm
Surface mining in the eastern USA disturbs hundreds of hectares of land by removing ecologically diverse eastern deciduous forests, resulting in the disruption and degradation of underlying soil resources. In order to restore soil function and stability to these ecosystems (Brevik et al., 2015), a sufficient medium for plant growth must be re-established through reclamation. The most ideal material to place on the surface is the pre-existing soil material (Skousen et al., 2011; Zipper et al., 2013). Currently, brown and gray sandstones often are employed as substitutes for topsoil (Emerson et al., 2009; Skousen et al., 2011). The brown sandstone substitute materials have a pH from 4.5 to 5.5 compared to the gray sandstone materials with a pH of 7.5 to 8.0 (Wilson-Kokes et al., 2013a, b). These low pH conditions in brown mine soil are more conducive to tree growth, while the higher pH of gray mine soils is better for many seeded grasses (Zipper et al., 2013). Both mine soils tend to contain high levels of rock fragments, which translates into poor water-holding capacity and poor nutrient relations in these topsoil substitutes (Haering et al., 2004). However, the temporal dynamics of the physical, chemical, and biochemical properties of topsoil substitutes following reclamation are not sufficiently understood.
Microorganisms are known to play an important role in re-establishing soil organic matter content and restoring ecosystem services following surface mining or other land disturbances (Anderson et al., 2008; Machulla et al., 2005; Haney et al., 2008; Ingram et al., 2005; Zipper et al., 2011). However, their biochemical activities can be slow to re-establish (Chodak, 2009) and may take several decades to reach stable conditions normally found in native soils (Chatterjee et al., 2009; Insam and Domsch, 1988). Soil carbon amendments stimulate microbiological activity (Bendfeldt et al., 2001; Elkins et al., 1984; Lindemann et al., 1984), as can the establishment of herbaceous plants using the additions of fertilizers and lime (Chaudhuri et al., 2015). However, as easily decomposable organic matter is rapidly consumed, an overall decline of microbial activity results if no further additions of external nutrients and organic matter are applied because a reservoir of soil organic matter is lacking in newly created mine soils (Stephens et al., 2001; Stroo and Jencks, 1982). Compaction of soil materials and planting of competitive forage species have been shown to arrest the re-colonization of native hardwood tree species and slow natural succession on these reclaimed sites (Franklin et al., 2012; Groninger et al., 2007), which also tends to diminish microbial diversity and activity. Hydroseeding to introduce lime, fertilizer, mulch, and seed is already a common practice in surface mine reclamation, and using lower rates of seed, fertilizer and lime, as well as using different herbaceous species, aids the development of forests on these sites (Franklin et al., 2012; Showalter et al., 2010). Hydroseeding a non-competitive, tree-compatible herbaceous forage mix can still meet the requirements for soil stabilization and conditioning, and may also be useful for re-establishing microbial communities that aid in nutrient cycling (Zipper et al., 2011). The objective of this study was to determine tree growth and soil properties including microbial biomass carbon (MBC) and potentially mineralizable nitrogen (PMN) in brown and gray mine soils and to determine the influence of a hydroseed treatment on these properties.
Arch Coal's Birch River Operation is located near Cowen in Webster County,
West Virginia, approximately 100 km northeast of Charleston
(38
In January 2007, a 4.9 ha plot was created using two types of sandstone
overburden. Half of the area was constructed with weathered brown sandstone,
the other half with unweathered gray sandstone. Overburden materials were
end-dumped into conjoining piles that were approximately 1.5 m deep
throughout the plot. To limit compaction, a bulldozer made only one pass over
the piles to strike off the tops, resulting in approximately 1.2 m depth of
rough-graded material. In spring 2007, 12 species of tree seedlings were
purchased from the West Virginia State Tree Nursery and planted on 2.4 m
centers by Williams Forestry, a professional tree planting company, at a
stocking rate of 1680 trees per ha (Table 1). In the fall of 2008, both ends
of the plot were hydroseeded with a seed mix of compatible herbaceous species
at a rate of 36 kg ha
Survival and growth of trees as well as soil physical and chemical properties
was measured from 2007 to 2012 as reported in Wilson-Kokes et al. (2013b).
Tree growth was assessed by measuring height and stem diameter at 2.5 cm
above ground, and a tree volume index was calculated as
height
Soil samples were collected from the top 15 cm at four randomly selected
points along transects within each treatment combination in July of 2007 to
2012. No surface organic layer formed at this site as either a litter layer
or as an A horizon during the course of this study. Field soils were air
dried and sieved through a 2 mm sieve to separate the fine soil fraction
(< 2 mm) from the coarse or rock fraction (
Soil pH was measured in a 1 : 2 mixture of 5 g of soil and 10 mL of
distilled deionized water (DDI) water with a Fisher
Scientific Accumet pH meter model 915 (Thermo Fisher Scientific Inc.,
Pittsburgh, PA). Electrical conductivity (EC) was measured using a 1 : 2
mixture on a Mettler Toledo S230 EC meter (Mettler-Toledo International Inc.,
Columbus, OH). Nutrients were determined using a Mehlich 1 extraction
solution (0.05 M HCl and 0.025 M H
Species and number of trees planted in 2007 at the Arch–Birch River mine in Webster County, West Virginia.
Biochemical measurements were made only on soil samples taken in 2012. Total
carbon (TC) and nitrogen (TN) were measured with a Leco TruSpec CHN elemental
analyzer (LECO Corp., St Joseph, MI). A 0.10 g sample of air-dried soil was
weighed into foil cups and combusted at 950
Species and rate of application of ground cover hydroseeded in 2008 at the Arch–Birch River mine in Webster County, West Virginia.
Microbial biomass carbon (MBC) of the soil was determined using the
chloroform fumigation extraction method from Brookes and Joergensen (2006).
Two triplicate sets of 10 g (dry weight) samples of field moist soil were
weighed into 125 mL glass serum bottles. One set of triplicate samples was
designated as a control and did not undergo chloroform fumigation. Fumigated
samples were exposed to 2 mL of amylene-stabilized chloroform. Air-tight
rubber stoppers were used to cap bottles prior to creation of a vacuum by
pulling air from each bottle, followed by incubation for 24 h in the dark
prior to extraction. Both fumigated and control samples were extracted with
25 mL of 0.5 M K
Mean tree volume index
Detailed annual results of tree survival and growth on the site have been
previously reported (Wilson-Kokes et al., 2013b). In brief, these data showed
that after six growing seasons (2007–2012), tree survival was significantly
higher in brown mine soil treatments compared to gray mine soils (83 vs.
72 %; data not shown). Average tree volume index
(height
Traditionally, aggressive herbaceous vegetation is thought to out-compete
tree seedlings for nutrients, water, and solar energy when seeded at high
densities (Fields-Johnson et al., 2012). However, when tree-compatible
species such as birdsfoot trefoil (
Mean ground cover on four soil treatments in 2012 at the Arch–Birch River mine in Webster County, WV (Wilson-Kokes et al., 2013b).
Treatments with hydroseeding initially had significantly higher herbaceous
cover with 30 % for brown and 22 % for gray than those treatments
without hydroseeding (3 to 12 %, Table 4). Gray mine soils alone had the
lowest average total cover at 11 %, while all other treatment
combinations averaged 27 to 39 % total cover (Table 4). Perennial
ryegrass (
Birch River mine soils exhibited similar differences in physical and chemical
properties as those observed in other studies of brown and gray sandstone
mine soils (Angel et al., 2008; Emerson et al., 2009). The pH values for
brown mine soils had the lowest mean values of all treatment combinations,
5.0 for brown and 5.5 for brown-hydroseed (Table 5). Gray mine soils ranged
in pH from 7.0 to 7.9. The mean pH ranges for brown mine soils in this study
fell within the typical range for mine soils created from weathered
sandstones (pH 4.5 to 5.5), while the gray mine soils were slightly below to
within the typical range for unweathered sandstones (pH 7.5 to 8.0) (Haering
et al., 2004; Wilson-Kokes et al., 2013a; Zipper et al., 2013). Compared to
initial values measured in 2008, EC values measured in 2012 had not declined
substantially, with average ECs ranging from 0.06 to 0.12 dS m
2008 and 2012 soil properties of samples from four soil treatments at the Arch–Birch River mine in Webster County, WV (Wilson-Kokes et al., 2013b).
Percent fines were significantly greater for brown (47 to 58 %) vs. gray (25 to 42 %) mine soils, another common result in similar studies. Miller et al. (2010) found that gray sandstone rocks exhibited a higher durability in a slake-durability test and were highly resistant to weathering during the freeze–thaw test. Zipper et al. (2011) reported that mine soils derived from gray sandstone would continue to have higher coarse fragments and lower percent fines than brown sandstone mine soils as they age because of resistance to degradation. High percentages of coarse fragments reduce water-holding capacities, which can negatively influence a site's productivity and tree growth (Rodrigue and Burger, 2004).
Brown mine soils had significantly greater total C and soil organic C than
gray mine soils, while carbonate C and coal C were similar among mine soil
types (Table 6). Total N was not significantly different among treatments and
ranged from 68 to 104 mg kg
Hydroseed application had a significant effect on MBC in brown mine soils and
PMN in both mine soils (Table 7). MBC in hydroseed plots on brown mine soils
had a mass of 17.5 mg kg
Carbon and nitrogen fractions in mine soil samples after six growing seasons at the Birch River Operation in Webster County, WV.
Biochemical properties and ratios for mine soils after six growing seasons at the Birch River Operation in Webster County, WV.
Rice et al. (1996) suggested that the ratio of MBC to total C (MBC : TC), as well as PMN to total N (PMN : TN), may provide an index of soil organic matter dynamics and soil quality. Some have suggested that mine soils have a large capacity for C storage (Shukla and Lal, 2005; Ussiri et al., 2006; Wick et al., 2009a, b), and these ratios should gradually decline with maturation and time to a stable ratio. In agricultural soils, MBC normally constitutes from 1 to 4 % of the total C, while PMN comprises about 2 to 6 % of total N (Anderson and Domsch, 1989; Jenkinson, 1988; Sparling, 1992). In mine soils, caution should be exercised in using MBC : TC because C may exist as carbonate C and coal C. Over time, the carbonates and coal will weather and degrade, but initially high levels may be found in fresh mine soils. Therefore, in our study we used two ratios: MBC : TC and MBC to soil organic C (MBC : OC). The MBC : OC ratio should be more accurate to correct the error associated with overestimating C contents with C fractions that are not utilized by microorganisms.
The ratio of MBC : TC in brown mine soils was significantly lower than the other three treatments (0.46 vs. an average of 0.93 % for the others; Table 7). This result is seemingly contradictory because the brown mine soils with more total C (Table 6) should presumably have more MBC. However, coal C made up almost half of the total C in these mine soils (Table 6), thereby making almost half of the total C largely unavailable to microbial immobilization. To account for this factor, the ratio of MBC : OC should provide a better ratio to assess the percentage of utilizable C used for microbial biomass. Using this ratio, the brown treatment was still lowest at 1.08 % with brown hydroseed, gray was next (1.82 and 2.02 %, respectively), and the gray hydroseed was significantly higher at 2.52 %. These values are on the low side but within those reported for agricultural soils (1 to 4 %; Anderson and Domsch, 1989).
The PMN : TN ratios of non-hydroseed mine soils in our study were 70 to
80 % less than the PMN : TN ratio found in the hydroseed treatments
(Table 7), indicating that much less of the N found in the soil was in the
active fraction and was therefore not as available to plants and microbes
(Stephens et al., 2001). Even though fertilizing with
N–P
Hydroseed treatment had a significant effect on biochemical properties of the mine soils due to the earlier fertilization and seeding, which stimulated vegetation growth (Table 4). The resulting greater vegetation and litter provided available C and N for microflora and fauna that contributed to the higher MBC found in the hydroseed treatments. A higher PMN : TN indicated that more of the TN in hydroseed treatments was available to microbes. With this being the only difference between the treatments, we assume that herbaceous vegetation cover was an influential factor in MBC and PMN in these mine soils. Nitrogen-fixing legumes were introduced by hydroseed application, which could have influenced the PMN : TN ratio. The effect of legumes on N cycling is unclear since the total N content of the mine soils with and without hydroseed was not different and the PMN on hydroseed plots was significantly greater in both mine soils. The herbaceous cover contributed by seeded legumes was very low compared to the seeded grasses; therefore, it was assumed that the N differences were likely due to the earlier N fertilization.
Mummey et al. (2002) investigated biochemical properties and spatial relationships with plant communities and found that greater MBC, soil organic matter and N depletions were concentrated at the base of plant stems. A similar trend could be occurring at the Birch River site where non-hydroseed plots had little initial soil organic C and N to produce plant growth. Our sampling methods did not take this into account, as soil samples were taken randomly at varying distances from tree bases. We may have diluted our samples by mixing samples near trees with samples further away from trees, thereby resulting in lower MBC and PMN. Had we sampled closer to tree bases, which were more abundant in hydroseed areas, we may have found our MBC and PMN values on non-hydroseed plots to be more similar to our hydroseed treatments.
Vegetation that rapidly decomposes and is recycled in mine soils appears to be beneficial for microbial activity. Herbaceous cover may promote a more homogenous soil environment, which could promote root expansion by trees and shrubs (Mummey et al., 2002). Other studies on reclamation and biochemical properties have demonstrated the positive impact of mulching and other organic matter additions on biological soil properties of reclaimed mine sites (Anderson et al., 2008; Machulla et al., 2005; Pallavicini et al., 2014; Showalter et al., 2010).
Our study was conducted 6 years after trees were planted and hydroseeding was performed, and as such provided a snapshot evaluation of reclamation progress at the site. Future studies at this site documenting changes of biochemical properties over time would better assess the nutrient cycling capabilities and restoration of soil quality at the site. In this manner, the success of reclamation practices and the return of ecosystem services could be evaluated. Other studies have examined these dynamic biochemical properties over time and demonstrated the development and evolution of microbial populations and diversity, which were used to indicate reclamation success on a site (Akala and Lal, 2001; Anderson et al., 2008; Chaudhuri et al., 2015; Insam and Domsch, 1988). According to a chronosequence study of reclaimed sites in West Virginia by Stephens et al. (2001), reclaimed mines saw a pulse in microbial activity and biomass in the first 10 years after reclamation, followed by a steady decline during the following years. This pulse was attributed to the rapid consumption of nutrients provided by fertilizers and the quick turnover of herbaceous vegetation and cycling of nutrients. Without additional inputs of nutrients, the plant community was slower to develop; lower amounts of organic materials were added to the soil, thereby resulting in reduced organic matter pools and nutrient cycling because few readily available nutrients were available for plant and microbial uptake. While PMN and MBC were higher in the hydroseed treatment in this study, it is possible that this site was only experiencing a temporary pulse in microbial activity, which will decline as it approaches 10 years of age.
A diverse and active microbial population is essential for sustained primary
productivity in ecosystems. Microbes are responsible for the majority of
plant litter decomposition and facilitate nutrient cycling through
immobilization and mineralization of soil organic compounds. Any drastic land
disturbance dramatically alters and disrupts the integrity of the existing
plant community, removes the soil resources, and destroys the soil microbial
components of an ecosystem. In order to re-establish land capability,
reclamation practices must re-establish a soil resource capable of supplying
water and nutrients for the plant community and must provide the capacity to
support a soil microbial community. With appropriate soil replacement, soil
amendments and seeding, the process for developing an ecosystem on the site
begins and gradually ecosystem function and stability can occur with time and
maturation of the system. Reclamation practices that re-establish the soil,
rapidly introduce organic matter, and promote soil microbial populations
should be implemented. Brown mine soils were a better growth medium for trees
and had higher total and organic C contents compared to gray mine soils.
Hydroseeding at a rate of 35 kg ha
The authors thank Keith O'Dell of Arch Coal for establishing the plots and for his financial support, and Lindsey Bishop and Greg Klinger for their assistance in the field and laboratory. Funding for this project came from Arch Coal, Birch River, and from funds appropriated under the Hatch Act. Edited by: J. Vanderborght