Greenhouse gases (GHGs) trap the sun’s heat and keep the earth’s surface warm. Without this natural “greenhouse” effect, average temperatures at the earth’s surface would fall below freezing. However, since the past few decades, there have been noticeable increases in atmospheric concentrations of GHGs like carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFC’s), perfluorocarbons (PFC’s) and sulfur hexafluoride (SF6). Human activities that release GHGs into the atmosphere have been identified as responsible for this increase. Carbon dioxide, one of the major GHGs, is produced when generating energy from fossil fuels such as coal.
About 5.4 billion tons of coal is burned each year worldwide, and it is the single largest fuel source for the generation of electricity world wide. About 40% of the electricity in the world and 50% in U.S. comes from coal. Coal burning generates about a third of the world’s CO2 emissions. The carbon (C) emissions resulting from power generation have been implicated as a contributor to global warming. Coal is currently produced in 26 states in the U.S. (Fig. 1). However, coal mining has a number of environmental impacts. In the United States alone, coal mining and coal use emitted 2104 Teragram CO2 equivalent in 2004. Such anthropogenic perturbations of the global C cycle directly affect global climate and ecosystem functions. The CO2 released to the atmosphere from fossil fuel combustion, mining, and other anthropogenic activities is widely documented and accepted as a principal cause of the projected increases in GHG emissions resulting in climate change. However, coal’s low cost compared to other energy sources encourages its use in the countries with large deposits.
Mining causes drastic change to the original soil profile and loss of soil organic carbon (SOC) (Fig. 2). During the process of mining, the topsoil (about 30 centimeters [cm]) is removed and stored separately. The overburden, composed of rock and heavy geologic material on top of coal, is then removed and placed into already mined pits. During the process of reclamation, the overburden is graded and topsoil is put back on top of the overburden to a depth of usually 30cm. The topsoil is graded to the original contour of the land. An initial dose of fertilizers and mulch is then applied before seeding the land with a mixture of grasses and/or legumes. The land under reclamation essentially remains undisturbed.
Potential for Carbon Sequestration
Reclamation of mine soils (RMSs) leads to the establishment of biomass, which in turn results in the accretion of SOC to the restoration of the mine ecosystem. The SOC thus accumulated in RMSs, not only replenishes SOC losses but may also offset CO2 emission from coal mining and use. The soil C sequestration in RMSs depends on factors and processes that determine net primary productivity and its addition to the soil, and those that affect soil organic matter (SOM) accretion and decomposition in the soil. The net balance of C input (via plant litter) and C loss (via decomposition) reflects the changes in the SOC pool of RMSs. To elicit a gain in C storage, therefore, there must be an increase in the amount of C entering the soil as plant residues, and a decrease in the rate of SOM decomposition. Realizing the potential of RMSs as a sink for C requires understanding of soil processes that affect SOC sequestration.
The less fertile mine soil, which is in its infant stage of soil development, offers greater opportunity for the recycling of organic waste material and the maximization of C sequestration potential than that of agricultural soils through adoption of land use with high C sequestration potential. Natural recovery of mine soil can take as long as 200 years. Therefore, proper reclamation of mine soil is an important process to sequester large amounts of atmospheric C and return land to a stable state.
Potential C sequestration for RMSs has not been estimated on the national scale. However, studies at C-MASC have shown that SOC sequestration potential of the RMSs ranges from 18 to 30 Mg C ha-1 for 0 to 30cm soil depth, at an average rate of 0.7 to 3 Mg C ha-1 yr-1.
Potential Carbon Dioxide Offset from Reclaimed Mine Soils
|Table 1. Potential CO2 offset rate from different land uses established in reclaimed mine soils.|
|Potential land uses||Potential CO2 offset rate (Mg ha-1 yr-1)||Estimated using data from|
|Forest||biomass||6.35||Kant and Kreps, 2004|
|5.451||Akala and Lal, 2000|
|8.812||Ussiri and Lal, 2006|
|8.753||Singh et al., 2006|
|total 4||9.40||Sperow, 2006|
|soil||5.391||Akala and Lal, 2000|
1 0-30 cm depth
2 Black locust forest for 0-50 cm depth
30-20 cm depth under 5 year old Albizia lebbeck, and A. procera plantation
4Total includes soil, biomass and litter
Reclaimed mine soils represent a huge potential sink for CO2. Potential average CO2 offset rates by mine soil C sequestration are 6.32, 5.32, 3.56 Mg ha-1 yr-1 in forest, pasture and agriculture ecosystem, respectively (Table 1). However, the potential CO2 sequestration rate in forest ecosystem (includes biomass, soil and litter) is 9.4 Mg ha-1 yr-1.
If we estimate that reclaimed forest mine soils in USA (3.2 million hectares) can offset 30 Teragrams (Tg) of CO2 each year (estimated based on the C sequestration rate of 2.56 Mg ha-1 yr-1 for forest ecosystem from Sperow, 2006), RMSs in the United States could offset approximately 1.5 Petagrams (Pg) of CO2 produced by coal combustion over 50 years.
Management Options to Achieve Maximum Potential Carbon Sequestration and Offset Carbon Dioxide Emissions form Coal Mining and Use
Researchers and scientists are studying into ways RMSs can be best managed to capture C from the atmosphere. Proper consideration should be given at different steps in the process of reclaiming mine soil as outlined in Table 2 to enhance C sequestration in mine soil.
Grading, topsoiling and soil compaction
In order for mine soil reforestation to be successful, it is essential that the top soil material have chemical, physical, and biological properties that are suitable for plant or grass species to be established, that surface materials have sufficient depth for rooting (at least one meter (m) is recommended for forest species and 0.3 m for grass), and that the top soil is placed on the surface without excessive compaction by mining and reclamation equipment. The reclamation of mine soil should involve final grading limited to the extent needed for the stability of slopes. Once the spoil material is placed, heavy equipment should not pass over it except for a final grading.
The RMSs must not be compacted to ensure successful establishment and growth of vegetation. Soil compaction in RMSs occurs due to use of heavy equipments during the reclamation process. Compacted minesoil reduces water infiltration, plant available water, root growth, soil structure, pore space for water movement, and inhibits plant growth. Natural soil processes that can reduce soil compaction in RMSs are slow. Therefore, practices that reduce soil compaction need to be introduced. Compaction can be minimized by grading when soil is dry, by using small dozers, and planting deep-rooted perennials. The proper storage and replacement of topsoil over spoil for at least 30 to 50 cm depth is one of the most crucial activities for assuring establishment of vegetation. Mixing of topsoil with topsoil substitute and inoculation with beneficial soil microbes (like N-fixing and P-solubilizing microbes) during final grading can improve the quality of growth medium. Top soil storage for long period prior to respreading will cause seeds to lose viability. Therefore, where feasible, topsoil should be moved directly from the mining area to the reclamation area. Using native soil for top soiling may function as a donor seed bank that provides an important source of genetic plant material. Subsoiling can be done before planting seed to break up compacted soil layers. This improves drainage, water infiltration, and aerates subsoil layers to encourage root penetration. The depth of sub-soiling can range from 0.3 to 0.6 m. Sub-soiling works best in dry soil. Reducing soil compaction increases root development and SOC sequestration in RMSs through increased biomass productivity.
Species selection, seeding, and planting
|Table 2. Management options to increase C sequestration in surface mine soil|
Success in establishing vegetation quickly and cost-effectively is the requirement of successful mine reclamation and is essential to mine profitability. The keys to a successful vegetation program are the selection of plant species that are suited to the properties of that minesoil, quick establishment of plant cover, control of erosion, and persistence through time. Vegetation to be seeded or planted on RMSs can be native or introduced. However, selected vegetation should minimize erosion and return the land to a productive use. Seed mixture is commonly used for establishing vegetation in RMSs. Composition of a seed mixture for specific mine reclamation site can be obtained from Public Affairs Office, Office of Surface Mining. Effective erosion control can be achieved by less competitive ground covers of annual grasses including foxtail millet (Setaria italica L.) and rye (Secale cereale L.), the perennial ryegrass (Lolium perenne L.) and redtop (Agrostis gigantean L.), and of legume species kobe lespedeza (Lespedeza striata var. Kobe) and birdsfoot trefoil (Lotus corniculatus L.). Among woody species, white pine (Pinus strobus L.) is widely planted because it is well adapted to acidic soils and grows quickly to meet the 5-year bond release requirement. Plants with rapid biomass accumulation are efficient in C sequestration. Existing vegetation types around the mine sites can be selected to represent the diversified plant community in RMSs. Test plots and extension/outreach sites can be established to select and demonstrate species suitable for the specific location and climate. However, vegetation on RMSs may not be successful if plant rooting is restricted by severe soil compaction and by the presence of acid or toxic materials. The tolerance of plant species to acidic or toxic mine soil conditions varies among species. For example, ryegrass (Lolium perenne L.) is tolerant to Cu toxicity and is suitable for metal mine tailings. Grasses such as deer-tongue grass (Panicum clandestinum L.) and switchgrass (Panicum virgatum L.) tolerate more acidic conditions than tall fescue (Festuca arundinacea L.) and perennial ryegrass (Lolium perenne L.). Kenland and Pennscott varieties of red clover (Trifolium pratense L.) are more adapted to acid mine spoil conditions than Chesapeake and Mammoth varieties.
Studies have shown that RMSs can establish a diverse native community fairly quickly, if appropriate soil management and reclamation measures are used. Most bioenergy crops are also adaptable to RMSs. The U.S. DOE has identified species capable of alleviating energy constraints and sequestering C as perennial herbaceous crops and short-rotation woody crops. Perennial crops are highly productive with a high capacity to sequester C over a 40 to 60 year period. In addition, the massive and deep-root systems in perennial bioenergy crops allow direct transfer of SOC into the subsoil, making it less prone to mineralization. For example, switchgrass has 4 to 5 times more belowground biomass than corn (Zea mays L.) and can add up to 2.2 Mg C ha-1 yr?1.
Vegetation aids in stabilizing the soil surface from erosion, enhances sequestration of atmospheric CO2 into soil, increases SOM and biological activity, improves physical, chemical and biological quality of RMSs, and enhances profile development. Vegetation is also a factor in the hydrologic cycle and affects both surface and groundwater. Therefore, proper revegetation or land use adopted after grading of RMSs is an important step in the overall reclamation process and plays a major role in restoring the quality of RMSs.
The establishment of vegetation and C sequestration in RMSs are affected by top-soil depth, soil compaction, soil toxicity, vegetation species selected, seedbed preparation, time of seeding and seeding rates, fertilizers and soil amendments, mulching, and weed competition. Most native hardwood species grow well on RMSs. The critical factors that affect survival and growth of trees are the quality of top soil and spoil, soil compaction, slope, aspect, position and competition from ground cover grasses. A key to successful reforestation with hardwood is selection of the species suited to RMSs. For example, sycamore (Platanus occidentalis L.), green ash (Fraxinus pennsylvanica L.), and red maple (Acer rubrum L.) are more tolerant of sites that are compacted, poorly drained, or have minesoils primarily derived from siltstones or shales.
Available nutrients and fertilizer recommendations
Nutrient deficiencies are one of the most universal attributes of RMSs. All new many old RMSs require significant fertilizer application for the establishment and maintenance of vegetation. So the nutrient required to sustain plant growth over time must come from initial fertilization and subsequent symbiotic N-fixation by legumes.
Poor correlation of soil tests with actual plant growth coupled with extreme variability in rock, gravel type and amount, and mineral solubility make fertilizer recommendation very difficult in RMSs. Fertilizer recommendation depends on available nutrients, pH, soil type, and loss. However, N application rate for new establishment should not exceed 170 kg ha-1 to avoid suppression of legumes, but not less than 85 kg ha-1 to support the grass establishment. The P application rate should be from 250 to 300 kg ha-1 and K application rate 100 to 125 kg ha-1 for new seedlings.
Soil amendments provide a better medium for plant growth by increasing macro- or micro-nutrients and SOM, bringing soil pH towards neutral, improving hydrologic properties, and reducing erosion. Common soil amendments used in RMSs are as follows:
Biosolids: RMSs are generally deficient in nutrients (especially N and P) and SOC. Spoil materials usually have unfavorable chemical (such as high pH and low cation exchange capacity [CEC]) and physical (high bulk density, poor soil structure) characteristics. These can be improved by the addition of organic materials or biosolids, which are the nutrient-rich organic materials resulting from the treatment of domestic sewage at a wastewater treatment facility. They improve the physical and chemical soil environment, enhance the ability of RMSs to increase biomass growth and consequent C sequestration. The process of tree growth using nutrients from organic waste converts into C with long residence time in wood and its products. The C within these wood products can then be included in sequestered C calculations for the mining company.
Compost, manure: These organic amendments provide a ready source of C and usually N for microorganisms in the RMSs. Manure from livestock waste provides a valuable source of SOM and nutrients. Manure should be plowed into the soil after application. Part of the manure readily decomposes and provides both C and N to soil microorganisms and N to higher plants. If salinity is a problem in RMSs, manure application should be carefully assessed as it might exacerbate the salt problem. Compost is derived from the biological decomposition of any solid waste. Application of compost improves micronutrient fertility, soil structure, water infiltration rate, water holding capacity, and CEC, and decreases bulk density of RMSs.
Mulching: Mulches are organic materials applied to the soil surface, usually after seeding the RMSs. The common mulches include wood residue or chips, straw, and native hay. Mulching provides temporary surface cover and improves soil microclimate for vegetation establishment. Application of surface mulch in RMSs improves soil water-holding capacity for revegetation efforts, improves plant stand, minimizes water loss, and increases availability of nutrients. Straw mulch applied at the rate of 6 Mg ha-1 to the surface after sowing grasses and legumes can increase plant stand. Studies have shown that application of straw and wood fiber mulches together greatly improves plant establishment and long-term vigor.
Liming: If the soil pH of RMSs is less than 4.0 and the soluble salts are high, it requires liming. In general, RMSs are fairly coarse textured and low in buffering capacity, so applications of agricultural lime at 2.5 to 12.4 Mg ha-1 (depending on initial pH) are typically sufficient to achieve a target pH of 6.5 for hay or pasture land uses. However, liming rates for RMSs are based on soil pH and vegetation species. Forest species, particularly pines, thrive at lower pH levels. Liming above pH 5.5 should be avoided.
Coal-combustion byproduct: Coal-combustion byproducts such as fly ash, bottom ash and flue gas desulfurization (FGD) can be used for restoring RMSs. These products neutralize soil acidity, provide micronutrients necessary for plant growth, increase drought tolerance, improve texture, infiltration rates, and water-holding capacities of extremely coarse textured and nutrient-poor RMSs.
Usually additional water is required in RMSs after seeding to enhance seed germination, plant establishment, and biomass growth. Generally up to 3.8 cm of irrigation water is applied in spring or early summer. The decision to use artificial water sources must be carefully evaluated. Irrigation is most important during the first few growing seasons. Sometimes, it may be less expensive to reseed a stand and wait for the next wet season than to set up an irrigation system.
Establishing ground cover to control soil erosion is the first objective of any land reclamation effort. Reclamation of coal-mine sites demands special consideration of challenging environments, particularly in mountainous areas. The most commonly used erosion control practices used in RMSs are rock riprap, lining with a jute mat (to prevent erosion until the grass becomes established), concrete drainage channels, and large rock-lined drains (to channel water down steep hillsides of mining area). Short-term degradable erosion control blankets consisting of an evenly distributed layer of 100% agricultural straw stitched to a single or double netting structure can be used to provide erosion control and assist with vegetation establishment for up to 12 months on RMSs.
These management practices lead to successful restoration of vegetation diversity and enhance biomass productivity in RMSs. The process of biomass growth converts the atmospheric CO2 into longer term wood and C products which are stored above and below ground within the tree increasing the potential C sink. The C within these wood products can then be included in sequestered C calculations for the mining company. Therefore, these C sequestration activities can off-set emissions from mining activities. The C-MASC, the Ohio State University in collaboration with Chicago Climate Exchange (CCX), a voluntary, legally binding commitment to reduce GHGs, is developing a protocol to trade credits of C sequestered in RMSs. Trading of C credits can provide incentive for mine landowners to adopt management systems which increase C sequestration in RMSs.
Current Carbon Sequestration Activities in the United States
The strategy of C sequestration is in accord with the President's Global Climate Change Initiative. It also supports the goals of the International Framework on Climate Change. Different federal agencies, state reforestation programs, academic research institutions and projects, private sectors, and non-governmental organizations are engaged in enhancing C sequestration in RMSs.
The Department of Energy (DOE) is funding C sequestration projects in RMSs to explore C sequestration potential of terrestrial ecosystem, and the degree to which C sequestration in RMSs can offset CO2 emissions and provide additional income to landowners through trading of C credits. DOE through National Energy Technology Laboratory (NETL) has formed a nationwide network of regional partnerships called Regional Carbon Sequestration Partnerships in the United States. These partnerships engage state agencies, universities, and private companies to create a nationwide network that will help determine the best approaches for terrestrial C sequestration. The DOE Center for Research on Enhancing C Sequestration in Terrestrial Ecosystems (CSiTE) focuses on soil disturbed by mining, highway construction, or poor management practices. The Office of Surface Mining (OSM) has entered into a memorandum of understanding (MOU) with the DOE, which establishes a framework for cooperation between OSM and DOE’s Office of Fossil Energy to promote a market-based approach to reclaim abandoned mine lands through reforestation. OSM’s Mid-Continent Regional Coordinating Center has also developed a Reforestation Initiative to examine methods to promote post-mining land use involving reforestation on active and abandoned surface coal mines.
State reforestation program establishes forests in abandoned mine lands of Alabama, Colorado, Indiana, Kentucky, Ohio (funded by DOE and Ohio Air Quality Development Authority), Oklahoma, Pennsylvania, and Virginia to enhance C sequestration. Academic research institute and projects (The Ohio State University, Stephen F. Austin State University, West Virginia University), national laboratories (Oak Ridge National Laboratory, Pacific Northwest National Laboratory, Los Alamos National Laboratory), non-governmental organizations (The Nature Conservancy) and private sectors (Allegheny Energy, American Electric Power, Cinergy) are also actively participating in enhancing C sequestration in RMSs. These GHG mitigation activities in RMSs conducted by different institutions will help slow GHG emissions in the United States.
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